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This chapter discusses the “four signals” of bone histology: ontogeny, phylogeny, mechanics, and environment. These “signals” help us to make sense of the kinds of variation seen in the bone tissues of extinct tetrapods. They have been shown empirically to be reliable guides to the interpretation of the most basic question in the paleohistology of bone: Why is this tissue formed the way it is, and why does it differ from the tissue of this other animal? Along with some calibration methods based on teeth in many mammalian groups, bone histology forms the basis of skeletochronology, the only currently available universal line of evidence that provides an absolute age on the skeletons of extinct vertebrates.

This chapter addresses some of the questions and issues associated with the “four signals” of bone histology, with the aim of actuating further research. It discusses how knowledge in skeletochronology, linear and mass growth rates, and timing of the reproductive maturity of extinct tetrapods can be applied to life-history reconstructions.

In Chapter 5 we have seen that Pleistocene herpetofaunas are often used to suggest paleoenvironmental conditions. The present chapter discusses the validity of this use. With the exception of behavioral responses (and a few quasi-physiological adaptations in reptiles), herpetological species are poorly adapted to resist temperature changes. Or, to put it another way, amphibians and reptiles lack the complex internal physiological mechanisms that interact to regulate temperature in endotherms. This has led to the assumption by many that herpetological species, because of their supposed greater sensitivity to temperature changes, are much better indicators of local thermal conditions than are birds and mammals. Moreover, most Quaternary amphibians and reptiles represent extant species whose ecological tolerances and habitat preferences are well known, whereas many Pleistocene endotherms, especially large mammals, are extinct species whose ecological traits and specific habitat requirements are not totally understood. This suggests that Pleistocene herpetofaunas should give more refined information about specific habitats than endothermic faunas. Also, it can be argued that most amphibian and reptile species are more spatially confined than endothermic ones (especially birds and large mammals) and that Pleistocene herpetological species indicate paleoenvironmental conditions of more restricted areas. Bailón and Rage (1992) address this subject in the light of their European experience: “They (amphibians and reptiles] are unable to compensate for large climatic variations as endotherms (birds and mammals) can; therefore each amphibian and reptile species can live only in a definite climate.” (p. 95). Moreover,… Amphibians and reptiles present another advantage: whereas endotherm vertebrates are represented in Quaternary fossiliferous localities by a not negligible percentage of extinct species, practically all fossil amphibians and reptiles of that age belong to extant species. If a species is extinct its ecological requirements cannot be known with certainty and are open to speculation whereas living species present in fossiliferous assemblages afford accurate information, (p. 95)… They also state that the size of reptile home ranges is small, and that as these species are not the preferred prey of predaccous birds, they are probably not an admixture of species that live in a wide area around the locality. Let us examine these assumptions.