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Postcanine Microstructure in Cricodon Metabolus

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Postcanine Microstructure in Cricodon Metabolus [Palaeontology, 2016, pp. 1–11] POSTCANINE MICROSTRUCTURE IN CRICODON METABOLUS, A MIDDLE TRIASSIC GOMPHODONT CYNODONT FROM SOUTH-EASTERN AFRICA by CHRISTOPHE HENDRICKX, FERNANDO ABDALA and JONAH CHOINIERE Evolutionary Studies Institute, University of the Witwatersrand, Johannesburg, South Africa; [email protected], [email protected], [email protected] Typescript received 9 March 2016; accepted in revised form 31 August 2016 Abstract: Cricodon metabolus is a trirachodontid cynodont dentine layers in each tooth. This study reveals that the enamel from the Anisian (Middle Triassic) of eastern and southern thickness varies along the tooth row in Cricodon as the enamel Africa. It has labiolingually expanded (gomphodont) postca- layer of the gomphodont postcanines is 11.5 times thicker than nines but also a sectorial tooth in the last postcanine locus. that of the sectorial crown. It is likely that this difference reflects In this paper, we examine the crown microstructure of iso- occlusal stresses and fewer replacements in gomphodont post- lated sectorial and gomphodont postcanines belonging to the canines relative to sectorial teeth. Approximately 100 incre- holotype specimen of this taxon using scanning electron mental growth lines of von Ebner are present in the dentine microscopy. The enamel of both teeth is prismless and com- layer, indicating that the deposition of the dentine by odonto- posed of discontinuous columnar divergence units, support- blasts occurred for three months before the animal’s death. ing the consistent presence of synapsid columnar enamel in cynognathians. Abundant tubules and numerous irregularly Key words: Trirachodontidae, Gomphodontia, postcanine, spaced incremental lines are also visible in the enamel and enamel, dentine, microstructure. G OMPHODONTS were one of the most diverse groups of The external morphology of gomphodont postcanines cynodonts and one of the dominant Middle to Late Trias- is relatively well known from investigations of tooth mor- sic vertebrates in South America and southern Africa. phology, occlusion, replacement, wear, function and They appeared in the Olenekian and radiated into three ontogeny (e.g. Seeley 1894, 1895, 1908; Ziegler 1969; main clades: Diademodontidae, Trirachodontidae and Hopson 1971, 2005, 2014; Crompton 1972; Osborn 1974; Traversodontidae (e.g. Battail 1983; Abdala & Ribeiro Grine 1976, 1977, 1978; Goni~ & Goin 1987, 1988, 1991; 2003; Hopson 2005, 2014). Gomphodonts are distinctly Abdala et al. 2002, 2006; Liu & Sues 2010; Martinelli heterodont and bear conical anterior incisors, large 2010; Sues & Hopson 2010). The tooth crown pointed canines, and molariform teeth that sometimes microstructure of gomphodonts, on the other hand, has include sectorial postcanines (Reisz & Sues 2000; Abdala only been cursorily studied in Diademodon, Massetog- et al. 2006). The postcanines of members of the clade nathus, Exaeretodon and Boreogomphodon (Moss 1969; Gomphodontia are morphologically diverse but share a Grine 1978; Grine et al. 1979; Osborn & Hillman 1979; gomphodont morphology, that is they are labiolingually Stern & Crompton 1995; Sander 1997; Abdala & Mal- enlarged and show tooth-on-tooth occlusion (e.g. Seeley abarba 2007). In addition, the internal tooth structure has 1895, 1908; Crompton 1972; Reisz & Sues 2000; Abdala never been investigated in a definitive trirachodontid et al. 2006; Hopson 2014). Differentiation of the three taxon, and all previous studies on crown microstructure main gomphodont clades is based mainly on postcanine in gomphodonts are restricted to a single crown type of heterodonty and morphology. Diademodontids have con- the tooth row (i.e. they do not compare enamel of differ- ical, ovoid gomphodont and sectorial postcanine teeth, ent tooth morphologies). whereas trirachodontids typically have ellipsoid gom- Cricodon metabolus is a trirachodontid represented by phodont teeth and sectorial teeth, and traversodontids incomplete cranial and postcranial material from the Ani- possess rectangular to trapezoidal uppers and quadrangu- sian (Middle Triassic) Manda Formation of Njalila, lar lower teeth, with sectorial postcanines in some juve- Ruhuhu Basin, Tanzania (Crompton 1955). Specimens niles (Abdala et al. 2006; Liu & Abdala 2014). referred to this taxon are also known from the latest © The Palaeontological Association doi: 10.1111/pala.12263 1 2 PALAEONTOLOGY Anisian Subzone C of the Cynognathus Assemblage Zone of the Burgersdorp Formation, Karoo Basin, South Africa (Abdala et al. 2005) and from the contemporaneous Ntawere Formation of the Luangwa Basin of north-eastern Zambia (Peecook et al. 2013; Hopson & Sidor 2015). This study aims to