Abstract
Osteohistological data are commonly used to study the life history of extant and extinct tetrapods. While recent advances have permitted detailed reconstructions of growth patterns, physiology and other features using these data, they are most commonly used in assessments of ontogenetic stage and relative growth in extinct animals. These methods have seen widespread adoption in recent years, rapidly becoming a common component of the taxonomic description of new fossil taxa, but are often applied without close consideration of the sources of variation present or the dimensional scaling relationships that exist among different osteohistological measurements. Here, we use a combination of theoretical models and empirical data from a range of extant and extinct tetrapods to review sources of variability in common osteohistological measurements, their dimensional scaling relationships and the resulting interpretations that can be made from those data. In particular, we provide recommendations on the usage and interpretation of growth mark spacing/zonal thickness data, when these are likely to be unreliable, and under what conditions they can provide useful inferences for studies of growth and life history.
Highlights
The use of osteohistological data, derived from thin-sectioned bone crosssections, to characterize the development, growth, and life history of extinct tetrapods has seen widespread adoption and advancement in recent years
Under Model 1, where ΔR (i.e. LAG spacing) remains at a constant rate, the other measures show an increasing rate due to dimensional scaling, in which a circle’s area increases by the square of the radius, and corresponding mass increases by the cube (e.g. A = πR2, C = 2πR, BM = ∼R3). This cannot be interpreted as total body growth rate remaining constant or consistently fast/ slow. Due to these dimensional scaling relationships and the resulting nature of bone growth, the rate of growth must increase in order for LAG spacing to remain constant
Such a pattern of consistent reduction in LAG spacing could be superficially interpreted as a decrease in growth rate, yet as this model demonstrates, it is representative of a constant rate of new bone tissue apposition where it is being measured
Summary
The use of osteohistological data, derived from thin-sectioned bone crosssections, to characterize the development, growth (via change in body mass), and life history of extinct tetrapods has seen widespread adoption and advancement in recent years. While in general terms growth zone spacing will decrease from the inner to outer cortex over the total ontogenetic period in tetrapods [4,11,22,23], this pattern varies and depends on functional and allometric patterns that are specific to the bone and taxon being analysed, as well as the ability to resolve patterns due to growth stage and individual growth variation All of these factors may be dynamic and shift through the ontogeny of an individual, as it encounters changes in resource availability and environment All of these factors may be dynamic and shift through the ontogeny of an individual, as it encounters changes in resource availability and environment (e.g. [11]), biomechanical changes through ontogeny [24], reproduces [25,26] and/or responds to pathology [27]
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