Ontogeny

The term ontogeny was coined by the German zoologist Ernst Haeckel in his 1866 work Generelle Morphologie der Organismen (General Morphology of Organisms), published while he was a professor at the University of Jena. In the same book, Haeckel also introduced the term phylogeny for the evolutionary history of a lineage, establishing a conceptual pair that has remained foundational in biology. Haeckel is best remembered in this context for his biogenetic law, summarized by the phrase "ontogeny recapitulates phylogeny," which proposed that the developmental stages of an embryo replay the adult forms of its evolutionary ancestors in chronological order. Although this strict recapitulation theory was challenged almost immediately — notably by Wilhelm His and later by Karl Ernst von Baer's competing laws of embryology — and is rejected by modern biologists as an overgeneralization, the broader insight that developmental data contain phylogenetic information remains valid. Embryos of related species do share certain features early in development that reflect common ancestry, even though they do not pass through the adult stages of ancestral organisms.

One of the most consequential applications of ontogenetic research in paleontology is the resolution of taxonomic inflation caused by naming different growth stages as separate species or genera.

Pachycephalosaurus case study:

In a landmark 2009 paper published in PLoS ONE, Jack Horner and Mark Goodwin presented multiple lines of evidence — cranial histology, comparative morphology, and CT scanning — that Dracorex hogwartsia (named in 2006 and characterized by a flat skull with prominent spikes), Stygimoloch spinifer (named in 1983 and characterized by a small dome with elongate horns), and Pachycephalosaurus wyomingensis (named in 1943 and possessing a massive rounded dome) represent a single ontogenetic series of the same taxon. The key evidence included the presence of an open intrafrontal suture in Stygimoloch skulls (indicative of subadult status, as this suture functions as an intramembranous bone growth site and remains unossified during active cranial vault expansion), highly vascularized metaplastic bone in the dome of Stygimoloch (indicating rapid growth), and the dense, less vascularized tissue of the Pachycephalosaurus dome (indicating growth had slowed). The squamosal horns, which were long and pointed in Dracorex and Stygimoloch, showed histological evidence of erosion and remodeling, ultimately producing the blunt, robust nodes seen in adult Pachycephalosaurus. The study demonstrated that these cranial features underwent extreme modification through metaplasia — a process in which dense fibrous connective tissues are transformed directly into bone — and that the changes formed a growth continuum rather than discrete developmental steps.

Triceratops and Torosaurus:

In 2010, John Scannella and Jack Horner proposed that Torosaurus was not a separate genus but the fully mature form of Triceratops, arguing that the solid frill of Triceratops developed fenestrae (openings) and elongated as the animal reached old age. However, this hypothesis has been contested. In 2012, Nicholas Longrich and Daniel Field published a rebuttal in PLoS ONE, analyzing 35 specimens and concluding that Torosaurus is a distinct genus based on the presence of immature Torosaurus specimens that are inconsistent with the synonymy hypothesis. The debate remains unresolved and illustrates the complexity of applying ontogenetic interpretations to taxonomy.

The case of Nanotyrannus lancensis dramatically illustrates both the power and the pitfalls of ontogenetic reasoning in paleontology. For decades, the question of whether Nanotyrannus represented a distinct small-bodied tyrannosaur or a juvenile Tyrannosaurus rex was one of the most contentious debates in the field. In 2020, Holly Woodward and colleagues published osteohistological evidence in Science Advances indicating that the specimens in question were ontogenetically immature, which supported the juvenile T. rex interpretation.

However, in late 2025, the debate took a dramatic turn. A study by Lindsay Zanno, James Napoli, and colleagues, published in Nature, analyzed the remarkably complete "Dueling Dinosaurs" fossil — a Nanotyrannus specimen locked in combat with a Triceratops. The team found an external fundamental system (EFS) in the Nanotyrannus bones — a series of tightly packed growth rings indicating that the animal had finished growing. At full maturity, this specimen was approximately half the length and one-tenth the body mass of a mature T. rex. Additionally, the Nanotyrannus skull contained more tooth sockets than any T. rex of any age, and CT scans revealed distinct routes for cranial nerves and sinuses that are established during embryonic development and remain fixed throughout life. A subsequent study published in Science in December 2025 further supported the validity of Nanotyrannus as a separate genus. The researchers also described a second species, Nanotyrannus lethaeus, based on the specimen known as "Jane."

This case powerfully demonstrates that ontogenetic maturity assessment — not merely body size — is the crucial factor in taxonomic determinations, and that even well-supported ontogenetic hypotheses may be overturned by new evidence.

