A NEW PLIENSBACHIAN ICHTHYOSAUR FROM DORSET, ENGLAND

A NEW PLIENSBACHIAN ICHTHYOSAUR FROM DORSET, ENGLAND by CHRISTOPHER MC GOWAN and ANGELA C. MILNER ABSTRACT. The first ichthyosaur to be recorded from the Pliensbachian Stage of the English Lower Liassic is described as Leptonectes moorei sp. nov., extending the geological range of Leptonectes to the Pliensbachian. According to criteria for assessing the maturity of ichthyosaurs, it is concluded that L. moorei is an immature individual of a relatively small, slender and short snouted species close to the earlier long-snouted L. tenuirostris (Conybeare) which ranges from the Rhaetian to the Sinemurian. The presence of a short-snouted leptonectid in the Pliensbachian suggests two contrasting patterns of rostral morphology within the clade Leptonectidae in the latest Early Jurassic, rostral reduction within the genus Leptonectes and rostral elongation in Excalibosaurus and Eurhinosaurus. I N January, 1995, Mr Chris Moore, of Charmouth, collected a small, partial ichthyosaur skeleton from the Belemnite Marls (Lower Pliensbachian) on the foreshore at Seatown, Dorset. The specimen bears a striking resemblance to the Rhaetian and Lower Liassic species Leptonectes tenuirostris (Conybeare) (McGowan 1989a, 1996), except for its shortened snout and the absence of an emarginated radius. The specimen was subsequently purchased by The Natural History Museum (BMNH) and the description of this new Lower Liassic species is the subject of the present paper. MATERIALS AND METHODS Measurements were made with dial calipers, recorded to the nearest 0 1 mm. The characters used, and the ratios derived from them, are summarized in Table 1 (for more details see McGowan 1974a, 1976). Abbreviations used are: BATGM, Bath Geological Museum (the ichthyosaurs, which are part of the historic Moore Collection, have been on loan to the National Museum of Wales, Cardiff, for the last several years); BMNH, The Natural History Museum, London; SCM, Somerset County Museum, Taunton. SYSTEMATIC PALAEONTOLOGY Class REPTILIA Order ICHTHYOSAURIA de Blainville, 1835 Family LEPTONECTIDAE Maisch, 1998 Genus LEPTONECTES McGowan, 1996 Leptonectes moorei sp. nov. Text-figures 1 4 Derivation of name. In honour of Mr Chris Moore, who discovered the specimen. Holotype. BMNH R 14370, an incomplete skeleton comprising the skull, fore paddles and anterior trunk. The only specimen. [Palaeontology, Vol. 42, Part 5, 1999, pp. 761 768] The Palaeontological Association

762 PALAEONTOLOGY, VOLUME 42 TEXT-FIG. 1.Leptonectes moorei sp. nov.; BMNH R 14370; complete specimen as preserved in left lateral aspect. The right forefin is preserved in ventral aspect and the left forefin in dorsal aspect. The belemnites, Passaloteuthis sp. (above snout and touching right mandible) and Pseudohastites sp. (below posterior end of mandible and left of scale bar), are also preserved on the slab; 0 25. Locality and horizon. The specimen was found on the foreshore, just to the west of Seatown, Dorset, in the uppermost part of the Belemnite Marls, 1 m below the Belemnite Stone band. This places it in the Lower Pliensbachian, close to the top of the Lower Liassic (see Arkell 1933). Diagnosis. Snout and mandible slender; snout short, snout ratio <0 70 and probably <0 64; prenarial ratio <0 56, but probably not <0 43; premaxillary ratio <0 48 and probably <0 44; orbit large, occupying most of the post-rostral segment of the skull, orbital ratio >0 20 and probably >0 25. Humerus with constricted shaft, widely expanded distally, with prominent leading edge facet; forefin probably with four digits; phalanges relatively large, well spaced distally; number of elements in longest digit probably not >15. Distinguished from L. tenuirostris and L. solei by relatively shorter snout and larger orbit; ulna longer than radius: from L. tenuirostris by the lack of a notch on the leading edge of the radius: from L. solei by relatively much larger orbit. DESCRIPTION The partial skeleton (Text-figs 1 2), which lies on its right side, comprises a skull, both forefins, the pectoral girdle (much of which is overlain by the forefins), several ribs, and a short section of the vertebral column, largely hidden from view beneath the ribs and matrix. The anterior of the two forefins is interpreted as the right one, seen in ventral

