Saturday, May 31, 2008

The Origin of High Growth Rates in Archosaurs

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. 2008. Armand de Ricqlès, Kevin Padian, Fabien Knoll and John R. Horner. Annales de Paléontologie 94: 57-76.

Abstract [edit]: 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.