Bone Histology (Osteohistology):

The primary method involves thin-sectioning fossil bone and examining it under a microscope to identify growth marks. Lines of arrested growth (LAGs) are dark bands formed when growth slows or ceases, typically on a seasonal cycle, analogous to tree rings. Counting LAGs provides a minimum age estimate for the individual. The spacing between successive LAGs reflects growth rate — wider spacing indicates faster growth. The external fundamental system (EFS), a layer of closely spaced LAGs at the bone periphery, indicates that growth has effectively ceased and the animal has reached skeletal maturity. However, recent comparative studies in extant taxa have shown discrepancies between LAG counts and known ages depending on sectioning methodology (petrographic ground sections versus stained microtomized sections), highlighting the need for continued methodological refinement.

Morphometrics:

Quantitative comparison of skeletal proportions across specimens of varying sizes reveals allometric growth patterns. Juvenile dinosaurs typically exhibit relatively larger heads, larger orbits, and shorter snouts compared to adults. Peter Dodson's pioneering 1975 work on lambeosaurine hadrosaurs demonstrated that extended neoteny (retention of juvenile features) and late-stage allometric growth could vastly increase the morphological disparity between different growth stages within a single species, causing researchers to misidentify juveniles as separate taxa.

CT Scanning and 3D Modeling:

Non-destructive computed tomography allows visualization of internal skeletal structures, suture fusion states, and pneumatic features. Micro-computed tomography (μCT) and synchrotron radiation micro-CT (SrμCT) are increasingly used to assess ontogenetic maturity without damaging valuable fossils. These techniques were instrumental in the Pachycephalosaurus study (revealing the open intrafrontal suture) and the Nanotyrannus study (revealing distinct cranial nerve pathways).

Dental Development:

Incremental growth lines in embryonic teeth, known as von Ebner lines, correspond to daily pulses of mineralization and have been used to estimate incubation periods in dinosaurs. Tooth replacement patterns and morphological changes in dentition also provide ontogenetic information.

Ontogenetic data have proven essential for understanding macroevolutionary patterns in dinosaurs. Heterochrony — evolutionary changes in the timing or rate of developmental processes — has been proposed as a major mechanism driving morphological diversification. The paedomorphic (juvenilized) skulls of both sauropods and birds have been suggested to have evolved through retention of ancestral juvenile features into adulthood. Ontogenetic shifts between bipedalism and quadrupedalism have also been hypothesized in several dinosaurian lineages, including ornithischians and sauropodomorphs.

Osteohistological growth data have illuminated the evolution of gigantism in sauropods. Research has shown that accelerated, uninterrupted growth and a lack of developmental plasticity are traits that evolved near the origin of sauropod gigantism, while accelerated but seasonally interrupted growth was already present in smaller (1–2 tonne) non-sauropodan sauropodomorphs. Among non-avian theropods, changes in rate and timing of growth — including truncation, prolongation, and acceleration — have been identified as important mechanisms by which body size evolved.

As reviewed by Chapelle, Griffin, and Pol (2025) in Biology Letters, several key challenges face the field of dinosaur ontogeny research:

Decoupling of size and age: Multiple taxa, including Plateosaurus, Massospondylus, Coelophysis, and Tyrannosaurus rex, show evidence that individuals of similar body size can differ significantly in age, a phenomenon linked to osteohistological growth plasticity influenced by environmental factors. This means body size alone is an unreliable proxy for ontogenetic maturity.

Sequence polymorphism: Different individuals of the same species can undergo morphological changes in different sequences during ontogeny, complicating the construction of reliable ontogenetic series.

Inconsistent terminology: Terms such as "juvenile," "subadult," and "adult" lack standardized definitions across the literature, leading to subjective interpretations and cross-study incoherence.

Sampling bias: Immature specimens are relatively rare in the fossil record, and growth series are known for only a few model taxa (e.g., Maiasaura, Psittacosaurus, Massospondylus, Coelophysis). Increasing sample sizes through continued fieldwork and reexamination of museum collections remains critical.

Methodological validation: Extant comparative datasets remain limited. Building comprehensive datasets from living archosaurs (birds and crocodilians) with known ages and environmental histories is essential for ground-truthing paleontological methods.

These challenges notwithstanding, ontogenetic research continues to transform our understanding of dinosaur biology, biodiversity, and evolution, and it remains one of the most active and consequential frontiers in modern paleontology.