Keywords: Achelata, Ibacus, Ypresian, Ghazij formation, distribution range, Neotethys

The fossil record of slipper lobsters (Scyllaridae) is very poor, even though the earliest species of the modern slipper lobsters (Neoscyllaridae) are known from the Early Cretaceous. Among the clades "subfamilies" of slipper lobsters, perhaps the most remarkable is Ibacinae (currently monogeneric), which contains species with a very wide carapace (cephalothoracic shield) and very deep incisions. Fossil species assigned to Ibacinae were reported from the Oligocene and Pleistocene. However, a recent reappraisal of Scyllaridae phylogeny revealed that the genus to which these two fossil occurrences belong is not closely allied to Ibacinae.

Therefore Ibacinae was left with no known fossil record until the present study. The present study describes a new species which can be confidently assigned to Ibacinae (closely allied to Ibacus) from the Ypresian (early Eocene) from the Ghazij formation in Shaisuro, Dera Ghazi Khan District, Pakistan. This new species, ?Ibacus mazariorum corresponds to the oldest recorded occurrence of Ibacinae. The Ghazij formation was deposited on the north-western margin of the India-Pakistan plate during a critical period, shortly before the collision between of the India-Pakistan plate with Asia. Consequently, it is possible that Ibacus, and closely allied species may have remained in the same biogeographic area since Eocene.

Considering the extant and fossil distribution of Ibacinae, additional fossil discoveries may be found in existing collections and fossiliferous outcrops from Neotethys, Indian and Pacific oceans.

Introduction

Slipper lobsters (Scyllaridae) are easy to recognize decapod crustaceans: contrary to almost all other crustaceans, their antennae do not have a long flagellum, but a rather short flagellum formed of a few podomeres [START_REF] Holthuis | Marine lobster of the world[END_REF][START_REF] Lavalli | Infraorder Palinura Latreille[END_REF][START_REF] Haug | The evolution of a key chararacter, or how to evolve a slipper lobster[END_REF].

Besides, in the case of modern species, (Neoscyllarida Haug, Audo, Charbonnier, Palero, Petit, Abi Saad & Haug, 2016: a clade recently erected to distinguish modern slipper lobsters and their closest fossil relatives), the antenna is even more peculiar: very wide, flattened, and asymmetrical. These antennae resemble a shovel, hence, Scyllaridae are sometimes called "shovel-nosed lobster". However, contrary to what this name implies, antennae are probably not used in burying, but rather to protect antennulae, to steer the animal during swimming and to flip upside-down opponents during antagonistic behaviour [START_REF] Spanier | Why do shovel-nosed (slipper) lobsters have "shovels[END_REF][START_REF] Lavalli | Behavior and sensory biology of slipper lobsters[END_REF].

Scyllaridae have a quite poor fossil record compared to other decapod crustaceans [START_REF] Audo | New Nisto of Slipper Lobster (Decapoda: Scyllaridae) from the Hadjoula Lagerstätte (Late Cretaceous, Lebanon)[END_REF]. Their known fossil diversity is rather low, discontinuous stratigraphically and geographically (see supplementary material table 1).

Based upon the available fossil evidence, we know that slipper lobsters have diverged from other achelates as early as the Late Jurassic [START_REF] Haug | Development in Mesozoic scyllarids and implications fort he evolution of Achelata (Reptantia, Decapoda, Crustacea)[END_REF]. Witnesses of the evolution of slipper lobsters (with a mosaic of primitive and derived characters) are discovered as late as Late Cretaceous [START_REF] Haug | The evolution of a key chararacter, or how to evolve a slipper lobster[END_REF][START_REF] Charbonnier | Fossil Crustacea of Lebanon[END_REF], but the diversification of Neoscyllaridae occurred earlier, no later than the Early Cretaceous (see supplementary material table 1). This diversification gave rise to at least five clades (see phylogenetic tree in [START_REF] Yang | Phylogenetic relationships, character evolution, and taxonomic implications within the slipper lobsters (Crustacea; Decapoda: Scyllaridae)[END_REF]. Four of these clades correspond to the subfamilies defined by [START_REF] Holthuis | A revision of the family Scyllaridae (Crustacea Decapoda Macrura). I. Subfamily Ibacinae[END_REF]: Arctidinae [START_REF] Holthuis | A revision of the family Scyllaridae (Crustacea Decapoda Macrura). I. Subfamily Ibacinae[END_REF], Scyllarinae Latreille, 1825, Theninae Holthuis, 1985and Ibacinae Holthuis, 1985. The fifth clade is formed by Evibacus Smith, 1869 + Parribacus Dana, 1852, formerly assigned to Ibacinae [START_REF] Holthuis | A revision of the family Scyllaridae (Crustacea Decapoda Macrura). I. Subfamily Ibacinae[END_REF], but which are more closely related to Scyllarinae and Theninae than to Ibacus Leach, 1815 [START_REF] Yang | Phylogenetic relationships, character evolution, and taxonomic implications within the slipper lobsters (Crustacea; Decapoda: Scyllaridae)[END_REF]). Among these aforementioned clades or "subfamilies", at least three are known in the fossil record (supplementary material table 1): (1) Arctidinae and (2) Scyllarinae which supposedly first occur in Early Cretaceous; (3) the unnamed clade containing Parribacus and Evibacus, which is represented in the fossil record by two species of Parribacus in Oligocene [START_REF] Glaessner | Vorkommen fossiler Dekapoden (Crustacea) in Fish-Schiefern[END_REF] and Pleistocene [START_REF] Hu | Crustacean Fossils of Taiwan[END_REF], both previously assigned to Ibacinae. Theninae has not yet been identified in the fossil record. With the exclusion of Parribacus from Ibacinae, it appears that no true Ibacinae was discovered in the fossil record before the present study. This situation is slightly surprising as Ibacinae could have been easily identified since it contains the most remarkable species, with the widest and most flattened (in proportion) carapaces (cephalothoracic shields).

