Forerunners of Mammals: Radiation • Histology • Biology

4. The Paleobiology and Bone Microstructure of Pelycosaurian-Grade Synapsids

91 4 4.1. Phylogeny-corrected stratigraphic ranges of basal synapsids superimposed onto the international Permo-Carboniferous global stratigraphy. Absolute dates of geologic stages (269–306) are in millions of years (Ma). Black bars represent observed ranges, white bars represent phylogeny-corrected ranges (“ghost lineages”), and gray bars represent uncertain ranges. Topology and stratigraphic ranges are based primarily on Huttenlocker (2008, unpublished), supplemented by data from Reisz, Berman, and Scott (1992), Laurin (1993), Modesto (1994), Laurin and Reisz (1995), Sidor and Hopson (1998), Kissel and Reisz (2004), and Campione and Reisz (2010). The Paleobiology and Bone Microstructure of PelycosaurianGrade Synapsids Adam K. Huttenlocker and Elizabeth Rega Modern reptilian-grade amniotes (e.g., snakes, lizards, crocodilians) and the lineage that gave way to mammals (Synapsida) diverged more than 320 Mya during the Carboniferous Period. Also called pelycosauriangrade synapsids or “pelycosaurs,” basal synapsids were the dominant terrestrial vertebrates of the Late Pennsylvanian through Early Permian periods. Given their key phylogenetic position near the divergence, the adaptive evolution of their skeletal structures are especially significant, as the anamniote-amniote transition was classically characterized by a transition from a predominantly amphibious to a more terrestrial mechanical regime and lifecycle (Romer 1957, 1958; Germain and Laurin 2005). Basal synapsids have been subject to extensive gross morphological, phylogenetic, and functional analyses which have revealed a diversity of body sizes and diets—including high fiber herbivory, as well as insectivory and apex-level carnivory—and attendant somatic and dental adaptations (e.g., Modesto and Reisz 1992; Maddin 2008). In the present chapter, we integrate existing information regarding basal synapsid gross osteology with new data regarding bone microstructure and biophysical modeling under both normal and pathological conditions. Importantly, histomorphological analysis underscores the diversity of lifestyles, growth patterns, and ecophysiological adaptations achieved early on in the synapsid lineage . For related topics omitted in the present review (in the interest of brevity), we recommend these additional readings: Kemp (1982, 2005), Hotton et al. (1986), Schultze and Trueb (1991). Institutional Abbreviations This chapter uses abbreviations for the following institutions: Carnegie Museum, Pittsburgh (CM); Field Museum, Chicago (FMNH UC); Harvard Museum of Comparative Zoology, Cambridge (MCZ); Sam Noble Oklahoma Museum of Natural History, Norman (OMNH); Royal Ontario Museum, Toronto (ROM); former University of California Vertebrate Paleontology Collections, Los Angeles (UCLA VP); University of California Museum of Paleontology, Berkeley (UCMP); Burke Museum of Natural History, Seattle (UWBM). Introduction Huttenlocker and Rega 92 Basal synapsids are of significant interest to paleontologists because their record is relatively well-documented through geologic time and their evolutionary history holds clues to the origins of mammalian structure and physiology (Reisz 1986; Hopson 1991; Laurin 1993; Modesto 1994; Berman et al. 1995; Laurin and Reisz 1995; Sidor and Hopson 1998; Rubidge and Sidor 2001).The two major morphological grades of nonmammalian synapsids (introduced in chapter 1) are pelycosaurian-grade synapsids and therapsids. The pelycosaurian-grade synapsids, characterized by hyperelongated neural spines, reached the peak of their diversity during the Early Permian Period, approximately 300–269 Mya (Fig. 4.1; see Plate A). Their early diversification is best documented in PermoCarboniferous localities of Euramerica which were arranged primarily along a paleoequatorial transect, representing subtropical to seasonally wet environments that varied greatly in local conditions, vegetation, and physiography, and indicating that early synapsids were well adapted to handle a modest array of habitats and environmental conditions (but perhaps not to the degree of their therapsid descendants; see chapters 1 and 11 of this book). At least six well-known families of pelycosaurs were established by the Early Permian (Fig. 4.1) (chapter 1 of this book), a few of which eventually achieved distributions across North and South America, Europe, and Africa (Romer and Price 1940; Reisz 1986; Piñeiro et al. 2003). These included the caseasaurian families Eothyrididae and Caseidae, and eupelycosaurian families Varanopidae, Ophiacodontidae, Edaphosauridae, and The Fossil Record of Basal Synapsids and Their Physical Environment Paleobiology and Bone Microstructure of Pelycosaurian-Grade Synapsids 93 Sphenacodontidae. Some rare sphenacodontians from North America and Europe, the “haptodontines,” have been interpreted as a paraphyletic -grade demonstrating trends toward the sphenacodontid-therapsid condition (Laurin 1993; Kissel and Reisz 2004). Their lack of hyperelongated neural spines coupled with their phylogenetic position between edaphosaurids and sphenacodontids supports the hypothesis that the dorsal sail arose more than once in early synapsids (i.e., first in edaphosaurids and later in post-haptodontine sphenacodontids). Kemp (1982, 2005) and Reisz (1986) have provided comprehensive reviews of early...