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

Hedwig and the Angry New Paper

Don't know if a) that title has been used yet or b) this paper has been
mentioned yet.

No, I do not have a copy (our University Library having cancelled the
subscription due to budget cuts.) Yes, I would love one...

Ricqlès, A. de, K. Padian, F. Knoll and J.R. Horner. 2008. On the origin of
high growth rates in archosaurs and their ancient relatives: Complementary
histological studies on Triassic archosauriforms and the problem of a
?phylogenetic signal? in bone histology. Annales de Paléontologie 94: 57-76.


Three possible hypotheses could explain the polarity of the histological
features of basal archosauriform and archosauromorph reptiles: either, the
fibrolamellar complex is basal; or, the lamellar-zonal complex is basal or
finally, the condition varied, and each complex evolved more than once in
these early groups. The answer to this question would have broad
implications for our understanding of the physiological, ecological, and
behavioral features of the first archosaurs. To this end, we sampled the
bone histology of various archosauriforms and basal archosaurs from the
Triassic and Lower Jurassic: erythrosuchids, proterochampsids, euparkeriids,
and basal ornithischian dinosaurs, including forms close to the origin of
archosaurs but poorly assessed phylogenetically. The new data suggest that
the possibility of reaching and maintaining very high growth rates through
ontogeny could have been a basal characteristic of archosauriforms. This was
partly retained (at least during early ontogeny) in most lineages of
Triassic pseudosuchians, which nevertheless generally relied on lower growth
rates to reach large body sizes. This trend to slower growth seems to have
been further emphasized among Crocodylomorpha, which may thus have
secondarily reverted toward more generalized reptilian growth strategies.
Accordingly, their ?typical ectothermic reptilian condition? may be a
derived condition within archosauriforms, homoplastic to the generalized
physiological condition of basal amniotes. On the other hand,
ornithosuchians apparently retained and even enhanced the high growth rates
of many basal archosauriforms during most of their ontogenetic trajectories.
The Triassic may have been a time of ?experimentation? in growth strategies
for several archosauriform lineages, only one of which (ornithodirans)
eventually stayed with the higher investment strategy successfully.

Our data again raise the problem of a possible ?phylogenetic signal? being
carried by bone histology. Bone histology is highly correlated to
?functional? characters as size and growth rates which are intensely
involved in species-specific ?life-history traits?, are under intense
scrutiny by selective pressures and may accordingly evolve very rapidly.
This rapid evolutionary rate would in turn produce patterns of
species-specific variations that could ?erase? higher-order taxonomic
signals in bone tissue. In other words, this fast turnover would introduce
autapomorphies (and homoplasies) at the level of apical (terminal) taxa that
could blur the wider ?phylogenetic signal?. Thus, the search for generalized
apomorphic (or plesiomorphic) conditions of bone histological
character-states at supraspecific levels may often be deceptive.
Nevertheless, bone tissue phenotypes can reflect a phylogenetic signal at
supraspecific levels if homologous elements are used, and if ontogenetic
trajectories and size-dependent differences are taken into consideration.

Thomas R. Holtz, Jr.
Email: tholtz@umd.edu   Phone: 301-405-4084
Office: Centreville 1216                        
Senior Lecturer, Vertebrate Paleontology
Dept. of Geology, University of Maryland
Fax: 301-314-9661               

Faculty Director, Earth, Life & Time Program, College Park Scholars
Fax: 301-405-0796

Mailing Address:        Thomas R. Holtz, Jr.
                        Department of Geology
                        Building 237, Room 1117
                        University of Maryland
                        College Park, MD 20742 USA