Nathalie Camelin
email: nathalie.camelin@univ-avignon.fr

Boris Detienne
email: boris.detienne@univ-avignon.fr

Stéphane Huet
email: stephane.huet@univ-avignon.fr

Dominique Quadri
email: dominique.quadri@univ-avignon.fr

Fabrice Lefèvre
email: fabrice.lefevre@univ-avignon.fr

Unsupervised Concept Annotation Using Latent Dirichlet Allocation and Segmental Methods

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Introduction

Spoken dialogue systems in the field of information query are basically used to interface a database with users using speech. When probabilistic models are used in such systems, good performance can only be reached at the price of collecting a lot of field data, which must be transcribed and annotated at the semantic level. It becomes then possible to train efficient models in a supervised manner. However, the annotation process is costly and as a consequence represents a real difficulty hindering the widespread development of these systems. Therefore any means to avoid it would be profitable as portability to new tasks, domains or languages would be greatly facilitated.

To give a full description of the architecture of a dialogue system is out of the scope of this paper. Instead we limit ourselves to briefly recall that once a speech recognizer has transcribed the signal it is common (though avoidable for very simple tasks) to use a module dedicated to extract the meaning of the user's queries. This meaning representation is then conveyed to an interaction manager that decides upon the next best action to perform considering the current user's input and the dialogue history. One of the very first steps to build the spoken language understanding (SLU) module is the identification of literal concepts in the word sequence hypothesised by the speech recogniser. An example of a semantic representation in terms of literal concept is given in Figure 1. Once the concepts are identified they can be further composed to form the overall meaning of the sentence, for instance by means of a tree representation based on hierarchical semantic frames.

To address the issue of concept tagging several techniques are available. Some of these techniques now classical rely on probabilistic models, that can be either discriminative or generative. Among these, the most efficiently studied this last decade are: hidden Markov models, finite state transducers, maximum entropy Markov models, support vector machines, dynamic fields (CRF). In [START_REF] Hahn | Comparing stochastic approaches to spoken language understanding in multiple languages[END_REF] it is shown that CRFs obtain the best performance on a tourist information retrieval task in French (ME-DIA [START_REF] Bonneau-Maynard | Semantic annotation of the french media dialog corpus[END_REF]), but also in two other comparable corpora in Italian and Polish.

To be able to apply any such technique, basic con- cept units have to be defined by an expert. In the best case, most of these concepts can be derived straightforwardly from the pieces of information lurking in the database tables (mainly table fields but not exclusively). Some others are general (dialogic units but also generic entities such as number, dates, etc). However, to provide an efficient and usable information to the reasoning modules (the dialogue manager in our case) concepts have to be fine-grained enough and application-dependent (even general concepts might have to be tailored to peculiar uses). To that extent it seems out of reach to derive the concept definitions using a fully automatic procedure. Anyhow the process can be bootstrapped, for instance by induction of semantic classes such as in [START_REF] Siu | Semi-automatic acquisition of domain-specific semantic structures[END_REF] or [START_REF] Iosif | Unsupervised combination of metrics for semantic class induction[END_REF]. Our assumption here is that the most time-consuming parts of concept inventory and data tagging could be obtained in an unsupervised way even though a final (but hopefully minimal) manual procedure is still required to tag the classes so as to manually correct automatic annotation.

Unlike the previous attempts cited above which developed ad hoc approaches, we investigate here the use of broad-spectrum knowledge extraction methods. The notion most related to that of concept in SLU is the topic, as used in information retrieval systems. Anyhow for a long time, the topic detection task was limited to associate a single topic to a document and thus was not fitted to our requirements. The recently proposed LDA technique allows to have a probabilistic representation of a document as a mixture of topics. Then multiple topics can co-occur inside a document and the same topic can be repeated. From these characteristics it is possible to consider the application of LDA to unsupervised concept inventory and concept tagging for SLU. A shortcoming is that LDA does not modelize at all the sequentiality of the data. To address this issue we propose to conclude the procedure with a final step to introduce specific constraints for a correct segmentation of the data: the assignments of topics proposed by LDA are modified to be more segmentally coherent.

The paper is organised as follows. Principles of automatic induction of semantic classes are presented in Section 2, followed by the presentation of an induction system based on LDA. The additional step of segmentation is presented in Section 3 with two variants: stochastic word alignment (GIZA) and integer linear programming (ILP). Then evaluations and results are reported in Section 4 on the French MEDIA dialogue task.

Automatic induction of semantic classes 2.1 Context modeling

The idea of automatic induction of semantic classes is based on the assumption that concepts often share the same context (syntactic or lexical). Implemented systems are based on the observation of cooccurring words according to two different ways. The observation of consecutive words (bigrams or trigrams) enables the generation of lexical compounds supposed to follow syntactic rules. The comparison of right and left contexts considering pairs of words enables to cluster words (and word compounds) into semantic classes.

In [START_REF] Siu | Semi-automatic acquisition of domain-specific semantic structures[END_REF] and [START_REF] Pargellis | Metrics for measuring domain independence of semantic classes[END_REF], iterative systems are presented. Their implementations differ in the metrics chosen to evaluate the similarity during the generation of syntactic rules and semantic classes, but also in the number of words taken into account in a word context and the order of successive steps (which ones to generate first: syntactic rules or semantic classes?). An iterative procedure is executed to obtain a sufficient set of rules in order to automatically extract knowledge from the data.

While there may be still room for improvement in these techniques we decided instead to investigate general knowledge extraction approaches in order to evaluate their potential. For that purpose a global strategy based on an unsupervised machine learning technique is adopted in our work to produce semantic classes.

Implementation of an automatic induction system based on LDA

Several approaches are available for topic detection in the context of knowledge extraction and information retrieval. They all more or less rely on the projection of the documents of interest in a semantic space to extract meaningful information. However, as the considered spaces (initial document words and latent semantics) are discrete the performance of the proposed approaches for the topic extraction tasks are pretty unstable, and also greatly depend on the quantity of data available. In this work we were motivated by the recent development of a very attractive technique with major distinct features such as the detection of multiple topics in a single document. LDA [START_REF] Blei | Latent dirichlet allocation[END_REF] is the first principled description of a Dirichlet-based model of mixtures of latent variables. LDA will be used in our work to annotate the dialogue data in terms of topics in an unsupervised manner. Then the relation between automatic topics and expected concepts will be addressed manually.

Basically LDA is a generative probabilistic model for text documents. LDA follows the assumption that a set of observations can be explained by latent variables. More specifically documents are represented by a mixture of topics (latent variables) and topics are characterized by distributions over words. The LDA parameters are {α, β}. α represents the Dirichlet parameters of K latent topic mixtures as α = [α 1 , α 2 , . . . , α K ]. β is a matrix representing a multinomial distribution in the form of a conditional probability table β k,w = P (w|k). Based on this representation, LDA can estimate the probability of a new document d of N words d = [w 1 , w 2 , . . . , w N ] using the following procedure.

A topic mixture vector θ is drawn from the Dirichlet distribution (with parameter α). The corresponding topic sequence κ = [k 1 , k 2 , . . . , k N ] is generated for the whole document accordingly to a multinomial distribution (with parameter θ). Finally each word is generated by the word-topic multinomial distribution (with parameter β, that is p(w i |k i , β)). After this procedure, the joint probability of θ, κ and d is then:

p(θ, κ, d|α, β) = p(θ|α) N i=1 p(k i |θ)p(w i |k i , β) (1)
To obtain the marginal probability of d, a final integration over θ and a summation over all possible topics considering a word is necessary:

p(d|α, β) = p(θ|α)   N i=1 k i p(k i |θ)p(w i |k i , β)   (2)
The framework is comparable to that of probabilistic latent semantic analysis, but the topic multinomial distribution in LDA is assumed to be sampled from a Dirichlet prior and is not linked to training documents. This approach is illustrated in Figure 2.

Training of the α and β parameters is possible using a corpus of documents, with a fixed number of topics to predict. A variational inference procedure is described in [START_REF] Blei | Latent dirichlet allocation[END_REF] which alleviates the intractability due to the coupling between θ and β in the summation over the latent topics. Once the parameters for the Dirichlet and multinomial distributions are available, topic scores can be derived for any given document or word sequence.

