Mesozoic Fishes 4 - Homology and Phylogeny, G. Arratia, H.-P. Schultze & M. V. H. Wilson (eds.): pp. 433-442, 2 figs. ©2008 by Verlag Dr. Friedrich Pfeil, Miinchen, Germany - ISBN 978-3-89937-080-5

A review of the Mesozoic Record of the Carcharhiniformes

Charlie J. UNDERWOOD & David J. WARD

Abstract

Although the Carcharhiniformes represent one of the most diverse and important groups of sharks alive today, their early history is very poorly known. Reinterpretation of previously figured Jurassic and Cretaceous fossils, along with collection of new specimens from the Cretaceous of the British Isles, has allowed the early record of this order to be reinterpreted. Whereas members of only one carcharhiniform family have been previously recorded from the Jurassic, and two from the Cretaceous, it is considered here that fossils of two families are known from Jurassic rocks, and at least five families from the Cretaceous. The relative timing of familial appear-ances is consistent with the predictions derived from cladistic analyses, although some of the cladogenic events can now be shown to have been earlier than previously recognised.

Introduction

The Carcharhiniformes sensu COMPAGNO (1973) represent one of the most diverse groups of selachians. Modern examples of this order vary in size from under 30 centimetres to over five metres in length, and are distributed from the ocean floor to intertidal areas, with some species being oceanic pelagics and oth-ers entering fresh water. Although much of the radiation within carcharhiniform families occurred within the Caenozoic, a number of families first appeared within the Jurassic and Cretaceous. Despite their im-portance within modern marine environments and their phylogenetically predicted early radiation (e.g., SHIRAI 1996), the early record of the group is very poorly known. There are eight extant families: the Carcharhinidae, Hemigaleidae, Leptochariidae, Proscylliidae, Pseudotriakidae, Scyliorhinidae, Sphyrnidae, and Triakidae. Only the Scyliorhinidae and the Triakidae have previously been recognised in the Mesozoic, although a number of carcharhiniform teeth have been figured that do not closely match those of extant members of those families (UNDERWOOD 2006).

Material

Much of the material considered here has been figured in other publications, although the taxonomic affini-ties of a number of these specimens have been reassessed for this study as described in the text below. In addition, a number of taxa are considered here that have not yet been figured elsewhere. A number these taxa form part of extensive faunas currently under study by the authors. These specimens were recovered from a number of horizons of phosphatic chalk from the Santonian to Lower Campanian of southern Eng-land, and from a Coniacian phosphatic greensand from Northern Ireland. In addition, a triakid tooth from the British Cenomanian found in the collections of the Natural History Museum, London, is figured.

All specimens figured here are deposited in the Natural History Museum, London (prefix BHNM P.) or in the Liverpool Museum (National Museums and Galleries on Merseyside) (prefix LIVCM).

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Jurassic Carcharhiniformes

The earliest records of Carcharhiniformes are from the Bathonian, where several genera are recognised (e.g., UNDERWOOD & WARD 2004, CAPPETTA, pers. comm. 2004). Earlier reported records are now known to have been based on misidentified teeth of palaeospinacid sharks (DELSATE et al. 2002). Some of these earliest records represent members of the Scyliorhinidae GILL, 1862 (represented today by the catsharks or spotted dogfish), including the taxa Palaeoscyllium tenuidens UNDERWOOD & WARD, 2004 (Fig. 1 A) and an additional unnamed form from the Bathonian of England. The familial position of other taxa is less clear. The genera PraeproscyIlium UNDERWOOD & WARD, 2004 (Fig. 1E,F) and Eypea UN-DERWOOD & WARD, 2004 (Fig. 1G) both have dental morphologies, with regard to the form of both the crown and the root, closer to extant members of the Proscylliidae FOWLER, 1941 (finback catsharks) than Scyliorhinidae, and are here considered to be probable members of that family. Fragmentary teeth of an additional, larger, ?carcharhiniform lack diagnostic morphological characters.

