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Growth and Life Habits of the Triassic Cynodont Trirachodon, Inferred from Bone Histology

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Growth and Life Habits of the Triassic Cynodont Trirachodon, Inferred from Bone Histology Growth and life habits of the Triassic cynodont Trirachodon, inferred from bone histology JENNIFER BOTHA and ANUSUYA CHINSAMY Botha, J. and Chinsamy, A. 2004. Growth and life habits of the Triassic cynodont Trirachodon, inferred from bone histol− ogy. Acta Palaeontologica Polonica 49 (4): 619–627. Growth pattern and lifestyle habits of the Triassic non−mammalian cynodont Trirachodon are deduced from bone histol− ogy and cross−sectional geometry. Several skeletal elements of Trirachodon were examined in order to document histological changes during ontogeny, as well as histovariability in the skeleton. The bone histology of all the elements consists of a moderately vascularized, periodically interrupted, fibro−lamellar bone tissue. This suggests that the overall growth of Trirachodon was probably rapid during the favourable season, but decreased or ceased during the unfavourable season. As the environment is thought to have been semi−arid with seasonal rainfall, it is possible that Trirachodon was sensitive to such environmental fluctuations. Some inter−elemental histovariability was noted where the number and prominence of growth rings varied. Limb bone cross−sectional geometry revealed a relatively thick bone wall and sup− ports earlier proposals that Trirachodon was fossorial. Key words: Cynodonts, Trirachodon, lifestyles, bone histology, growth patterns. Jennifer Botha [[email protected]], Natural History Collections Division, South African Museum, Iziko Museums of Cape Town, P.O. Box 61, 8000, South Africa (corresponding author); Anusuya Chinsamy [[email protected]], Zoology Department, University of Cape Town, Rondebosch, 7701, South Africa. Introduction few studies that have examined Trirachodon have focused on its morphology (Seeley 1895b; Crompton and Ellen− Trirachodon is a herbivorous non−mammalian cynodont berger 1957; Kemp 1982), which has led to a rather limited whose remains have been found in the Early to Middle Trias− understanding of its biology. sic Cynognathus Assemblage Zone, of the Beaufort Group, Given that bone histology is well recognized as providing Karoo Supergroup of South Africa (Rubidge 1995). The cra− pertinent information about the biology of extinct vertebrates nium of Trirachodon is similar to Diademodon, a contempo− (e.g., Amprino 1947; Ricqlès 1969, 1972, 1974, 1976, 1980; rary non−mammalian cynodont, and is characterized by a Chinsamy 1990, 1993a, 1995, 1997; Reid 1996; Horner et al. short, narrow snout; wide orbital region; slender zygomatic 2000; Ricqlès et al. 2001, 2003), we applied this methodol− arches and antero−dorsally placed eyes (Seeley 1895a; Kemp ogy to Trirachodon. Although the organic components of 1982). However, Trirachodon, with a maximum body length bone (which include osteocytes, vascular canals and colla− of 50 cm, is much smaller than the 2 m long Diademodon and genous fibres) are destroyed during fossilization, their struc− has fewer gomphodont (molariform) postcanine teeth, which tural organization usually remains intact, thereby allowing are broader transversely and anteroposteriorly shorter than the bone tissue microstructure of the fossil to be discerned those of Diademodon (Seeley 1895b; Crompton and Ellen− (Francillon−Vieillot et al. 1990). Comparing the bone micro− berger 1957; Kemp 1982). structure with that of living animals allows various aspects Trirachodon had a more mammal−like posture than the such as growth, individual age and the lifestyle habits of ex− earlier, more basal cynodont genera such as the Permian tinct animals to be interpreted (e.g., Enlow and Brown 1956, Procynosuchus.InTrirachodon though, the forelimb still 1957). Several early bone microstructure studies on isolated had a sprawling orientation (Kemp 1982). The hindlimb pos− skeletal remains of non−mammalian therapsids have been ture was semi−erect (Kemp 1982), which would have im− conducted (e.g., Enlow and Brown 1956, 1957). However, in proved the locomotor efficiency of the animal, possibly al− the late 1960s and 1970s Armand de Ricqlès undertook a lowing for more sustained activity (Carrier 1987; Pough et al. systematic assessment of the bone microstructure of a variety 1996). Other derived mammalian characteristics include a of non−mammalian therapsids including dinocephalians and bony secondary palate and precise postcanine tooth occlu− dicynodonts (Ricqlès 1972), and therocephalians, gorgonop− sion, both of which would have increased food−processing sians, and cynodonts (Ricqlès 1969). Although his analyses efficiency (Kemp 1982). Compared to other non−mammalian were mainly on isolated fragments of specimens identified cynodonts, Trirachodon fossils