In recent years, several studies have been carried out in language processing based on LDA. For instance, [START_REF] Tam | Unsupervised language model adaptation using latent semantic marginals[END_REF] worked on unsupervised language model adaptation; [START_REF] Celikyilmaz | Lda based similarity modeling for question answering[END_REF] ranked candidate passages in a questionanswering system; [START_REF] Phan | Learning to classify short and sparse text & web with hidden topics from large-scale data collections[END_REF] implemented LDA to classify short and sparse web texts. The outer plate represents documents, while the inner plate represents the repeated choice of topics and words within a document.

where p(z n | θ) is simply θ i for the unique i such that z i n = 1. Integrating over θ and summing over z, we obtain the marginal distribution of a document:

p(w | α, β) = p(θ | α) N ∏ n=1 ∑ z n p(z n | θ)p(w n | z n , β) dθ.
(3)

Finally, taking the product of the marginal probabilities of single documents, we obtain the probability of a corpus:

p(D | α, β) = M ∏ d=1 p(θ d | α) N d ∏ n=1 ∑ z dn p(z dn | θ d )p(w dn | z dn , β) dθ d .
The LDA model is represented as a probabilistic graphical model in Figure 1. As the figure makes clear, there are three levels to the LDA representation. The parameters α and β are corpuslevel parameters, assumed to be sampled once in the process of generating a corpus. The variables θ d are document-level variables, sampled once per document. Finally, the variables z dn and w dn are word-level variables and are sampled once for each word in each document.

It is important to distinguish LDA from a simple Dirichlet-multinomial clustering model. A classical clustering model would involve a two-level model in which a Dirichlet is sampled once for a corpus, a multinomial clustering variable is selected once for each document in the corpus, and a set of words are selected for the document conditional on the cluster variable. As with many clustering models, such a model restricts a document to being associated with a single topic. LDA, on the other hand, involves three levels, and notably the topic node is sampled repeatedly within the document. Under this model, documents can be associated with multiple topics.

Structures similar to that shown in Figure 1 are often studied in Bayesian statistical modeling, where they are referred to as hierarchical models (Gelman et al., 1995), or more precisely as conditionally independent hierarchical models (Kass and Steffey, 1989). Such models are also often referred to as parametric empirical Bayes models, a term that refers not only to a particular model structure, but also to the methods used for estimating parameters in the model (Morris, 1983). Indeed, as we discuss in Section 5, we adopt the empirical Bayes approach to estimating parameters such as α and β in simple implementations of LDA, but we also consider fuller Bayesian approaches as well. [START_REF] Blei | Latent dirichlet allocation[END_REF]). The grey circle is the only observable variable.

In our work LDA is employed to annotate each user's utterance of a dialogue corpus with topic. Utterances longer than one word are included in the training set as its sequence of words. Once the model has been trained, inference on data corpus assigns the topic with the highest probability to each word in a document. This probability is computed from the probability of the topic to appear in the document and the probability of the word to be generated by the topic. As a consequence we obtain a full topic annotation of the utterance.

Notice that LDA considers a user utterance as a bag of words. This implies that each topic is assigned to a word without any consideration for its immediate context. An additional segmental process is required if we want to introduce some context information in the topic assignment.

3 Segmental annotation

Benefits of a segmental annotation

The segmental annotation of the data is not a strict requirement for language understanding. Up to quite recently, most approaches for literal interpretation were limited to lexical-concept relations; for instance this is the case of the Phoenix system [START_REF] Ward | Understanding Spontaneous Speech[END_REF] based on the detection of keywords. However in an NLP perspective, the segmental approach allows to connect the various levels of sentence analysis (lexical, syntactic and semantic). Even though, in order to simplify its application, segments are generally designed specifically for the semantic annotation and do not have any constraint on their relation with the actual syntactic units (chunks, phrasal groups, etc). To get relieved of such constraints not only simplifies the annotation process itself but as ultimately the interpretation module is to be used inside a spoken dialogue system, data will be noisy and generally bound the performance of the syntactic analysers (due to highly spontaneous and ungrammatical utterances from the users, combined with errors from the speech recognizer).

Another interesting property of segmental approach is to offer a convenient way to dissociate the detection of a conceptual unit from the extraction of its associated value. The value corresponds to the normalisation of the surface form (see last column in 1); for instance if the segment "not more than" is associated to the concept comparative-payment, its value is "less than". The same value would be associated to "not exceeding" or "inferior to". Value extraction requires a link between concepts and words based on which the normalisation problem can be addressed by means of regular expressions or concept-dependent language models (even allowing integrated approaches such as described in [START_REF] Lefèvre | Dynamic bayesian networks and discriminative classifiers for multi-stage semantic interpretation[END_REF]). In the case of global approaches (not segmental), value extraction must be dealt with directly at the level of the conceptual unit tagging, as in [START_REF] Mairesse | Spoken language understanding from unaligned data using discriminative classification models[END_REF]. This additional level is very complex (as some values may not be enumerable, such as numbers and dates) and is only affordable when the number of authorised values (for the enumerable cases) is low.

To refine the LDA output, the topic-to-word alignment is discarded and an automatic procedure is used to derive the best alignment between topics and words. While the underlying probabilistic models are pretty comparable, the major interest of this approach is to separate the tasks of detecting topics and aligning topics with words. It is then possible to introduce additional constraints (such as locality, number of segments, limits on repetitions etc) in the latter task which would otherwise hinder topic detection. Conversely the alignment is self-coherent and able to question the associations proposed during topic detection with respect to its own constraints only. Two approaches were designed to this purpose: one based on IBM alignment models and another one based on integer linear optimisation.

Alignment with IBM models (GIZA)

Once topic assignments for the documents in the corpus have been proposed by LDA, a filtering process is done to keep only the most relevant topics of each document. The χ max most probable topics are kept according to the probability p(k|w i , d) that topic k generated the word w i of the document d. χ max is a value fixed empirically according to the expected set of topics in a sentence. Then, the obtained topic sequences are disconnected from the words. At this point, the topic and word sequences can be considered as a translation pair to produce a word-topic parallel corpus. These data can be used with classical approaches in machine translation to align source and target sentences at the word level. Since these alignment models can align several words with a single topic, only the first occurrence is kept for consecutive repetitions of the same topic. These models are expected to correct some errors made by LDA, and to assign in particular words previously associated with discarded topics to more likely ones.

In our experiments the statistical word alignment toolkit GIZA++ [START_REF] Franz | A systematic comparison of various statistical alignment models[END_REF] is used to train the so-called IBM models 1-4 as well as the HMM model. To be able to train the most informative IBM model 4, the following training pipeline was considered: 5 iterations of IBM1, 5 iterations of HMM, 3 iterations of IBM3 and 3 iterations of IBM4. The IBM4 model obtained at the last iteration is finally used to align words and topics. In order to improve alignment, IBM models are usually trained in both directions (words towards concepts and vice versa) and symmetrised by combining them. For this purpose, we resorted to the default symmetrization heuristics used by MOSES, a widely used machine translation system toolkit [START_REF] Koehn | Moses: Open source toolkit for statistical machine translation[END_REF].

Alignment with Integer Linear

Programming (ILP)

Another approach to the re-alignment of LDA outputs is based on a general optimisation technique. ILP is a widely used tool for modelling and solving combinatorial optimisation problems. It broadly aims at modelling a decision process as a set of equations or inequations (called constraints) which are linear with regards to so-called decision variables.

An ILP is also composed of a linear objective function. Solving an ILP consists in assigning values to decision variables, such that all constraints are satisfied and the objective function is optimised. We refer to [START_REF] Der-San Chen | Applied Integer Programming: Modeling and Solution[END_REF] for an overview of applications and methods of ILP. We provide two ILP formulations for solving the topic assignment problem related to a given document. They both take as input data an ordered set d of words w i , i = 1...N , a set of K available topics and, for each word w i ∈ d and topic k = 1...K, the natural logarithm of the probability p(k|w i , d) that k is assigned to w i in the considered document d. Model [ILP ] simply finds the highest-probability assignment of one topic to each word in the document, such that at most χ max different topics are assigned.

[ILP ] : max

N i=1 K k=1 log(p(k|w i , d)) x ik (3) K k=1 x ik = 1 i (4) y k -x ik ≥ 0 i, k (5)
K k=1 y k ≤ χ max (6) x ik ∈ {0, 1} i, k y k ∈ {0, 1} k
In this model, decision variable x ik is equal to 1 if topic k is assigned to word w i , and equal to 0 otherwise. Constraints (4) ensure that exactly one topic is assigned to each word. Decision variable y k is equal to 1 if topic k is used. Constraints (5) force variable y k to take a value of 1 if at least one variable x ik is not null. Moreover, Constraints (6) limit the total number of topics used. The objective function (3) merely states that we want to maximize the total probability of the assignment. Through this model, our assignment problem is identified as a p-centre problem (see [START_REF] Revelle | Location analysis: A synthesis and survey[END_REF] for a survey on such location problems).

Numerical experiments show that [ILP ] tends to give sparse assignments: most of the time, adjacent words are assigned to different topics even if the total number of topics is correct. To prevent this unnatural behaviour, we modified [ILP ] to consider groups of consecutive words instead of isolated words. Model [ILP seg] partitions the document into segments of consecutive words, and assigns one topic to each segment, such that at most χ max segments are created. For the sake of convenience, we denote by p(k|w ij , d) = j l=i log(p(k|w l , d)) the logarithm of the probability that topic k is assigned to all words from i to j in the current document.