Carcharhiniform material from Late Jurassic rocks suggests that taxa similar to those living in the Ba-thonian were present. Palaeoscyllium WAGNER, 1857 is known from both skeletal and dental remains (see LEIDNER & THIES1999, UNDERWOOD 2002). The shape of the body and head, position and shape of the fins, morphology and heterodonty of the teeth, and shape of the placoid scales all more closely resemble those of extant scyliorhinids than members of other carcharhiniform families, agreeing with a scyliorhinid affinity for the genus. Other Late Jurassic taxa are far less well known, but suggest the presence of several genera. Unnamed teeth from the Kimmeridgian of France (CANDONI 1995) appear to belong to a species of Praeproscyllium (UNDERWOOD 2004) or an allied genus. A single tooth from the Kimmeridgian of Portugal (KRIWET 1997) appears to show some similarity to Praeproscyllium, but may represent a further, unnamed, genus. Poorly preserved skeletal remains of Macrourogaleus hassei (WOODWARD 1889) appear to represent a scyliorhinid with a single dorsal fin (KRIWET & KLUG 2004). A well-preserved skeleton of what is probably an additional Tithonian carcharhiniform taxon is currently under study (THIES pers. com. 2003). Although sometimes regarded as a carcharhiniform (THIES & CANDONI 1998), we consider Corysodon SAINT-SEINE, 1949 to be an orectolobiform based on the morphology of the tooth root and the shape and positions of the fins.

Cretaceous Carcharhiniformes

Teeth of small carcharhiniform sharks are frequently abundant and diverse elements within Cretaceous marine shark assemblages, with high diversity assemblages being present within many Late Cretaceous assemblages (e.g., CAPPETTA 1980, NOUBHANI & CAPPETTA 1997, and several samples currently under study by the authors). In addition, there are several known occurrences of well-preserved skeletal remains, particularly from the Upper Cretaceous lithographic limestones of Lebanon (CAPPETTA 1980), which in many cases confirm the affinities of the isolated teeth. Despite this, the very small size of many of the isolated teeth has resulted in them commonly being overlooked. The lack of detailed study of the

Fig. 1. Teeth of representative Jurassic and Cretaceous Carcharhiniformes. White scale bars for all species = 500 pm. > All photographs SEM images except Q. A, Palaeoscyllium tenuidens UNDERWOOD & WARD, 2004, BMNH P. 66045, labial view of lateral tooth, Bathonian of Watton Cliff, Dorset. B, Palaeoscyllium sp., BMNH P. 66293, labial view of anterior tooth, fluvial Barremian of Yaverland, Isle of Wight. C,D, Pseudoscyliorhinus sp., BMNH P. 66366, labial (C) and lingual (D) views of lateral tooth. Santonian of Boxford, Berkshire. E, Praeproscyllium oxoniensis UNDERWOOD & WARD, 2004, BMNH P. 66054, labial view of anterior tooth, Bathonian of Wood-eaton Quarry, Oxfordshire. F, Praeproscyllium oxoniensis UNDERWOOD & WARD, 2004, BMNH P. 66055, labial view of posterior tooth, Bathonian of Woodeaton Quarry, Oxfordshire. G, Eypea leesi UNDERWOOD & WARD, 2004., BMNH P. 66058, labial view of anterior tooth, Bathonian of Watton Cliff, Dorset. H,I, Pteroscyllium sp., BMNH P. 66395, lingual (I) and labial (H) views of lateral tooth. Santonian of Winterbourne, Berkshire. J,K, Lep-tocharias sp., BMNH P. 66418, labial (J) and lingual (K) views of tooth. Santonian of Winterbourne, Berkshire. L,M, Leptocharias sp., BMNH P. 66420, labial (L) and lingual (M) views of tooth. Santonian of Winterbourne, Berkshire. N,0, ?triakidae indet., LIVCM 1998.48.E, labial (O) and lingual (N) views of tooth. Hauterivian of Speeton, Yorkshire. P, Paratriakis sp. BMNH P. 66415, Santonian of Winterbourne, Berkshire. Q, Pachygaleus sp., BMNH P. 13346, labial view of tooth. Cenomanian of southern England. R,S,T, Ihoxodon sp., BMNH P. 66422, occlusal (R), labial (S) and lingual (T) views of tooth. Santonian of Winterbourne, Berkshire.

