rhodesiensis, consists of disarticulated specimens that provide morphological information for one or a few elements (not whole individuals) at a time 23. The other known stratigraphically constrained bonebed, that of M. Similar to early sauropodomorphs, most of these taxa (e.g., Asilisaurus, Silesaurus) are known from multiple horizons and localities. rhodesiensis focused on constructing a growth curve for the taxon and did not link the age and size data with external morphological variation in the same specimens 22. A pioneering osteohistological analysis of M. High variation in morphological ontogenetic trajectories has been reported in early theropod dinosaurs, most notably Coelophysis bauri, Megapnosaurus rhodesiensis, as well as silesaurid dinosauriforms, although these studies either did not consider osteohistology 18, 19, 20 or found it uninformative for assessing chronological age 21. Similar variation has recently been reported in the osteohistology of another early sauropodomorph, Massospondylus 14, although in both studies the relatively large sample was taken from multiple horizons (and therefore, populations separated in both time and space) and morphological variation was not discussed. Osteohistology of the Triassic sauropodomorph Plateosaurus revealed high intraspecific variation in body size for a given ontogenetic age, which was interpreted as indicative of developmental plasticity 17. The earliest dinosaurs have been suggested to have extremely high levels of intraspecific variation in growth pattern and ontogenetic trajectory-potentially higher than even living crocodylians. To better contextualize the ancestral degree of ontogenetic variation within avian-line archosaurs, its reduction in some taxa, and the biological processes underscoring this disparity in variation, it is necessary to examine multiple types of ontogenetic data from their extinct relatives, especially non-avian dinosaurs. ![]() ![]() The difference between extant crocodilians and birds suggests that a transition or transitions to reduced ontogenetic variation occurred within avian-line archosaurs. In contrast, some birds are thought to have lower levels of such growth variation, potentially as a consequence of their sustained rapid growth 6, 15, 16 and ability to compensate for periods of short-term nutrient limitation with faster growth, which may return a bird to a normal growth trajectory before it reaches maturity 3. ![]() Indeed, similar levels of variation are widespread among non-avian and non-mammalian amniotes 9, 10, 11, 12, 13 and may be plesiomorphic for Tetrapoda as a whole 14, 15. Crocodylians exhibit a relatively high degree of morphological, histological, and body size variation during growth 5, 6, 7, 8. Levels of intraspecific variation in postnatal ontogeny differ among vertebrate groups, with a clear difference between the two clades of living archosaurs, Crocodylia and Aves. Therefore, intraspecific variation in ontogenetic trajectories-the changes undergone during life history-is evolutionarily critical because variation in growth timing, rate, duration, and body size at maturity can lead to differential survival, fitness, and fecundity of the members of a population 2, 3, 4. Intraspecific variation is the fuel that powers evolution by natural selection, and the life history of an organism is a major determinant of its body size, morphology, ecology, and ultimate reproductive success 1. Poor correlations among size, age, and morphological maturity strongly support the presence of unique, highly variable growth trajectories in early dinosaurs relative to coeval archosaurs and their living kin. Twenty-four histologically sampled individuals were less than a year to at least four years old and confirm the right-skewed age distribution of the Coelophysis assemblage. ![]() The Triassic theropod Coelophysis bauri, known from a bonebed preserving a single population of coeval individuals, provides an exceptional system to assess whether highly variable growth patterns were present near the origin of Dinosauria. Recent work hints that early dinosaurs possessed elevated levels of such variation compared to other archosaurs, but comprehensive data uniting body size, bone histology, and morphological variation from a stratigraphically constrained early dinosaur population are needed to test this hypothesis. Intraspecific variation in growth trajectories provides a fundamental source of variation upon which natural selection acts.
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