Early tetrapod jaw biomechanics

[Ben Creisler <bcreisler@gmail.com>]

From: Ben Creisler
bcreisler@gmail.com


A follow-up to some non-dino articles I posted yesterday. After
further checking, it turns out that the paper about the fishlike jaws
of early tetrapods was posted online back in March even though the
press release came out yesterday. The article has not been mentioned
yet on the DML, so a bit belated, here is the ref:


Philip S. L. Anderson, Matt Friedman, and Marcello Ruta (2013)
Late to the Table: Diversification of Tetrapod Mandibular Biomechanics
Lagged Behind the Evolution of Terrestriality.
Integrative and Comparative Biology (advance online publication)
doi: 10.1093/icb/ict006
http://icb.oxfordjournals.org/content/early/2013/03/22/icb.ict006.abstract?sid=89b58eb3-a51b-4d90-ae92-649230362d87


The origin of digit-bearing tetrapods in the Middle to Late Devonian
(ca. 393–360 Mya) and their subsequent invasion of land represent a
classic case of a major evolutionary radiation driven by new
ecological opportunities. In this and other examples, exploration of
new eco-space is hypothesized to correlate with functional innovation
and adaptive divergence of phenotypes. Simultaneous changes in all
morphofunctional systems are rare in major evolutionary transitions
and may be non-existent. Here, we focus on the mandibles of early
tetrapods and their kin as a model system to test whether shifts in
functional innovation were coeval with some major events in tetrapod
history. To this end, we quantified mechanical variation in the
mandibles of tetrapodomorphs ranging in age from Early Devonian to
earliest Permian. Biomechanical disparity is stable from the Devonian
to the Early Pennsylvanian, even though the origin of weight-bearing,
digited limbs, and the initial phases of the colonization of land
occurred during this interval. An appreciable increase in functional
variation is detected in the latest Pennsylvanian and earliest
Permian, when stem and crown amniotes began to explore new regions of
mechanical morphospace, a pattern partly attributable to the origin of
herbivory. We find no difference in the rate of functional change
between tetrapodomorph “fish” and early digited tetrapods, although
two independent shifts are detected among Devonian stem tetrapods more
crownward than Acanthostega. Instead, the most profound shifts in
evolutionary rate are nested well within the tetrapod crown and are
associated with amniotes (particularly diadectomorphs and some
synapsids). The substantial temporal gap between the origin of
postcranial features associated with terrestriality, such as limbs
with functional elbow/knee and wrist/ankle joints capable of
weight-bearing, and the onset of divergence in jaw biomechanics
provides a compelling example of “functional modularity” during a
major adaptive radiation.