Macroevolutionary patterns of theropod gigantism as revealed through osteohistology and growth curve reconstruction

CULLEN, THOMAS; CANALE, JUAN IGNACIO; APESTEGUÍA, SEBASTIÁN; SMITH, NATAN; MAKOVICKY, PETER

Alberta

Congreso; 7th Annual Meeting Canadian Society of Vertebrate Palaeontology; 2019

Canadian Society of Vertebrate Palaeontology

Many theropod clades contain extremely large-bodied, or gigantic, representatives, and the repeated but independent evolution of gigantism has been a topic of longstanding interest. Data derived from bone histology combined with methods for growth reconstruction have played a key role in investigations into this issue, and several previous studies have used histological data to suggest that multiple theropod taxa achieved large body size through similarly drastic increases in their growth rates when compared to smaller relatives. Despite these insights, concerns remain with respect to the source of histological data used for these reconstructions, the degree of variation present in the proxies used for growth estimation, and the ubiquity of the mechanisms underlying gigantism in theropods. Disagreements exist within the literature regarding which element of the skeleton is ?best? for growth curve reconstructions, with sampling of weight-bearing versus non weight-bearing bones often, though not exclusively, being at the center of these debates. Regarding the issue of variation, potentially con- founding issues remain at several scales of comparison, such as between bones of an individual (intra-skeletally), between individuals (intra-specifically), and between species (inter-specifically). Understanding the range of variation that may exist at each of these scales is critical if we are to confidentially perform reconstructions of age and growth in extinct taxa using osteohistological data. To address these issues, we perform multi-element histological sampling of multiple individuals representing a phylogenetically broad range of theropod taxa to both expand our quantitative record of osteohistological variation among closely related taxa, and to test if particular elements consistently preserve the most complete growth record across individuals and taxa. We then perform a series of growth-curve reconstructions with these data to address the question of how theropods achieved large body size in multiple evolutionarily independent events. We find considerable variability in metrics such as growth mark count and remodelling between elements, with a relatively strong ontogenetic component to observed patterns. In skeletally mature individuals (possessing an external fundamental system, or EFS, in the periosteal margins of their bones) the bulae can undergo substantial secondary remodelling, severely hampering age estimation and growth curve reconstruction, whereas femora continue to preserve extensive primary growth records. In skeletally-immature individuals, the bulae have not remodelled to nearly the same extent, and thus may provide a more-complete growth record when compared to femora of the same individual due to medullary expansion in the latter. Comparisons of growth curve reconstructions (performed primarily through femoral data, but with additional curves from other elements made for comparison) between large-bodied theropods produced divergent results, with some taxa, such as tyrannosaurids, appearing to achieve large body size through a considerable increase in growth rate, and others, such as carcharodontosaurids, appearing to achieve large body size through considerable increase in growth period while maintaining a slower overall rate of growth. These differences suggest that not all theropods achieved gigantism via the same means, and that a variety of physiological strategies may have been employed depending on both environmental and phylogenetic parameters.