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isolated dental remains of Cretaceous Carcharhiniformes has resulted in a very poor understanding of the generic, and probably familial, diversity.

Early Cretaceous marine selachian faunas are generally poorly known, with only Albian faunas be-ing relatively well represented in the literature. Despite this, scattered records suggest the presence of a number of taxa in Berriasian to Aptian rocks. The Scyliorhinidae are represented by PalaeoscyIlium, which has been recorded from the Valanginian (REES 2005), Barremian (SWEETMAN & UNDERWOOD 2006) (Fig. IB), and Albian (UNDERWOOD & MITCHELL 1999). Cretascyliorhinus UNDERWOOD & MITCHELL, 1999 is known from a number of Albian sites (e.g., CAPPETTA 1977), as are a number of scyliorhinids of uncertain generic affinity (e.g. UNDERWOOD & MITCHELL 1999, WARD in press). Additional Valangin-ian scyliorhinids are mentioned, but not described or figured (CAPPETTA 1990: 35).

Protoscyliorhinus HERMAN, 1977 has been recorded from a number of Early Cretaceous sites, ranging from Barremian (BIDDLE & LANDEMAINE 1988) to Albian (BIDDLE 1993) in age. The dental morphology of Protoscyliorhinus, with a "U"-shaped root lacking a clearly defined basal face, lacking clearly defined lateral cusplets, and having a flat labial surface, is here considered to more closely resemble that of extant members of the Proscylliidae than Scyliorhinidae, and so it is here tentatively considered to represent a member of the former. A second genus of uncertain affinity, Pteroscyllium CAPPETTA, 1980 is known from the English Aptian (UNDERWOOD 2004) and Albian (UNDERWOOD & MITCHELL 1999). Santonian ex-amples of this genus are known from well-preserved skeletal remains from the Lebanon (CAPPETTA 1980) that demonstrate a scyliorhinid-like overall morphology and dentition. However, the dental morphology is unlike that of any known scyliorhinid (UNDERWOOD 2004), and more closely resembles that of some more primitive taxa of Lamniformes. It is therefore considered here that this genus probably requires the erection of a new family, although it is considered that the definition of this would be premature without study of the skeletal remains.

A single tooth recorded from the Hauterivian of northern England appears to suggest the presence of a species belonging to the Triakidae GRAY, 1851 (the family that includes the tope and smoothhounds) (UNDERWOOD et al. 1999: 292) (Fig. 1N,0) . The crown and root morphology is extremely similar to that of many modern triakids and unlike that of anything else known from the early Cretaceous. The histology could not be observed, and more material is needed to confirm the affinity of this taxon.

Although commonly abundant and diverse, much work is needed before the generic diversity of Late Cretaceous scyliorhinids can be realistically assessed. Scyliorhinus BLAINVILLE, 1816 s.s. is known from the Campanian and Maastrichtian (HALTER 1994), but the majority of species referred to Scyliorhinus lack the distinctive dental characteristics of extant members of the genus (UNDERWOOD 2006). Palaeoscyllium is present within the British Santonian and Campanian (pers. observ.) and Cretascyliorhinus is known from a number of Upper Cretaceous stratigraphical intervals (see UNDERWOOD & MITCHELL 1999). Pseudoscyliorhinus MULLER & DIEDRICH, 1991 is known from the Cenomanian (MULLER & DIEDRICH 1991), Turonian (as 'Scyliorhinus' reussi HERMAN, 1977), Coniacian, Santonian, and Campanian (pers. obs.) (Fig. 1C,D). Some Late Cretaceous scyliorhinid species have tooth morphologies resembling extant genera and may prove to be congeneric. Other species have a tooth morphology unlike any living scyliorhinid species and will almost certainly require the erection of new genera.

