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Non-Dino articles: Tetrapod vertebrae and temnospondyls

From: Ben Creisler

A few recent non-dinosaur-related papers that may interest some people:

Stephanie E. Pierce, Per E. Ahlberg, John R. Hutchinson, Julia L.
Molna, Sophie Sanchez, Paul Tafforeau, and Jennifer A. Clack (2013)
Vertebral architecture in the earliest stem tetrapods.
Nature (advanced online publication 13 January 2013)
DOI: 10.1038/nature11825

The construction of the vertebral column has been used as a key
anatomical character in defining and diagnosing early tetrapod groups.
Rhachitomous vertebrae—in which there is a dorsally placed neural arch
and spine, an anteroventrally placed intercentrum and paired,
posterodorsally placed pleurocentra—have long been considered the
ancestral morphology for tetrapods. Nonetheless, very little is known
about vertebral anatomy in the earliest stem tetrapods, because most
specimens remain trapped in surrounding matrix, obscuring important
anatomical features. Here we describe the three-dimensional vertebral
architecture of the Late Devonian stem tetrapod Ichthyostega using
propagation phase-contrast X-ray synchrotron microtomography. Our
scans reveal a diverse array of new morphological, and associated
developmental and functional, characteristics, including a possible
posterior-to-anterior vertebral ossification sequence and the first
evolutionary appearance of ossified sternal elements. One of the most
intriguing features relates to the positional relationships between
the vertebral elements, with the pleurocentra being unexpectedly
sutured or fused to the intercentra that directly succeed them,
indicating a ‘reverse’ rhachitomous design. Comparison of Ichthyostega
with two other stem tetrapods, Acanthostega and Pederpes, shows that
reverse rhachitomous vertebrae may be the ancestral condition for
limbed vertebrates. This study fundamentally revises our current
understanding of vertebral column evolution in the earliest tetrapods
and raises questions about the presumed vertebral architecture of
tetrapodomorph fish and later, more crownward, tetrapods.

For new release with video:



Rainer R. Schoch (2013)
The evolution of major temnospondyl clades: an inclusive phylogenetic analysis.
Journal of Systematic Palaeontology (advance online publication)

Phylogenetic analysis of a large dataset (72 taxa, 212 characters)
focuses on the in-group relationships of temnospondyls, the largest
lower tetrapod clade. Representatives of all clades and grades were
considered, spanning the entire stratigraphical range of temnospondyls
from the Early Carboniferous through to the Early Cretaceous. Several
major groups are defined phylogenetically (node or branch-based)
rather than by apomorphies. The following groups were unequivocally
found to be monophyletic: Edopoidea (node), Dvinosauria (stem, excl.
Brachyopidae), Dissorophoidea (node), Eryopidae (stem), and
Stereospondyli (node). The latter encompass three well-defined,
branch-based taxa: Rhinesuchidae, Trematosauria and Capitosauria.
Trematosauria (stem) contain Trematosauroidea (node), which includes
the classic trematosaurids, metoposaurids, and possibly part of the
rhytidosteids (Peltostega) but their in-group relationships remain
unsettled; most other short-snouted stereospondyls (chigutisaurids,
brachyopids, Laidleria and the plagiosaurids) are probably
monophyletic and likely nest in some form with trematosauroids.
Capitosauria (stem) include the Capitosauroidea (node) spanned by
Parotosuchus and Mastodonsaurus, with the successive stem taxa
Edingerella, Benthosuchus, Wetlugasaurus and Watsonisuchus. In all
variant analyses, edopoids form the basalmost temnospondyl clade,
followed by a potential clade (or grade) of small terrestrial taxa
containing Balanerpeton and Dendrerpeton (‘Dendrerpetontidae’). All
taxa higher than Edopoidea are suggested to form the monophyletic stem
taxon Eutemnospondyli, tax. nov. The remainder of Temnospondyli fall
into four robust and undisputed clades: (1) Dvinosauria; (2)
Zatracheidae plus Dissorophoidea; (3) Eryopidae; and (4)
Stereospondyli. These taxa are together referred to as Rhachitomi
(node). Eryopidae and Stereospondylomorpha are probably monophyletic,
here referred to as Eryopiformes (tax. nov.). The position of
Dissorophoidea + Zatracheidae is still ambiguous; it may either form
the sister taxon of Dvinosauria, or nest between Dvinosauria and
Eryopiformes, whereas there is no support for Euskelia (Dissorophoidea
+ Eryopidae) after basal taxa of each clade are better understood.



Florian Witzmann & Rainer R. Schoch (2012)
Reconstruction of cranial and hyobranchial muscles in the Triassic
temnospondyl Gerrothorax provides evidence for akinetic suction
Journal of Morphology (advance online publication)
DOI: 10.1002/jmor.20113

The cranial and hyobranchial muscles of the Triassic temnospondyl
Gerrothorax have been reconstructed based on direct evidence (spatial
limitations, ossified muscle insertion sites on skull, mandible, and
hyobranchium) and on phylogenetic reasoning (with extant basal
actinopterygians and caudates as bracketing taxa). The skeletal and
soft-anatomical data allow the reconstruction of the feeding strike of
this bottom-dwelling, aquatic temnospondyl. The orientation of the
muscle scars on the postglenoid area of the mandible indicates that
the depressor mandibulae was indeed used for lowering the mandible and
not to raise the skull as supposed previously and implies that the
skull including the mandible must have been lifted off the ground
during prey capture. It can thus be assumed that Gerrothorax raised
the head toward the prey with the jaws still closed. Analogous to the
bracketing taxa, subsequent mouth opening was caused by action of the
strong epaxial muscles (further elevation of the head) and the
depressor mandibulae and rectus cervicis (lowering of the mandible).
During mouth opening, the action of the rectus cervicis muscle also
rotated the hyobranchial apparatus ventrally and caudally, thus
expanding the buccal cavity and causing the inflow of water with the
prey through the mouth opening. The strongly developed depressor
mandibulae and rectus cervicis, and the well ossified, large
quadrate-articular joint suggest that this action occurred rapidly and
that powerful suction was generated. Also, the jaw adductors were well
developed and enabled a rapid mouth closure. In contrast to extant
caudate larvae and most extant actinopterygians (teleosts), no cranial
kinesis was possible in the Gerrothorax skull, and therefore suction
feeding was not as elaborate as in these extant forms. This
reconstruction may guide future studies of feeding in extinct aquatic
tetrapods with ossified hyobranchial apparatus.