Richardoestesia gilmorei, Currie et al., 1990

Hendrickx, Christophe & Mateus, Octávio, 2014, Abelisauridae (Dinosauria: Theropoda) from the Late Jurassic of Portugal and dentition-based phylogeny as a contribution for the identification of isolated theropod teeth, Zootaxa 3759 (1), pp. 1-74 : 27-30

Richardoestesia aff. R. gilmorei Currie et al., 1990

Referred material. ML 939 ( Fig. 10 ).

Locality and horizon. Cliffs of Valmitão South, Lourinhã, Portugal. Amoreira-Porto Novo Member, Lourinhã Formation, Late Kimmeridgian, Upper Jurassic.

Description. The crown is entirely preserved but shows an important spalled surface extending on the apical part of the mesial margin of the tooth. A small piece and some denticles of the distal carina are missing but most of them are intact and well-preserved. The tooth only preserved the basal part of the root.

Crown. The crown is small ( CH of 5.1 mm), slightly elongated (CBH of 1.82) and strongly compressed labiolingually (CBR of 0.5; Table 3 ). The tip is strongly recurved distally and the apex is pointed, mostly due to the wear facet. The mesial carina is missing and might have been worn on the tip of the crown. The distal carina is serrated and bears denticles from the cervix to the apex.

In lateral view, the crown is straight along the basal part, then abruptly curves distally at two-thirds of its height at an angle of 55° to the vertical, forming an acute backward tip. The most basal part of the crown is slightly constricted mesio-distally, but the constriction only occurs on the mesial margin of the crown, the distal margin being straight along the first fourth of the crown. The distal carina is universally concave, but the carina curves above the straight basal margin, and the distal part of the carina is straight. The mesial margin is convex above the cervix only on the basal half of the crown, the other half remaining flat due to the wear facet. A convex surface delimited by a longitudinal groove mesially and a flattened or slightly concave surface distally appears on both lingual and labial faces. This large mesial ridge follows the same curvature of the crown and its mesio-distal width decreases towards the tip. It starts one-third of the way from the base of the crown on the labial face and from the apical part of the root on the lingual surface. Both lingual and labial grooves are narrow and reach the wear facet at the tip.

In mesial view the crown tip is straight and curves neither labially nor lingually. Both labial and lingual faces are weakly convex and the crown-base width is slightly narrower than the mid-crown width. The crown remains, however, strongly compressed labio-lingually all along its height, and the crown width slightly decreases from the mid-crown to the tip.

In distal view, the most basal part of the serrated carina is straight and vertical but then curved all along the rest of the crown with the bow directed lingually. The distal carina is slightly oriented labially (we regarded the labial face of the crown as the face towards which the distal carina was displaced, at it is almost always the case in theropods; pers. obs.), and the labial face adjacent to the carina is flat whereas the lingual surface near the carina is concave.

In apical view, the basal part of the mesial margin is strongly convex and the wear facet situated on the distal part forms a narrow flat surface revealing the enamel and the dentine layers. In basal view, the crown-base forms an “eight-shaped” in cross section ( Fig. 10G ) due to the basal concavity on both labial and lingual side of the crown. The concave surface on the lingual face is shallow, triangular in shape and extends on one-third of the crown whereas the one on the labial face is slightly deeper and ends at the cervix level. The mesial part of the crown is labio-lingually wider (1.2 mm) than the distal part (1 mm). The dentine layer is thicker in the centre of both labial and lingual sides, giving an even well-pronounced “eight-shaped” to the pulp cavity, thinner distally.

Denticles. Only the distal carina is preserved and serrated, and the morphology of the denticles varies along the carina. With 10 denticles per 1 mm basally and at the mid-crown and 9 apically, the denticles slightly increase in size near the apex. The basal denticles are longer mesio-distally than baso-apically. In lateral view, they are tongue-shaped with their external margin strongly convex, parabolic and symmetrically rounded or slightly pointing towards the tip of the crown ( Fig. 10I ), giving them an asymmetrical outline. Although the basal denticles become mesio-distally shorter towards the root and the mid-crown, they share a same baso-apical width than denticles at mid-height of crown. On the other hand, the apical denticles are short and baso-apically larger than the basal ones. The most apical denticles are cartouche-shaped with their external margin symmetrically or asymmetrically convex. These denticles are also mesio-distally short and just form a small symmetrical bump at the apex in lateral view. In apical view, the lingual and dorsal surfaces of the body of the denticles are convex, and the denticle tip is chisel-like in shape.

The interdenticular sulci of basal denticles are absent or very short. When present, they are shallow and straight, extending perpendicular to the distal margin on the labial and lingual faces from between the denticles. The interdenticular sulci are totally absent in the apical denticles. The interdenticular space of distal denticles is narrow, slightly larger in the apical denticles, and usually filled with sediment.

Surface. The enamel texture of the crown surface is irregular and shows finely wrinkled non-oriented structures on both sides ( Fig. 10H ). Except for the presence of those microscopic sculptures, there is no other ornamentations on the crown surface.