Protoscyliorhinus, and hence possible representatives of the Proscylliidae, is recorded from the Ceno-manian of Lithuania and Turonian of France (HERMAN 1977: 259). This is very similar in overall tooth morphology to the Palaeogene genus Foumtizia NOUBHANI & CAPPETTA, 1997, which likewise may be a proscylliid.

A number of species of Pteroscyllium have been recorded from the Late Cretaceous (Fig. 1H,I), with the last records of the genus being from the Maastrichtian (NOUBHANI & CAPPETTA 1997). Skeletal remains of two species are known from the Santonian of Lebanon (CAPPETTA 1980), and the body outline and dentition are well known. The well-developed covering of denticles obscures much of the cranial and postcranial skeleton, and these are therefore not so well known.

Although there is no recorded fossil record of the Leptochariidae GRAY, 1851 (represented today by a single species, the barbeled houndshark), it is here considered to be represented within Late Cretaceous assemblages. Isolated teeth recovered from several British Santonian sites (Fig. 1J-M) have a crown and characteristic root morphology of Leptocharias Smith in MULLER & HENLE, 1838. It is considered here that other unrecognised occurrences of this genus also occur in the fossil record, with teeth of the Palaeogene 'Scyliorhinus' ptychtus NOUBHANI & CAPPETTA, 1997 also strongly resembling Leptocharias and here provisionally assigned to it.

436

Pn eproscyllium Eypea

P Un

'alaeoscyllium named genus

Unnamed Unnamed

Macrourogaleus (:+? unr

Pteroscyllium

Carcharhinidae:

Leptochariidae

genus-genus^ amed genus)

probable unn amed genera

Protoscyliorhinus

Cretascyliorhinus -various unnamed genera-

Pseudoscyliorhinus

Pteroscyllium -

genus indet.-Pachygaleus

Paratriakisr

Foumtizia

Scylidrhinus Casieria

Microscyliorhinus Porodermoides

Prerriontreia

Palaeogaleus -Archaedtriakis-

Squatigaleus -Triakis

Khouribgaleus Galeorhinus

? Loxodon-Physogaleus

Abdounia Danogaleus

Leptocharias -

Aal Baj Bat Cal Oxf Kim Tit Ber Val Hau Ber Apt Alb Cen Tur Con San Cam Maa

M. Jurassic L. Jurassic Early Cretaceous Late Cretaceous Fig. 2. Known Mesozoic and Palaeocene ranges for carcharhiniform genera, including unnamed genera mentioned in the text. Note that the apparent Palaeocene radia-tion may be an artefact of the intensive study of material of this age from Moroccan phosphorites.

Triakids are typically common within Late Cretaceous assemblages, with well-preserved skeletons being known from several European sites (e.g., CAPPETTA 1980, MULLER 1989). Material referred to the extant genus Galeorhinus BLAINVILLE, 1816 is known from a number of species ranging from the Cenomanian (Popov & Lapkin 1999, pers. observ., see Fig. 1Q) onWARDs. Despite this, the tooth morphol-ogy of Cretaceous nominal Galeorhinus species (teeth robust with a convex anterior edge, short main cusp and swollen or folded labial crown edge) suggest that these species probably represent Pachygaleus CAP-PETTA, 1992. An additional, unnamed triakid is also present in the Cenomanian of Germany (MULLER & DIEDRICH 1991). Paratriakis HERMAN, 1977 (Fig. IP) first appears in the latest part of the Cenomanian (HERMAN 1977). Within the latter parts of the Late Cretaceous, other genera appear alongside Pachygaleus and Paratriakis. Palaeogaleus GURR, 1962 is first recorded in the Santonian (pers. observ.), and continues into the Palaeogene. Withirt the Campanian and Maastrichtian, Archaeotriakis CASE, 1978 and Squatigaleus CAPPETTA, 1989 are recorded respectively. These both have highly derived dental morphologies and may be closely related. Their teeth to some extent resemble the extant Pseudotriakis BRITO CAPELLO, 1868, but the triakid affinity suggested by CAPPETTA (1989) is followed here.

