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Growth, allometry, and age/size distribution of the Late Triassic theropod dinosaur Coelophysis bauri: preliminary results

An account of the age and size distribution among the Late Triassic theropod Coelophysis bauri.

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Abstract

We statistically extracted size (= age) classes from Coelophysis bauri metrics. Using these size classes, we generated a hypothetical growth curve for C. bauri based on femur lengths (N=56). This curve is similar in shape to that of some birds (e.g. Gallus gallus) with the differences largely in time scale, and also to previously reported histologically derived growth data for C. kayentakatae. Age and mass distributions were then calculated based on the number of individuals in each age class and on femur dimensions. Hypothetically, Coelophysis’ growth rate was very high for the first year. Sexual dimorphism apparently onset between years one and two. After one year growth slowed in the gracile morph while the robust morph is first apparent and grew aggressively for another year; slow growth then continued in both morphs. Robust and gracile morphs probably represent males and females respectively based on their sexual dimorphism index (SDI = robust size / gracile size = 1.34). Both age and mass distributions are of hyperbolic form. Very small ~ one-year-olds weighing ~2 kg and ~1.4 m long comprise 40 % of the population, ~11 % are adults weighing ~14 kg and ~2.7 m long, ~2 % reach 25 kg and 3.1 m length.


Our allometry study, based on the 15 suitably complete specimens, agrees with previous studies showing that orbits and hind limbs show negative allometry while skull and neck lengths are positive. Allometric growth constants (this study) relative to total length are: skull length, 1.62; cervical series, 1.31; sacrum, 1.26; caudal series, 0.78; forelimb inc. hand, 1.3; hand, 1.52; hindlimb inc. foot, 0.92; foot, 0.91. Relative to skull length: orbit diameter, 0.28; height at quadrate, 0.22; height at prefrontal, 0.32. The cervical series shows complex allometry (log-transformed data are better fit by a polynomial than linear regression); growth rate being high in juveniles and progressively lower in adults. Thus, relative to adult proportions, juveniles had very short, high faces with large orbits, very short necks, short torsos, long tails, short arms, very small hands, long legs and large feet.