Physiological implications of the bone histology of Syntarsus rhodesiensis (Saurischia: Theropoda)

Femora of Syntarsus rhodesiensis specimens of differing ontogenetic stages were sectioned and prepared for histological study. Features such as the structure of the cortical bone tissue, the degree of vascularisation and haversian substitution, and the general pattern of bone deposition through ontogeny, are described. The question of the bearing of bone histology on interpretation of the probable physiology of Syntarsus is also discussed. INTRODUCTION Interpretations of the physiological patterns of extinct animals have long fascinated palaeontologists who have sought to understand the fossils as once living organisms. Anatomical features such as locomotor apparatus (Bakker, 1971, 1972, 1980), brain size (Hopson, 1978n9), predator prey ratios (Bakker, 1971, 1972, 1980), and bone histology (De Ricqles, 1974, 1976, 1980) are examples of what has been studied to try to gain insight into the type of physiology that extinct animals once experienced. It has become increasingly clear that the histological structure of fossil bone can provide better clues to physiology than most other methods (Bennett & Ruben, 1986). Work on fossil bone histology began relatively early in the nineteenth century (Nopcsa & Heidsieck, 1933), and subsequently interest in the field has developed considerably (e.g. Enlow & Brown, 1956, 1957, 1958; Peabody, 1961; Enlow, 1963, 1969; Currey, 1962). Palaeohistology is still considered a relatively new field of study, but it has now broadened its horizons from being initially mainly comparative and descriptive to include considerations of functions and physiology (De Ricqles, 1974, 1976, 1980; Reid, 1984, 1987; Buffrenil, Farlow, and De Ricqles, 1986). There is still much debate over the physiological meaning of different tissue types (Reid, 1984, 1987; Bouvier, 1977 ; De Ricqles, 1976, 1980, 1983), but the general consensus is that bone histology may provide some clues as to the animal's growth rate, metabolic rate, activity levels, and hence some indications concerning its heat production and thermoregulatory ability. In this study an attempt was made to deduce aspects of the possible physiology that Syntarsus once experienced, as reflected in its bone histology. The general bone histology is described initially and thereafter follow speculations on the animal's thermoregulatory ability, possible evidence for sexual dimorphism, and also the probable growth strategy it employed. MATERIALS AND METHODS Femora of the early Jurassic theropod dinosaur, Syntarsus rhodesiensis, were obtained from the collections of the Natural History Museum of Zimbabwe. Gross femoral dimensions were used to assign the femora to approximate "age-classes" following Raath ( 1977). The femora were also sorted on the basis of trochanteric development into two variants, robust and gracile, which Raath ( 1977) thought reflected sexual dimorphism. He suggested that the robust femora may belong to females. Specimens were embedded in Epofix cold mounting resin which was prepared according to the manufacturer's specifications: The specimens were sectioned below the 4th trochanter and further along the shaft. Thin sections were subsequently prepared using a "Micro-Trim" sectioncutting machine. The sections were then ground on a Buehler Ecomet III grinder using 400, 600 and I 000 grit carborundum paper discs, until judged to be of acceptable thinness. Aluminium oxide powder sprinkled onto a cloth-covered lap-wheel was used for final polishing. Water was used as a coolant. Gross examination of prepared slides was carried out under a Zeiss Stereomicroscope. More detailed examination was carried out under a Zeiss Photomicroscope 2. RESULTS Bone Histology of Syntarsus rhodesiensis. The bone of S. rhodesiensis comprises a distinctive cortical region enclosing a large central medullary cavity. It seems that the medullary cavity was devoid of trabeculae, since no traces of trabeculae have been observed in bone from either young or old animals. The cortex is stratified into distinct types of bone signifying what is interpreted as periods of slowed and fast growth (fig. IA). The region of rapid growth consists of highly vascular fibro-lamellar bone. Primary osteons that occur are mainly longitudinal in form, but circumferential vessels are also found. The blood vessels tend to anastomose