Sylvain Duffaud, Laboratoire de Paléontologie des vertébrés, U.R.A. 1761, case 106, Université Pierre et Marie Curie, 4, place Jussieu, 75252 Paris cedex 05 France.
The fossiliferous site of Champ-Garimond was discovered more than sixty years ago. This name was given to honour Saturnin Garimond who, interested in fossils, was the first to observe the presence of huge bones in a very restricted area of a field near Fons-outre-Gardon (Languedoc, southern France). These bones were later identified as dinosaur remains.
In the early 60’s, a team from the Université Montpellier II conducted a study on the microremains. The site where the sediment was taken from is extremely reduced in surface. It represents the latest part of the Cretaceous which crops out in this part of the basin. Champ-Garimond has yielded a rather rich and diverse fauna including gasteropods, fishes (a Lepisosteidae, a Teleostean and an undeterminate fish, perhaps Amiidae), turtles (two species at least), crocodilians (a Crocodilidae, and an Alligatoridae), dinosaurs and two teeth of an Eutherian (Ledoux et al, 1966).
The Lissamphibia are represented by few remains : two vertebrae, a single fragmentary humerus and four partly broken dentaries of salamanders (belonging to at least two, possibly three, taxa) ; and distal ends of two humeri, showing the presence of two frog species.
The gasteropod assemblage indicates a Valdo-Fuvelian or Begudo-Rognacian age. On the basis of the presence of the charophyte Amblyochara media, the site is considered as being Valdo-Fuvelian (Campanian) in age (Babinot et al, 1983).
Batrachosauroididae indet. Stereophotograph 1 : lateral view of the right side of the vertebra ; the picture is taken slightly from the above. Magnification X6. Stereophotograph 2 : anterior view of the vertebra. Magnification X6.
Indeterminate CaudataRemains of Caudata from the Mesozoic are rare ; it is interesting to note in the Champ-Garimond fossils the presence of a second taxa of salamander, represented at least by a single vertebra and perhaps by two broken dentaries. The vertebra is slender, small, and elongate. The centrum is opisthocoelous, slightly depressed dorso-ventrally and slightly arched dorsally in lateral view. The condyle is hollowed by a broad and not deep depression, but with a central pit. Zygapophyseal ridges are well-developed (the vertebra may have appeared rectangular in dorsal view) and pass through the dorsal rib-bearers. A horizontal ridge connects the ventral rib-bearer to the cotyle and the condyle. These characteristics are shared by the families Plethodontidae and Salamandridae. On the basis of their stratigraphical and present geographical distribution, an attribution to the family Salamandridae could be tentatively proposed (Salamandridae are mostly Europeans, and known since the Paleocene). But the pattern of the ossification of the centrum is not as what is usually observed in this family, and could remind in some way the plethodontid pattern (see Estes, 1969). Although this Caudata belongs to one of these two families, it however remains “indeterminate”. Nevertheless, it is one of the earliest recording of a « modern » salamander. It would be interesting to reach a familial determination of this “indeterminate Caudata” of the late Cretaceous of Europe ; this would be possible only if new fossils, particularly cranial remains, are found. |
The vertebra is robust, opisthocoelous, and approximately 4.5mm long. Its proportions may indicate a very anterior position in the vertebral column.
The centrum is hourglass shaped with very strong cotyle and condyle. It bears a well-developed but narrow median keel, running from the cotyle to the condyle, with its ventral limit nearly straight. The condyle is very prominent, with a deep central notochordal fossa. Neither subcentral nor neural nerve foramina are present. The neural arch is massive, wider than the centrum. The posterior part of the neural arch is missing.
No well developed zygapophyseal ridges are present ; a horizontal ridge connects the ventral rib-bearer to the condyle and the cotyle. A transversal crest units the dorsal rib-bearers to the neural spine.
The rib-bearers, strongly diverging, are connected by a vertical bony lamina.
Within the Caudata, true opisthocoely is found only in the three families Batrachosauroididae, Plethodontidae and Salamandridae. The prominent ring-shaped condyle with its deep notochordal fossa is a distinctive feature of the family Batrachosauroididae.
Batrachosauroidids are known from upper Cretaceous to Pliocene in North-America, and a probable member of the group, Palaeoproteus, is found in the Paleocene and the Eocene of Europe. Older batrachosauroidids have been described (Nessov, 1981) or reported (Ensom et al, 1991, Evans & Milner, 1995) from Eurasia.
Palaeoproteus, the tertiary European member of the family, has been tentatively assigned to the Batrachosauroididae mainly on the basis of the morphology of its atlas, and because of the resemblance between the dentaries of Palaeoproteus and Prodesmodon. In other aspects, it is unlike the other members of the family (“nothing like Palaeoproteus has yet been found in America”, Estes, 1981). It strongly differs from the Batrachosauroididae from Champ-Garimond in having amphicoelous vertebrae with peculiar prominent hypapophyses.
