Box The Nyrany Tetrapod Communities

One of the most diverse faunas of Late Carboniferous tetrapods is from Nyrany, a small mining town in the Czech Republic. Fossil tetrapods were first reported from coal mines in this area in the 1870s (A. R. Milner, 1980), and since then many hundreds of specimens have been collected and studied.

Carboniferous Temnospondyl Fossils
I Skeleton of the temnospondyl Isodectesshowing excellent preservation of the delicate bones, and of the body outline: (a) dorsal slab; (b) ventral slab of the same specimen. (The original specimen is 48 mm long and from the Upper Carboniferous of the USA; courtesy of Andrew Milner.)

The fossil tetrapods nearly all came from a 300-m-thick sequence of coalified shales and mudstones near the base of the Nyfany Gaskohle Series (Westphalian D, Late Carboniferous in age, c. 300 Myr ago). These sediments were laid down in an enclosed lake under gentle conditions, and they contain remains of plants such as Calamites, a giant horsetail that grew in up to 1m of water. There are also rare fossils of small sharks, acanthodians and actinopterygians, as well as water-living arthropods and terrestrial millepedes. At the time of deposition of these beds, the lake was small and poorly aerated, and the sediments represent a fairly rapid accumulation.

The fossil tetrapods are generally very well preserved, and they occasionally show traces of soft parts here, and in similar localities elsewhere (see illustration I). The cadavers seem to have sunk to the bottom rapidly, with relatively little decomposition and no scavenging. It may be that the animals swam a little too deep in the lake, and encountered anoxic bottom waters that suffocated them.

A census of most of the 700 or so Nyfany tetrapod specimens currently housed in museums around the world (A. R. Milner, 1980) shows that there were 20 species of basal tetrapods, with representatives of most major groups, and four species of amniotes. These fall into three main ecological associations (see illustration II).

1 Open-water/lacustrine assocation: three very rare forms from Nyrany, an eogyrinid anthracosaur and two baphetids presumably fished in the open water.

2 Terrestrial/marginal association: representatives of 13 species lived on or close to the shores of the lake. These include primitive temnospondyls, anthracosaurs, an aistopod, four microsaurs and three primitive amniotes.

3 Shallow-water/swamp-lake association: the remaining seven tetrapods from Nyfany all appear to have been partially aquatic, and to have swum rapidly about in shallow parts of the lake where plants grew in the water and where the bottom was covered with plant debris. They include two temnospondyls, a branchiosaur, two nectrideans, a microsaur and an aistopod. Most of these presumably fed on small fishes or small tetrapods.

II A Late Carboniferous tetrapod community, based on the Nyrany locality, Czech Republic. Four main habitats are indicated, with representative vegetation and tetrapods, from left to right: open water (eogyrinid, Baphetes); shallow lake (Ophiderpeton, Sauropleura, Microbrachis, Scincosaurus); lake margin (Gephyrostegus, Amphibamus, Aornerpeton, Ricnodon, etc.); possibly upland (Scincosaurus). The food web on the left shows what eats what (the arrows run from the base of the food chains—the plants—through various invertebrates and fishes to the predatory tetrapods, and terminating at the top of the diagram with the 'top' carnivores that feed on other tetrapods). (Based on A.R. Milner, 1980 and other sources.)

II A Late Carboniferous tetrapod community, based on the Nyrany locality, Czech Republic. Four main habitats are indicated, with representative vegetation and tetrapods, from left to right: open water (eogyrinid, Baphetes); shallow lake (Ophiderpeton, Sauropleura, Microbrachis, Scincosaurus); lake margin (Gephyrostegus, Amphibamus, Aornerpeton, Ricnodon, etc.); possibly upland (Scincosaurus). The food web on the left shows what eats what (the arrows run from the base of the food chains—the plants—through various invertebrates and fishes to the predatory tetrapods, and terminating at the top of the diagram with the 'top' carnivores that feed on other tetrapods). (Based on A.R. Milner, 1980 and other sources.)

Nectrideans Carboniferous Period

Fig. 4.13 Aquatic nectrideans,Sauropleura,skeleton (a) and caudal vertebrae in lateral view (b),and Diplocaulus (c-f): (c) life restoration; (d) anterior view of head; (e) dorsal view of skull; (f) sequence of growth stages, from juvenile (top left) to adult (bottom right), showing the growth of the projecting 'horns'. The numbers 20,40,60, etc., are measurements, in millimetres, of total body lengths. [Figures (a,b) after A.C.Milner, 1980; (c-e) after Cruickshank and Skews, 1980; (f) after Olson, 1951.]