[ILP seg] : max

N i=1 N j=i K k=1 p(k|w ij , d) x ijk (7) i j=1 N l=i K k=1 x jlk = 1 i (8) N i=1 N j=i K k=1
x ijk ≤ χ max (9)

x ijk ∈ {0, 1} i, j, k
In this model, decision variable x ijk is equal to 1 if topic k is assigned to all words from i to j, and 0 otherwise. Constraints (8) ensure that each word belongs to a segment that is assigned a topic. Constraints ( 9) limit the number of segments. Due to the small size of the instances considered in this paper, both [ILP ] and [ILP seg] are well solved by a direct application of an ILP solver.

4 Evaluation and results

MEDIA corpus

The MEDIA corpus is used to evaluate the proposed approach and to compare the various configurations. MEDIA is a French corpus related to the domain of tourism information and hotel booking [START_REF] Bonneau-Maynard | Semantic annotation of the french media dialog corpus[END_REF]. 1,257 dialogues were recorded from 250 speakers with a wizard of Oz technique (a human agent mimics an automatic system). This dataset contains 17k user utterances and 123,538 words, for a total of 2,470 distinct words.

The MEDIA data have been manually transcribed and semantically annotated. The semantic annotation uses 75 concepts (e.g. location, hotel-state, time-month. . . ). Each concept is supported by a sequence of words, the concept support. The null concept is used to annotate every words segment that does not support any of the 74 other concepts (and does not bear any information wrt the task). On average, a concept support contains 2.1 words, 3.4 concepts are included in a utterance and 32% of the utterances are restrained to a single word (generally "yes" or "no"). Table 1 gives the proportions of utterances according to the number of concepts in the utterance.

# concepts 1 2 3 [4,72] % utterances 49.4 14.1 7.9

28.6

Table 1: Proportion of user utterances as a function of the number of concepts in the utterance.

Notice that each utterance contains at least one concept (the null label being considered as a concept). As shown in Table 2, some concepts are supported by few segments. For example, 33 concepts are represented by less than 100 concept supports. Considering that, we can foresee that finding these poorly represented concepts will be hard for LDA. [1,100[ [100,500[ [500,1k[ [1k,9k[ [9k,15k] 33 21 6 14 1 (null)

Table 2: Number of concepts according to their occurrence range.

Evaluation protocol

Unlike previous studies, we chose a fully automatic way to evaluate the systems. In [START_REF] Siu | Semi-automatic acquisition of domain-specific semantic structures[END_REF], a manual process is introduced to reject induced classes or rules that are not relevant to the task and also to name the semantic classes with the appropriate label. Thus, they were able to evaluate their semi-supervised annotation on the ATIS corpus. In [START_REF] Pargellis | Metrics for measuring domain independence of semantic classes[END_REF], the relevance of the generated semantic classes was manually evaluated giving a mark to each induced semantic rule.

To evaluate the unsupervised procedure it is necessary to associate each induced topic with a MEDIA concept. To that purpose, the reference annotation is used to align topics with MEDIA concepts at the word level. A co-occurrence matrix is computed and each topic is associated with its most co-occurring concept.

As MEDIA reference concepts are very finegrained, we also define a high-level concept hier-archy containing 18 clusters of concepts. For example, a high-level concept payment is created from the 4 concepts payment-meansOfPayment, paymentcurrency, payment-total-amount, payment-approxamount; a high-level concept location corresponds to 12 concepts (location-country, location-district, location-street, . . . ). Thus, two levels of concepts are considered for the evaluation: high-level and fine-level.

The evaluation is presented in terms of the classical F-measure, defined as a combination of precision and recall measures. Two levels are also considered to measure topic assignment quality:

• alignment corresponds to a full evaluation where each word is considered and associated with one topic;

• generation corresponds to the set of topics generated for a turn (no order, no word-alignment).

System descriptions

Four systems are evaluated in our experiments.

[LDA] is the result of the unsupervised learning of LDA models using GIBBSLDA++ tool 1 . It assigns the most probable topic to each word occurrence in a document as described in Section 2.2. This approach requires prior estimation of the number of clusters that are expected to be found in the data. To find an optimal number of clusters, we adjusted the number K of topics around the 75 reference concepts. 2k training iterations were made using default values for α and β.

[GIZA] is the system based on the GIZA++ toolkit 2 which re-aligns for each sentence the topic sequence assigned by [LDA] to word sequence as described in Section 3.2.

[ILP ] and [ILP seg] systems are the results of the ILP solver IBM ILOG CPLEX 3 applied to the models described in Section 3.3.

For the three last systems, the value χ max has to be fixed according to the desired concept annotation. As on average a concept support contains 2.1 words, χ max is defined empirically according to the number of words: with i = [[2, 4]]: χ max = i with 1 http://gibbslda.sourceforge.net/ 2 http://code.google.com/p/giza-pp/ 3 http://www-01.ibm.com/software/integration/optimization/cplexoptimizer/ i = [[5, 10]] words: χ max = i -2 and for utterances containing more than 10 words: χ max = i/2. For the sake of simplicity, single-word utterances are processed separately with prior knowledge. City names, months, days or answers (e.g. "yes", "no", "yeah") and numbers are identified in these oneword utterances.

Results

Examples of topics generated by [LDA], with K = 100 topics, are shown in Table 3.

Plots comparing the different systems implemented w.r.t. the different evaluation levels in terms of F-measure are reported in Figures 3,4, 5 and 6 (high-level vs fine-level, alignment vs generation).

The [LDA] system generates topics which are correctly correlated with the high-level concepts. It can be observed that the bag of 75 topics reaches an F-measure of 61.5% (Fig. 3). When not enough topics are required from [LDA], induced topics are too wide to fit the fine-grained concept annotation of MEDIA. On the other hand if too many topics are required, the performance of bag of high-level topics stays the same while a substantial decrease of the F-measure is observed in the alignment evaluation (Fig. 4). This effect can be explained by the automatic alignment method chosen to transpose topics into reference concepts. Indeed, the increase of the number of topics makes them co-occur with many concepts, which often leads to assign them to the most frequent concept null in the studied corpus.

From the high-level to fine-level concept evaluations, results globally decrease by 10%. An additional global loss of 10% is also observed for both the generation and alignment scorings. In the fine- level evaluation, a maximum F-measure of 52.2% is observed for the generation of 75 topics (Fig. 5) whereas the F-measure decreases to 41.5% in the alignment evaluation (Fig. 6).

To conclude on the [LDA] system, we can see that it generates topics having a good correlation with the high-level concepts, seemingly the best representation level between topics and concepts. From these results it seems obvious that an additional step is needed to obtain a more accurate segmental annotation, which is expected with the following systems.

The [GIZA] system improves the results. It is very likely that the filtering process helps to discard the irrelevant topics. Therefore, the automatic alignment between words and the filtered topics induced by [LDA] with IBM models seems more robust when more topics (a higher value for K) is required from [LDA], specifically in high-level concept alignment (Fig. 4).

Systems based on the ILP technique perform better than other systems whatever the evaluation. Considering [LDA] as the baseline, we can expect significant gains of performance. For example, an Fmeasure of 66% is observed for the ILP systems considering the high-level concept generation for 75 topics (Figure 4), where the maximum for [LDA] was 61.5%, and an F-measure of 55% is observed (instead of 50.5% for [LDA]) considering the highlevel concept alignment.

No significant difference was finally measured between both ILP models for the concept generation evaluations. Even though [ILP seg] seems to obtain slightly better results in the alignment evaluation. This could be expected since [ILP seg] intrinsically yields alignments with grouped topics, closer to the reference alignment used for the evaluation.

It is worth noticing that unlike [LDA] system behaviour, the results of [ILP ] are not affected when more topics are generated by [LDA]. A large number of topics enables [ILP ] to pick up the best topic for a given segment among in a longer selection list. As for [LDA], the same losses are observed between high-level and fine-level concepts and generation and alignment paradigms. Nevertheless, an Fmeasure of 54.8% is observed at the high-level concept in alignement evaluation (Figure 4) that corresponds to a precision of 56.2% and a recall of 53.5%, which is not so low considering a fully-automatic high-level annotation system.

Conclusions and perspectives

In this paper an unsupervised approach for concept extraction and segmental annotation has been proposed and evaluated. Based on two steps (topic inventory and assignment with LDA, then resegmentation with either IBM alignment models or ILP) the technique has been shown to offer performance above 50% for the retrieval of reference concepts. It confirms the applicability of the technique to practical tasks with an expected gain in data production.