Discussion. ML 939 is interpreted as a shed tooth as it lacks most of the root and the pulp cavity is slightly excavated.

The presence of a basal constriction between the crown and root has been observed in basal most theropods like Eoraptor lunensis ( Sereno et al. 1993) and many coelurosaurs such as the tyrannosauroid Proceratosaurus ( Rauhut et al. 2010) , the compsognathid Compsognathus ( Zinke & Rauhut 1994) , the ornithomimosaur Pelecanimimus ( Pérez-Moreno et al. 1994) , alvarezsaurids ( Perle et al. 1993), basal oviraptorosaurs ( Osmólska et al. 2004), therizinosaurs (e.g., Russell & Dong 1993; Zhao & Xu 1998; Kirkland et al. 2005), troodontids (e.g., Currie et al. 1990; Baszio 1997; Norell et al. 2000; Currie & Dong 2001; Sankey et al. 2002; Averianov & Sues 2007), the dromaeosaurids Microraptor ( Xu et al. 2000) , and many basal avialans such as Archaeopteryx and Cathayornis ( Hou 1997; Feduccia 2002).

Nevertheless, the presence of an eight-shape outline of the crown-base in cross-section is a common feature of many deinonychosaurs such as Saurornitholestes ( Currie et al. 1990; Sankey et al. 2002), Tsaagan ( Norell et al. 2006) , Pyroraptor ( Allain & Taquet 2000; Gianechini et al. 2011b), Buitreraptor ( Gianechini et al. 2011b) and the enigmatic theropod Richardoestesia gilmorei ( Currie et al. 1990) . With perhaps the exception of Berberosaurus (MNH Pt339), the base crown of non-maniraptoriform theropods like coelophysoids, ceratosaurs, megalosauroids, allosauroids and most of tyrannosauroids can be subcircular, ovoid, elliptical, lenticular or bean-shaped but not eight-shaped (pers. obs.). This also seems to be the case in more derived coelurosaurs such as Compsognathidae (e.g., Zinke 1998: fig. 2; Dal Sasso & Maganuco 2011: fig. 44 to 48), therizinosaurs ( Clark et al. 1994: fig. 12; Zhao & Xu 1998: fig. 1), Oviraptorosaurs (Balanoff et al. 2009: fig. 2-7) and perhaps Ornitholestes hermanni (AMNH 619). The latter possesses a median concave surface on the labial surface of some crowns, but does not seem to have any on the lingual one, giving a bean-shaped outline of the crown base in cross section (pers. obs.). The tyrannosaurid Alioramus altai (IGM 100-1844) and the neovenatorid Orkoraptor burkei ( Novas et al. 2008; Benson et al. 2010) are two exceptions; the latter possesses a particularly developed median depression on both labial and lingual sides of the crown. To our knowledge, it represents the second non-coelurosaurian theropods with an eight-shaped cross section of the crown (the other one being Berberosaurus ), and other neovenatorids such as Neovenator (MIWG 6348), Aerosteon (Sereno et al. 2008) , Fukuiraptor ( Azuma & Currie 2000; Currie & Azuma 2006; Molnar et al. 2009) and Australovenator ( Hocknull et al. 2009) do not display this peculiarity. An eight-shaped outline of the base crown was also reported in the coelophysoid Liliensternus by Gianechini et al. 2011 ( Fig. 3c ). Nevertheless, based on the crown morphology of this taxon, it is more likely that the eight-shaped outline corresponds to a cross section in the root rather than at the base-crown. ML 939 has a low crown with small denticles and a mesiodistal constriction at the base and therefore contrasts with the elongated teeth of Neovenatoridae and Tyrannosauridae which bear large denticles and never show a mesio-distal constriction at the base crown (pers. obs.). Therefore, it is unlikely that this shed tooth belongs to a non-maniraptoriform theropod.

ML 939 serrations are particularly minute and the distal carina bears nine to ten denticles per one millimetre. Among deinonychosaurs, such condition only exists, to our knowledge, in the taxa Richardoestesia gilmorei and Richardoestesia isosceles (e.g., Currie et al. 1990; Sankey 2001; Sankey et al. 2002; Baszio 1997; Sankey 2008; Larson 2008; Larson & Currie 2013) but the dental morphology of the latter (i.e., teeth with no constriction, straight to slightly recurved, crown subtriangular in outline) strongly differs with that of ML 939. The external margins of the denticles are symmetrically rounded or slightly curved towards the tip of the crown, and the basal and mid-crown denticles have similar size on the distal carina, two conditions shared by Richardoestesia gilmorei ( Currie et al. 1990 fig. 8.4; Baszio 1997; Larson 2008). Although the presence of a longitudinal groove mesially positioned on the crown has never been noticed in Richardoestesia gilmorei , this feature seems to be present in some specimens assigned to this species (see Baszio 1997: Plate IV fig. 47; Sankey et al. 2002: fig. 5 n°6), and longitudinal grooves have already been observed in the genus Richardoestesia ( Currie et al. 1990; Sankey 2001; Rauhut 2002). Nevertheless, several differences exist between ML 939 and the teeth of the holotype of Richardoestesia gilmorei , namely, the presence of interdenticular sulci and mesio-distally elongated distal denticles, and the absence of a mesial carina reaching the cervix in ML 939. Although the mesial serration are usually restricted to the apicalmost part of the crown in R. gilmorei , the mesial carina always reaches the cervix in this taxon (Derek Larson pers. comm.).