The present study investigates a fossil from the Ypresian of Pakistan, which displays characters typical of Ibacinae, closely matching those of Ibacus, and assesses its significance: this discovery improves our knowledge of Scyllaridae fossil diversity and of the minimum age of Ibacinae.

Geological context

The studied specimen was collected from the Ghazij formation in Shaisuro (Fig. 1), 150 km south-west of Dera Ghazi Khan, Punjab Province [START_REF] Collins | New lower Tertiary crabs from Pakistan[END_REF]. The Ghazij formation crops out over several hundreds of kilometres near the suture zone between India-Pakistan plate and Asia [START_REF] Clyde | Stratigraphic response and mammalian dispersal during initial India-Asia collision: Evidence from the Ghazij Formation, Balochistan, Pakistan[END_REF]. This formation has an interesting origin: it was deposited on the north-western margin of the India-Pakistan plate about 50-56 MA ago (Ypresian, early Eocene), at a time when India-Pakistan plate was just beginning its collision with Asia [START_REF] Clementz | Early Eocene warming events and the timing of terrestrial faunal exchange between India and Asia[END_REF]; note that the timing of this collision is still poorly constrained: [START_REF] Zhu | Age of initiation of the India-Asia collision in the east-central Himalaya[END_REF]. This important collision is probably recorded by the formation. Indeed, the formation was initially deposited under marine conditions, then in paralic conditions and the sedimentation at the top of the formation is of terrestrial origin [START_REF] Clyde | Stratigraphic response and mammalian dispersal during initial India-Asia collision: Evidence from the Ghazij Formation, Balochistan, Pakistan[END_REF]. In this context, it is very likely that this variation in the sedimentation is a direct result of the collision between India-Pakistan plate and Asia and of the uplifting of the Ghazij formation.

Surprisingly, the specimen described herein was labelled as coming from the "top of the Ghazij shales, Shaisuro" (handwritten on the label placed with the specimen), and the locality is referred by Collins & Morris (1978: p. 961) as "top of the Ghazij formation". As explained in [START_REF] Clyde | Stratigraphic response and mammalian dispersal during initial India-Asia collision: Evidence from the Ghazij Formation, Balochistan, Pakistan[END_REF], the Ghazij formation ends up with sedimentation of terrestrial origin, so a marine slipper lobster is unlikely to come from the upper most part of the Ghazij formation. Rather, it may come from the top of the green shales, in the lower part of the Ghazij formation. The shales mentioned on the label may represent the uppermost part of the Ghazij formation in Shaisuro (opposed to the top of the whole Ghazij formation), which may explain the confusion. Unfortunately, no information is available on the precise horizon of the Ghazij formation from which this specimen comes from. No information can be gathered from the specimen itself either, as it is preserved in a nodule, and the surrounding sediment was removed.

The Ypresian of Shaisuro has also yielded 6 specimens of the brachyuran Glyphithyreus wetherelli [START_REF] Bell | Monograph of the fossil malacostracous crustacea of Great Britain Parts I. II. The Palaeontographical society[END_REF] [START_REF] Collins | New lower Tertiary crabs from Pakistan[END_REF]. In addition, three other brachyuran species are reported from Ghazij formation in other outcrops, Laeviranina sinuosa [START_REF] Collins | New lower Tertiary crabs from Pakistan[END_REF], Pyromaia inflata Collins & Morris, 1978, Pakicarcinus orientalis [START_REF] Collins | New lower Tertiary crabs from Pakistan[END_REF] [START_REF] Collins | New lower Tertiary crabs from Pakistan[END_REF][START_REF] Charbonnier | Early Paleogene decapod crustaceans from the Sulaiman and Kirthar Ranges, Pakistan[END_REF].