Future work will investigate the use of n-grams to increase LDA accuracy to provide better hypotheses for the following segmentation method. Besides, other levels of data representation will be examined (use of lemmas, a priori semantic classes like city names. . . ) in order to better generalise on the data.

Figure 1 :

 1 Figure 1: Graphical model representation of LDA. The boxes are "plates" representing replicates.The outer plate represents documents, while the inner plate represents the repeated choice of topics and words within a document.

  997

Figure 2 :

 2 Figure2: Graphical representation for LDA variables (from[START_REF] Blei | Latent dirichlet allocation[END_REF]). The grey circle is the only observable variable.

Figure 3 :Figure 4 :

 34 Figure 3: F-measure of the high-level concept generation as a function of the number of topics.

Figure 5 :

 5 Figure 5: F-measure of the fine-level concept generation as a function of the number of topics.

Figure 6 :

 6 Figure 6: F-measure of the fine-level concept alignment as a function of the number of topics.

  Semantic concept representation for the query "give me the rate since I'd like a room charged not more than fifty euros".

	words	concept	normalized value
	donnez-moi	null
	le	refLink-coRef	singular
	tarif	object	payment-amount-room
	puisque	connectProp	imply
	je voudrais	null
	une chambre	number-room	1
	qui coûte	object	payment-amount-room
	pas plus de	comparative-payment	less than
	cinquante	payment-amount-integer-room 50
	euros	payment-unit	euro
	Figure 1:

Table 3 :

 3 Examples of topics discovered by LDA (K = 100).

	Topic 0		Topic 13	Topic 18	Topic 35	Topic 33	Topic 43
	information	time-date	sightseeing	politeness	location	answer-yes
	words	prob. words	prob. words prob. words	prob. words prob. words	prob.
	d'	0.28	du	0.16 de	0.30 au		0.31	de	0.30 oui	0.62
	plus	0.17	au	0.11 la	0.24 revoir	0.27	Paris	0.12 et	0.02
	informations 0.16	quinze	0.08 tour	0.02 madame 0.09	la	0.06 absolument	0.008
	autres	0.10	dix-huit	0.07 vue	0.02 merci	0.08	près	0.06 autre	0.008
	détails	0.03	décembre 0.06 Eiffel 0.02 bonne	0.01	proche 0.05 donc	0.007
	obtenir	0.03	mars	0.06 sur	0.02 journée	0.01	Lyon	0.03 jour	0.005
	alors	0.01	dix-sept	0.04 mer	0.01 villes	0.004 aux	0.02 Notre-Dame 0.004
	souhaite	0.003 nuits	0.04 sauna 0.01 bientôt	0.003 gare	0.02 d'accord	0.004

ACKNOWLEDGEMENTS

This work is supported by the ANR funded project PORT-MEDIA (www.port-media.org) and the LIA OptimNLP project (www.lia.univ-avignon.fr).


Jean-Paul Saint

Simona Saint

Beltanelliformis brunsae Menner in Keller et al., 1974: an undoubted Ediacaran fossil from Neoproterozoic of Dobrogea (Romania)

Keywords: Flat discoid imprints, Ediacaran biota, Precambrian, Dobrogea, Romania Empreintes discoïdes plates, Biota édiacarien, Précambrien, Dobrogea, Roumanie

 This paleontological development confirms both the large geographical distribution of this species and the Ediacaran age of the Histria Formation.

INTRODUCTION

The period corresponding to the Late Precambrian (Neoproterozoic) and the Precambrian-Cambrian boundary is a key moment in the evolution of the biosphere and biodiversity. At this time, the metazoan organisms begin to organize in complex aquatic ecosystems prefiguring the "Cambrian revolution" from which the modern living aquatic world settles durably with its multiple forms of life [START_REF] Erwin | Ecological drivers of the Ediacaran-Cambrian diversification of Metazoa[END_REF].

The Late Proterozoic fossiliferous deposits distributed throughout the world have brought incomparable information, the most famous being that of Ediacara in Australia, which gave its name to the last period of the Precambrian, the Ediacaran (-630 to -542 million years). Since the highlighting of this very particular stand in the middle of the 20th century, a particular interest has been focused on these fascinating archaic forms of life, with a somewhat idealized vision of a world without predators relayed through the notion of "Garden of Ediacara" [START_REF] Mcmenamin | The Garden of Ediacara[END_REF]. Despite decades of discoveries and research, Ediacaran life remains in many respects somewhat enigmatic, both in terms of comparison with large taxonomic groups of current organisms and the ecology of these early complex ecosystems [START_REF] Brasier | Decoding the Ediacaran enigma[END_REF][START_REF] Mccall | The Vendian (Ediacaran) in the geological record: Enigmas in geology's prelude to the Cambrian explosion[END_REF][START_REF] Mcgabhann | -There is no such thing as the 'Ediacara Biota[END_REF]. Numerous studies have thus been devoted to progressively updated organisms, and contradictory discussions have been initiated on their biological affinity and way of life. For a long time, the "Ediacaran fauna" was naturally mentioned, the authors assimilating Ediacaran organisms to animal groups such as annelids, echinoderms, arthropods... More recently, other ways have been followed, interpreting certain Ediacaran organisms as protists [START_REF] Zhuravlev | Were Ediacaran Vendobionta multicellulars?[END_REF], as microbial colonies [START_REF] Steiner | Chuaria circularis Walcott 1899-''megasphaeromorph acritarch'' or prokaryotic colony?[END_REF][START_REF] Grazhdankin | -Ediacaran microbial colonies[END_REF], or even very controversially as fungi or lichens [START_REF] Retallack | Were the Ediacaran fossils lichens?[END_REF]Peterson et al. 2003;Handcliffe & Hancy 2013;Retallak 2013aRetallak , 2013b)). It has also been proposed the name of "vendozoa or vendobionta" [START_REF] Seilacher | Vendozoa: organismic construction in the Proterozoic biosphere[END_REF][START_REF] Seilacher A | Vendobionta and Psammocorallia: lost constructions of Precambrian evolution[END_REF] for these organisms that could constitute an apart "phylum", today without descendants. Abundant works were dedicated to the identification and creation of a large number of taxa giving an idea of a fairly large biodiversity of the Ediacaran biota. Moreover, new methods, such as surface scanners, modeling, geochemistry, cladistic phylogeny are being used to better understand the characteristics of certain Ediacaran fossils [START_REF] Antcliffe | Charnia at 50: developmental models for Ediacaran fronds[END_REF][START_REF] Laflamme | -Ediacaran fronds[END_REF][START_REF] Brasier | Evolutionary relationships within the Avalonian Ediacara biota: new insights from laser analysis[END_REF][START_REF] Laflamme | Osmotrophy in modular Ediacara organisms[END_REF][START_REF] Brasier | The architecture of Ediacaran fronds[END_REF][START_REF] Bykova | Stable carbon isotopes of sedimentary kerogens and carbonaceous macrofossils from the Ediacaran Miaohe Member in South China: Implications for stratigraphic correlation and sources of sedimentary organic carbon[END_REF]Dececchi et al. 2017;Cui et al.,2016;[START_REF] Schiffbauer | Fossils, proxies, and models: Geobiology at critical transitions in the Proterozoic-Paleozoic[END_REF].