M C GOWAN AND MILNER: JURASSIC ICHTHYOSAUR 763 TEXT-FIG. 2. Leptonectes moorei sp. nov.; BMNH R 14370; interpretive outline diagram of specimen in Text-fig. 1. CL, clavicle; CO, coracoid; FF, forefin; ICL, interclavicle; L., left; mr, mandibular ramus; R., right; SC, scapula. Scale bar represents 200 mm. TABLE 1. Characters recorded and ratios derived. Where relevant, measurements and ratios for the holotype of L. moorei are given in parentheses. Character or Ratio Skull length (328 mm) Jaw length (341 mm) Orbital diameter (107 2 mm) Snouth length (194 mm) Premaxillary length ( 127 mm) Prenarial length (156 mm) Snout depth at level of external naris ( 40 mm) Total body length (N/A) Elements in longest digit (probably 12) Tooth length Coracoid length ( 77 mm) Scapula length (117 5 mm) Orbital ratio (0 31) Snout ratio (0 57) Premaxillary ratio (0 37) Prenarial ratio (0 46) Description Distance between tip of snout and posterior edge of quadrate Distance between tip of dentary and posterior edge of angular Internal diameter of orbit, measured along its longitudinal axis Distance between tip of snout and anterior (internal) margin of orbit Distance between tip of snout and anterior tip of maxilla Distance between tip of snout and anterior margin of external naris Minimal depth of snout, measured at right angles to longitudinal axis of skull at level of anterior end of external naris Skull length, plus body length, measured along vertebral column, from atlas to tip of tail Number of elements in longest digit of forefin, counting from epipodials Maximum length, from tip of crown to base of root, measured between horizontals perpendicular to longitudinal axis of tooth Maximum anteroposterior length Maximum length, measured between horizontals perpendicular to longitudinal axis of element Orbital diameter divided by jaw length Snout length divided by jaw length Premaxillary length divided by jaw length Prenarial length divided by jaw length

764 PALAEONTOLOGY, VOLUME 42 view; the deltopectoral crest is formed by the prominent proximal process of the humerus, which lies towards the preaxial border. The left forefin seen in dorsal aspect lies more posteriorly. The comparatively small size of BMNH R 14370 the skull is only 328 mm long raises the question of its maturity. Johnson (1977) listed four criteria for assessing immaturity among ichthyosaurs. One of these is present, namely the surface texturing on the shaft of the humerus, which is like fine sandpaper. Furthermore, the sutures between the proximal fin elements are not fully closed (Text-fig. 4). The head of the right humerus appears to be flat; however, this is due primarily to its exposure in ventral aspect which accentuates the rather flattened profile. The head of the left humerus, in contrast, appears convex, which is the mature condition. Johnson s fourth criterion, the rudimentary or complete absence of notching in the leading edge of the preaxial elements of the fin, is inconclusive. This is because notching is present only in some species and is not size related, and the absence of any evidence of notching, as in BMNH R 14370, probably only reflects that notching is not a feature of the species. On balance, it appears that it is not a fully mature individual on the basis of the humerus texture and the state of the proximal fin element sutures. As noted earlier, aside from its abbreviated snout, Leptonectes moorei bears a remarkable resemblance to Leptonectes tenuirostris. Thus the humerus has a narrow shaft, widened distally, with a prominent leading-edge facet. The forefin has only four digits and the individual phalanges are relatively large and few in number. The orbit is large, occupying most of the postrostral segment of the skull, and the snout and mandible are both slender. It will be shown below that none of these features can be attributed to immaturity, so there is no possibility that the skeleton represents a juvenile individual of L. tenuirostris. L. solei, from the Sinemurian of Dorset, by far the largest of the three species referred to Leptonectes, is very similar to L. tenuirostris in having a long and slender rostrum, a humerus that is widely expanded distally, and phalanges that are rounded rather than polygonal (McGowan 1993). Consequently, aside from its elongated rostrum, it too has much in common with L. moorei. Unfortunately, the species is incompletely known because the holotype is rather poorly preserved, and there are shortcomings with the single referred specimen. There is also a considerable size discrepancy between the two species. Thus, the holotypes of L. moorei and L. solei have skull lengths of 328 mm and c. 1585 mm respectively. As