Taxonomic Implications of Relative growth in Lambeosaurine Hadrosaurs

Abstract

Dodson, P. (Laboratories of Anatomy, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19174) 1975. Taxonomic implications of relative growth in lambeosaurine hadrosaurs. Syst. Zool. 24:37-54.-The Oldman Formation (Campanian) of Alberta includes in its dinosaurian fauna 3 genera and 12 species of closely related lambeosaurine hadrosaurs. A biometric study of 36 skulls that ranged in size by a factor of three was undertaken; the data were fitted to the allometric equation. Bivariate plots reveal the validity of two genera, Corythosaurus and Lambeosaurus, with one species of the former, C. casuarius, and two of the latter, L. lambei and L. magnicristatus (quite rare), being accepted. Sexual dimorphism is recognized in each. Procheneosaurus includes juvenile individuals of each genus. Although 48 cranial characters were measured, only 5 or 6, confined to the crest, had any discriminatory value. The analogy between cassowaries and hadrosaurs in growth of the crest is observed. The sex ratio and maximum size of each sex is about equal for both common species. [Cretaceous; allometry; dinosaurs; Lambeosaurinae.] Reptiles differ from mammals and birds in that they grow continuously, though not at constant rates, throughout life. Therefore a population of reptiles is likely to contain individuals distributed over a much wider range of size than a population of birds or mammals that grow quickly to a definitive adult size; any field guide to reptiles (e.g. Conant, 1958) will verify this. If allometry and sexual dimorphism are also significant factors, a population of reptiles may show a great range of statistical variation. This contrasts sharply with the quite narrow range of variation that characterizes many aspects of mammalian anatomy, especially teeth (Simpson, Roe and Lewontin, 1960, pp. 89-95). An ecological consequence of continuous growth is that species of large reptiles in their juvenile stages fill niches that would be occupied by a number of mammalian (canids (Rosenzweig, 1966); bovids (Bell, 1969 ) ) or avian ( accipiters ( Storer, 1966 ) ) species of different sizes. This has been documented for Alligator mississipiensis (Mcllhenny, 1935); Crocodilus niloticus (Cott, 1961) and Varanus komodoensis (Auffenberg, 1972); diets change radically as these animals grow. Thus niche differentiation by size among closely related species of large reptiles does not seem to be a feasible strategy. These and other considerations are brought to bear on a fauna of 3 genera and 12 species of lambeosaurine hadrosaurs (crested duck-billed dinosaurs) from the Oldman Formation of southern Alberta. The Oldman Formation has yielded one of the richest Upper Cretaceous dinosaur faunas of the world, more than 320 articulated specimens having been recovered from a 200 ft. section as exposed along 15 miles of the Red Deer River (Russell, 1967; Dodson, 1971). Perhaps half a million years or less is represented, and evolution within the formation is undemonstrated. It is therefore assumed that the animals preserved were essentially contemporaneous. Lambeosaurine hadrosaurs constitute the most abundantly preserved element of the Oldman dinosaurian fauna, more than 40 skulls having so far been found. Lambeosaurines were extremely variable in cranial morphology (Fig. 1), particularly with regard to development of the crest. The crest was constructed of paired premaxillae and nasals, and enclosed the narial passages, which were greatly elaborated in comparison with the usual reptilian condition. Does the currently accepted taxonomy of