Recent works often take a more detailed look at the conditions of fossilization and burial (taphonomy) which make it possible to explain both the type of preservation and also to specify the original morphology of organisms [START_REF] Tarhan | New morphological diversity or preservational variability?: resolving the taphonomic context of Ediacaran assemblage-scale heterogeneity[END_REF][START_REF] Tarhan | Taphonomic controls on Ediacaran diversity: uncovering the holdfast origin of morphologically variable enigmatic structures[END_REF][START_REF] Elliott | New evidence on the taphonomic context of the Ediacaran Pteridinium[END_REF]. Thus, the intervention of microbial mats in the quality of preservation of Ediacaran organisms is put forward by many authors (Callow & Brasier 2009;[START_REF] Gehling | Textured organic surfaces associated with the Ediacara biota in South Australia[END_REF][START_REF] Mcilroy | Smothering of microbial mats by macrobiota: implications for the Ediacara biota[END_REF][START_REF] Laflamme | Microbial biofilms and the preservation of the Ediacara biota[END_REF][START_REF] Kenchington | Reading and writing of the fossil record: preservational pathways to exceptional fossilization[END_REF][START_REF] Tarhan | Microbial mat sandwiches and other anactualistic sedimentary features of the Ediacara member (Rawnsley quartzite, South Australia): implications for interpretation of the Ediacaran sedimentary record[END_REF]. Nevertheless, the interest of scientific community is naturally focused on the Ediacaran "system" with a particular regard on its palaeoecological significance in the ancient biosphere and its place in the general evolution of life [START_REF] Laflamme M | On the eve of animal radiation: phylogeny, ecology and evolution of the Ediacara biota[END_REF][START_REF] Peterson | The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records[END_REF][START_REF] Liu | First evidence for locomotion in the Ediacara biota from the 565 Ma Mistaken Point Formation[END_REF][START_REF] Retallack | First evidence for locomotion in the Ediacara biota from the 565 Ma Mistaken Point Formation[END_REF][START_REF] Serezhnikova | Colonization of substrates: Vendian sedentary benthos[END_REF][START_REF] Shen | The Avalon explosion: evolution of Ediacara morphospace[END_REF][START_REF] Grazhdankin | Patterns of evolution of the Ediacaran soft-bodied biota[END_REF][START_REF] Narbonne | Deep-water Ediacaran fossils from Northwestern Canada: taphonomy, ecology, and evolution[END_REF][START_REF] Antcliffe | A new ecological model for the ∼ 565 Ma Ediacaran biota of Mistaken Point, Newfoundland[END_REF][START_REF] Droser | The advent of animals: The view from the Ediacaran[END_REF]Liu et al. 2015a;[START_REF] Tarhan | An examination of the evolution of Ediacaran paleoenvironmental and paleoecological research[END_REF][START_REF] Dufour | Ediacaran pre-placozoan diploblasts in the Avalonian biota: the role of chemosynthesis in the evolution of early animal life[END_REF][START_REF] Retallack | -Ediacaran sedimentology and paleoecology of Newfoundland reconsidered[END_REF][START_REF] Bowyer | Controls on the evolution of Ediacaran metazoan ecosystems: A redox perspective[END_REF][START_REF] Briggs | Seilacher, Konstruktions-Morphologie, morphodynamics, and the evolution of form[END_REF]Budd & Jensen 2017;[START_REF] Reid | Possible evidence of primary succession in a juvenile-dominated Ediacara fossil surface from the Flinders Ranges, South Australia[END_REF]).

Many deposits have been identified throughout the world enriching the debate, but without really solving the enigmatic aspect of this life so little related to the modern world that will appear very soon after Precambrian-Cambrian boundary [START_REF] Shu | Cambrian explosion: Birth of tree of animals[END_REF][START_REF] Erwin | Ecological drivers of the Ediacaran-Cambrian diversification of Metazoa[END_REF]. Considering this wide range of opinions, any new discovery obviously provides essential data for the understanding of this disappeared ecosystem.

The comprehensive reviews of Ediacaran deposits around the world provide an interesting assessment of the state of knowledge [START_REF] Mccall | The Vendian (Ediacaran) in the geological record: Enigmas in geology's prelude to the Cambrian explosion[END_REF]Fedonkin et al. 2007).

However, in Europe, apart from the large outcrops of White Sea in Russia already known, some deposits in Ukraine, and the famous Charnwood site in England, the data are much rarer. The Precambrian sediments of Dobrogea are also potentially able to contain fossil remains and to provide eventually new lighting about Ediacaran life.

Several works have thus mentioned the presence in Central Dobrogea of some imprints or traces attributable to elements of the Ediacaran living world [START_REF] Oaie | Traces of organic activity in the Greenschist Series of central Dobrogea (Romania)[END_REF][START_REF] Oaie G | Necesitatea protejării urmelor de actitate organică de vârstă precambrian superior din cadrul seriei şisturilor verzi din Dobrogea centrală[END_REF][START_REF] Oaie | Sedimentological significance of mudstone microclast intervals in Upper Proterozoic turbidites, Central Dobrogea, Romania[END_REF][START_REF] Oaie | Sedimentology and source area composition for the Neoproterozoic-Eocambrian turbidites from East Moesia[END_REF][START_REF] Seghedi | Neoproterozoic terranes in the Moesian basement and in the Alpine Danubian nappes of the South Carpathians[END_REF][START_REF] Oaie | Signification sédimentologique des traces mécaniques et biogènes des dépôts précambriens et paléozoïques de Dobrogea centrale et du nord[END_REF][START_REF] Seghedi A | Palaeozoic Formations from Dobrogea and Pre-Dobrogea -An Overview[END_REF]. This organic remains are represented by two samples: a "medusoid" imprint identified as Nemiana simplex Palij 1976 for one, and multiple discoid imprints of possible Beltanelloides, for the other. Traces of activity were also observed and considered as belong to the Nereites ichnofacies. Like the other Ediacaran sites the presence of microbial mat at the surface of Precambrian beds was recently highlighted [START_REF] Martin | Evidence of microbial mats in the Ediacaran deposits of Dobrogea (Romania)[END_REF][START_REF] Martin | Traces of organic activity and microbial mats in the Ediacaran basement of the Moesian Platform[END_REF]. The discovery of Aspidella type disks, which may represent holdfasts of frondose organisms, has made it possible to add new information and to propose an idealized reconstitution based on the knowledge of the moment (Saint [START_REF] Martin | -Le monde ediacarien de Dobrogea[END_REF]. However, observations made on the available material and the previous proposed determinations should probably be revised in the light of the most recent works about Ediacaran organisms. In this sense, the present preliminary article proposes to examine a certain type of discoid imprints sampled from the Precambrian terranes of Central Dobrogea, to give update identification from comparisons with known data and to discuss more general consequences.

GEOLOGICAL SETTING

The studied specimens bearing discoid imprints were collected within sediments of the basement in the central Dobrogea area (Fig. 1). The central Dobrogea is characterized by large outcrops, especially in the valleys, of sediments belonging to Precambrian and more particularly to the Histria formation [START_REF] Seghedi | Field Guidebook, Central and North Dobrogea. IGCPProject No. 369 "Comparative evolution of Peri-Tethyan Rift Basins[END_REF] corresponding approximately to the "greenschist Formation" denomination of ancient authors. The Histria Formation, up to 5000 m thick as estimated from geological and geophysical data, consists of two coarse members of sandstone separated by a thinner member with pelites and siltites [START_REF] Seghedi | Field Guidebook, Central and North Dobrogea. IGCPProject No. 369 "Comparative evolution of Peri-Tethyan Rift Basins[END_REF][START_REF] Oaie | Sedimentologia şi tectonica seriei Şisturilor Verzi din Dobrogea Centrală şi prelungireaei în acvatoriul MăriiNegre[END_REF]. According to several works [START_REF] Seghedi | Field Guidebook, Central and North Dobrogea. IGCPProject No. 369 "Comparative evolution of Peri-Tethyan Rift Basins[END_REF][START_REF] Oaie | Sedimentologia şi tectonica seriei Şisturilor Verzi din Dobrogea Centrală şi prelungireaei în acvatoriul MăriiNegre[END_REF][START_REF] Oaie | Sedimentology and source area composition for the Neoproterozoic-Eocambrian turbidites from East Moesia[END_REF] considering sedimentological, structural and mineralogical features, the Histria Formation should be accumulated in a foreland basin setting, an interpretation consistent with results of geochemical and detrital zircon distribution data (Żelaźniewicz et al. 2001; Żelaźniewicz et al. 2009).

Mineralogical, petrographic and sedimentological data show a basin sourced by a continental margin dominated by an active volcanic arc [START_REF] Oaie | Sedimentology and source area composition for the Neoproterozoic-Eocambrian turbidites from East Moesia[END_REF][START_REF] Seghedi | Neoproterozoic terranes in the Moesian basement and in the Alpine Danubian nappes of the South Carpathians[END_REF]. The low-grade metamorphic ("greenschists") and weakly deformed clastic rocks of the Histria Formation were considered as flyschoid-like sediments (Kraütner et al. 1988) corresponding to median to distal turbiditic sequences [START_REF] Oaie | Sedimentological significance of mudstone microclast intervals in Upper Proterozoic turbidites, Central Dobrogea, Romania[END_REF][START_REF] Oaie | Sedimentology and source area composition for the Neoproterozoic-Eocambrian turbidites from East Moesia[END_REF][START_REF] Seghedi | Neoproterozoic terranes in the Moesian basement and in the Alpine Danubian nappes of the South Carpathians[END_REF][START_REF] Balintoni | Peri-Amazonian provenance of the Central Dobrogea terrane (Romania) attested by U/Pb detrital zircon age patterns[END_REF].

A Late Proterozoic-Early Cambrian estimated age for the sediments of Histria Formation is documented both by geochemical K/Ar datation in the order of -572 million years [START_REF] Giuşcă | Asupra virstei absolute a formatiunilor cristaline din vorlandul orogenului carpatic[END_REF]) and palynological assemblages [START_REF] Kräutner | Precambrian of Dobrogea[END_REF]).