a consequence of these problems, it is not possible to make the same detailed comparisons between the new species and L. solei, as it was for L. tenuirostris. Skull (Text-fig. 3) The skull is exposed from the left side, with the left ramus of the mandible in its natural position. However, there has been some shearing and displacement, and the right mandibular ramus is preserved from its ventral aspect. What appears to be part of the left side of the rostrum has sheared away from the rest, and lies immediately below the level of the remainder of the rostrum. The quadrate has been displaced, reducing the accuracy of the measurement of the skull length. There is also some difficulty in determining the anterior tip of the maxilla, so the premaxillary length is only approximate. The architecture of the skull is dominated by the large orbit, which approaches a perfect circle as in Leptonectes tenuirostris, and by the slenderness and brevity of the rostrum. Values for the orbital and snout ratios are 0 31 and 0 57 respectively. Ichthyosaurus breviceps similarly has a large orbit and abbreviated snout (McGowan 1974b), with observed ranges for the orbital and snout ratios of 0 28 0 33 (n ¼ 6) and 0 50 0 56 (n ¼ 6) respectively. The external naris is a prominent feature, as it is in I. breviceps, being broadly petaloid and it has more than a simple, smooth, outline. The ventral border has a smoothly rounded lappet of bone extending dorsally into the naris. The posterior end of the lappet continues as a dorsally directed process with slightly notched dorsal and posterior margins, which lies immediately anterior to the posterior narial margin formed by the lacrimal. There is also a narrow crack-like extension anterior to the lappet, although this could be a preservational feature. The naris therefore bears some resemblance to the more complex narial opening of Leptonectes tenuirostris (see McGowan 1989a). The teeth are neither markedly small or slender, in contrast with the condition in L. tenuirostris and in L. solei where the slender teeth are relatively small compared with the skull. Thus in BATGM M3566, a specimen of L. tenuirostris of similar size to the holotype (based on the lengths of the humerus and scapula), the longest exposed tooth is only 9 6 mm long, compared with 15 6 mm in the present specimen. Even in the largest specimen of L. tenuirostris (McGowan 1989a, text-fig. 3), an individual that is at least 50 per cent. bigger than the holotype, the longest tooth is only 13 mm. The teeth have the typical conical shape of other ichthyosaurs, e.g. Ichthyosaurus, about half of the total length of the tooth comprising the crown. The root is slightly bulbous and coarsely fluted; in comparison the crown lacks distinct surface sculpture and is smooth. Forefins (Text-fig. 4) The humerus of L. moorei closely resembles that of L. tenuirostris, with all the features enumerated above. As in that species, there is a prominent deltopectoral crest, but no obvious dorsal process. There is a small raised process distally,

M C GOWAN AND MILNER: JURASSIC ICHTHYOSAUR 765 TEXT-FIG. 3. Leptonectes moorei sp. nov.; BMNH R 14370; skull in left lateral aspect omitting displaced elements of the right mandibular ramus. A, angular; D, dentary; J, jugal; L, lacrimal; L., left; MX, maxilla; N, nasal; PMX, premaxilla; PO, postorbital; Q, quadrate; R., right; SA, surangular; SP, splenial; SQ, squamosal; sr, sclerotic ring. Scale bar represents 50 mm. Inset, detail of narial margin; 1. on the ventral aspect, just anterior to the peak marking the separation between the radial and ulnar facets. The process is reminiscent of those seen on the distal humerus of Ophthalmosaurus, although the ventral process in that taxon is close to the postaxial margin of the element. In marked contrast with the condition in L. tenuirostris, the radius of L. moorei lacks a notch in its leading edge, the latter being remarkably straight. In this regard L. moorei compares more closely with L. solei, where the radius is similarly not notched. The trailing edge of the radius, and the leading edge of the ulna, are both slightly concave, and it is possible that they may have enclosed a narrow, elliptical, foramen between them. However, such a foramen is distinctly different from the small circular one enclosed between the two epipodials in L. tenuirostris. The ulna is marginally larger than the radius, whereas the reverse is true for L. tenuirostris and for L. solei. Asin L. tenuirostris, there