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Re: News item: temnospondyl body impressions
Temnospondyls apparently had smooth, unarmored, skin.
I don't know where the surprise is.
There are two entirely different entities that are called "scales".
One is more or less plesiomorphic for vertebrates: bony plates in the
dermis. This is found today in most actinopterygians, the coelacanths, at
least the Australian lungfish, and in most caecilians (for which they are a
plesiomorphy). It is also found in almost all temnospondyls and
"lepospondyls". The amniote line lost the dorsal scales, but retained the
ventral scales, which, having already migrated from the dermis to the
subcutis, became long and rod-shaped and are called gastralia. (And were
subsequently lost again and again, so that today only the crocodiles and the
tuataras retain them.)
Unless very large and thick, such scales should not be visible on any but
extremely detailed imprints of the body surface.
The other is an autapomorphy of... probably Sauropsida: thickened and
hardened folds of the epidermis. No bone, next to no fossilization
potential. Not expected in a temnospondyl or amphibian.
Meanwhile, it's reassuring to hear that some stem-group amphibians
seem to have shared a derived soft-tissue condition with the crown group.
That is not the case.
And if temnospondyls are stem-group amphibians, they should be called
amphibians. But the discussion is still ongoing. Here are the latest two
Jason Anderson, Diane Scott, Robert Reisz & Stuart Sumida: A new
stem[-]batrachian (Temnospondyli: Amphibamidae) from the Early Permian of
Texas, SVP meeting abstracts 2007, 40A
The on-going [sic] controversy over lissamphibian origins is fueled by their
highly derived morphology with respect to archaic fossil forms. Here we
report a new specimen that bridges the gap between amphibamid temnospondyls
and basal frogs and salamanders. The specimen is missing the zeugopods, most
of the manus, and the ventral-most portion of the skull. The skull is broad
and rounded, but it has a lighter, more strut-like construction than is
typical for temnospondyls. The extremely large interpterygoid vacuities are
bordered by a pterygoid that just fails to reach the lateral skull margin
except by a dorsal projection, a palatine, and a narrow vomer that bears
denticles arranged in three rows on a ridge along the medial margin of the
large choana. Marginal teeth are tiny, monocuspid, pedicellate cones. A very
large otic notch, with articulation scars for the tympanic annular
cartilage, closely approaches the orbital margin and creates a narrow
postorbital bar. There are 17 presacral vertebrae, and the caudal vertebrae
are poorly ossified and rapidly taper out. Ribs are short, laterally
projecting elements with spatulate distal tips. The olecranon is ossified. A
basale commune is present in the pes, which has a phalangeal formula of
?-2-3-4-3. This specimen has a mosaic of amphibamid characters and
synapomorphies of both frogs and salamanders. The overall impression of the
skull is frog-like, except the frontals are not coossified with the
parietals. Pedicellate teeth are [among tetrapods] known only from
lissamphibians, amphibamids, and possibly one branchiosaur. The vomers, with
the lack of fang-pit pairs and [the] raised patch of denticles in rows[,]
are especially batrachian. The vertebral count is transitional between
*Amphibamus* (21) and *Triadobatrachus* (14). The tail appears to be in the
process of being lost and the phalangeal count suggests that this specimen
may be the most basal frog; however, the basale commune is unique to
salamanders. The presence of a basale commune in an amphibamid suggests that
preaxial digital development was more widespread among batrachian
sister[-]groups, and may have been primitively present in frogs.
The paper should be out soon.
I haven't seen the specimen, but from this abstract I wonder if it's adult;
if not, its similarity to frogs & salamanders might be exaggerated. For
example, rows of teeth on the vomer are replaced by fang-pit pairs in the
ontogeny of the amphibamids *Amphibamus* and *Platyrhinops*, and the ribs
grow with positive allometry in temnospondyl ontogeny. I also wonder if it's
a case of *Effigia*-style convergence to several clades at once -- the
specimen has not only synapomorphies of frogs and salamanders, but also
autapomorphies of either. Salamanders don't have that kind of reduced tail
(which might again be a feature of immaturity), and frogs don't have the
fusion between distal tarsals 1 and 2 that is characteristic of salamanders.
Well, we'll see. Articulation scars for the cartilage that holds the eardrum
in frogs would be fascinating. Incidentally, some basal salamanders have
exactly 17 presacral vertebrae, and this is the maximum for apparently all
salamanders except those with reduced limbs and lengthened bodies; so far,
the 21 of *Amphibamus* were the minimum for temnospondyls.
Damien Germain: Anatomy of lepospondyls and origin of lissamphibians, SVP
meeting abstracts 2007, 80A
This is claimed to a poster in the abstracts volume. It was meant to be a
talk. In the end, Damien didn't go to the SVP meeting and didn't present
anything. Thus, the following abstract is all anyone outside the lab knows
about his hopefully soon finished thesis, and those inside the lab, other
than him, don't know much more either. At least one paper is in press,
Lissamphibians are one of the last clade[s] of extant tetrapod whose origin
in the fossil record is still currently very [much] debated. Three main
hypotheses are [being] discussed: a monophyletic origin within dissorophoid
temnospondyls, a monophyletic origin within lepospondyls and a polyphyletic
origin with urodeles and anurans evolving from two clades of dissorophoid
temnospondyls[*] and caecilians from microsaur[ian] lepospondyls. The
anatomy of two lepospondyls have [sic] been studied. The late Permian
diplocaulids from Morocco are redescribed and the systematics of nectrideans
is discussed. Moreover, they are the last lepospondyls known in the fossil
record. An aistopod from the Upper Carboniferous of France is also
described. It exhibits some anatomical features that were previously known
only among Lissamphibians [sic] and some dissorophoids. This study reveals
that some synapomorphies used to unit[e] Lissamphibians and temnospondyls
are homoplas[t]ic. A developmental study of ossification sequences of
urodeles reveals that these developmental sequences are highly variable and
should be used very carefully to solve the problem of lissamphibian origin.
Finally, a phylogenetic analysis of a large scale of Paleozoic tetrapods is
performed and shows that each hypothesis concerning the origin of
Lissamphibians is weakly supported. The main causes of these incongruences
are various. An extendsive homoplasy reigns among Paleozoic tetrapods. The
lack of "transitional" fossils provides huge morphological gaps between
crown-lissamphibians and their putative sister-taxa and temporal gaps in the
beginning [of] the Carboniferous (the [sic] Romer's gap) where all groups of
Paleozoic tetrapods diversified, with a very poor fossil record, and in the
Permo-Triassic boundary, after which the first Lissamphibians appeared
whereas all their putative sister-groups had disappeared well before. Some
new axes of research are thus proposed in order to solve the origin of
Lissamphibians despite the lack of fossils that could exhibit enough
features to anchor Lissamphibians in one (or more) Paleozoic group.
* This is Carroll's concept, where the frogs are amphibamids and the
salamanders are branchiosaurids. According to the abstract by Anderson et
al., both are amphibamids.
So, let's come back in a year or two... :-)