Testing size-frequency distributions as a method of ontogenetic aging: a life history assessment of hadrosaurid dinosaurs from the Dinosaur Park Formation of Alberta, Canada, with implications for hadrosaurid paleoecology

Hadrosaurid dinosaurs, the dominant large-bodied terrestrial herbivores in
most Laurasian Late Cretaceous ecosystems, have an exceptional fossil
record consisting of many species known from partial ontogenetic series
making them an ideal clade with which to conduct life history studies.
Previous research considered the Dinosaur Park Formation (DPF) of Alberta
as an attritional, or time-averaged, sample and interpreted size-frequency
distribution of long bones collected from the DPF with three size classes
to suggest that hadrosaurids from the DPF attained near-asymptotic body
size in under three years. This conflicted with previously published
osteohistological estimates of 6+ years for penecontemporaneous
hadrosaurids from the Two Medicine Formation (TMF) of Montana suggesting
either extreme variation in hadrosaurid growth rates or that
size-frequency distributions and/or osteohistology and growth modeling
inaccurately estimate ontogenetic age. We tested the validity of the
previously proposed size-age relationship of hadrosaurids from the DPF by
significantly increasing sample size and combining data from
size-frequency distributions and osteohistology across multiple long bones
elements. The newly constructed size-frequency distributions typically
reveal four relatively distinct size-frequency peaks that, when integrated
with the osteohistological data, aligned with growth marks. The yearling
size class was heavily underrepresented in the size-frequency
distribution. If not due to preservation, this suggests that either
juvenile (< 2 years of age) hadrosaurids from the DPF had increased
survivorship following an initially high nestling mortality rate, or that
yearlings were segregated from adults. A growth curve analysis revealed
asymptotic body size was attained in approximately 7 years, which is
consistent with hadrosaurids from the TMF. The data suggest size-frequency
distributions of attritional samples underestimate age and overestimate
growth rates, but when paired with osteohistology can provide unique life
history insights.