Abstract
AbstractHadrosaurid 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 3 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-bone 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.
Highlights
Hadrosaurs, commonly known as duckbilled dinosaurs, were the dominant largebodied terrestrial herbivores in many latest Cretaceous ecosystems (Horner et al 2004)
The newly constructed size–frequency distribution of the Dinosaur Park Formation (DPF) hadrosaurid sample revealed four relatively distinct size–frequency peaks that formed a positive parabolic distribution consistent with patterns exhibited by attritional assemblages
If not due to preservation, this may suggest that hadrosaurids from the DPF had high survivorship until 2 years of age if they managed to survive the initial high nestling mortality or that juveniles were segregated from the main population or adult-dominated herd and largely not present in the DPF region during this ontogenetic interval
Summary
Hadrosaurs, commonly known as duckbilled dinosaurs, were the dominant largebodied terrestrial herbivores in many latest Cretaceous ecosystems (Horner et al 2004) They had a nearly global geographic distribution (Lund and Gates 2006; Prieto-Márquez 2010b) that included latitudinal extremes (Case et al 2000; Gangloff and Fiorillo 2010) and were one of the most diverse and abundant dinosaur groups in the Late Cretaceous (e.g., Horner et al 2004; Prieto-Márquez 2010a,b). Three distinct size– frequency peaks were identified and interpreted as discrete size classes Two of these size–frequency peaks were attributed to juveniles and subadults, whereas the broad third size–frequency peak was inferred as a clustering of overlapping size classes of adult individuals found within the DPF sample. Assuming a closely correlated relationship between size and age, this study suggested that hadrosaurids from the DPF attained asymptotic body size within 3 years of age
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