History of discovery
The holotype of ''Platyhystrix'' (AMNH FARB 4785) was first discovered in the Early Permian Cutler formation in Rio Arribas Co, New Mexico in 1881 by American paleontologist, E.D. Cope. The holotype consisted of a few fragmented neural spines, and was initially listed under the species name, ''Zatrachys apicalis''. In 1910, American paleontologist E.C. Case reclassified the neural spines as belonging to a new species of the pelycosaurian reptile '' Ctenosaurus'', ''Ctenosaurus rugosus'' (the specific name means "wrinkled, shriveled"), since they resembled spines from Texas belonging to ''C. koeneni'', described by Friedrich von Huene. In Case’s description of the ''Platyhystrix'' holotype, he initially classified it as part of a new reptile specimen, but still noted tubercles along the neural spines which were similar to the projections found on amphibian skulls. It was S.W. Williston who created the genus in 1911, and placed it within the Temnospondyli order once fractured skull elements were described in 1916.Description
Skull
Compared to other dissorophid temnospondyls, ''Platyhystrixs skull is rather large (over 19 cm long along midline), as well as long and narrow when analyzed in dorsal view. There is a wide variety in dermal sculpturing which occurs along the dorsal and lateral portions of the skull. Large ridges and tubercle-like processes are present along the dorsal half of the orbital rim, edges of the skull table, and areas which adjoin the cheek. Nodular-like processes are most pronounced on the posterior portion of the skull roof, on the postorbital, squamosal, supratemporal, and tabular. The central dorsal portion of the skull is characterized by a reticulated pitting pattern, which becomes finer as it extends towards the nasals. These kinds of dermal ornamentation are what diagnose ''Platyhystrix'' from other members of Dissorophidae. Based on marginal dentition and preserved portions of the premaxilla, ''Platyhystrix'' may have had upwards of 65 teeth on either side of the upper jaw, in the form of simple, pointed pegs. Other diagnostic features of the skull include: long and narrow nasals, whose length is equivalent to approximately one third of the midline length of the skull; posteriorly closed otic notch; parietal is large and extends anteriorly beyond margin of orbit; parietals are longer than the frontals; postfrontal length is greater than twice its width and equal to the length of the supratemporal; cheek is steeply inclined and meets the skull table at nearly right angle.Dorsal Blades
The most notable characteristic of ''Platyhystrix'' is its elongated neural spines. Initially, these “spines” were thought to be an extension of the neural arch above the transverse process. However, the reassignment of ''Platyhystrix'' to the armored Dissorophidae clade and the blade’s extensive ornamentation led Vaughn in 1971 to reinterpret this feature as an osteoderm that was fused to the true neural spine instead. This reclassification suggested a superficial convergence with the neural spines present in synapsids. Histological analysis revealed that the dorsal blades of ''Platyhystrix'' do indeed share histological features and were likely homologous with the internal osteoderm series present in other dissorophids. This evidence points to the blades being of dermal origin, and are a novel example of dermal-endochondral co-ossification in a Paleozoic tetrapod. It is estimated that the notable sail was made up of a range of 11-15 laterally compressed and distally expanded blades. Most of the distal length of these dorsal blades is covered with ridges and pustules, similar to the dermal ornamentation seen on the skull. Similar to ''Edaphosaurus'', a paired set of lateral tubercles can be found proximally on the blade, and while some spines curve anteriorly, the rest exhibit severe curving toward the pelvis. The purpose of the blades still remains unknown, although many theories have been postulated over the years. The use of the sail for thermoregulation, sexual dimorphism, and species identification are all hypotheses that have been applied to other sail-bearing taxa. A speculation more specifically related to ''Platyhystrix'' is that these neural spines could have served to stiffen the vertebral column in order to adapt to a more terrestrial lifestyle.Paleobiology
''Platyhystrix'' may have been preyed upon by larger temnospondyls such as '' Eryops'', or by larger carnivorous reptiles, which were becoming more common and diverse in the drier climate of the Permian. The skull was large and strongly built, with a frog-like face. ''Platyhystrix'' had a compact body, reaching long including the tail, and its short, sturdy legs indicate a mainly terrestrial life. ''Platyhystrix'' appeared rather unusual: the dorsal vertebrae were extraordinarily lengthened, and in life they probably formed a skin-covered sail. This structure was possibly for thermal regulation, as in other animals of similar appearance, such as the pelycosaurs '' Dimetrodon'' and '' Edaphosaurus''. The back of ''Platyhystrix'' was also covered with thick hard plates, similar to those of its near relative, '' Cacops''.Stratigraphic and geographic range