are relatively scarce and the only to generic level, they nevertheless provided an impor− Acta Palaeontol. Pol. 49 (4): 619–627, 2004 http://app.pan.pl/acta49/app49−619.pdf 620 ACTA PALAEONTOLOGICA POLONICA 49 (4), 2004 tant understanding of the range of bone tissue types present recovered from a bone bed in the Aliwal North district and among the non−mammalian therapsids. Until now, however, have all been diagnosed as representing Trirachodon kanne− Trirachodon has yet to be studied. meyeria. Several individuals of different sizes were identi− It has previously been suggested that Trirachodon was a fied, which probably represent different ontogenetic ages. fossorial animal, based on skeletal remains preserved inside The CGP1/79 radius and ulna belong to a single individual. burrow casts recovered from the Driekoppen Formation in The largest tibia in the study, SAM−PK−5881c, is designated South Africa (Groenewald et al. 2001) and the Omingonde as adult on the basis of the size and well−finished bone sur− Formation in Namibia (Smith and Swart 2002). As studies on faces. This tibia was not directly associated with any other the cross−sectional geometry of bone have shown that a di− limb bones. Measurements of the complete tibia SAM−PK− rect relationship exists between an animal’s lifestyle and the 5881c were used to estimate the total lengths of the incom− structural design of its bones (Wall 1983; Stein 1989; Fish plete tibiae. Based on ratios from tibia SAM−PK−5881c, the 1993; Bou et al. 1990), here we combine the histological ratio of diameter to length for the tibiae SAM−PK−5881b and analysis with an assessment of the cross−sectional geometry NMQR3282b was calculated. The estimated total length of of Trirachodon limb bones. the tibiae was then divided by the total length of tibia SAM− PK−5881c, and a percentage of adult size was thus obtained Institutional abbreviations.—NMQR, National Museum, (Table 2). Femur NMQR3282a is similar in size to tibia Bloemfontein; SAM−PK, South African Museum, Iziko Mu− NMQR3282b. Few percentage adult estimations could be seums of Cape Town; CGP, Council for Geoscience, Pretoria. calculated as few elements were complete and a fully articu− lated skeleton of Trirachodon was unavailable for study. As long bones undergo the least secondary remodeling in Materials and methods the midshaft region (Chinsamy 1990, 1991, 1995; Francil− lon−Vieillot et al. 1990; Horner et al. 1999), all the elements Trirachodon remains have been recovered from the Cyno− were thin sectioned in this region. The ribs were also sec− gnathus Assemblage Zone, of the Beaufort Group, Karoo tioned in the midshaft region. As a consequence of their frag− Supergroup of South Africa (Rubidge 1995), and are cur− mentary nature, only the proximal parts of the scapulae were rently housed in various institutions in South Africa. For our sectioned. Most of the limb bones were incomplete, but it analysis, eleven skeletal elements, including femora, tibiae, was possible to thin section the proximal regions of the femur scapulae, ribs, a radius and an ulna, were selected to consider (SAM−PK−5881a) and tibiae (SAM−PK−5881b, SAM−PK− both ontogenetic and inter−elemental histological variability 5881c) as well. The thin sectioning technique follows that of (Table 1). Chinsamy and Raath (1992). Terminology used is sensu The femur NMQR3282a and tibia NMQR3282b were Francillon−Vieillot et al. (1990), Reid (1996), and Starck and found together with two lower jaws of similar size in a block Chinsamy (2002). of matrix, which allowed these elements to be identified as Several studies have shown that vascularization in bone Trirachodon. As these elements were found with two lower tissue differs among taxa (Enlow and Brown 1957; Currey jaws, they may either both belong to one individual or they 1960; Chinsamy 1991, 1993b) and also among different ele− could be from two different individuals (Table 1). The vari− ments (Horner et al. 2000; Curry 1999; Ray et al. in press). ety of skeletal elements designated as SAM−PK−5881, were The vascularization of the bone tissue was assessed by mea− Table 1. The Trirachodon specimens examined in this study and their localities. The NMQR3282 elements were found in a matrix that included two similar sized lower jaws. The SAM−PK−5881 elements were recovered from a bone bed in the Aliwal North district and have all been diagnosed as a single species, Trirachodon kannemeyeria. They represent several individuals of various sizes and probably stages in ontogenetic age. The CGP1/79 elements belong to a single individual. District Specimen number Skeletal element Portion sectioned Kestell NMQR3282a femur midshaft NMQR 3282b tibia midshaft Aliwal North SAM−PK−5881a femur midshaft/proximal SAM−PK−5881b tibia midshaft/proximal SAM−PK−5881c tibia midshaft/proximal SAM−PK−5881d rib midshaft SAM−PK−5881e rib midshaft SAM−PK−5881f
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