The Carcharhinidae JORDAN & EVERM ANN, 1896 (here taken to include genera such as Rhizoprionodon WHITLEY, 1929 and Loxodon MULLER & HENLE, 1838) comprise probably the most morphologically and ecologically diverse family of modern sharks, including the whaler or requiem sharks. Despite this, much of the radiation within this group appeared to occur during the Neogene (e.g., CAPPETTA 1987), and the recorded fossil record does not extend earlier than the Palaeocene (e.g., MAISEY et al. 2004). Samples from the Santonian currently under study by the authors, however, yielded rare teeth assignable to Loxo-don (Fig. 1R-T) (UNDERWOOD 2006). This would represent the first known Mesozoic occurrence of this family. Although dentally very similar to the genera Rhizoprionodon and Scoliodon MULLER & HENLE, 1837, these teeth have been provisionally assigned to Loxodon on the basis of crown morphology and lack of an incipient posterior cusplet (HERMAN et al. 1991; pers. observ. of modern dentitions).

Environmental palaeoecology

Modern Carcharhiniformes are present within almost all marine environments (e.g., COMPAGNO 1988), with some entering freshwater. Morphological diversity is highest in shelfal settings, with both large and small taxa commonly being present. Some deeper water environments have a high species diversity, but these are typically dominated by small, morphologically conservative forms (e.g., LAST & STEVENS 1994). Within the Neogene to Recent, medium to large (two metre plus) species are known within several carcharhiniform families (Carcharhinidae, Hemigaleidae, Pseudotriakidae, and Sphyrnidae). Despite this, there is no evidence for any Mesozoic carcharhiniforms of this size.

Although palaeoenvironmental coverage of published selachian faunas is very incomplete, fossil Car-charhiniformes have been recorded from a wide range of shelfal palaeoenvironments. The almost complete lack of studies on sharks from Mesozoic deep oceanic facies results in a general lack of information on deep water taxa. Middle Jurassic Carcharhiniformes are recorded from a number of different palaeoenvironments (UNDERWOOD & WARD 2004), with different taxa being recorded from different facies. Within the study area, Eypea dominated offshore settings, with Palaeoscyllium in shallow water carbonates and Praeproscyl-lium in lagoons. This indicates that, even at this early stage in the evolution of the order, considerable environmental specificity was present. Although all of these palaeoenvironments were marine to some degree, molluscan faunas from some of the lagoonal deposits suggests reduced salinity and indicates the ability of at least this genus of early carcharhiniform to survive in brackish conditions. Other Jurassic taxa are known from neritic mudstones or lagoonal plattenkalks, with Palaeoscyllium formosum being present in both, although it is probable that many of the plattenkalk faunas are allochthonous (VIOHL 1996), or at least parautochthonous and derived from a range of surrounding habitats.

The majority of Early Cretaceous Carcharhiniformes are known from offshore marine sediments. De-spite this, rare examples of scyliorhinid teeth are known from freshwater facies. Teeth of Palaeoscyllium were recorded from a channel sandstone within the fluvial Barremian Wessex Formation of southern England (SWEETMAN & UNDERWOOD 2006). It is unclear whether this represents a euryhaline or truly freshwater species, but it is assumed, by analogy with modern carcharhiniforms, that the former is more likely. No other species of the Scyliorhinidae, fossil or extant, is known to enter freshwater.

Although Late Cretaceous Carcharhiniformes are known from many sites; the great majority of these are within offshore, fine grained (mudstone and chalk) facies. A small number of taxa, mostly triakids, are

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known from shallow water sands and silts. Diversity is highly variable between sites, typically being low within mudstones and chalks of the American Interior Seaway and higher within the northern European Chalk facies. Diversity is highest within phosphatic deposits, such as from the southern margin of Tethys (e.g., NOUBH ANI & CAPPETTA 1997). Even more diverse carcharhiniform assemblages are present within the phosphatic chalks of the British Santonian and Campanian (with up to 12 carcharhiniform species in some samples, currently under study by the authors), where the Carcharhiniformes form part of a very diverse assemblage of small-toothed sharks, with over 90 % of teeth recovered being under 3 mm high.