Within the family, the vertebrae of Batrachosauroides are very similar to the vertebra found at Champ-Garimond in their overall morphology : opisthocoely, large uncalcified depression in condyle, lack of basapophyses, divergent rib-bearers, and general massive structure with a very strong, ring-like, partially ossified condyle.
The Batrachosauroididae from Champ-Garimond appears to be more closely related to Batrachosauroides, from the early Eocene and the Miocene of North-America, than to the coeval Opisthotriton and Prodesmodon of North-America, or to the European paleocene form Palaeoproteus.
Most Batrachosauroididae seem to have been of aquatic habits, with elongated body form and reduced limbs, with the probableexception of Batrachosauroides gotoi (lower Eocene, North-America), whose vertebrae bear strongly divergent rib-bearers, and show no trace of an extensive development of muscle crests. Batrachosauroides dissimulans (lower Miocene, North-America), the other species of the type-genus, displays somewhat more robustly built vertebrae, but with closely appressed rib-bearers ; this could indicate less developed limbs than in B. gotoi (Estes, 1981).
The Batrachosauroididae from Champ-Garimond possess widely-separated rib-bearers ; it doesn’t seem to show any special aquatic adaptation.
Palaeogeography and early history of the group : various hypotheses.
The opening of the Atlantic ocean was initiated by the middle Cretaceous ; at that time, the separation of North-America and Europe was not completed but deeply engaged. Assuming that the Batrachosauroididae from Champ-Garimond is closely related to Batrachosauroides, the presence of these two taxa in both the late Cretaceous of Europe and the early Eocene of North-America could be explained either by a direct crossing, posterior to this opening, or by a vicariant process.
The presence of a Batrachosauroididae at Champ-Garimond (=Fons in Estes) was previously reported by Estes (1981) but no description was done until now. A vicariant pattern has then been proposed to explain the repartition of the group at the end of the Cretaceous (Le Loeuff, 1991).
By showing that the family was well diversified at the end of the Cretaceous, this description of an european Batrachosauroididae supports the hypothesis of an ancient origin of the group. This hypothesis has been recently secured by the attribution of specimens from Las Hoyas (early Cretaceous, Spain ; Evans & Milner, 1995) to the family (other batrachosauroidids have been reported from the late Jurassic of England -Ensom & al, 1991- and the upper Cretaceous of Asia -Nessov, 1981- , but in both these papers attribution to the family is not discussed). The family was thus probably present prior to the separation of the two continents.
Nevertheless, if a vicariant pattern is retained to explain the history of the “Batrachosauroides group” (i.e., Batrachosauroides + “Champ-Garimond”), then one must admit that some Batrachosauroides related forms were present and have never been found as fossils from the early to the middle Cretaceous in Europe, and from the early Cretaceous to the basal Eocene in North-America.
Concerning micro-vertebrate remains, the European fossil record from the early and the middle Cretaceous is scarce : Lazarus taxa are likely to be found. On the contrary, late Cretaceous and Paleocene from North-American are relatively well-documented (numerous salamanders are known from these strata): although not impossible, this makes the presence of a Lazarus taxa more unlikely, and weakens the vicariant hypothesis for the “Batrachosauroides group”.
On the other hand, various studies on the mammalian faunas from Europe and North-America have evidenced strong similarities in the paleocene and eocene faunas from the two continents. Faunal exchanges have been shown to occur during the Paleocene and possibly at the end of the Cretaceous. It is therefore not impossible that an amphibian may have passed from Europe to North-America.
If accepted, this implies the existence of a full terrestrial way : ecological requirements of the Amphibians are extremely restrictive as they need fresh-water (at least at the breading period) and can’t survive in a marine or even brackish environment.
Various authors have proposed vicariant patterns to explain similarities between late Cretaceous faunas from Europe and North-America. Concerning the family Batrachosauroididae, this idea is re-emphasised here by the differences recognised between coeval relatives in the two continents, perhaps due to a period of separation. Vicariance has thus probably led to the late Cretaceous situation as seen here. But during the upper Cretaceous or early Paleogene, a direct crossing may have occurred from Europe to America, which would explain the presence in America of a form, Batrachosauroides, the closest known relatives of which are European and 20 Ma older.
This remains a highly hypothetical scenario : mainly because of the scarcity of the fossil record ; and because it is founded in part on the absence of a taxa in the Cretaceous/Paleocene of North America, which does not constitute a positive proof ; and finally because of our lack of knowledge of the early history of the group.
The specimens of Las Hoyas have been assigned to the family but no full description or illustration is available at the moment. Their description may provide useful informations about a possibly less derived state of the vertebrae, and a possible phylogenetic tendency to develop robust vertebrae, or not. This will help in testing the alternative hypothesis of a convergence.
Acknowledgements
I am grateful to J.C. Rage for his help and useful comments. Thanks to Zeresenay Alemseghed for his advices. Thanks to C. Abrial for the stereophotographs.
References
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Estes, R. ; 1981. Gymnophiona, Caudata. Handbuch der Paläoherpetologie, Part 2. Gustav Fisher Verlag, Stuttgart, XV + 115pp.
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