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THE YOUNG ONES — SMALL TEMNOSPONDYLS FROM THE ARCADIA FORMATION A. A. Warken and M.N. Hutchinson Warren, A. A. and Hutchinson, M.N. 1990 3 31: The young ones — small temnospondyls from the Arcadia Formation. Mem Qd Mus. 28(1): 103-106 Brisbane. ISSN 0079-8835. An assemblage of small temnospondyl (Amphibia, Labyrinthodontia) skuJIs from ihe Arcadia Formation of Queensland is [he only such collection Irom the Early Triassic Using non-morphometric characters we have been able to identity, from among these specimens, juvenile capitosaurs and a rhytidosteid, whereas two skulls of similar size and superficially similar shape have been determined as mature dissorophoids. We caution against [he use of skull proportion? in labyrinthodont taxonomy and demonstrate that the iremalo'taurian group of labyrimhodonts can be considered to be neotenic in at least one character I Amphibia, Labyrinthodontia, temnospondyls, Triussic, Arcadia Formation, capimsuur\, rhytidosteid, juveniles. A.A. Warren and M.N. Hutchinson, Department of Zoology, la Trohe University, Bundoora, Victoria 3083. Australia; 20 June, 1988. One problem common to palaeontological and neontologicil studies of the Class Amphibia s the difficulty of determining to which known adult species a juvenile might belong. Small labyrinthodont amphibians of the order Temnospondyli are commonly found at several localities in the Fermo-Carboniferous of Europe and the middle Pennsylvanian of Illinois. Originally assigned to the labyrinthodont Order Phyllospondyli, or branchiosaurs, these were recognised by Romer (1939) as having the characteristics of small or larval temnospondyls. While some of these Palaeozoic forms may now be assigned to various genera within the Eryopoidea and Trimeroraehoidea, most remain sheltered beneath the enlarged umbrella of the Dissorophoidea. The Early Triassic Arcadia Formation of Queensland has yielded a series of labyrinthodont fossils belonging to various families of temnospondyls. Most common components of the labyrinthodont fauna are members of the families Capitosauridac (Warren 1980; Warren & Hutchinson, 1988), Rhytidosteidae (Howie, 1972a; Warren & Black, 1985; Warren & Hutchinson, 1987), Brachyopidae (Howie, 1972b; Warren & Hutchinson, 1983) and Chigutisauridae (Warren, 1981). Rare and fragmentary specimens of the Trematosauridae (Warren, 19S5b) and Plagiosauridae (Warren, 1985a) have also been found. In addition, the material collected from the Arcadia Formation includes a number of small skulls of rather uniform size and shape which initially proved difficult to place in a known family. These presumed juveniles are the smallest (youngest?) individuals to be recorded from the Triassic. Much larger juveniles of near-adult proportions have been described in the Triassic species Benihositchus sushkin't (Bystrow & E fremov . 1 940) and Parowsuchus peabodyi (Welles &Cosgriff, 1965). When considering the relationships of the Queensland juveniles we need to look at the families of Triassic temnospondyls known from Australia and must also consider the possibility that, as in the Palaeozoic, some specimens may be adults of small temnospondy! species such as those found within the Dissorophoidea. CAP1TOSAURIDAE The first enlightenment came in 1984 when wc discovered at the Duckworth Creek locality some one centimetre long skulls (QMF 12290, QMF ] 229 1 ) in close proximity to remains of moderately-sized temnospondyls (QMF 1 228 1 , QMF 12282). Although the characteristic capitosaurid skull shape was not evident, so that the larger skull showed the proportions of a lydekkerinid while the smaller resembled a branchiosaur (Fig. 1). we soon realised that skulls of both sizes shared several characters of the Family Capitosauridae. In both we were able to recognise capitosaurid features such as the hamate process of the tower jaw, ttansverse ridges on the parasphenoid, inclusion of frontal bones in the

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The Young Ones - Small Temnospondyls from the Arcadia Formation

A A Warren
Memoirs of The Queensland Museum 28: 103-106 (1990)

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