[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

New ref - Carrier & Farmer



A few weeks back I mentioned a new paper by David Carrier and Colleen Farmer
published in American Zoologist, as part of their on-going work on
respiration in modern and extinct archosaurs. Here's the latest part, and
the one (I suspect) of most general interest to the dinosaur list
readership:

Carrier, D.R. & C.G. Farmer. 2000. The evolution of pelvic aspiration in
archosaurs. Paleobiology 26: 271-293.

Summing up their previous arguments (also represented here): both birds and
crocs use pelvic muscles in active breathing, unlike most other vertebrates.
Although they use these muscles in different ways, C&F propose that the
common ancestor of all archosaurs did use these muscles in what they call
"curiassal breathing": retraction of muscles attached to the gastralia pull
thes backwards and downwards, enlarging the visceral cavity and allowing the
lungs to fill with air.

In this paper they explore variations on this theme among different groups
of advanced archosaurs.  They acknowledge that many of these ideas are
speculative, and are at present difficult to test, but present them for
futher discussion.  Here are the examples:

Crocodylomorpha: modern crocs have a diaphragmatic hepatic (liver) piston,
as discussed in recent years by Ruben, Jones & colleagues.  This system is
aided by a derived mobile pubis, which is actually pulled back and forth by
pelvic muscles during breathing.  They note that the common ancestor of
crocodylomorphs seem to have had a primitively immobile pubis, but that a
mobile pubis evolves convergently in the sphenosuchian _Hesperosuchus_ and
in the common ancestor of Protosuchia and Mesoeucrocodylia.  This suggest to
them that diaphragmatic breathing was ancestral in Crocodylomorpha (and
maybe deeper in Pseudosuchia?), but that the more advanced condition evolved
multiple times.  This would further suggest that diaphragmatic breathing did
not evolve initially in response to an aquatic habit, as it appears in the
(primitively terrestrial) basal crocodylomorphs.

Pterosaurs: once again, these guys seem to be a mix of things.  Like
advanced birds, they have a nototarium and a synsacrum, suggesting a pelvic
pump (which would be consistent with independant evidence for an air sac
system).  However, pterosaurs have a neomorph bone, the prepubis, which
resembles the mobile pubes of crocodyliforms.  C&F suggest that pterosaurs
may have also had a croc-like diaphragmatic system.  Weird critters, all
around.

Ornithischians: this clade lost gastralia; C&F suggest that guts full of
plant matter may have hindered the curiassal breathing system.  This leaves
three possibilities to them: 1) ornithischians used only rib breathing; 2)
they used rib and diaphragm breathing; 3) they used rib breathing and
something else.  Although they cannot eliminate possibilities 1 & 2, they
suggest that there is evidence for possibility 3 in several groups of
ornithischians, particularly Neoceratopsia, Ornithopoda & Stegosauria:
Neoceratopsians, especially ceratopsids: these guys have a large plate-like
prepubic process, extremely reduced pubes proper (with a very reduced
connection to the ilium), limited contact between the ischium & ilium, and a
forwards-curving ischium.  They suggest that the pubis & ischium rocked fore
and aft relative to the ilium, and that the pubis & ischium rotated relative
to each other.  As the ischiocaudalis muscle was flexed, the ischium would
rotate backwards, rotating the pubis downward, increasing the volume of the
body cavity.  As the rectus abdominus muscle (attached to the ventralmost
part of the ischium) flexed, it pulled the ischium forward, the pubis
upward, and pushed the bad air out.
Ornithopods, especially iguanodontians: these guys have a prepubic process,
which is very plate-like in iguanodontians, and their pubes have very
reduced contact with the ilium & ischium.  C&F suggest that they would be
pulled outwards as rotational flaps to expand the belly width.  However, in
order to create such a motion they have to hypothesize a new muscle
(running from the supraacetabular crest to the cranial margin of the
prepubic process).
Stegosaurs: somewhat similar to ornithopods, although the prepubic process
is less plate-like.  They envision a similar rotational system to that in
ornithopods.

Theropods: already discussed in my earlier posting.

Much that is interesting in this paper, although (as mentioned above)
difficult to test.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-314-7843