Jack Horner: nests, Maiasaura and dinosaur growth

Jack Horner is associated with a major shift in how palaeontologists study dinosaur reproduction. Fossils from the Two Medicine Formation of Montana brought eggs, nests and young Maiasaura into the same research story. Horner’s work with Robert Makela helped turn those finds into testable questions about nesting and growth. The fossils are important, but they do not preserve a complete account of how any adult behaved.

Egg Mountain and the Maiasaura record

In the late 1970s, Horner and colleagues investigated a site near Choteau, Montana, that became known as Egg Mountain. The deposits preserved nests and young dinosaurs in repeated associations. Horner and Makela described Maiasaura peeblesorum in 1979 and argued that juveniles found in a nesting context indicated a close relationship between the young and the nest site.

Each observation answers a limited question. Eggshell and nest structures show that eggs were laid there. Juvenile bones show which animals were present and, through growth patterns, can help estimate their developmental stage. Repeated nests in a breeding area make accidental association less likely. Researchers then infer behaviour from the combination of these features and the sedimentary setting.

From nest association to parental care

The best-known interpretation is that some young Maiasaura remained in or near nests after hatching and may have depended on adults. Their size and skeletal development were used to argue that very young individuals were not fully independent. Later work on bone histology and tooth wear has added detail about growth and possible food needs. These are lines of evidence that strengthen or refine a behavioural model; they are not direct observations of feeding or guarding.

A nest can be affected by flooding, burial and animal movement. Juveniles in one place could have gathered for several reasons. To distinguish care from other explanations, researchers compare multiple nests, age distributions, bone growth, trackways and the local geology. The broader guide to dinosaur eggs and nests explains why a clutch, a nesting ground and parental care are separate claims.

Growth is part of the story

Horner’s research programme helped make dinosaur growth a central palaeontological subject. Thin sections of fossil bone can preserve growth marks and tissue structure. When researchers sample multiple individuals, they can compare how bones changed as animals grew. The method has limits: preservation is uneven, growth can vary with age and environment, and a bone is not a direct measurement of body mass or behaviour.

Maiasaura has remained an important example because its record includes a large series of young and adults from a well-studied formation. Such a sample lets researchers ask how body proportions and bone tissues changed through development. It also encourages caution: estimates from one species cannot automatically be applied to every dinosaur.

A collaborative career and a useful caution

Horner’s public profile often centres on the nesting finds, but the work depended on co-authors, students, preparators, field assistants and the Museum of the Rockies collection. Makela’s role in the early Maiasaura research deserves explicit credit. Subsequent investigators have revisited the same material with new analytical tools and sometimes different interpretations.

The strongest conclusion is precise: the Montana deposits preserve colonial nesting and an unusual association of nests with young Maiasaura; several anatomical and developmental findings support prolonged nest association and probable parental care. The degree of care, its duration and its presence in other dinosaur groups remain questions for evidence. See the Maiasaura profile for the fossil record and the guide to dinosaur growth for methods used to reconstruct development.