Paleontologists describe giant amphibian that lived 260 million years ago
AGÊNCIA FAPESP/DICYT A new analysis by a group of paleontologists offers a more detailed portrait of one of Brazil’s most interesting fossil species, Australerpeton cosgriffi, a giant relative of modern amphibians that lived during the Permian period 260 million years ago. Despite its link to today’s frogs and salamanders, A. cosgriffi was 2.5 m long and resembles a crocodile, with a long tapering snout and scaly body.
The new description of the species, which emphasizes its unique cranial morphology, was published in the Zoological Journal of the Linnean Society last month. Its first author is Estevan Eltink, who earned his PhD at the University of São Paulo’s Paleontology Laboratory in Ribeirão Preto, Brazil, with support from FAPESP. A previous paper on a recently discovered new specimen of A. cosgriffi was published by the Journal of Vertebrate Paleontology in May 2014.
The paper published last month was also co-authored by Max Langer, another researcher at the same lab as Etlink and the principal investigator for the Thematic Project entitled “The origin and rise of dinosaurs in Gondwana (late Triassic-early Jurassic)”. Langer supervised Eltink’s PhD research. The other co-authors are Eliseu Dias from Western Paraná State University, Sérgio Dias-da-Silva from the Federal University of Santa Maria (Rio Grande do Sul), and Cesar Schultz from the Federal University of Rio Grande do Sul (UFRGS).
According to Eltink, the first fossils of the species were found in Serra do Cadeado, Paraná, in the 1970s and 1980s by the late Mário Costa Barberena, also a paleontologist at UFRGS. Barberena provided the first formal description of A. cosgriffi in 1998, dealing precisely with its cranial morphology. Other studies of its postcranial material, i.e., the rest of the skeleton apart from the skull, were reported in the following decade.
“Although the species has already been studied a good deal, Barberena’s description was superficial, requiring a more detailed review of its anatomy and an in-depth comparison of its structures,” Eltink said. There was also considerable uncertainty regarding its proper place in the “family album” of primitive amphibians.
To be more precise, A. cosgriffi belongs to the order Temnospondyli, a large group of animals that enjoyed considerable evolutionary success in terrestrial and aquatic environments for more than 200 million years. Before they disappeared, a subgroup gave rise to modern amphibians, according to the prevailing hypothesis among paleontologists.
A. cosgriffi is the most complete temnospondyl ever collected in South America. Its very long, pointed snout – the technical term is “longirostrine” – resembles that of animals that existed in what is now Russia.
However, there was also a possibility that the species was more closely linked to contemporary temnospondyls from South Africa with a short snout, i.e., a brevirostrine mandible. The researchers set out to elucidate this question by means of a detailed analysis of the skulls from this species discovered so far and by comparing their morphology to those of other temnospondyls.
The researchers reassessed the systematic position of A. cosgriffi using a new matrix of 221 characteristics. The most important feature in placing the species on the family tree of primitive amphibians was the unique shape of its ear, especially the tympanic cavity, which helped classify it as a relative of the South African forms known as Rhinesuchidae.
“In the Russian forms, represented by the Platyoposaurinae with their strikingly longirostrine mandible, this trait is a result of convergent evolution,” Eltink said.
Convergent evolution is the process whereby two lineages of animals that are not closely related independently evolve similar morphological adaptations as a result of natural selection in similar environments or ecological niches.
The question, of course, is this: convergence to what end? Both the shape of A. cosgriffi’s snout and its dentition suggest a diet of fish, which must also have been the case for the Russian species.
A modern animal that superficially resembles A. cosgriffi is the gharial (Gavialis gangeticus). Also called the Indian gavial, its elongated narrow snout reduces water resistance when it snaps its jaws underwater to catch fish. “Although the groups are phylogenetically distant, their cranial morphology is similar,” Eltink said.
“Thus, similar feeding habits often lead to similar cranial morphology. Among the temnospondyls, for example, this morphology appears in three different lineages: Archegosauridae, Rhinesuchidae, and Trematosauridae.”
Observing the overall evolutionary history of tetrapods (all vertebrates except fish), he went on, “the elongated shape of the skull appears convergently in various aquatic and semi-aquatic lineages, so it seems to be quite plastic”.
A predator of this size would require a very spacious semi-aquatic habitat. At the time the species was alive on Earth, the continents were assembling to form the supercontinent Pangea, joining up what is now the northwestern coast of Africa and the eastern coast of South America.
This process formed a transitional environment between continental river areas and drier regions, and it is possible to imagine a coastline with dunes and freshwater lagoons in which A. cosgriffi swam and fed on fish. Indeed, it may have been an environment not unlike Lençóis Maranhenses in northeastern Brazil.