With a strongly labiolingually compressed profile of the crown, ML 939 was coded as a lateral tooth. The cladistic analysis performed on the dentition-based dataset recovered ML 939 as a close relative of Richardoestesia gilmorei ( Fig. 1 ). The clade encompassing those two taxa is defined by two ambiguous synapomorphies: a weak constriction occurring at the base crown (characters 63) and subequal number of distal denticles basally and at the mid-crown (character 99). The analysis performed on the supermatrix recovered it as a Dromaeosauridae along with Richardoestesia ( Fig. 2 ; Appendix, Fig. A10 ).

Richardoestesia gilmorei is a common species in the Late Cretaceous of Northern America and teeth belonging to this taxon, or referred to it, have been found in the Santonian Milk River Formation, the Campanian Belly River Group, the Campanian-Maastrichtian Horseshoe Canyon Formation, and the Maastrichtian Scollard Formation of Alberta, the Frenchman Formation of Saskatchewan ( Canada), the Hell Creek Formation of Montana and the Lance Formation of Wyoming (e.g., Currie et al. 1990; Baszio 1997; Longrich 2008; Sankey 2008; Larson 2008; Larson et al. 2010; Larson & Currie 2013). Given the results of the cladistic analysis, R. gilmorei likely belongs to Dromaeosauridae than any other theropod clade.

Small theropod teeth from the Upper Jurassic of Portugal have already been assigned with caution to the genus Richardoestesia by Zinke (1998). Nevertheless, they strongly differ from ML 939 by being extremely elongated and weakly recurved, resembling the elongated and subtriangular teeth assigned to Richardoestesia sp. by Baszio (1997), and Richardoestesia isosceles by Sankey (2001). Following the cladistic analysis and the diagnosis of teeth belonging to Richardoestesia sp. (and R. gilmorei in particular) given by Currie et al. (1990), Baszio (1997) and Longrich (2008), and since the presence of teeth similar to those of Richardoestesia isosceles has already been reported in the Late Jurassic of Portugal ( Zinke 1998), ML 939 is ascribed to the possible dromaeosaurid Richardoestesia , which extends the stratigraphic range of the taxon back to the Jurassic. ML 939 is similar to R. gilmorei teeth in many aspects, but this taxon has only been recorded in the Late Cretaceous of North America, more than 90 million years after the Jurassic/Cretaceous boundary. We therefore consider that ML 939 belongs to a close relative of Richardoestesia gilmorei .

Figures

FIGURE 1. Strict consensus cladogram of seven most parsimonious trees recovered from analysis of dentition based characters. Initial analysis was a New Technology Search using TNT v.1.1 of a data matrix comprising 141 dentition-based characters for one outgroup (Eoraptor), 59 nonavian theropod taxa, as well as ML 327, ML 939, ML 962 and ML 966. Tree length = 703 steps; CI = 0.331; RI = 0.564. Bremer support values are in bold and bootstrap values are in italic. For silhouette attribution, see Appendix.

FIGURE 2. Strict consensus cladogram of 49 most parsimonious trees recovered from analysis of a supermatrix of 1972 discrete characters after the deletion of the two wildcard taxa Erectopus and Piatnitzkysaurus. The supermatrix includes a dentition-based datamatrix of 141 discrete characters and six recent datasets based on whole theropod skeleton (Xu et al. 2009; Brusatte et al. 2010; Martinez et al. 2011; Senter 2011; Pol and Rauhut 2012; Carrano et al. 2012). Initial analysis was a New Technology Search using TNT v.1.1 for one outgroup (Eoraptor), 57 non-avian theropod taxa and ML 327, ML 966, ML 939 (coded as lateral teeth), and ML 962 (coded as a mesialmost tooth). Tree length = 3552 steps; CI = 0.563; RI = 0.628. For silhouette attribution, see Appendix.

FIGURE 3. Isolated tooth (ML 327) of an Abelisauridae in lingual (A), mesial (B), labial (C), distal (D), apical (F), basal (G) and mesio-lingual (H) and labio-distal views. Apical denticles of the distal carina in labial view (E). Abbreviations: dca, distal carina; esp, enamel spalling; ids, interdenticular sulcus; idsp, interdenticular space; lgr, longitudinal groove; mca, mesial carina; tun, transversal undulation; wfa, wear facet.

FIGURE 10. Isolated tooth (ML 939) of Richardoestesia aff. R. gilmorei in lingual (A), distal (B), labial (C), mesial (D), apical (F) and basal (G) views. Mid-crown denticles of the distal carina in labial (E, I) views, and enamel texture in lingual (H) view. Abbreviations: cs, concave surface; dca, distal carina; ent, enamel texture; esp, enamel spalling; ids, interdenticular sulcus; lad, labial depression; lgr, longitudinal groove.