Phylogenetic implications

Both morphological and molecular data have been used to reconstruct neoselachian phylogenies, and these studies have typically included several Carcharhiniforme taxa. An analysis of interrelationships within the Carcharhiniformes by COMPAGNO (1988) included a greater variety of taxa, but resolution within the group was poor. In all phylogenies, the Scyliorhinidae are placed as a sister group to all other (included) Carcharhiniforme families (e.g, SHIRAI 1996, MAISEY et al. 2004, WINCHELL et al. 2004) or at least close to the base of the clade (COMPAGNO 1988). WINCHELL et al. (2004) included two genera of the Scyliorhinidae and concluded that the family is paraphyletic, with Apristurus GARMAN, 1913 being more closely related to other carcharhiniform families than Scyliorhinus. The Proscylliidae are also considered to be close to the base of the carcharhiniform clade, and are generally considered more derived than the Scyliorhinidae (e.g., SHIRAI 1996, MAISEY et al. 2004). They appear to be close to, and possibly form a monophyletic group with, Pseudotriakis (COMPAGNO 1988, SHIRAI 1996).

The relative positions of Leptocharias and the Triakidae are uncertain (SHIRAI 1996), and it is probable that the latter is paraphyletic (MAISEY 1984, COMPAGNO 1988, SHIRAI 1996). The Leptochariidae and Triakidae together form paraphyletic sister groups to the more derived clade containing the Carcharhinidae, Sphyrnidae GILL, 1862 and Hemigaleidae HASSE 1879 (e.g., COMPAGNO 1988, SHIRAI 1996, MAISEY et al. 2004, WINCHELL et al. 2004). The Hemigaleidae appear to be a sister group to the Carcharhinidae (including Sphyrnidae) (e.g. SHIRAI 1996), with the Carcharhinidae being paraphyletic if the Sphyrnidae are excluded and Rhizoprionodon and Galeocerdo MULLER & HENLE, 1838 included (e.g., NAYLOR 1992).

The Middle Jurassic occurrences of scyliorhinids and probable proscylliids agree well with these groups being basal within the carcharhiniform clade. The absence of pseudotriakids from the Jurassic (and subsequent) fossil record may either be due to a consistently deep water habit of the family, or possibly due to a relatively recent derivation of the Pseudotriakidae from the Proscylliidae.

The Cretaceous fossil record of the Triakidae and probable Leptochariidae again fits well with the sequence of familial originations inferred from phylogenetic studies, although the poor Early Cretaceous neoselachian record and the difficulty in recognition of leptochariids on dental remains suggests that there is a good possibility of earlier specimens being recorded in the future.

The presence of Loxodon in the Cretaceous suggests a far earlier radiation of the 'crown group carcharhi-nids' than previously realised, as this is generally considered to be more derived than the Hemigaleidae. Although none of these other groups are currently known from the Cretaceous, examples of Hemigaleidae, Carcharhinidae and Galeocerdo are known from the Palaeocene and Eocene (CAPPETTA 1987, MAISEY et al. 2004).

A summary of the known ranges of Mesozoic carchariniform genera is given in Fig. 2.

Acknowledgements

We would like to thank Andy GALE, Ian JARVIS and Peter WOODROOF for his help in the original collecting of the British Late Cretaceous material considered here. Steven SWEETMAN is thanked for bringing our attention to the Wessex Formation material. Alison LONGBOTTOM and staff at the Natural History Museum, London and thanked for their help. We would also like to thank the referees whose comments greatly improved this paper. This work was partly carried out with the assistance of University of London grant GL2CU.

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Authors' addresses: Charlie J. UNDERWOOD, School of Earth Sciences, Birkbeck College, Malet Street, London WC1E7HX, United Kingdom; e-mail: c.Underwood@bbk.ac.uk David J. WARD, Crofton Court, 81 Crofton Lane, Orpington, Kent, BR51HB, United Kingdom; e-mail: david@fossil.ws.

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