LARGE NUTRIENT FORAMINA IN FOSSIL FEMORA INDICATE INTENSE LOCOMOTOR AND METABOLIC ACTIVITY IN TRIASSIC ARCHOSAUROMORPHS AND THE PSEUDOSUCHIAN LINEAGE

SEYMOUR, R. S.; EZCURRA, M. D.; HENDERSON, D.; JONES, MARC E. H.; MAIDMENT, SUSANNAH C. R.; MILLER, C. V.; NESBITT, S. J.; SCHWARZ, D.; SULLIVAN, C.; WIBERG, E.

Congreso; SVP meeting; 2019

The size of the principal nutrient foramen on long bones can indicate the sizeof the nutrient artery and hence an index of blood flow rate (Qi) to the boneshaft. Because blood flow is essential for bone remodelling to repair microfracturescaused by locomotion, the foramen size and Qi are functionallyrelated to the level of locomotion. In extant mammals, for example, Qiincreases with body mass with an exponent of 0.86, which is almostindistinguishable from the exponent (0.87) for exercise-induced maximumaerobic metabolic rate. Extant non-avian reptiles, which generally do notremodel their long bones, have Qi values about 10 times lower than mammals,and considering their low blood pressure, bone perfusion is about 50 timeslower. Extant bipedal cursorial birds have Qi values that also scale with bodymass to the 0.89 power, but are almost twice the values for cursorialquadrupedal mammals, because bipeds place about twice the stress on theirfemora, compared to quadrupeds. These results for living animals show howthe size of the nutrient foramen relates to intensity of locomotion.Nutrient foramina on well-preserved fossil femora can be photographed,measured and compared to data from living species. We searched worldcollections and sampled about 200 fossil femora with preservedforamina. The present study is based on 34 specimens of Triassic, nonarchosaurianarchosauromorphs and pseudosuchians, dating from 250 Mya to48 Mya. The Qi values for these animals are not significantly different fromextant mammals and birds, and reveal a similar scaling exponent of0.90. They are significantly above the Qi data for extant non-avian reptiles,which are ectothermic and rely extensively on anaerobic metabolism duringbrief bouts of intense activity. These results support the hypothesis that thecrocodylian lineage of archosaurs were originally highly active animals,relying on aerobic metabolism for sustained locomotion. They required highrates of blood flow to all parts of the body, including the bones. These resultscomplement the cardiovascular evidence of a previously enigmatic, 4-chambered heart in extant ectothermic crocodiles. A completely divided heartis essential for high aerobic metabolic rates in birds and mammals to separatesystemic and pulmonary blood pressures and pump oxygenated blood at highrates to the tissues. As a result, this evidence bolsters the idea that the heartmorphology in extant crocodiles is a retention of an ancestral trait that nolonger functions to support endothermy.