Material and Methods

This study is based on the sample NHMUK In.48229 housed in the palaeontological collections of the Natural History Museum, London, United Kingdom (NHMUK). This specimen was collected by Ernest Sheppard Pinfold F.G.S., who was a geologist of the Attock Oil Company (Claire Mellish, pers. comm. 2017). The fossil is preserved in three dimensions within a concretion and was probably only minimally flattened during diagenesis.

The specimen was imaged in cross-polarized light [START_REF] Bengtson | Teasing fossils out of shales with cameras and computers[END_REF][START_REF] Haug | Imaging and documentating gammarideans[END_REF]) and with image stacking to enhance the contrast and ensure the whole specimen is in focus. In addition, the dorsal aspect of the specimen was imaged at two slightly diverging angles to produce stereo pairs with an exaggerated stereographic effect (Fig. 2A).

The morphological terminology follows that used by [START_REF] Holthuis | A revision of the family Scyllaridae (Crustacea Decapoda Macrura). I. Subfamily Ibacinae[END_REF], except that the term "ridge" is replaced by "carina".

Abbreviations: CL= Carapace Length, measured from the posterior margin of ocular incision to the posterior margin (parallel to median line); CW= Carapace width, measured as the maximum width, perpendicular to median line.

Systematic palaeontology

"Class" Malacostraca Latreille, 1802 "Order" Decapoda Latreille, 1802 "Suborder" Pleocyemata [START_REF] Burkenroad | The evolution of the Eucarida, (Crustacea, Eumalacostraca), in relation to the fossil record[END_REF] Achelata [START_REF] Scholtz | Phylogenetic systematics of the reptantian Decapoda (Crustacea, Malacostraca)[END_REF] Carapace ornamentation. -carapace smooth medially, visibily ornamented laterally with pits, which may correspond to the erosion of tubercles (see Charbonnier et al. 2014: 333, for typical example of variations due to the preservation).

Etymology. -dedicated to the Mazari tribe, which inhabits the area where the fossil was discovered.

Type Material. -Holotype NHMUK In.48229, by monotypy (Fig. 2). Discussion. -?Ibacus mazariorum nov. sp. is ascribed to Neoscyllarida based upon its wide, dorsoventrally flattened carapace with marked branchial and median carina and large ocular incisions placed on the anterior margin. Similar disposition, with a large carapace can also be found in polychelidan lobsters, brachyurans and stomatopods. However, the disposition of carina and grooves in these taxa is completely different, notably, because they do not possess any anterior submedian carina, neither have a branchial groove extending along the median carina as much as in ?Ibacus mazariorum nov. sp. As a whole, the whole pattern of grooves and carinae on the carapace is only compatible with some genera of slipper lobsters (Scyllaridae). Within Scyllaridae, such a wide carapace is only documented in the Neoscyllarida, other groups having more elongated carapaces. More precisely, the ocular incisions (or orbits) which open anteriorly, are nearer to each other than to the anterolateral angle, the raised branchial and postrostral carinae are diagnostic of Ibacus [START_REF] Leach | The Zoological Miscellany being description of new, or interesting animals[END_REF][START_REF] Holthuis | A revision of the family Scyllaridae (Crustacea Decapoda Macrura). I. Subfamily Ibacinae[END_REF]. However, due to the fragmentary condition of this fossil, it is not possible to verify if this species has the typical deep cervical incision of Ibacus. It is therefore more reasonable to assign this species with some doubts to Ibacus. ?Ibacus mazariorum is the only known fossil species with distinct affinities with Ibacus. It is easily distinguished from all other species of Ibacus by its particularly narrow frontal margin, narrower than the ocular incision width, wider in all other species.

Geographic and Stratigraphic

Discussion

Minimum age of Ibacinae

?Ibacus mazariorum is the only known occurrence of a representative of Ibacinae. This occurrence proves that Ibacinae already diverged from their nearest Scyllaridae relatives before the early Eocene. This new data may help calibrate the divergence time between the "subfamilies" of Scyllaridae for future works similar to that proposed by [START_REF] Palero | Phylogenetic relationships between spiny, slipper and coral lobsters (Crustacea, Decapoda, Achelata)[END_REF].

Distribution range of Ibacinae and Ibacus in past and extant environments

?Ibacus mazariorum is either a representative of Ibacus or a very close relative to this genus.

The distribution range of Ibacus is currently limited to the Indian ocean and West-Pacific [START_REF] Holthuis | A revision of the family Scyllaridae (Crustacea Decapoda Macrura). I. Subfamily Ibacinae[END_REF][START_REF] Holthuis | Marine lobster of the world[END_REF]. ?Ibacus mazariorum used to live on the north-western margin of India-Pakistan plate, in the Neotethys ocean. Its distribution matches that of extant species of Ibacus. It is undoubtedly a case of phylogenetic conservatism of the distribution range.