On other hand, U/Pb ages based on detrital zircon suggest a maximum late Ediacaran depositional age [START_REF] Żelaźniewicz | More evidence on Neoproterozoic terranes in Southern Poland and southeastern Romania[END_REF](Żelaźniewicz et al. , Balintoni et al. 2011). In addition, the discovery in the fine grained members of a "medusoid imprint" identified as Nemiana simplex Palij [START_REF] Oaie | Traces of organic activity in the Greenschist Series of central Dobrogea (Romania)[END_REF][START_REF] Oaie G | Necesitatea protejării urmelor de actitate organică de vârstă precambrian superior din cadrul seriei şisturilor verzi din Dobrogea centrală[END_REF], a typical Ediacaran fossil, has been invoked to confirm this Neoproterozoic age. However, the characteristics of this sample do not quite match with the original conception of this species [START_REF] Palij | Remains of a skeletonless fauna and traces of life ability from deposits of the Upper Precambrian and Lower Cambrian of Podolia[END_REF][START_REF] Palij | Soft body Metazoa and fossil animal's traces of the Vendian and Early Cambrian[END_REF].

Descriptions or discoveries of other fossil remains undoubtedly belonging to the socalled "Ediacaran Biota" would be an additional argument for confirmation of the proposed ages.

The Histria succession is very rich in sedimentary structures represented by large trains of ripples marks well preserved on some stratification planes [START_REF] Oaie G | Necesitatea protejării urmelor de actitate organică de vârstă precambrian superior din cadrul seriei şisturilor verzi din Dobrogea centrală[END_REF][START_REF] Oaie | Sedimentological significance of mudstone microclast intervals in Upper Proterozoic turbidites, Central Dobrogea, Romania[END_REF][START_REF] Oaie | Sedimentology and source area composition for the Neoproterozoic-Eocambrian turbidites from East Moesia[END_REF]. The frequent presence of surfaces marked by various wrinkled structures suggests the implication of microbial mats (Saint [START_REF] Martin | Evidence of microbial mats in the Ediacaran deposits of Dobrogea (Romania)[END_REF][START_REF] Martin | Traces of organic activity and microbial mats in the Ediacaran basement of the Moesian Platform[END_REF] resulting in the formation of MISS (Microbially Induced Sedimentary Structures), often described elsewhere in the deposits of the upper Proterozoic, especially during the Ediacaran period [START_REF] Arouri | Reconnaissance sedimentology and hydrocarbon biomarkers of Ediacarian microbial mats and acritarchs, lower Ungoolya Group, Officer Basin[END_REF][START_REF] Bouougri | Siliciclastic biolaminites indicative of widespread microbial mats in the Neoproterozoic Nama Group of Namibia[END_REF][START_REF] Lan | Proliferation of MISS-forming microbial mats after the late Neoproterozoic glaciations: Evidence from the Kimberley region, NW Australia[END_REF][START_REF] Kumar | Microbially induced sedimentary structures (MISS) from the Ediacaran Jodhpur Sandstone, Marwar Supergroup, western Rajasthan[END_REF][START_REF] Parihar | New report of Arumberia banksi (Ediacaran affinity) Mat Structures from the Girbhakar Sandstone of Marwar Supergroup, Bhopalgarh area, Jodhpur, Western Rajasthan, India[END_REF][START_REF] Kolesnikov | Arumberiamorph structure in modern microbial mats: implications for Ediacaran palaeobiology[END_REF][START_REF] Tarhan | Microbial mat sandwiches and other anactualistic sedimentary features of the Ediacara member (Rawnsley quartzite, South Australia): implications for interpretation of the Ediacaran sedimentary record[END_REF].

MATERIAL AND METHODS

The specimen 1, actually exposed in the gallery of the National Museum of Geology (Bucharest), was collected near the town of Gura Dobrogei, on the banks of the Casimcea River where the Histria Formation is well outcropping. Mentioned as representing a "medusoid" imprints [START_REF] Oaie | Traces of organic activity in the Greenschist Series of central Dobrogea (Romania)[END_REF]) close to the genus Beltanelloides [START_REF] Oaie G | Necesitatea protejării urmelor de actitate organică de vârstă precambrian superior din cadrul seriei şisturilor verzi din Dobrogea centrală[END_REF], until now this specimen has never been described, although it is an important element of appreciation of possible remains testifying an Ediacaran life in Romanian sediments.

The specimen 2, housed in the collections of the French National Museum of Natural History of Paris (MNHN.F.A68682), was sampled at outcrops of the Histria Formation on the edge of Sinoe Lake, near the antic town of Histria during a field campaign carried out as part of a bilateral research program between the MNHN and the Romanian Geoecomar Institut.

In order to appreciate the variations of imprint dimensions, diameter of round imprints or long axis of oval imprints, were measured. Considering the deformations that mostly affect the specimen 1, it was considered preferable to measure in addition the surface of each discoid imprint. The surface measurement was so performed using the appropriate functions of the open software Image J.

A section intersecting the surface bearing the footprints and the underlying sediment was made in specimen 2 in order to observe in thin section the main petrographic characteristics.

DESCRIPTION

The specimen 1of pelitic nature is a roughly rectangular plate measuring approximately 14 cm in length and 7 cm in width (Fig. 2a). The oxidized ferruginous surface displays 37 visible discoid flat imprints, of which 22 are integrally preserved and can be measured (Fig. 2b). Each discoid individual is smooth at the center and shows very fine concentric ridges towards the periphery. Discoid individuals are distributed contiguously or very slightly apart. In some cases, some discoid imprints appear to partially cover the neighboring imprint. All the imprints are affected by a unidirectional deformation according to an elongation giving them an oval shape reflecting the posterior tectonic constraints. Other earlier deformations have undoubtedly affected the surface of the sediment: small differences in elevation are observed between individuals with stretching of the wrinkled peripheral structures (Fig. 2b). The length of the long axis of the oval imprints ranges from 1.34 to 1.94 cm with an average of 1.66 cm. These measures show overall certain homogeneity of size.

The specimen 2 constitutes a block cut according to the dominant fracturation allowing showing both the surface and an oblique section of the original sedimentation (Fig. 2c). The surface of the block, strongly ferruginized to a thickness of about 1 mm, is approximately rectangular with a length of about 20 cm over a width of 7 to 8 cm. Discoid imprints on the surface are usually contiguous and also may overlap slightly one to other. They display the same character as the first specimens with a smooth part in the center and a finely wrinkled periphery (Fig. 2d). Unlike specimen 1, the discoid imprints are very slightly deformed with a roughly circular outline. The diameter is quite heterogeneous with values distributing between 1.03 and 2.50 cm for an average of 1.55 cm. The discoid imprints itself concern only a very thin thickness (Fig. 3a). The specimen shows a succession of fine grained beds and very thin beds of coarser siliciclastic sediment. Within these sequences are individualized two bodies of one centimeter thickness with slightly coarser grains structured in micro-HCS (Fig. 3b). The characteristics of the sedimentation revealed by the polished section show that, like most identical fossils around the world, discoid imprints are represented in positive hyporelief, in bed sole position. Given the outcrop conditions, which essentially show the top surfaces of the beds, it is very difficult, if not almost impossible, to observe these fossils in place. This could explain the small number of found samples.

The measurement of the surfaces shows a disparity of average size between the two samples, the individuals of the specimen 2 showing a greater heterogeneity of distribution and on average a larger surface (Fig. 4). This reflects a fairly large variability in size within a sample or between two samples.

ASSIGNMENT AS BELTANELLIFORMIS BRUNSAE

The assignment of the studied samples to the Ediacaran fossils mentioned in the abundant literature dedicated to discoidal impressions comes up against the already old problems of a nomenclature mainly related in fact to taphonomic aspects. If we only refer to remains exhibiting exactly the same characteristics, namely the more or less contiguous presence of flat discoid imprints with fine concentric wrinkles at the periphery and a smooth central part, two main designations have been adopted:

