Platyrhinops Lyelli (Amphibia: Temnospondyli) J
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Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 100, 275–295, 2010 (for 2009) Morphology and systematics of the Pennsylvanian amphibian Platyrhinops lyelli (Amphibia: Temnospondyli) J. A. Clack1 and A. R. Milner2 1 Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK 2 Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK ABSTRACT: The morphology of the amphibamid temnospondyl amphibian Platyrhinops lyelli from the Middle Pennsylvanian of Linton, Ohio is reassessed from previously described and undescribed specimens. Newly reported or newly significant features include the presence of bicuspid marginal teeth, anteriorly widened frontals, elongate choanae, a broad rhomboidal sphenethmoid and a pair of flattened blade-like ceratohyals. One specimen shows an unusual distribution of tooth sizes along the premaxillaries. No derived characters can be found to justify reference of the species lyelli to the genus Amphibamus as represented by its type species A. grandiceps, but Platyrhinops does belong to the ‘Amphibamus branch’ of the Amphibamidae. KEY WORDS: Amphibamidae, Carboniferous, dentition, Dissorophoidea, Ohio Platyrhinops lyelli is based on a small number of amphibian cellate teeth in A. grandiceps, concluding that it was a juvenile specimens from the mid-Pennsylvanian locality of Linton, dissorophid. Ohio, USA. It belongs to the clade of temnospondyl amphib- In the context of lissamphibian origins, Trueb & Cloutier ians known as the Dissorophoidea – more specifically, the (1991) distinguished four families of dissorophoids; one family Family Amphibamidae – which have long been part of the in particular – the Dissorophidae – included the genera Dole- debate over the origin of modern amphibians. Recent work has serpeton, Tersomius and Amphibamus, amongst others. The focused on this family, with several new taxa having been other three families were the Trematopidae, the Micromeler- described in the past few years. Inclusion of Platyrhinops lyelli petontidae and the Branchiosauridae. The latter two families in this debate has relied on old or incomplete descriptions: this are those containing the mainly paedomorphic forms generally paper aims to clarify its anatomy so that it can more accurately known as branchiosaurs. be included in ongoing cladistic, systematic and morphological Milner (1993) and Clack & Milner (1994) attempted to studies. clarify the systematic position of some of the dissorophoid taxa For many years, Platyrhinops lyelli was considered con- and recognised that Amphibamus grandiceps and A. lyelli did generic with Amphibamus grandiceps, a small temnospondyl not share any characters that could unite them in a single from the Late Carboniferous of Mazon Creek, Illinois. Both genus. They placed the species lyelli within the next available have a history of being perceived as frog-like and being generic name (see synonymy history below), Platyrhinops. possible early relatives of frogs. From its first description, They reinstated the Family Amphibamidae for Doleserpeton, Platyrhinops (as ‘Pelion’) was noted as remarkably frog-like by Tersomius, Amphibamus and Platyrhinops. Milner (1993) Wyman (1858), Cope (1868) and Moodie (1916). Under the placed Amphibamidae, Branchiosauridae and Lissamphibia in generic name ‘Miobatrachus’, Amphibamus grandiceps was an unresolved trichotomy, with the Dissorophidae, Micro- more specifically implicated in the origin of modern amphib- melerpetontidae and Trematopidae, in that order, as succes- ians by Watson (1940), who pointed out many potential sively stem-ward families. similarities between it and anurans. Reig (1964) suggested Since then, many anatomical and phylogenetic studies of the ‘branchiosaurs’, a group of poorly known larval temnospon- dissorophoids have been undertaken, resulting in a growing dyls, as lissamphibian ancestors. Branchiosaurs and amphib- consensus over the content and intra- and inter-relationships amids, then and now, are considered as closely related (e.g. of the Amphibamidae, especially with respect to their closeness Schoch & Milner 2008). Carroll (1964) and Estes (1965) agreed to modern amphibians. This work is summarised below in that dissorophoids, and Amphibamus grandiceps in particular, section 8. were close to the ancestors of lissamphibians. At the time, the inter- and intra- relationships of the dissorophids as a family remained unresolved. Institutional abbreviations Bolt (1969) described the small dissorophoid temnospondyl Doleserpeton from the Early Permian of Oklahoma, and AMNH – American Museum of Natural History, New York, erected the monotypic family Doleserpetonidae for it. This USA animal