Using Osteohistology to Determine the Taxonomic Validity of the Late Cretaceous Dinosaur Nanotyrannus lancensis Bakker et al., 1988 (Theropoda: Tyrannosauridae)
Abstract
Nanotyrannus lancensis is a putative Tyrannosaurid species from the Late Cretaceous (Maastrichtian) of North America, distinguished from the coeval Tyrannosaurus rex by body size, number of teeth and some minor cranial characters. However, not all workers have accepted N. lancensis as a valid taxon, instead proposing that specimens identified as N. lancensis represent juvenile T. rex. In this study, I examined the osteohistology of a tibia, a pubis, and three ribs of a single N. lancensis individual recovered from the Lance Formation of eastern Wyoming in order to determine the degree of skeletal maturity of the individual. All elements sectioned have a cortex primarily comprised of zonal fibrolamellar bone tissue with primarily longitudinal vascularization, with cancellous bone comprising the interior of the element. Bone tissue becomes less vascularized in zones closest to the sub-periosteal region, but this region still remains vascularized. Innermost portions of the cortex display Haversian remodeling in all thin sections. Lines of arrested growth (LAGs) and associated annuli are present in all elements sectioned: 6-8 in a rib (HRS08507); 1 in the tibia (HRS08421); 11-13 in the pubis (HRS01514). Extensive Haversian remodeling, resulting in dense Haversian tissue, is especially prevalent in the pubis and ribs (HRS08514, HRS08467), which presumably resulted in earlier LAGs being destroyed during ontogeny. No medullary cavity migration appears to have occurred, though medullary expansion probably destroyed the earliest LAGs. Some elements possess double and triple LAGs, which were considered one growth cessation event. All growth zones decrease in size outwardly. No external fundamental system (EFS) was observed in any element sectioned.
I conclude that the individual studied was at minimum 12-13 years old when it died, although earlier LAGs were probably destroyed by remodeling and medullary expansion. The individual had not ceased growth as evidenced by the vascularized sub-periosteal layer; however, the longitudinal vascularization and decrease in vascularization in sub-periosteal zones indicate that the individual’s growth was slowing. This, in addition to extensive Haversian remodeling, indicate that the individual was not a juvenile. I conclude that Nanotyrannus lancensis Bakker et al., 1988 is a distinct taxon from Tyrannosaurus rex Osborn, 1905.
Using Osteohistology to Determine the Taxonomic Validity of the Late Cretaceous Dinosaur Nanotyrannus lancensis Bakker et al., 1988 (Theropoda: Tyrannosauridae)
Cedarville, OH
Nanotyrannus lancensis is a putative Tyrannosaurid species from the Late Cretaceous (Maastrichtian) of North America, distinguished from the coeval Tyrannosaurus rex by body size, number of teeth and some minor cranial characters. However, not all workers have accepted N. lancensis as a valid taxon, instead proposing that specimens identified as N. lancensis represent juvenile T. rex. In this study, I examined the osteohistology of a tibia, a pubis, and three ribs of a single N. lancensis individual recovered from the Lance Formation of eastern Wyoming in order to determine the degree of skeletal maturity of the individual. All elements sectioned have a cortex primarily comprised of zonal fibrolamellar bone tissue with primarily longitudinal vascularization, with cancellous bone comprising the interior of the element. Bone tissue becomes less vascularized in zones closest to the sub-periosteal region, but this region still remains vascularized. Innermost portions of the cortex display Haversian remodeling in all thin sections. Lines of arrested growth (LAGs) and associated annuli are present in all elements sectioned: 6-8 in a rib (HRS08507); 1 in the tibia (HRS08421); 11-13 in the pubis (HRS01514). Extensive Haversian remodeling, resulting in dense Haversian tissue, is especially prevalent in the pubis and ribs (HRS08514, HRS08467), which presumably resulted in earlier LAGs being destroyed during ontogeny. No medullary cavity migration appears to have occurred, though medullary expansion probably destroyed the earliest LAGs. Some elements possess double and triple LAGs, which were considered one growth cessation event. All growth zones decrease in size outwardly. No external fundamental system (EFS) was observed in any element sectioned.
I conclude that the individual studied was at minimum 12-13 years old when it died, although earlier LAGs were probably destroyed by remodeling and medullary expansion. The individual had not ceased growth as evidenced by the vascularized sub-periosteal layer; however, the longitudinal vascularization and decrease in vascularization in sub-periosteal zones indicate that the individual’s growth was slowing. This, in addition to extensive Haversian remodeling, indicate that the individual was not a juvenile. I conclude that Nanotyrannus lancensis Bakker et al., 1988 is a distinct taxon from Tyrannosaurus rex Osborn, 1905.
