Utegenia shpinari

Some characters of Utegenia shpinari organization with notes on Amniote origin.

By Dmitry V. Malakhov

Laboratory of Paleozoology, Institute of Zoology, Akademgorodok, 32, Almaty, 480032, Kazakhstan

E-mail: dragon@nursat.kz

Utegenia shpinari (Seymouriamorpha) has been attributed to the family Discosauriscidae (Kuznetsov, Ivakhnenko, 1981). The species considered to be the oldest member of the family (Malakhov, 2000a) accordingly to age of the locality deposits (Radchenko, 1959), determined as Late Carboniferous - Early Permian. The species possesses very interesting characters, which permit to suggest it as a "model species"; of amniote ancestor. The set of paedomorphic features (see below) may evident, that Utegenia was an aquatic animal. Moreover, these features are encountered in all species of the family, but they may be less visible in those species inhabited more humid biotopes (European genus Discosauriscus, and, probably, Chinese Urumqia luidaowanensis). So, the suggestion of Klembara (1995), that all known specimens are of early postmetamorphic age, and adults were terrestrial, seems to be overestimated.

The list of paedomorphic characters in Utegenia.

1 Unossified endocranium
2 Reduced number of dermal skull bones
3 Poorly developed sculpture on dermal skull bones
4 Larval i.e. parabolic skull shape
5 "Immature" lateral line system
6 Unossified epiphyses of limb bones
7 Unossified mesopodium
8 Unossified terminal caudal vertebrae
9 The only ilia are represented in sacrum

1. Unossified endocranium is an attribute of all discosauriscs. The largest specimens in rare case may have ossified basioccipitale, exooccipitale and basisphenoid (Ivakhnenko, 1987; Ivakhnenko, 1981; Kuznestov, Ivakhnenko, 1981; Klembara, 1997; Laurin, 1996a, 1996b). However, the unossified endocranium is a common condition. Palatal bones, pterigoids and vomers are well ossified in all discosauriscs, beginning from late larvae stage. This is well corresponded to premetamorphic urodelens(Lebedkina, 1979), which have endocranial ossification after metamorphosis.

2. The number of dermal skull bones is less than in most of known Labyronthodonts. Moreover, Klembara (1997) described additional ossifications in the skull of Discosauriscus which look like small bones, mostly developing in the suture zone between parietals and surrounding bones (squamosals).

3. The well developed sculpture on dermal bones surface was common for most of Paleozoic amphibians. It is known, however, that the sculpture become readily observed in only postmetamorphic specimens (Shishkin, 1987). In Utegenia skull the sculpture is rather less developed and this is larval feature.

4. As it was hypothesized by Bystrow (1935) the elongated skull characterizes advanced amphibian species (which were mature), whereas the parabolic skull shape reflects "ancestor" condition, which was inherited from Sarcopterygians. In the other hand, parabolic skull is surely larval character, also typical for modern amphibians (Anura and Urodela). Their paedomorphic origin and recent trends are obvious.

5. The lateral line organs in amphibians leave traces on the bone surface of only postmetamorphic specimens (Boy, 1974). In Utegenia skull the infraorbital canal is absent. This may evident that lateral line organs still didn't approach bone surface, i.e. this feature could be also considered as a larval.

6 and 7. Unossified limb elements (mesopodium and epiphyses of limb bones) are well corresponded to the sequence of the ossification in tetrapod limb (Carroll, 1997), and represent a larval condition. These elements remain unossified for quite long time in immature animal. Benton (1997) considered unossified ankle and wrist of branchiosaurs as paedomorphic characters.

8. There is clear cranio-caudal direction in the order of vertebrae ossification in Utegenia. In even largest specimens of our collection (#164\K-1988) terminal caudal vertebrae have the only paired neural arches ossified. Intercentrae and pleurocentrae remain invisible on the rock surface, i.e. these elements were cartilaginous.

9. The presence of only paired ilia is detected in all specimens of our collection, where the sacral region preserved. Other discosauriscs (Riabinin, 1911; Ivakhnenko, 1981; Fakui et al., 1984; Klembara, Bartik, 2000; Laurin, 1996a) might have three pair of bones, but they remain unfinished, and sacrum is not fused.

The integument of Utegenia

The integument structure of Utegenia possesses very exciting characters. Cornfield scales are represented along with dermal ventral shield. The last looks somewhat reduced if compare with many other Paleozoic amphibians, since it spread in only abdomen, and does not occupy neither legs nor tail. Cornified scalation were previously noted by Ivakhnenko (1987), however, he didn't provide a clear description of scales. In my material cornified scales appear like round reddish-brown spots, arranged into fish-like pattern. Each scale has a diameter about 2 mm. Rows of the keratinous scales are spread through the dorsal and lateral body surfaces. It's possible, that keratinous scales were presented in abdomen too, but I couldn't recognize any sign of them because of waves of dermal scalation. Such an appearance is very closed to that described for Uranocentrodon senekalensis (Findlay, 1968). For my knowledge, there are only three cases of keratinous component preservation in the fossil amphibians. The third case is Ariekanerpeton sigalovi (Ivakhnenko, 1981, 1987) - the discosauriscid known from Sarytaypan locality of Tajikistan.

Summary

The charatcers of Utegenia, like paedomorphic development and keratinous scales in the integument, permit to consider it as a "model" species, which reflects those features, which a true ancestor of amniotes must have. The paedomorphosis is of great evolutionary value, as a source of innovations in the origin of many taxa (Raff, Kaufman, 1986; Smirnov, 1991, 1999; Carroll, 1997; Kuzmin, 1999; Dodson, Tumarkin, 1998; Maderson et al, 1982; Laurin, 1998). The paedomorphic trends are suggested to be related with aquatic or semiaquatic life style (Vorobjeva, 1992). The main doubt to accept the paedomorphosis of Utegenia is an absence of gills in all known age stages. However, this situation may results from cartilaginous nature of gill skeleton, which does not remain in fossill condition. Like a model species Utegenia has reptiliomorph organization, which, however, was not highly adopted, i.e. evolutionary restricted. "Larval" structure of Utegenia still does not reveal all the potential of genotype. In the other hand, one of the main terrestrial adaptations - keratinized integument, has already been developed.

Literature cited

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Author's papers

Malakhov D., Dujsebayeva T. 2000.   Pattern of Scalation of Late Carboniferous-Early Permian seymouriamorph Utegenia shpinari (Seymouriamorpha; Discosauriscidae). Abstr. IV Asian Herpetol. Meeting. Chengdu .p.118.
Malakhov D. 2000a. Some Speculations on the Life Style of Permo-Carboniferous Seymouriamorph Tetrapods (Discosauriscidae):          Immaturity or Paedomorphosis? Russ. Journ. Herp. 7(3): 227-231
Malakhov D. 2000b. The Topography of the Lateral Line Organs on the Skull of Utegenia shpinari (Seymouriamorpha:          Discosauriscidae). Biota. 1(2): 77-82.
Malakhov D. V., and T. N. Dujsebayeva. 2001. Cornified Scalation of Utegenia shpinari (Seymouriamorpha: Discosauriscidae) and Radiation of the Family during Late Paleozoic. Biota (in press).