not only had pedicellate teeth but they were bicuspid: BMNH – Department of Palaeontology, Natural History these were lissamphibian-like features. Later, Bolt (1974) Museum (previously British Museum (Natural History)), Lon- described morphological features of Amphibamus, Tersomius don, UK and other dissorophids, separating them from the Doleser- CM – Carnegie Museum, Pittsburgh, USA petontidae. Bolt (1979) described bicuspid and possibly pedi- FMNH – Field Museum of Natural History, Chicago, USA Downloaded 2010 from (forhttps://www.cambridge.org/core 2009) The Royal Society. University of Edinburgh. of Athens, on doi:10.1017/S1755691010009023 08 Oct 2021 at 02:06:44, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1755691010009023 276 J. A. CLACK AND A. R. MILNER MB – Museum fur Naturkunde, Humboldt Universita¨t, Romer 1947), an embolomere (Romer 1930 as ?Leptophractus Berlin, Germany lancifer Romer, 1963), a lepospondyl (Moodie 1916) and even MCZ – Museum of Comparative Zoology, Harvard Univer- doubtfully as a captorhinomorph (Romer 1966). sity, Cambridge, Massachusetts, USA In the early twentieth century, the Linton material was USNM – National Museum of Natural History, Washington extensively reviewed by Moodie, who described a fourth D.C., USA specimen, another incomplete postcranial skeleton (USNM YPM – Peabody Museum, Yale University, New Haven, USA 4461), as an example of the nectridean Diceratosaurus puncto- lineatus (Moodie 1909b, 1916). In 1931, Steen transferred the type of ‘Tuditanus’ mordax 1. Systematic history of the Platyrhinops material and a newly described well-preserved skull (BMNH R.2670) to a new genus Platyrhinops. At the time, no other good skull of Platyrhinops lyelli has a complex synonymy, the holotype and Platyrhinops was known from Linton and the type material some of the referred specimens having long and distinct had just been confused with Saurerpeton by Romer, so Steen’s taxonomic histories. In the following brief account, the pre- Platyrhinops mordax was more correctly recognised as a primi- 1964 histories of five of the most significant specimens are tive dissorophid quite distinct from Pelion lyelli (Romer 1947; reviewed separately in the interest of clarity, followed by an Gregory 1950). integrated history of subsequent research. Thus, by 1947, the type of Pelion lyelli was confusingly Following the description of Baphetes planiceps by Owen in associated with basal dvinosaurian material, most other speci- 1854, the second Carboniferous amphibian specimen to be mens were variously misidentified or undetermined, and only recognised as such was a skeleton collected by Dr John V. Steen’s skull of Platyrhinops was relatively correctly assigned Lauderdale in 1857 from the mine dump at Linton, Ohio. The to the Dissorophidae. One consequence of this confusion was type specimen of Colosteus scutellatus had in fact been de- that the Linton material had ceased to be seriously considered fi scribed earlier, in 1856, but as a sh (see Hook 1983 for in relation to the origin of Lissamphibia. When Watson (1940) details). Lauderdale’s specimen was reported as a ‘batrachian and Romer (1947) had emphasised the derived, possibly frog- reptile’ by Wyman in late 1857. Although only poorly visible in like features of Amphibamus grandiceps (as ‘Miobatrachus ventral aspect, it was immediately observed to be frog-like in romeri’) from Mazon Creek, no consideration was given to the general appearance and initially named Raniceps lyelli by contemporaneous Linton material. In 1950, Gregory argued Wyman in 1858. However, because the genus Raniceps was that Amphibamus grandiceps was more primitive and less preoccupied, a new generic name Pelion was created for it by frog-like than Watson had suggested, and that although its Cope in 1868. The main slab of the holotype specimen relationship to frogs was plausible, there were no positive (AMNH 6841) bore an almost complete skeleton of a small features to support it. The resemblances noted by other amphibian visible in ventral aspect and it was immediately authors were argued by Gregory to be similarities associated recognised as strikingly frog-like in its possession of elongate with small size. limbs and a broad, rounded skull with large palatal vacuities. The acid-etching of the holotype of Pelion lyelli by Dr Wyman (1858), Cope (1868, 1875) and Moodie (1916) all Donald Baird in the 1950s revealed a skull roof like that of the commented on its frog-like appearance, and Moodie went so small Permian dissorophid Tersomius, and it was immediately far as to place it in the Order Salientia. Cope (1875) and later clear that none of Romer’s referred material belonged with the authors placed it in its own family, the Peliontidae. While