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ROBIN HEWISON BOOK 2007.Qxp THE SKULL AND MANDIBLE OF THE STEREOSPONDYL LYDEKKERINA HUXLEYI , (TETRAPODA: TEMNOSPONDYLI) FROM THE LOWER TRIASSIC OF SOUTH AFRICA, AND A REAPPRAISAL OF THE FAMILY LYDEKKERINIDAE, ITS ORIGIN, TAXONOMIC RELATIONSHIPS AND PHYLOGENETIC IMPORTANCE By Robin H. Hewison Oldstream Cottage, 24 Park St. Dunster, Somerset TA24 6SR [email protected] (01643 821525) ABSTRACT. The structure of the skull and mandible of the basal stereospondyl Lydekkerina huxleyi (Lydekker) Broom 1915 , from the Lower Triassic Lystrosaurus Assemblage Zone (Middle Beaufort Group of the Karoo Basin) of South Africa, is re-described in detail on the basis of a study of the holotype skull, BMNH R 507, other previously described material, and some undescribed material, in particular, a small and relatively complete skull, UMZC T110, and an almost complete right mandible, believed to be BP/1/1373. Three new autapomorphies are proposed for Lydekkerina huxleyi : a bi-tuberculated quadrate boss; the presence of a pair of tubercles on the palatal surface of the exoccipitals; and the presence of dentary type teeth within the denticle fields on all three coronoid bones. New, composite reconstructions of the skull and mandible are given, and the genus and species are diagnosed. A reappraisal of the family Lydekkerinidae by means of a chronological review of the morphology of the skull and mandible of all the genera and species that have been assigned to the Lydekkerinidae, concludes that that the content of the family should be restricted to Lydekkerina huxleyi , Broomulus dutoiti, Limnoiketes paludinatans, Deltacephalus whitei , and Eolydekkerina magna. Putterillia platyceps is considered to be a junior synonym of L. huxleyi , and Lydekkerinia kitchingi, Lydekkerina panchetensis and Luzocephalus kochi to be nomina vana . Broomistega putterilli , formerly Lydekkerina putterilli , is adjudged to be a rhinesuchid, and ‘ Parotosuchus ’ madagascariensis (Watsonisuchus madagascariensis ) a mastodonsaurid. It is argued that Chomatobatrachus halei and Luzocephalus blomi are allied more to rhytidosteids than they are to lydekkerinids, and are sufficiently different from each other, and from other forms, to be classified within separate monotypic families. Indobenthosuchus panchetensis and Cryobatrachus kitchingi are assessed as Stereospondyli incertae sedis . The lydekkerinids are considered to be small-sized adult stereospondyls derived from Upper Permian rhinesuchid temnospondyls by the paedomorphic process of progenesis, and the family Lydekkerinidae, to be a specialised basal offshoot of an adaptive radiation of Upper Permian and Lower Triassic stereospondyls. Undoubted members of the family are confined to southern areas of Gondwana viz. South Africa, Madagascar and Australia, but fragmentary remains of ‘lydekkerinids’ have been reported from India, Russia and Brazil. KEY WORDS: Lydekkerina , Lydekkerinidae, stereospondyls, Rhinesuchidae, mastodonsaurids, rhytidosteids, mandible, Lower Triassic, Lystrosaurus Assemblage Zone of South Africa, Permo-Triassic extinction, progenesis, adaptive radiation, Gondwana. INTRODUCTION Lydekkerina huxleyi (Lydekker) Broom 1915, is a small, short-faced, basal stereospondyl from the Lower Triassic Lystrosaurus Assemblage Zone of South Africa. It is by far the most commonly preserved temnospondyl within this assemblage and is known from over 200 skulls and a considerable amount of postcranial material. The holotype, BMNH R507, and two other specimens from the same locality, (R506 and R508), were originally described as Bothriceps huxleyi by Lydekker in 1889, and again in 1890, along with two additional specimens, R504 and R505. In 1915, however, Broom removed this species 1 from the genus Bothriceps (a brachyopid temnospondyl), and established for it the new genus Lydekkerina. The general structure of the skull and mandible of L. huxleyi has become well known through the researches of Watson (1912, 1913, 1919, 1951), Broom (1915), Haughton (1925), Broili & Schröder (1937), Parrington (1948), Warren & Black (1985), Jupp & Warren (1986), Shishkin, Rubidge & Kitching (1996) and Jeannot, Damiani & Rubidge (2006), yet certain important morphological features of the skull and mandible still remain unknown. The holotype skull itself remained virtually unknown until very recently, and our only previous knowledge of it comprised the two original, very brief descriptions by Lydekker (1889 and 1890), and his figure of the skull roof (1890, fig. 41), which showed merely the outline of the skull and the position and apparent sizes of the nares, orbits and otic embayments. Although Watson (1919) referred directly to the structure of the holotype skull, it is clear from his text, and a personal knowledge of the specimens