Morphological variation in small theropods and its meaning in systematics: evidence from Syntarsus rhodesiensis

Paper describing how morphological variation in Syntarsus rhodesiensis demonstrates sexual dimorphism, not taxonomic diversity.

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Because they are small and their bones fragile, small theropods are usually found as poorly preserved fossils. It is, therefore, often difficult to identify morphological features that clearly and unequivocally characterize individual small theropod taxa, to distinguish them from each other. This legacy of fragility has unquestionably handicapped studies of small theropod systematics in the past. Several authors have recently published cladistic analyses of archosaur lineages, including the theropods, but the picture remains somewhat murky as their cladograms are not easily reconciled with each other. Perhaps this is also partly attributable to study material that is incomplete and poorly preserved.

The Early Jurassic (terminal Karoo) Syntarsus rhodesiensis is a small theropod taxon based on material that is both quantitatively adequate and qualitatively excellent as far as preservation is concerned. Partial remains of more than 30 individuals have been recovered from one of three known localities in the fine-grained aeolian Forest Sandstone Formation of Zimbabwe, southern Africa. The fossiliferous bed in this unusually rich locality suggests that a single event caused the catastrophic mass death of a socially gregarious group.

Syntarsus shows clear and consistent morphological variation that is bimodally distributed in the sample examined. The variation affects particularly the trochanters and muscle scars of the femur, but it has also been observed in other elements of the skeleton. I conclude that the morphological variation shown by the known Syntarsus sample reflects clear sexual dimorphism, and not taxonomic diversity. The dimorphism manifests as gracile and robust morphs, with robusticity acquired only after attainment of a body size taken to mark the onset of sexual maturity. Based on the population structure reflected in the Chitake River Syntarsus sample, I suggest that the robust morph might represent females.