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.
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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).