Beltanelliformis brunsae Menner, in [START_REF] Keller | New finds of fossils in the Precambrian Valdai Series along the Syuzma River[END_REF][START_REF] Keller | New finds of fossils in the Precambrian Valdai Series along the Syuzma River[END_REF][START_REF] Narbonne | -Ediacaran biota of the Wernecke Mountains, Yukon, Canada[END_REF][START_REF] Steiner | Chuaria circularis Walcott 1899-''megasphaeromorph acritarch'' or prokaryotic colony?[END_REF][START_REF] Yuan | Macroscopic carbonaceous compressions in terminal Proterozoic shale: A systematic reassessment of the Miaohe biota, South China[END_REF][START_REF] Narbonne | The Canadian Cordillera[END_REF][START_REF] Ivantsov | Revision of the Problematic Vendian Macrofossil Beltanelliformis (=Beltanelloides, Nemiana)[END_REF]Ivantsov 2017) or Beltanelloides sorichevae [START_REF] Sokolov | The Vendian Stage in Earth History[END_REF][START_REF] Sokolov | Organic world of the Earth on the way to Phanerozoic differentiation, Vest[END_REF][START_REF] Glaessner | The dawn of animal life. A biohistorical study[END_REF][START_REF] Sokolov | The Vendian system[END_REF][START_REF] Fedonkin | Vendian faunas and the early evolution of Metazoa[END_REF]Fedonkin & Runnegar 1992;[START_REF] Sokolov | Essays on the Advent of the Vendian System[END_REF]Fedonkin & Vickers-Rich,2007;Leonov 2007aLeonov , 2007b;;[START_REF] Leonov | Upper Vendian assemblages of carbonaceous micro-and macrofossils in the White Sea Region: systematic and biostratigraphic aspects[END_REF][START_REF] Leonov | A find of Vendian fossils in the beds of the Dal'nyaya Taiga Group of the Patoman Plateau[END_REF]. However, the adoption of a systematic status is complicated for several reasons according to the authors' conception and the supposed nature of these fossils: 1) the same designation has been used for variable preservation modes; 2) different names have been assigned to the same type of fossil; (3) different names have been assigned by some authors to fairly similar discoid imprints which are supposed to be different in nature but corresponding for other authors to the same original type of organism. In general, these are rather gregarious forms preserved in a bug-shaped manner or flat imprints, assuming originally a rather globular form. [START_REF] Narbonne | -Ediacaran biota of the Wernecke Mountains, Yukon, Canada[END_REF] had already distinguished among the family of these discoid remains a "Beltanelliformis-type", characterized by the presence of concentric peripheral fine wrinkles, and a "Nemiana-type", more globose and smooth, corresponding to two taphonomic processes of the same original organism of undefined nature [START_REF] Narbonne | The Canadian Cordillera[END_REF]. On the other hand, Leonov (2007a) attributes these two types of preservation to two different organisms: a "Beltanelloides" form that would be attributable to a planktonic spherical organism and a "Nemiana" form that would result from the dwelling imprint of a benthic bag-shaped organism. It should be noted that this distinction is based in part on measurements showing a significant difference between the two "morphotypes". The differences in size between our two specimens show that it cannot be a discriminating argument (Fig. 4). The dilemma has been convincingly summarized by the recent comprehensive revision of [START_REF] Ivantsov | Revision of the Problematic Vendian Macrofossil Beltanelliformis (=Beltanelloides, Nemiana)[END_REF], which demonstrates that, due to the synonymy and anteriority aspects, our fossils must be better related to Beltanelliformis brunsae Menner in [START_REF] Keller | New finds of fossils in the Precambrian Valdai Series along the Syuzma River[END_REF]. The name Beltanelloides sorichevae is thus not valid, as indicated by [START_REF] Narbonne | -Ediacaran biota of the Wernecke Mountains, Yukon, Canada[END_REF] and other authors having often pointed out in synonymy the two species (Fedonkin & Runnegar 1992). As a result, the studied specimens are determined as follow (for complete diagnosis and synonymy, refer to [START_REF] Ivantsov | Revision of the Problematic Vendian Macrofossil Beltanelliformis (=Beltanelloides, Nemiana)[END_REF]):

Regnum incertae

Genus Beltanelliformis Menner in [START_REF] Keller | New finds of fossils in the Precambrian Valdai Series along the Syuzma River[END_REF] Beltanelliformis brunsae Menner in [START_REF] Keller | New finds of fossils in the Precambrian Valdai Series along the Syuzma River[END_REF] It should be noted that in this work Beltanelloides sorichevae [START_REF] Sokolov | Most ancient deposits of the Early Cambrian and sabelliditids[END_REF]Nemiana simplex Palij, 1976 are clearly synonymized with Beltanelliformis brunsae.

DISCUSSION

The problem of systematic assignment is closely linked, not only to the taphonomic processes themselves, but also to the inferred original organic nature of these fossils.

Thus, the remains comparable to our specimens could be considered at the same time to be fossil bodies, fossil traces or megascopic compression (Hofmann 1992a(Hofmann , 1992b;;Runnegar & Fedonkin 1992, Fedonkin & Runnegar 1992, Runnegar 1992a, 1992b;[START_REF] Jensen | A critical look at the Ediacaran trace fossil record[END_REF]. Like many other discoidal elements of the Ediacaran biota, Beltanelliformis was first considered as "medusoid" organism [START_REF] Sokolov | The Vendian Stage in Earth History[END_REF][START_REF] Palij | Soft body Metazoa and fossil animal's traces of the Vendian and Early Cambrian[END_REF][START_REF] Fedonkin | The Vendian White Sea biota (Precambrian skeletonless fauna of the northern Russian Platform), Trudy Geologiceskogo Instituta[END_REF][START_REF] Sokolov | The Vendian as the terminal system of the Precambrian[END_REF]). As mentioned above, "Nemianatype" preservation has been attributed to bug-shaped organisms. For a long time an affinity with benthic coelenterates such as anemones was considered. Various reconstructions have favored this option, these fossils being clearly ranked among the coelenterates [START_REF] Gureev | Vendiata-primitive Precambrian Radialia[END_REF][START_REF] Fedonkin | Systematic description of Vendian metazoa[END_REF][START_REF] Fedonkin | Vendian faunas and the early evolution of Metazoa[END_REF][START_REF] Fedonkin | -Vendian body fossils and trace fossils[END_REF][START_REF] Seilacher | Trace fossils in the Ediacaran-Cambrian transition: Behavioral diversification, ecological turnover and environmental shift[END_REF]). On other hand, according to Leonov (2007a), the "Beltanelloides-type" remains would correspond to spherical floating organisms, formed of a thin and flexible envelope fallen to the bottom and whose compaction would explain the fine concentric lines around the periphery. In the same sense, Ivantsvov et al. ( 2014) consider that the presence of basically plastic prediagenetic distortions, as observed for Romanian specimens, suggests that the envelope of Beltanelloides was also elastic. If we adopt the idea of a single type of organism, we must reconcile the different types of preservation. [START_REF] Narbonne | -Ediacaran biota of the Wernecke Mountains, Yukon, Canada[END_REF] proposed a scenario arguing a continuum, but they give no conclusion as to the exact nature of the organism.

Flat discoid fossils, like that of Dobrogea, have often been interpreted as compressions of more or less spherical organisms, of which, in some cases, there are only traces or organic films, or more rarely both. As a result, they have been compared with other Chuaria-type Ediacaran fossils known as carbon compressions, algal or microbial in origin [START_REF] Hofmann | -Proterozoic carbonaceous compressions ("metaphytes" and "worms[END_REF][START_REF] Steiner | Chuaria circularis Walcott 1899-''megasphaeromorph acritarch'' or prokaryotic colony?[END_REF][START_REF] Steiner | Evidence of organic structures in Ediacara-type fossils and associated microbial mats[END_REF][START_REF] Leonov | Comparative taphonomy of the Vendian genera Beltanelloides and Nemiana as a key to their true nature[END_REF][START_REF] Ragozina | Macrophytes and organically preserved microfossils in the Vendian complex of the southeastern White Sea area, Russia[END_REF][START_REF] Grazhdankin | The Ediacaran White Sea biota in the Central Urals[END_REF][START_REF] Dong | On the morphological and ecological history of Proterozoic Macroalgae[END_REF]Grazhdankin et al. 2007;Leonov 2007aLeonov , 2007b;;[START_REF] Leonov | Upper Vendian assemblages of carbonaceous micro-and macrofossils in the White Sea Region: systematic and biostratigraphic aspects[END_REF][START_REF] Moczydłowska | The Ediacaran microbiota and the survival of Snowball Earth conditions[END_REF][START_REF] Ragozina | -Association of macro-and microfossils in the Vendian (Ediacaran) postglacial successions in Western Mongolia[END_REF]Bykova 2017;[START_REF] Ye | A systematic description of new macrofossil material from the upper Ediacaran Miaohe Member in South China[END_REF], although this affinity is doubtful for other authors [START_REF] Narbonne | -Ediacaran biota of the Wernecke Mountains, Yukon, Canada[END_REF]. According to Xiao et al. (2017), stable carbon isotope values for samples of Beltanelliformis preserved as carbonaceous macrofossil do not allow to discriminate between interpretations of this organism as a colonial bacterium or a eukaryotic alga. However, the most recent studies dedicated to biomarkers from Beltanelliformis specimens similar to those of Romania, but with a preserved organic film, favor a microbial origin, probably cyanobacterial (Bobrovsky et al. 2016, Bobrovsky et al. 2017, Ivantsov 2017).