However, with only one specimen in one locality, it is not possible to know if the size of the distribution range varied for Ibacus and its closest relatives.

As a result, we may have better chances to complete our poor knowledge of fossil Ibacinae by investigating existing collections and fossiliferous outcrops from Mediterranean Sea, Neotethys, Indian and Pacific oceans. Besides, at the time, the nearby Mediterranean part of Neotethys was still connected to the Eastern Neotethys [START_REF] Golonka | Plate-Tectonic maps of the Phanerozoic[END_REF]. Therefore, it is surprising that Ibacinae does not occur in present-day Mediterranean sea. .

Conclusions

1. ?Ibacus mazariorum is the only known occurrence of a fossil Ibacinid; 2. ?Ibacus mazariorum completes our knowledge about fossil Scyllaridae, a group with a particularly poor fossil record.

3. This species belongs to Ibacus Leach, 1815, or is a very closely allied species;

"

  Family" Scyllaridae[START_REF] Latreille | Familles naturelles du règne animal, exposées succinctement et dans un ordre analytique, avec l'indication de leurs genres[END_REF] Neoscyllarida[START_REF] Haug | The evolution of a key chararacter, or how to evolve a slipper lobster[END_REF] "Subfamily" Ibacinae[START_REF] Holthuis | A revision of the family Scyllaridae (Crustacea Decapoda Macrura). I. Subfamily Ibacinae[END_REF] "Genus" ?Ibacus Leach, 1815 Type Species. -Ibacus perroni Leach, 1815, by monotypy.Original diagnosis (Leach 1815). -Shell broader than long, subdepressed, on each side very deeply fissured; eyes inserted at the anterior margin; peduncle thick, recurved, acuminated towards its extremity. External pedipalpes with the two first joints externally crested; second joint internally, with many fissures; flagrum tripartite; the last segment manyjointed. Legs ten, short; two anterior pair abruptly thicker than the others; fifth pair pair didactyle; thumb short. Tail with five membranaceous lamellae, which are crustaceous at their base. Interior antennae three-jointed, terminated by two many-jointed setae: external antennae squamiform, four-jointed. [sic] Emended diagnosis (Holthuis 1985: rearranged from key to genera). -Exopods of all maxillipeds with a multi-articulate flagellum; Carapace strongly depressed, with a deep cervical incision in the lateral margin. Mandible with a simple or two-segmented palp ("subfamily" Ibacinae); orbits anteriorly open, placed on the anterior margin of the carapace. Cervical incision usually open, at least at the base. Carapace without posteromedian tooth. Atlantic and Indo-West Pacific; Dorsal surface of the body smooth and punctate, not tuberculate, sometimes pubescent. Carapace with postrostral and branchial carinae distinct. Distance between the orbits as long as or shorter than the distance separating each orbit from the nearest anterolateral angle. Fifth abdominal somite with posteromedian spine. Mandibular palp consisting of a single segment. (Note that the key originally included biogeographic characters which are herein preserved).?Ibacus mazariorum nov. sp.

Fig. 2

 2 Fig.2

  Range. -Ghazij formation, Shaisuro, Mazari tribal area, ca 150 km south-west of Dera Ghazi Khan, Punjab Province, 28° 27' N., 69° 50' E. Ypresian stage, Eocene (locality 3 in Collins & Morris 1978).

4.FiguresFigure 1

 1 Figures

Figure 2 ?

 2 Figure 2 ?Ibacus mazariorum nov. sp. from the Ypresian of Shaisuro. A, holotype in dorsal view, stereopair, cross-polarized light; B, interpretative line-drawing of dorsal view; C, frontal view, cross-polarized light; D, interpretative line-drawing of frontal view. Abbreviations: a= branchiocardiac groove, asmc= anterior submedian carina, bc= branchial carina, c= postcervical groove, e1e= cervical groove, ei?= cervical incision, fm= frontal margin, ic?= intercervical carina, inc= intestinal carina, mc= median carina, o= ocular incision, oc= orbital carina, po= postorbital carina, poc= posterior carina. Scale bars: 10 mm. Pictures: D. Audo, © NHMUK.

Acknowledgements

This study would not have been possible without the help of Claire Mellish, senior curator of the collection of graptolites and fossil arthropods at the NHMUK. I am indebted to the peoples who provided important comments that improved greatly an earlier version of my manuscript: Àlex Ossó (Tarragona) for bringing to my attention the article of Feldmann & Schweitzer (2017); Ferran Palero (Blanes) who reminded me of the publication rejecting the monophyly of Ibacinae sensus Holhtuis (1985); and the two reviewers, Carolin Haug (Munich) and Sylvain Charbonnier (Paris) for their insightful suggestions and corrections.

Supplementary material