Preliminary statistical analysis defining the juvenile robust and gracile forms of the Triassic dinosaur Coelophysis

A mathematical analysis defining two distinct body types of Coelophysis from Ghost Ranch, probably representing males and females.

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Samples of Coelophysis and some other theropods exhibit robust and gracile morphs. In previous work, the morphs have been studied by means of allometric and other bone-to-bone comparative methods that yield interpretation-dependent results. We use probability plots to compare biometric data for Coelophysis bauri to the Normal probability density function. This produces an unambiguous definition of the robust and gracile forms in terms of the statistical distribution, mean, and standard deviation of specific bones or bone series sizes. Although our sample size is only adequate for preliminary analysis, size distributions in Coelophysis comprise three component modes. The most parsimonious explanation of the modes is that they represent juveniles, and robust and gracile adult morphs. Resolution of the component modes indicates that juveniles constitute ~40% of the population, and the robust and gracile adults each constitute ~30%, strongly supporting sexual dimorphism as the biological basis of the two adult morphs.

Some examples from the preliminary analysis are (mean / standard deviation, lengths in mm): robust: skull 245/26, femur 210/16, 3rd metatarsal 118/9, gracile: skull 169/13, femur 166/6, 3rd metatarsal 86/2, juvenile: skull 97/14, femur 128/7, 3rd metatarsal 78/3.

Probability plotting of larger samples should yield information on the onset of sexual maturity and determine whether there was a maximum adult size. Statistical analysis may prove generally useful in separating and defining ontogenetic stages and morphs; we have also determined three-mode size distributions in the "Shake-n-Bake" theropods of the Willow Springs, AZ locality, Syntarsus rhodesiensis, and Allosaurus fragilis.