Fig. 4.13 Aquatic nectrideans,Sauropleura,skeleton (a) and caudal vertebrae in lateral view (b),and Diplocaulus (c-f): (c) life restoration; (d) anterior view of head; (e) dorsal view of skull; (f) sequence of growth stages, from juvenile (top left) to adult (bottom right), showing the growth of the projecting 'horns'. The numbers 20,40,60, etc., are measurements, in millimetres, of total body lengths. [Figures (a,b) after A.C.Milner, 1980; (c-e) after Cruickshank and Skews, 1980; (f) after Olson, 1951.]

Anthracosauria Skeleton
Fig. 4.14 The aistopod Aornerpeton: (a) reconstructed skeleton; (b-d) skull in lateral,dorsal and ventral views; (e) trunk vertebra in dorsal (left) and lateral (right) views. (After Gregory, 1948, courtesy of the American Journal of Science.)
Temnospondyl Skeleton
Fig. 4.15 The early anthracosaur Proterogyrinus: (a—c) skull in lateral, dorsal and ventral views; (d) restoration of the skeleton; (e) the anthracosaur Pholiderpeton. [Figures (a-d) after Holmes, 1984; (e) after Panchen, 1972.]

4.4.4 'Anthracosauria'

The anthracosaurs, a paraphyletic group that arose in the Early Carboniferous, and survived into the Early Triassic, include a number of moderate-sized fish-eaters. Some were apparently terrestrial, whereas others became secondarily adapted to life in the water.

Proterogyrinus from the Lower Carboniferous of West Virginia, USA (Holmes, 1984) and Scotland is about 1m long and has an elongate skull (Figure 4.15(a-d)). The skull table, the square area at the back of the skull (Figure 4.15(a, c)), is set off from the cheek area, and there is a line of weakness between the two units that presumably allowed the skull to flex during jaw opening, as in osteolepiforms. Proterogyrinus has large vertebrae, a short neck and a flat-sided tail. The limbs are well developed for moving rapidly on land, but the flattened tail shows that Proterogyrinus could swim well. Later anthracosaurs, such as Pholiderpeton from the Upper Carboniferous of England (Panchen, 1972), were even more clearly adapted for an aquatic lifestyle, with their long slender bodies, small limbs and deep tail fin (Figure 4.15(e)).

4.4.5 Vertebral evolution

One of the most startling patterns of evolution observed among the basal tetrapods occurs in the backbone. In sarcopterygian fishes, there are three main components of each vertebra, a pleurocentrum and an intercentrum encompassing the notochord below and

Intercentrum
Fig. 4.16 Divergent evolution of the vertebrae in batrachomorphs, in which the intercentrum comes to dominate, and in lepospondyls and reptiliomorphs, where the pleurocentrum dominates. Examples of vertebrae from key taxa are shown around a simplified phylogenetic tree. (Based on various sources.)

a neural arch above (Figure 4.16). Then, in the early evolution of tetrapods, the pleurocentrum became the main element of the vertebra in reptiliomorphs and amniotes, and the intercentrum became the main element in temnospondyls and lissamphibians.

This split in vertebral evolution among tetrapods is documented in successive fossils. The vertebrae of Eusthenopteron and Acanthostega are similar in that the intercentrum is the dominant element, a crescent-shaped structure in anterior view, wedge-shaped in lateral view, that lies in front of the smaller pleurocentrum, composed of two short elements, one on each side. In 'anthracosaurs', the intercentrum and pleurocentrum may be of equal size, and then the in-tercentrum reduces to a small wedge in seymouri-amorphs, and reduces even further in amniotes, becoming either a thin plate or disappearing altogether. In temnospondyls, on the other hand, the intercentrum expands and the pleurocentrum reduces to a small wedge.

The vertebrae of two groups, the lepospondyls and lissamphibians, have been much debated. Lep-ospondyls have holospondylous vertebrae (see section 4.4.3), but is the fused centrum the pleucrocentrum or the intercentrum? As expected from their phylogenetic position (see Box 4.5), the centrum of lepospondyls is composed of the pleurocentrum: while this is not clear in nectrideans and aistopods, many microsaurs have a small intercentrum tucked under the pleurocentrum, and the two elements fuse to form a single centrum. The single centrum in lissamphibians is presumably composed largely of the pleurocentrum, as their closest relatives, the dissorophoids, are unusual among temnospondyls in that the pleurocentrum is much larger than the intercentrum.

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