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
BioStor
