One associated effect of being 'warm-blooded' is a relatively fast growth-rate. Mammals (and birds, who are also 'warm-blooded') tend to grow much faster than 'cold-blooded' vertebrates, like fish and reptiles. This fast growth rate is in turn associated with a particular type of bone growth pattern, called fibrolamellar bone (FLB). Both mammals and birds have FLB, and Shelton and his colleague investigated its presence in an early fossil relative of mammals, Ophiacodon.
Ophiacodon is distantly related to the large sail-back reptile, Dimetrodon, that people may be familiar with, and lived in North America around 280-300 million years ago. Although it didn't look particularly mammal-like (you could be forgiven for calling it a 'lizard', it has many characters that link it with mammals. The highly vascularized tissue had previously been observed in Ophiacodon, but its presence had been written off to ecological factors, like its aquatic lifestyle, rather than to its fundamental physiology. Shelton looked at a number of bones of Ophiacodon of individuals of different ages, and found that the more likely explanation is that Ophiacodon was at least partly 'warm-blooded'
'It is surprising that even in some of the earliest representatives of our own lineage from 300 million years ago, there already was a tendency towards warm-bloodedness', said Sander. Ophiacodon probably isn't directly ancestral to modern mammals, and its 'warm-blooded' characters show that it may have evolved it in parallel with mammals, which itself is interesting.
Says Shelton, "Warm-bloodedness is one of the great inventions of evolution, but we do not completely understand its advantages. We can never fully appreciate the physiology of an extinct organism, but studies like ours will help."
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AUTHOR CONTACT INFORMATION
Stienmann Institute, Bonn, Germany
OPHIACODON (BASAL SYNAPSID) BONE HISTOLOGY AND THE ORIGIN OF MAMMALIAN ENDOTHERMY
SHELTON, Christen, Stienmann Institute, Bonn, Germany
The origin of mammalian endothermy has long been held to reside within the early therapsid groups. However, shared histological characteristics have been observed in the bone matrix and vascularity between the 'pelycosaur' (non-therapsid synapsid) Ophiacodon and the later Therapsida. Historically, this coincidence has been explained as simply a reflection of the presumed aquatic lifestyle of Ophiacodon or even a sign of immaturity. However, histological sampling of an ontogenetic series of Ophiacodon humeri and of additional juvenile to adult humeri and femora indicates the pervasive occurrence of fibrolamellar bone (FLB) in the autopodia of this pelycosaur. Thus, the findings reaffirm the initial observation of fast growing tissue and disprove that the highly vascularized cortex is simply a reflection of an early ontogenetic stage. The periosteal bone tissue demonstrates the classic histological characteristics of FLB. The cortex consists of dense radially arranged primary osteons in a woven bone matrix and remains highly vascularized throughout ontogeny, providing unequivocal evidence of fast skeletal growth. Overall, the FLB tissue in Ophiacodon is more derived or 'mammal-like' in terms of the development of primary osteons, bone matrix, and growth mark expression then what has been described thus far for any other 'pelycosaur' taxon. However, long bone histology is inconclusive as to the preferred ecology of Ophiacodon, aquatic vs. terrestrial. The findings suggest that the evolutionary beginnings of mammalian endothermy and high skeletal growth rates date back to the late Carboniferous, approximately 35 million years earlier than previously hypothesized.