Lystrosaurus

Botha-Brink, Jennifer & Angielczyk, Kenneth D., 2010, Do extraordinarily high growth rates in Permo-Triassic dicynodonts (Therapsida, Anomodontia) explain their success before and after the end-Permian extinction?, Zoological Journal of the Linnean Society 160 (2), pp. 341-365 : 352-354

LYSTROSAURUS

Three species of Lystrosaurus were examined in this study, namely the Late Permian Lystrosaurus maccaigi , and the Early Triassic L. murrayi and L. declivis (species identifications follow those of Grine et al., 2006; see also Botha & Smith, 2007). Although specimens of the Triassic species of Lystrosaurus are abundant, specimens of the large Permian species, L. maccaigi are rare. This study is the first to analyse L. maccaigi bone microstructure. A tibia (cortical thickness = 34%) and rib (SAM-PK-K10571), a humerus and ulna (NMQR3663), and a radius (NMQR3641; cortical thickness = 41%) from three individuals were assessed. The bone tissue of L. maccaigi consists of fibrolamellar bone. The primary bone tissue is uninterrupted in the radius (36% adult), but is interrupted by LAGs in the tibia (69% adult) and annuli in the humerus and ulna (72% adult). Enlarged channels are also observed in the humerus, ulna, and radius, similar to those seen in Dicynodon , but they narrow progressively in the mid and outer cortex to form a laminar network in NMQR3663 ( Fig. 4C ). The rib also consists of fibrolamellar bone, but although the section is from the midshaft region, the medullary cavity is completely filled with trabecular bone, similar to that reported by Ray et al. (2005) for the ribs of L. murrayi ( Ray et al., 2005: Fig. 5C, D ). Towards the periphery of the bone, a region of poorly vascularized parallel-fibred bone is observed. This too, has been reported in L. murrayi ( Ray et al., 2005: Fig. 5C ).

As species identifications in this study follow that of Grine et al. (2006) and Botha & Smith (2007), L. declivis is distinguished from L. murrayi as a separate species. The bone tissue of several L. declivis limb bones (BP1/14211; NMQR735; SAM-PK-11184; SAM-PK-K8013) were examined and compared with the bone tissue of L. murrayi . Ray et al. (2005) listed SAM-PK-11184 and SAM-PK-K8013 as L. murrayi , but based on the presence of prefrontal bosses, prominent frontonasal and longitudinal premaxillary ridges, and longer snouts in these specimens ( Cluver, 1971; Grine et al., 2006), they are identified as L. declivis in this study. The bone tissue of L. declivis is similar to that of L. murrayi and consists of highly vascularized fibrolamellar bone tissue. Two thick annuli and one LAG are observed in tibia NMQR735b ( Fig. 4D ). All the elements exhibit enlarged channels in the cortex. These are distributed uniformly throughout the cortex in radius BP/1/4211a and haphazardly in ulna BP/1/4211b. The channels become smaller towards the periphery to form longitudinal circumferential rows in the ulna. The channels in all the elements of NMQR735 are either arranged in circumferential longitudinal primary osteons or they form a laminar network throughout the cortex.

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Lystrosaurus NMQR 3678 is an adult and the largest Lystrosaurus specimen in this study. Although recovered from the Early Triassic Lystrosaurus Assemblage Zone it was found isolated and thus, cannot be identified to species level. However, because of its large size (total length exceeds the total length of the largest L. murrayi humerus known), we have tentatively identified it as L. declivis . This element was included in the study to emphasize that although this element is from an adult individual, enlarged channels are still present in places ( Fig. 4E ).

Figures

Figure 4. Permo-Triassic dicynodont bone histology. A, juvenile Dicynodon humerus SAM-PK-K5576d; B, subadult Dicynodon humerus NMQR3633a; C, subadult Lystrosaurus maccaigi ulna NMQR3663b; D, juvenile Lystrosaurus declivis tibia NMQR735b; D, E, adult Lystrosaurus humerus (possibly Lystrosaurus declivis) NMQR3678; F, late subadult Kannemeyeria femur NMQR2674b. Arrows indicate enlarged channels in all genera. Scale bars: D, E = 500 mm; A, B, C, F = 413 mm.

Figure 5. Phylogenetic distribution of mean channel density and enlarged vascular channels amongst therapsids. Mean channel density optimized using squared-change parsimony and equal branch lengths; enlarged vascular channels optimized using parsimony. Topology modified from Angielczyk (2007), Botha, Abdala & Smith (2007), Fröbisch (2007), and Liu, Li & Cheng (2002), Higher level taxonomy of anomodonts follows Kammerer & Angielczyk (2009).