Growth Rate

The study of growth rates in dinosaurs has its roots in the pioneering work of Armand de Ricqlès, a French paleontologist who in the late 1960s began systematically examining the microstructure of fossil bone to draw inferences about growth and physiology (De Ricqlès, 1968, 1969). Building on the foundational principle established by Italian anatomist Rodolfo Amprino in 1947 — that local bone microstructure reflects local growth rate (Amprino's rule) — de Ricqlès observed that dinosaur cortical bone was highly vascularized and composed predominantly of fibrolamellar tissue, features far more similar to the bone of fast-growing endothermic birds than to that of slow-growing ectothermic reptiles such as crocodilians. He concluded that dinosaurs had a physiology more closely approximating that of modern birds than of living reptiles, challenging the then-dominant view of dinosaurs as sluggish, cold-blooded animals.

The 1990s saw major advances in quantitative growth reconstruction. Anusuya Chinsamy (1993) produced the first attempted dinosaur growth curve, based on the basal sauropodomorph Massospondylus. Collaborations between de Ricqlès, Jack Horner, and Kevin Padian expanded the scope by sampling embryonic, juvenile, and adult specimens of ornithischians and theropods, including Troodon, Maiasaura, and Hypacrosaurus (Horner, De Ricqlès & Padian, 1999, 2000). In 2000, Gregory Erickson and Tatyana Tumanova published the first fully quantitative growth curve for a dinosaur — Psittacosaurus mongoliensis — using a novel method they called Developmental Mass Extrapolation (DME), which combined LAG counts with body-mass estimates derived from limb bone length allometry (Erickson & Tumanova, 2000).

Lines of Arrested Growth (LAGs): When a dinosaur long bone is sectioned transversely and viewed under a microscope, concentric dark lines are visible within the cortex, much like tree rings. These LAGs represent annual pauses or significant slowdowns in bone deposition, typically correlated with seasonal environmental stress. By counting LAGs, researchers estimate an individual's age at death; by measuring the spacing between successive LAGs, they calculate annual increments of bone growth, which can be converted to mass increments using allometric equations.

Bone Tissue Typology: The type of primary bone tissue in the cortex is a qualitative-to-semiquantitative proxy for growth rate. Fibrolamellar bone (FLB), with its woven collagen matrix and rich network of primary osteons and vascular canals, indicates rapid periosteal bone deposition. Lamellar-zonal bone, with highly organized parallel-fibered collagen and sparse vascularity, indicates slow growth. Most non-avian dinosaurs exhibit predominantly fibrolamellar bone throughout much of their cortex, signaling sustained rapid growth during ontogeny.

External Fundamental System (EFS): As an individual approaches skeletal maturity, the intervals between LAGs progressively narrow until a tightly packed cluster of growth lines — the external fundamental system (also called the outer circumferential layer, OCL) — forms at the periosteal surface. The presence of an EFS indicates that growth has effectively ceased and the animal has reached its asymptotic body size. Its absence in a specimen simply means the individual died before achieving full skeletal maturity (Woodward, Horner & Farlow, 2011).

Growth Curve Modeling: Age-mass data extracted from histological analysis are fitted to standard sigmoidal growth models (logistic, Gompertz, von Bertalanffy) to reconstruct species-level growth curves. The reliability of these curves improves with larger sample sizes, as demonstrated by the landmark Maiasaura study based on 50 tibiae (Woodward et al., 2015).

Erickson, Curry Rogers & Yerby (2001) — Dinosaurian Growth Patterns: Published in Nature (vol. 412, pp. 429–433), this study was the first to quantify growth rates across a phylogenetically and size-diverse sample of dinosaurs. The authors demonstrated that all sampled dinosaurs exhibited sigmoidal growth curves but grew at rates accelerated well beyond the primitive reptilian condition. Small dinosaurs achieved growth rates similar to marsupials; large species matched eutherian mammals and precocial birds; giant sauropods grew at rates comparable to those of similarly sized cetaceans. Critically, avian-level growth rates were not present in non-avian dinosaurs but were attained in a stepwise fashion after birds diverged from their theropod ancestors in the Jurassic.

Erickson et al. (2004) — Tyrannosaurid Growth: Also published in Nature (vol. 430, pp. 772–775), this study reconstructed growth curves for four tyrannosaurid species. Tyrannosaurus rex exhibited a maximum growth rate of approximately 2.1 kg per day during a dramatic adolescent growth spurt between roughly ages 14 and 18, during which the animal gained approximately 70% of its adult body mass. Skeletal maturity was reached within about two decades, and maximum lifespan was estimated at approximately 28 years. Compared to its close relatives (Albertosaurus, Daspletosaurus, Gorgosaurus), T. rex achieved its enormous size primarily through an acceleration of growth rate rather than an extension of the growth period — a clear case of peramorphosis via acceleration.

Woodward et al. (2015) — Maiasaura Population Biology: This Paleobiology study examined 50 tibiae of the hadrosaurid Maiasaura peeblesorum, constituting the largest histological sample ever analyzed for any fossil vertebrate at that time. The authors determined that Maiasaura reached sexual maturity at approximately 3 years and skeletal maturity after approximately 8 years. Growth rates and mortality rates were both highest during the first year of life, and the mean mortality rate increased sharply after skeletal maturity, consistent with senescence beginning around age 8.