explore the microstructure of a sectorial tooth and a gomphodont postcanine in the same specimen of the trirachodontid Cricodon metabolus (Crompton 1955), and to use these data to investigate the evolution of the dentition in gomphodont cynodonts. We also investi- gate the distribution of enamel microstructure in non- mammaliaform cynodonts and enamel thickness along the trirachodontid tooth row. Finally, incremental lines of von Ebner in the dentine of the gomphodont tooth are analysed to determine the temporal interval of dentine deposition. Institutional abbreviation. UMZC, University Museum of Zool- ogy, Cambridge, UK. MATERIAL AND METHOD Terminology Dental positional nomenclature follows Smith & Dodson FIG. 1. Sectorial postcanine of the holotype specimen of Cri- (2003) and Hendrickx et al. (2015). The anatomical codon metabolus (UMZC T905); right distal maxilla or left distal nomenclature used to describe and annotate external tooth dentary tooth. A, labial; B, lingual; C, distal; D, mesial; and E, sub-units follows the terminology and abbreviations of apical views. Abbreviations: aac, anterior accessory cusp; ce, cer- Hopson (2005) for gomphodont teeth, and Abdala et al. vix; mc, main cusp; pac, posterior accessory cusp; se, serration; (2006) for the sectorial postcanine. Descriptions of the Xs1, apicalmost horizontal section; Xs2, basalmost horizontal internal microstructure of the crown, especially enamel section. Scale bars represent: 5 mm (A–D); 3 mm (E). Colour microstructure, use the terminology provided by Sander online. (1999, 2000). For the planes of sectioning we follow the ter- minology given by Wood & Stern (1997) and Sander (1999, labiolingually and 2.4 mm mesiodistally in the apical hori- 2000), that is, transverse, sagittal (rather than longitudinal zontal section (section 1; Fig. 1A). The gomphodont tooth or vertical), horizontal (rather than cross) and tangential. is a left maxillary postcanine, of which a part of the lingual crown is broken (Fig. 2). Portions of the upper central cusp and the upper external cusp are also missing. The crown Material measures 10.31 mm labiolingually, 5.74 mm mesiodistally in its longest part at the cervix, and 4.34 mm apicobasally Material used for this study includes a gomphodont and on its mesial side, in the highest point of the central crest a sectorial postcanine of the holotype specimen of Cri- (Hendrickx et al. 2016). At the level of the external cusp, the codon metabolus UMZC T905 (Figs 1, 2). The teeth were crown measures 4.16 mm apicobasally and 3.71 mm found isolated but in close proximity to the rest of the mesiodistally at the cervix. skeleton, and belong to a single individual (Crompton 1955). The sectorial postcanine is a complete tooth from the distalmost end of the tooth row (either of the right Histological method maxilla or the left dentary). This crown measures 6.77 mm apicobasally from cervix to apex on its mesial Both teeth were embedded in epoxy resin and sectioned side (Fig. 1) and 5.91 mm in its distal side, with a length with a diamond saw. The two postcanines were each sec- and a width of 5.11 mm and 3.61 mm at the cervical line, tioned twice in horizontal section: through the complete respectively (Hendrickx et al. 2016). It also measures crown for the sectorial tooth, and through the upper exter- 2.9 mm labiolingually and 5.2 mm mesiodistally in the basal nal cusp for the gomphodont postcanine. The gomphodont horizontal section (section 2; Fig. 1A), and 1.5 mm tooth was also sagitally sectioned twice: immediately HENDRICKX ET AL.: CRICODON POSTCANINE MICROSTRUCTURE 3 where it reaches 33 lm. The enamel microstructure can be seen in tangential section of the mesial portion of the crown (Fig. 3D, E), and in horizontal section of the rest of the tooth (Fig. 3B, C). The enamel microstructure closely matches the typical synapsid columnar enamel (SCE sensu Sander 1997) composed of short and discontinuous colum- nar divergence units whose crystallites are here tilted at an angle of 60–70° from the enamel–dentine junction (EDJ) in horizontal section (Fig. 3C). Planes of crystallite divergence are clearly observed in some parts of the enamel layer (black arrow in Fig. 3C). The enamel layer of the sectorial tooth does not show any evidence of incremental lines in horizontal section. Odontoblast tubules are abundant in the dentine layer where they can easily be seen as pits or sinuous casts projecting from the horizontal section surface (Fig. 3D–F). The tubular density is homogeneous through- out the dentine surface, with 7–8 tubules per 10 lm2. FIG. 2. Upper gomphodont postcanine of the holotype speci- Gomphodont postcanine men of Cricodon metabolus (UMZC T905); left maxillary tooth. A, apical; B, labial; and C, mesial views. Abbreviations: ab, ante- The enamel layer thickness of the gomphodont tooth
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