Sciency Thoughts: An early Tetrapod from the Late Carboniferous of the Czech Republic.

Monday, 3 November 2014

An early Tetrapod from the Late Carboniferous of the Czech Republic.

In the 1870s the Bohemian palaeontologist Antonín Frič collected a number of early Tetrapod specimens from the Humboldt Mine at Nýřany in the Plzěn-Manětín Basin in what is now the Czech Republic. These were described in his ‘Fauna der Gaskohle und der Kalksteineder Permformation Böhmens’, which was published in several volumes from 1879 onwards. One of these fossils was described as Diplovertebron punctatum in the 1885 volume of the work, and was subsequently redescribed under several other names by different workers, including by Frič himself, with some confusion as to which specimens should be referred to this species. The specimens have been considered important to our understanding of the evolution of the early Amniotes (Vertebrates capable of laying a watertight egg on dry land or descended from one that was), but were difficult to access for outside palaeontologists for much of the 20^th Century due to political turmoil in Central Europe (for example an attempt to examine the specimens by Richard Carroll in 1968 was cut short after two days by the Russian invasion of Czechoslovakia).

In a paper published in the Journal of Vertebrate Palaeontology on 8 July 2014, Jozef Klembara of the Department of Ecology at Comenius Universityin Bratislava, Jennifer Clack of the University Museum of Zoology in Cambridge, Andrew Milner of the Department of Earth Sciences at The Natural History Museum in London and Marcello Ruta of the School of Life Sciences at the University ofLincoln redescribe a series of specimens from Nýřany, including Frič’s Diplovertebron punctatum under the name Gephyrostegus bohemicus.

Klembara et al. acknowledge that the name Diplovertebron punctatum has priority (was used first), which usually means that it should be the correct name for use in all subsequent publications, but note that current rules on taxonomy allow for the use of a more recently allocated name if the original name was allocated prior to 1899 and the other name has been used extensively in subsequent publications, and for this reason use Gephyrostegus bohemicus, a name first used by Otto Jaekel in 1902, and preferred in most 20^th Century publications on the species.

Gephyrostegus bohemicus. Photograph (A) and drawing (B) of skull and right lower jaw in dorsal view. Klembara et al. (2014).

Klembara et al. refer four specimens to the species, Frič’s original Diplovertebron punctatum specimen, a skull and partial post-cranial skeleton preserved as part and counterpart on two blocks, a partially disarticulated skull in part and counterpart described by Frič as Hemichthys problematica in 1895, a partially articulated skull, and an almost complete skeleton described by Brough and Brough as Gephyrostegus watsoni in 1967.

Gephyrostegus bohemicus. Photograph (A) and drawing (B) of skull and right lower jaw in ventral view. Klembara et al. (2014).

Gephyrostegus bohemicus has an elongate skull 25-58 mm in length with teeth on its palate as well as its jaw bones (a common condition in modern fish, but not terrestrial vertebrates). The teeth are conical and curve backwards, a state typical of small carnivores or insectivores which capture live prey then swallow it whole with little processing. Klembara et al. suspect that even the larges specimens are not adults, as they show incomplete ossification of the skeleton (in young vertebrates the skeleton is formed largely of cartilage, then ossifies – turns to bone – as the animal matures).

Gephyrostegus bohemicus. Photograph of galvanoplastic cast of skull and lower jaws in right lateral view produced by Frič and described as Hemichthys problematica. Klembara et al. (2014).

Gephyrostegus bohemicus was placed in the family Gephyrostegidae by Jaekel in his 1909 classification of Tetrapods. Two other fossils have been assigned to this family, both from the Late Carboniferous. Eusauropleura digitata is a North American species first described by Edward Drinker Cope in 1968, and which was most recently redescribed by Carroll in 1970 from two post-cranial (headless) skeletons. Klembara et al. feel that this species is not well enough understood for its relationships to be discussed with confidence at this time, though they do plan to re-examine it in the near future.

The third species currently assigned to the Gephyrostegidae is Bruktererpeton fiebigi, described from a partial skeleton from Germany in 1973. This species does appear to be very similar to Gephyrostegus bohemicus, although it does differ in several important details of the cranial anatomy, and Klembara et al. conclude that the two species are indeed closely related and should be placed in the same family.

Gephyrostegus bohemicus.Reconstruction of skull in dorsal (A) and ventral (B) views. Klembara et al. (2014).

Previous analyses of the relationships of the Gephyrostegidae to other early Tetrapods have suggested that the group are stem-Amiotes (i.e. more closely related to modern Reptiles, Mammals and Birds than to modern Amphibians), but their exact relationship to other groups has been hard to determine. Re-analysis of the group’s position with a better understanding of the anatomy of Gephyrostegus bohemicus and with Eusauropleura digitata excluded from the analysis strongly suggests that the Gephyrostegidae are closely related to the Seymouriamorpha, a group of early Tetrapods long thought to be stem-Amniotes, but now generally considered to be more closely related to modern Lisamphibians (Frogs, Salamanders etc.) than Reptiles, following the discovery of larval forms with external gills.