Sauropod Growth: Sauropod dinosaurs, the largest terrestrial animals in Earth's history, present particular challenges and interest for growth rate studies. Erickson et al. (2001) estimated that Apatosaurus reached its adult mass of approximately 25,000 kg in about 15 years, with a peak growth rate exceeding 5,000 kg per year. However, Lehman & Woodward (2008), using a different model based on average cortical bone apposition rates, estimated a much longer growth period of approximately 70 years for Apatosaurus, with a maximum rate of about 520 kg per year. This substantial discrepancy highlights the methodological uncertainties inherent in sauropod growth rate estimation, including difficulties in accurately counting LAGs in highly remodeled sauropod bone. More recent consensus, as summarized in a 2023 Current Biology review, suggests that sauropods generally reached asymptotic size in their third or fourth decade of life.

Dinosaur growth rates have been among the most influential lines of evidence in the long-running debate over dinosaur thermophysiology. De Ricqlès (1968, 1980) first used the fibrolamellar bone tissue of dinosaurs to argue for endothermy, and the quantitative work of Erickson and colleagues confirmed that dinosaur growth rates were far above typical ectothermic levels. However, the relationship between growth rate and metabolic strategy is more nuanced than a simple endotherm–ectotherm dichotomy.

In 2014, Grady et al. published an influential study in Science proposing that dinosaurs were "mesotherms" — organisms with metabolic rates intermediate between full endothermy and full ectothermy. By analyzing the allometric scaling of maximum growth rates across a broad vertebrate dataset, they found that dinosaurs fell between the endothermic and ectothermic regression lines. This mesothermic model suggests that dinosaurs generated internal heat but did not maintain body temperature as precisely as modern birds and mammals.

This hypothesis was not universally accepted. Several critiques (D'Emic, 2015; Myhrvold, 2016) pointed out methodological issues in Grady et al.'s analysis, including the choice of growth models and the handling of phylogenetic non-independence. Legendre et al. (2016), using bone growth rate data modeled from the equations of Montes et al. (2007), proposed that archosaurs ancestrally had high metabolic rates and that extant crocodilian ectothermy is actually a derived condition — implying that most dinosaurs, and especially theropods closely related to birds, were likely closer to true endotherms.

The current view among most researchers is that a single metabolic label cannot be applied to all dinosaurs. Given the extraordinary diversity of the clade — spanning body masses from less than 1 kg to over 70,000 kg — a range of metabolic strategies likely existed, and growth rate data support this diversity.

One of the most significant findings from growth rate research is the documentation of how the characteristically rapid, determinate growth seen in modern birds evolved from the dinosaurian ancestral condition. Most non-avian dinosaurs achieved sexual maturity before skeletal maturity, taking several years to decades to reach full size — a pattern more similar to extant crocodilians than to living birds. In contrast, nearly all extant birds reach skeletal maturity within their first year and achieve sexual maturity afterward (Ricklefs, 1968).

Histological analyses of stem birds have shown that this avian-style growth evolved gradually. The Jurassic Archaeopteryx and Early Cretaceous non-ornithuromorph birds such as Jeholornis, Confuciusornis, and various enantiornithines still required multiple years to reach adult size, like their non-avian relatives (Erickson et al., 2009; Chinsamy, Chiappe & Dodson, 1995; O'Connor et al., 2014). Avian-style growth has been confirmed in the derived ornithurines Hesperornis and Ichthyornis (Chinsamy, Martin & Dobson, 1998), and possibly also in some non-ornithurine ornithuromorphs such as Iteravis and Yanornis (O'Connor et al., 2015; Wang et al., 2019), suggesting that rapid determinate growth evolved independently or at least in a stepwise manner within the lineage leading to modern birds.

While bone histology–based growth rate estimation is the most powerful tool available for reconstructing life-history parameters in extinct vertebrates, it is subject to several important limitations. Early LAGs in the innermost cortex are frequently destroyed by medullary expansion and secondary remodeling, requiring mathematical retrocalculation to estimate lost growth records. Intraskeletal histovariability — differences in bone tissue and LAG counts among different skeletal elements within the same individual — can introduce error if sampling is restricted to a single element. Body-mass estimation from limb bone dimensions carries its own uncertainties, particularly for taxa with unusual body proportions.

Köhler et al. (2012) demonstrated that LAGs are also formed in extant endothermic mammals as part of a plesiomorphic energy-conservation strategy, definitively debunking the earlier assumption that the mere presence of LAGs indicated ectothermy. This finding clarified that LAGs record seasonal growth modulation regardless of metabolic strategy.

Future advances are expected from larger population-level sampling (following the Maiasaura model), integration of molecular paleontology techniques with traditional histology, and refined allometric models for body-mass estimation. The combination of these approaches holds considerable promise for producing more accurate and taxonomically comprehensive reconstructions of growth dynamics across the Dinosauria.