''Platyhystrix'' is primarily known from Early Permian formations from the southwestern United States. They are the most common rhachitomous amphibian fossils found in Wolfcampian age formations (~295-280 mya.), with one exception being El Cobre Canyon in north-central New Mexico, which has been debated as being dated around the early Permian or late Pennsylvanian (a stage of the Late Carboniferous). The best casting of neural spines comes from the Cutler Formation in southwestern Colorado. The part of the formation in which the specimens were collected were confidently identified as belonging to the Wolfcampian age of the Early Permian, with the sediment composition denoting the presence of an ancient lake or river. Additional neural spines have been found in sites scattered around New Mexico, southeastern Utah, and southwestern Colorado. Vaughn further describes one of the most productive regions of the EarlyPaleoecology
New Mexico, and other states in the Rocky Mountain regions, were situated about ten degrees north of the equator, on the western edge of Pangea during the Early Permian. Throughout the Wolfcampian stage, most of northern and central New Mexico was composed of rivers which emptied into a shallow sea. The rivers that deposited the Abo Formation were made of fine silt beds, and were up to 8 m deep and 50 m wide, and reflect semi-arid and semi-humid conditions suggested by paleosols found on adjacent floodplains. These river size estimations infer a yearly precipitation rate between 30–100 cm/year. The vertebrate and plant fossil composition of these areas also provide insights as to the relative paleoclimate which supported these communities. The infrequent paleoflora (conifers, seed ferns, and Walchia) found in these areas are all species known to be adapted to xerophytic (dry) conditions. It is also believed that these kinds of plants began to dominate the American Southwest as the Early Permian climate trended toward drier and more seasonal conditions. Vertebrate fossils found at these Wolfcampian sites consist of fish, amphibians, and reptiles, including charismatic pelycosaur reptiles such as ''Edaphosaurus''. The presence of large ectotherms, assuming that these extant reptiles are similar to modern day species, hints at a daytime temperature range between 25-41 °C, and not dropping below 5 °C in the winter season.Phylogeny
''Platyhystrixs relationship within and amongst the Dissorophidae family has been highly contested since its discovery and description. Carroll and DeMar spent the 1960s attempting to explain the relationships amongst the wide variety of genera within the family, and while their phylogenies depicted differing conclusions, they had reached a general consensus when it came to the basal versus advanced groupings. While the skull described by Berman in 1981 was able to provide better context, DeMar’s logic placed ''Platyhystrix'' as more structurally advanced than other Wolfcampian dissorophids. Carroll’s logic (based on vertebral armor evolution) placed ''P. rugosus'' at a more basal position amongst Permian dissorophids, with the closest sister taxon being '' Aspidosaurus'' of the late Pennsylvanian. In 2012, Schoch produced the most comprehensive dissorophid temnospondyl phylogeny to date, analyzing 25 taxa and 70 characters in total. The result of this analysis placed ''Platyhystrix'' and ''Aspidosaurus'' as successive sister taxa of all other dissorophids. ''Platyhystrix'' was separated from ''Aspidosaurus'' using two osteoderm characters, and this placement additionally agrees with the stratigraphy of fossil discovery. More recently, new phylogenetic analyses have been produced in order to better understand the evolution of modern day lissamphibians, due to the increasing consensus that their monophyletic group is derived from Temnospondyli. In 2019, Atkins, Reisz, and Maddin used characters relating to braincase simplification over time to construct a new phylogeny of lissamphibian origin. Based on the characters used in parsimony analysis, ''Platyhystrix'' was noted as having a much more basal position relative to Schoch’s findings, as a sister taxon to the Olsoniformes clade. Cladogram produced by Schoch (2012):References
* Vertebrate Palaeontology by Michael J. Benton and John Sibbick * The Simon & Schuster Encyclopedia of Dinosaurs and Prehistoric Creatures: A Visual Who's Who of Prehistoric Life by Barry Cox, Colin Harrison, R.J.G. Savage, and Brian Gardiner * Bibliography Of Fossil Vertebrates 1934-1938 by C. I. Camp {{Taxonbar, from=Q135586 Dissorophidae Carboniferous temnospondyls of North America Cisuralian temnospondyls of North America Permian geology of Texas Prehistoric amphibian genera Cutler Formation Fossil taxa described in 1910