are relatively few phalanges in each digit, the maximum number in articulation being eight or nine, giving a probable count of between 12 and 13 for the number of elements in the longest digit. Pectoral girdle (Text-figs 1 2) The left scapula and most of the clavicle are well exposed, together with the interclavicle, but both coracoids are overlain by the right forefin, obscuring much of their structure. The scapula is 117 5 mm long, and the coracoid has an anterior posterior length of c. 77 mm. The coracoid appears to be rounded, as is typical of Jurassic ichthyosaurs, and there is certainly an anterior notch, as in L. tenuirostris and in L. solei. MATURITY Although the holotype is not a large individual, equally it could not be described as small in comparison with the range of size known for Lower Jurassic ichthyosaurs. Thus, in terms of its forefin and pectoral girdle, it compares in size with SCM 8372, a specimen of L. tenuirostris which has a total body length of 2 6 m. Because this specimen has a crushed skull, few cranial comparisons can be made with the holotype, but these measurements compare quite closely. Their orbital diameters are 107 2 mm (holotype) and 118 mm, whilst the depth of the snout at the level of the external nares is c. 37 mm and 40 mm respectively. Their skull lengths, in contrast, are 328 mm and 600 mm respectively, reflecting the much attenuated snout of the holotype. Given the similarity in their non-rostral measurements, it is inconceivable that the

766 PALAEONTOLOGY, VOLUME 42 TEXT-FIG. 4. Leptonectes moorei sp. nov.; BMNH R 14370; forefins in ventral (right fin) and dorsal (left fin) aspect. dp, distal process; dpc, deltopectoral crest; FF, forefin; H, humerus; I, intermedium; L., left; l-ef, leading-edge facet; R., right; RA, radius; RE, radiale; U, ulna; UE, ulnare. Scale bar represents 50 mm. Inset, right radius and ulna showing partially closed suture between them; 0 33. holotype could represent an immature individual of L. tenuirostris. Furthermore, since the holotype is larger than any of the specimens referred to I. breviceps, as well as most of the individuals of the commonest Lower Liassic species, I. communis, it is equally unlikely that it represents an immature individual of any other taxon. Since the holotype is not small when compared with other Lower Liassic ichthyosaurs, and since only two of its features exemplify immaturity, it is best considered as a subadult individual. Its distinctive features are therefore considered to be characteristic of the species, and not attributable to immaturity. DISCUSSION In reviewing the species L. tenuirostris, McGowan (1989a) drew attention to two variant individuals (BMNH 2009 and BMNH 36182) in which the snout was especially long. This extension results in rostral ratios that lie beyond the observed ranges for the referred specimens. Some individuals of the species also have an overbite, where the tip of the rostrum extends beyond that of the mandible. All of this is suggestive of a degree of plasticity in the rostral region, and it has been suggested that L. tenuirostris may have been a close early relative of the Sinemurian Excalibosaurus costini, a putative ancestor for the swordfish-like ichthyosaur Eurhinosaurus, from the Toarcian (McGowan 1986, 1989b). A recent phylogenetic analysis of eight post-triassic ichthyosaur taxa by Maisch (1998) showed that Leptonectes, Excalibosaurus and Eurhinosaurus form a clade for which the name Leptonectidae was erected. Characters cited as diagnostic for the family include very long and slender snouts, elongate and slender fins with four or three primary digits, a tendency to develop overbite, very large orbits, a very short postorbital skull segment, a reduced fenestra temporalis less than half the size of the orbit, and the lack of distinct surface sculpture on the tooth crowns. Maisch proposed the latter two features as synapomorphies