he was studying, that he was referring, not to the holotype itself, but to two related specimens from the same locality, BMNH R505 and R506. Many years ago, in connection with this reappraisal of the Lydekkerinidae, the dorsal surface of the holotype skull was cleared of matrix at the British Museum of Natural History, and is described and illustrated here in as much detail as is possible. The palatal and occipital surfaces were not prepared at that time, due to the fragile nature of the skull, but improved preparation techniques have now made this possible (Jeannot et al 2006), and relevant information regarding the morphology of the palate and occiput have been incorporated into this account. A detailed study has also been made of a small, well ossified and virtually complete skull of Lydekkerina huxleyi , UMZC T110, (formerly D.M.S.Watson B.156), which consists of 10 interlocking pieces and some fragments, that enable many new features of the skull roof, palate, occiput, palatoquadrate complex, neurocranium, otic region and stapes to be studied, illustrated and described. This skull, is the ‘unnamed form’ that Cosgriff (1974) recognized as probably being conspecific with L. huxleyi , a view that is confirmed by this present study. The skull has been figured in various views and sections by Watson (1951, fig 15 A-E), but has not until now been described. A detailed study has also been made of other previously described material of L. huxleyi , as well as some undescribed material, of which an almost complete right mandible, that was once in the possession of D.M.S.Watson, and believed to be BP/1/1373 , has allowed many new features of the mandible to be described and illustrated. The new information resulting from these studies has made it possible to give a more informative diagnosis of Lydekkerina huxleyi , and to provide new and more detailed, composite reconstructions of its skull and mandible. L. huxleyi was the only member of the family Lydekkerinidae when this was established by Watson (1919), but since then, fourteen additional species have been referred to the family: Lydekkerina putterilli Broom, 1930; Putterillia platyceps Broom, 1930; Broomulus dutoiti (Broom) Romer, 1947; Limnoiketes paludinatans Parrington, 1948; Lydekkerina kitchingi Broom, 1950; Deltacephalus whitei Swinton, 1956; Lydekkerina panchetensis Tripathi, 1969; Indobenthosuchus panchetensis Tripathi, 1969; Cryobatrachus kitchingi Colbert & Cosgriff, 1974; Chomatobatrachus halei Cosgriff, 1974; Luzocephalus blomi Shiskin, 1980; Eolydekkerina magna Shishkin, Rubidge & Kitching, 1996; ‘ Parotosuchus ’ madagascariensis (cladogram of Damiani 2001); and Luzocephalus kochi Jeannot, Damiani & Rubidge, 2006. Almasaurus habbazi Dutuit (1972), was added in error by Warren & Black (1985 p323). L. huxleyi and L. kitchingi are known from an abundance of cranial and postcranial material, but the remaining species are known virtually only from isolated skulls, and for this reason, all definitions of species, genera and families that are presented here are based solely on cranial material. Watson’s original definition of the family Lydekkerinidae (Watson 1919) was based mainly upon BMNH R505 and R506, two specimens of L. huxleyi from the same locality as the holotype, but, with the growth of the family, later diagnoses of the Lydekkerinidae, came to be founded on genera, other than Lydekkerina . The diagnosis by Cosgriff (1974), for example, was based largely upon the skull structure of Chomatobatrachus halei and Cryobatrachus kitchingi , two new temnospondyls he had assigned to the family, whilst that of Shishkin et al (1996), was mainly derived from the structure of the skulls of Chomatobatrachus halei, Luzocephalus blomi and a newly described form, Eolydekkerina magna. The most recent diagnosis of the family, that of Jeannot et al (2006), was also based partly on 2 Chomatobatrachus, Luzocephalus , and Eolydekkerina , as well as Lydekkerina , and was essentially a synthesis based on previous work (eg Milner, 1991; Shishkin, et al 1996), combined with a study of the holotype skull and a large number of L. huxleyi specimens, mainly from South African collections. The content of the family Lydekkerinidae, its distribution, phylogenetic relationships and habits have been briefly summarised by Warren (2000), and dealt with in greater depth by Schoch & Milner (2000), and, although the membership of the family has been reviewed again recently by Jeannot et al (2006), there remains a great deal of uncertainty and disagreement about the content of the family, and its relationships. Apart from Lydekkerina huxleyi , only Eolydekkerina has achieved universal acceptance into the family. Of the remaining taxa, Broomulus , Putterillia , Lydekkerina putterilli and Limnoiketes
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