The known occurrences of Beltanelliformis brunsae are apparently limited exclusively to the Ediacaran period (=Vendian), being perhaps the most common and widely distributed Ediacaran fossil worldwide [START_REF] Narbonne | The Ediacara Biota: A Terminal Neoproterozoic experiment in the evolution of life[END_REF][START_REF] Mccall | The Vendian (Ediacaran) in the geological record: Enigmas in geology's prelude to the Cambrian explosion[END_REF]. In the Precambrian of Russia (Siberia, Urals, White Sea ...) and Ukraine, this common species seems characteristic of the "Upper Vendian" (see review in [START_REF] Ivantsov | Revision of the Problematic Vendian Macrofossil Beltanelliformis (=Beltanelloides, Nemiana)[END_REF]. In other parts of the world, known occurrences also correspond to the upper Neoproterozoic: Great Britain [START_REF] Pyle | -Integrated Ediacaran chronostratigraphy, Wernecke Mountains, northwestern Canada[END_REF]McIllroy et al. 2005;[START_REF] Liu | Reviewing the Ediacaran fossils of the Long Mynd[END_REF]), Canada and Newfoundland [START_REF] Hofmann | -Ediacaran (Precambrian) fossils from the Wernecke Mountains[END_REF]Liu et al. 2015b), China [START_REF] Zhao | -Discovery of a Miaohe-type Biota from the Neoproterozoic Doushantuo Formation in Jiangkou County, Guizhou Province, China[END_REF][START_REF] Wang | The correlation between macroscopic algae and metazoans in the Ediacaran: a case study on the Wenghui biota in northeastern Guizhou, South China[END_REF], Mongolia [START_REF] Ragozina | -Association of macro-and microfossils in the Vendian (Ediacaran) postglacial successions in Western Mongolia[END_REF], South America [START_REF] Aceñolaza | -Icno-asociaciones de la transición Precámbrico /Cámbrico en el noroeste de Argentina[END_REF]Drefahl & Silva 2007;[START_REF] Netto Rg | Evidences of life in terminal Proterozoic deposits of southern: a synthesis[END_REF]), Namibia (Wood et al. 2015), Australia (Xiao et al. 2013). Consequently, Romanian specimens extend westward the distribution field of the species known in East Europa. They also testify to an Ediacarian age for the Histria formation of central Dobrogea.

Occurrences of Beltanelliformis around the world generally concern sediments deposited in shallow water environment [START_REF] Narbonne | -Ediacaran biota of the Wernecke Mountains, Yukon, Canada[END_REF][START_REF] Fedonkin | Vendian faunas and the early evolution of Metazoa[END_REF][START_REF] Narbonne | The Ediacara Biota: A Terminal Neoproterozoic experiment in the evolution of life[END_REF][START_REF] Aceñolaza | -Icno-asociaciones de la transición Precámbrico /Cámbrico en el noroeste de Argentina[END_REF][START_REF] Grazhdankin | Patterns of distribution in the Ediacaran biotas: Facies versus biogeography and evolution[END_REF][START_REF] Pyle | -Integrated Ediacaran chronostratigraphy, Wernecke Mountains, northwestern Canada[END_REF][START_REF] Grazhdankin | The Ediacaran White Sea biota in the Central Urals[END_REF]McIllroy et al. 2005;[START_REF] Seilacher | Trace fossils in the Ediacaran-Cambrian transition: Behavioral diversification, ecological turnover and environmental shift[END_REF]Leonov 2007a;[START_REF] Narbonne | The Canadian Cordillera[END_REF][START_REF] Rozhnov | Development of the trophic structure of Vendian and Early Paleozoic marine communities[END_REF][START_REF] Rozhnov | From Vendian to Cambrian: the beginning of morphological disparity of modern metazoan phyla[END_REF]Liu,2011;[START_REF] Ivantsov | Revision of the Problematic Vendian Macrofossil Beltanelliformis (=Beltanelloides, Nemiana)[END_REF]Ivantsov 2017;Liu et al. 2015b;[START_REF] Netto Rg | Evidences of life in terminal Proterozoic deposits of southern: a synthesis[END_REF]Grytsenko 2016;[START_REF] Ragozina | -Association of macro-and microfossils in the Vendian (Ediacaran) postglacial successions in Western Mongolia[END_REF], sometimes in connection with storm deposits and hummocky cross-stratification (HCS) [START_REF] Narbonne | -Ediacaran biota of the Wernecke Mountains, Yukon, Canada[END_REF][START_REF] Pyle | -Integrated Ediacaran chronostratigraphy, Wernecke Mountains, northwestern Canada[END_REF][START_REF] Mccall | The Vendian (Ediacaran) in the geological record: Enigmas in geology's prelude to the Cambrian explosion[END_REF][START_REF] Narbonne | The Canadian Cordillera[END_REF]. If the cyanobacterial nature is proved, we can estimate that this would correspond to the photic zone. This trend is consistent with the microbial mat surfaces observed in the Histria Formation [START_REF] Martin | Evidence of microbial mats in the Ediacaran deposits of Dobrogea (Romania)[END_REF][START_REF] Martin | Traces of organic activity and microbial mats in the Ediacaran basement of the Moesian Platform[END_REF]) and the sedimentological features of the specimen 2.

CONCLUSIONS

The studied samples unambiguously present exactly the same characteristics as various samples described in the Ediacarian deposits in Podolia, White Sea or Siberia. Although the discussion of their origin is not yet fully decided, recent studies seem to show that the hypothesis of the cyanobacterial colony imprint can be favored. The attribution of these specimens to the Ediacaran morpho-species Beltanelliformis brunsae Menner in Keller eta al., 1974

  is probably the most acceptable. The most interesting consequence is the confirmation of the Ediacaran age of the Histria Formation and therefore of the various organic traces known in Central Dobrogea. The deposits of the Histria Formation therefore represent a certain potential for restitution of the Ediacarian life in the Romanian ground. Further systematic field investigations would better measure the abundance and the diversity of the Ediacaran fossils. bilateral program, PAI Brâncusi (Nouvelles données sur l'écosystème énigmatique du Précambrien supérieur). It is also a contribution to the ATM (Actions Thématiques du Muséum) program of the MNHN (Muséum national d'Histoire naturelle, Paris). During various field research, we received the friendly help of Jean-Pierre André, Lilian Cazes, Sylvain Charbonnier, Didier Merle and Philippe Richir. The sections and thin sections were performed by Séverin Morel (MNHN). BUDD G.E. & JENSEN S. 2017. -The origin of the animals and a 'Savannah' hypothesis for early bilaterian evolution. Biological Review 92: 446-473. BYKOVA N. 2017. -Paleoecology and Taphonomy of Ediacaran macrofossils from the Khatyspyt Formation, Olenek Uplift, Siberia. PhD, Virginia Polytechnic Institute and State University, 125 p. BYKOVA N., GILL B.C., GRAZHDANKIN D., ROGOV V. & XIAO S. 2017. -A geochemical study of the Ediacaran discoidal fossil Aspidella preserved in limestones: Implications for its taphonomy and paleoecology. Geobiology 15: 572-587. CALLOW R.H.T. & BRASIER M. 2009. -Remarkable preservation of microbial mats in Neoproterozoic siliciclastic settings: Implications for Ediacaran taphonomic models. Earth-Science Reviews 96: 207-219. CUI H., GRAZHDANKIN D.V., XIAO S., PEEK S., ROGOV V., BYKOVA N.V., SIEVERS N.E., LIU X.M. & KAUFMAN A.J. 2016. -Redox-dependent distribution of early macro-organisms: Evidence from the terminal Ediacaran Khatyspyt Formation in Arctic Siberia. Palaeogeography, Palaeoclimatology, Palaeoecology 461: 122-139. DECECCHI T.A., NARBONNE G.M., GREENTREE C. & LAFLAMME M. 2017. -Relating Ediacaran fronds. Paleobiology 43(2): 171-180. DREFAHL M. & SILVA R. C. 2007. -Ocorrências de fósseis ediacaranos na Formação Camarinha (Neoproterozóico Superior), Sudeste do Estado do Paraná. In: ISMAR de SOUZA CARVALHO et al. (eds). Paleontologia: Cenários da Vida. Rio de Janeiro: Editora Interciência Ltda., v. 1, pp. 3-11.

FIGURE CAPTIONS Figure 1

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Figure 2

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Figure 3 a.

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Figure 4

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Figure 4

 4 Figure 4

ACKNOWLEDGEMENTS

We address special thanks to Gheorghe Oaie, dear to our memory, who opened the way for the work on the organic remains of Neoproterozoic from Dobrogea and has always supported our investigations. We are also indebted to Antoneta Seghedi who shared with us his perfect knowledge of the field and facilitated the study of the specimen of the Museum of Geology. This work was carried out thanks to the French-Romanian