M C GOWAN AND MILNER: JURASSIC ICHTHYOSAUR 767 of the Leptonectidae. L. moorei shows the characters listed by Maisch with the exception of a long snout and overbite. Although the tips of the premaxillae extend beyond the dentaries, there has been some shearing of the rostrum, as noted earlier, and the apparent slight overbite may be due to this displacement. A more comprehensive analysis of ichthyosaurian genera by Motani (in press) also groups Leptonectes, Excalibosaurus and Eurhinosaurus as a clade. Some of the features they share include: a slender rostrum and mandible; forefin with no more than four or five digits; humerus widely expanded distally; and the distal end of humerus has a leading edge facet. If the rostral region of the holotype of L. moorei were extended such that the skull length was the same as that of SCM 8372 the similarly sized specimen of L. tenuirostris its cranial features would be similar to those of that species. Thus the snout and rostralorbital ratios would fall within the observed ranges for the referred specimens of L. tenuirostris, and, although the prenarial and premaxillary ratios would lie beyond that range, both variables would have values that were less than those for the two longsnouted variants (BMNH 2009 and BMNH 36182). Whilst the ancestry of L. moorei is unknowable, it is conceivable that it could have evolved from L. tenuirostris through rostral reduction. The temporal sequence is compatible with such a speculation, because L. tenuirostris appeared in the Rhaetian (McGowan 1989a), whereas L. moorei is from the Lower Pliensbachian, some 13 million years younger (see Gradstein et al. 1995). Thus the presence of a short snouted leptonectid in the upper Lower Jurassic suggests two contrasting patterns of rostral morphology within the clade, rostral reduction within the genus Leptonectes which ranges from the Rhaetian to the Pliensbachian, and rostral elongation in the Sinemurian Excalibosaurus and Toarcian Eurhinosaurus. Acknowledgements. We thank the collector, Mr Chris Moore, for making this specimen available to The Natural History Museum; and Julian Mulock who drew the figures. We also thank William Lindsay of The Natural History Museum Palaeontology Conservation Unit for preparing parts of the skull, Dr Neale Monks for identifying the belemnites and staff of the Museum s Photographic Unit for producing the photographs and composite figures. REFERENCES ARKELL, W. J. 1933. The Jurassic System in Great Britain. Clarendon Press, Oxford, 681 pp. BLAINVILLE, H. M. D. de 1835. Description de quelques espèces de reptiles de la Californie, précédée de l analyse d un systéme général d erpétólogie et d amphibiologie. Nouvelles Annales du Muséum d Histore Naturelle, 4, 236 296. GRADSTEIN, F. M., AGTERBERG, F. P., OGG, J. G., HARDEENBOL, J., VAN VEEN, P., THIERRY, J. and ZEHUI, H. 1995. A Triassic, Jurassic and Cretaceous time scale. Special Publication of the Society for Sedimentary Geology, 54, 95 126. JOHNSON, R. 1977. Size independent criteria for estimating relative age and the relationships among growth parameters in a group of fossil reptiles (Reptilia; Ichthyosauria). Canadian Journal of Earth Sciences, 14, 1916 1924. MAISCH, M. W. 1998. A new ichthyosaur genus from the Posidonia Shale (Lower Toarcian, Jurassic) of Holzmaden, SW- Germany with comments on the phylogeny of post-triassic ichthyosaurs. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen, 209, 47 78. MCGOWAN, C. 1974a. A revision of the longipinnate ichthyosaurs of the Lower Jurassic of England, with descriptions of two new species (Reptilia: Ichthyosauria). Royal Ontario Museum, Life Sciences Contributions, 97, 1 37. 1974b. A revision of the latipinnate ichthyosaurs of the Lower Jurassic of England (Reptilia: Ichthyosauria). Royal Ontario Museum, Life Sciences Contributions, 100, 1 30. 1976. The description and phenetic relationships of a new ichthyosaur genus from the Upper Jurassic of England. Canadian Journal of Earth Sciences, 13, 668 683. 1986. A putative ancestor for the swordfish-like ichthyosaur Eurhinosaurus. Nature, 322, 454 456. 1989a. Leptopterygius tenuirostris and other long-snouted ichthyosaurs from the English Lower Lias. Palaeontology, 32, 409 427. 1989b. Computed tomography reveals further details of Excalibosaurus, a putative ancestor for the swordfishlike ichthyosaur Eurhinosaurus. Journal of Vertebrate Paleontology, 9, 269 281. 1993. A new species of large, long-snouted ichthyosaur from the English lower Lias. Canadian Journal of Earth Sciences, 30, 1197 1204. 1996. The taxonomic status of Leptopterygius Huene, 1922 (Reptilia: Ichthyosauria). Canadian Journal of Earth Sciences, 33, 439 443.

768 PALAEONTOLOGY, VOLUME 42 MOTANI, R. in press. The phylogeny of the Ichthyopterygia. Journal of Vertebrate Paleontology. CHRISTOPHER MCGOWAN Department of Vertebrate Palaeontology Royal Ontario Museum Toronto Ontario M5S 2C6 Canada Typescript received 29 June 1998 Revised typescript received 8 March 1999 ANGELA C. MILNER Department of Palaeontology The Natural History Museum Cromwell Road London SW7 5BD, UK