A new traversodontid cynodont from the Santa Maria Formation (Ladinian-Carnian) of southern Brazil, with a phylogenetic analysis of Gondwanan traversodontids Author Abdala, F. Author Ribeiro, A. M. text Zoological Journal of the Linnean Society 2003 2003-12-31 139 4 529 545 https://academic.oup.com/zoolinnean/article-lookup/doi/10.1111/j.1096-3642.2003.00096.x journal article 10.1111/j.1096-3642.2003.00096.x 0024-4082 5439219 SANTACRUZODON HOPSONI GEN. ET SP. NOV. Holotype . MCN PV 2768: fragmentary skull with lower jaws ( Fig. 1 ). Referred specimens. MCN PV 2751, MCN PV 2752 ( Fig. 2B,C ), MCP 4044 PV: three lower jaws; MCN PV 2770: incomplete maxilla with postcanines ( Fig. 2A ); MCP 4034 PV: fragmentary skull and lower jaw with postcanines. Age. The Santa Cruz do Sul cynodonts most closely resemble predominantly Carnian forms such as Exaeretodon , and Ladinian forms such as Massetognathus ; thus a Late Ladinian age is assumed for the Santa Cruz do Sul Fauna of the Santa Maria Formation ( Abdala et al., 2001 ). Etymology. Named after the city of Santa Cruz do Sul , where the new species was discovered, plus don (tooth; Greek) and hopsoni in honour of Dr James A. Hopson, in recognition of his vast contribution to our knowledge of nonmammalian therapsids. Diagnosis. Santacruzodon hopsoni exhibits a combination of features observed in other traversodontid cynodonts and an autapomorphy. It features a ballshaped ventrally projecting suborbital process such as occurs in Dadadon isaloi Flynn et al., 2000 ; the incisors are flattened bucco-lingually as in Massetognathus pascuali , but showing a series of 7-9 marginal cuspules as in Arctotraversodon plemmyridon ( Hopson, 1984 ) ; the upper postcanines present an anterior small crest conformed by a series of cingular cuspules as in ‘ Scalenodon ’ attridgei Crompton, 1972 . As in many other cynodonts (e.g. Massetognathus , Exaeretodon ), S . hopsoni features three labial cuspules in the upper postcanines, but showing as autapomorphy the posterior cusp very large, representing more than half the length of the labial crest. Figure 1. Santacruzodon hopsoni gen et sp. nov. Holotype (MCN PV 2768). Skull in (A) latero-dorsal, (B) lateroventral views. (C) Close-up showing the upper incisor structure. (D) Drawing of the upper dentition; Lower jaw in (E) dorsal, (F) lateral views. Scale bars: A, B, D, E, F = 10 mm; C = 2 mm. Figure 2. Santacruzodon hopsoni gen et sp. nov. (A) Stereopair of upper postcanine series (MCN PV 2770) in occlusal view; Lower jaw (MCN PV 2752) in (B) dorsal (stereopair) and (C) lateral views. Scale bars = 10 mm. Description. Most of the material is poorly preserved, providing little information on bone sutures. The skull fragment of the holotype includes the rostrum and orbital region, but lacks the temporal portion ( Fig. 1A ). Measurements of the specimen are presented in Table 1 . The estimated skull length of this specimen is 80 mm . The rostrum is crushed dorsoventrally, without preservation of the dorsal surface, but with remnants of the ascending process of the premaxilla. A distinctive feature of the skull is a rounded, ventrally well-projected suborbital process of the jugal ( Fig. 1A ). A platform of the maxilla is present lateral to the postcanine series ( Figs 1B , 2A ). There are four arrowhead-shaped upper incisors, flattened labio-lingually with 9-11 marginal cuspules ( Fig. 1B, C ). The external aspect of the incisor is flat, whereas the internal face is more convex. A diastema is present between incisors and canine. The canine is poorly preserved, but seems to have been small in size. There is no diastema between the canine and postcanines. The postcanines are morphologically heterogeneous and vary from seven to ten in number on the different specimens ( Fig. 1B ). The anterior teeth are mostly triangular in occlusal outline, whereas the posterior ones become more enlarged bucco-lingually ( Figs 2A , 3A ). These last teeth are proportionally more developed anteroposteriorly than in other traversodontids and present a deep occlusal basin. The labial crest shows three cusps, the large posterior one representing more than half the length of the crest ( Fig. 3A ). The posterior transverse crest bears three cusps, with the lingual and middle ones positioned very close together and with a basin separating them from the labial cusp ( Fig. 3A ). In addition, there is an anterior cingular crest less developed in height than the posterior crest, formed by a series of cingular cusps (at least eight or nine; Fig. 3A ). The lower jaws present a shallow mandibular ramus with a fused symphysis ( Figs 1E, 1F , 2C ). The masseteric fossa extends anteriorly to the level of postcanine 7 to 9; the last two postcanines are covered laterally by the ascending coronoid process. Three procumbent and labio-lingually flattened lower incisors are present, each with 11 marginal cuspules. These teeth are larger than the upper incisors, and spoon-shaped, being convex labially, whereas its lingual face is remarkably concave ( Figs 1E , 2B, 2C ). A wide vertical ridge separating two shallow depressions is present on the lingual face of the lower incisors. The canine is relatively small and there is a short diastema between the canine and the first postcanine. There are 9-10/11 lower postcanines, with the anterior transverse crest formed by two cusps: the higher labial cusp lies slightly anterior to the wider lingual cusp ( Fig. 3B ). In addition, the postcanines feature a labial crest with 2-3 cusps following the main anterior one ( Fig. 3B ), and a posterior cingular crest being, in some cases, as high as the anterior transverse crest. Table 1. Skull and dentition measurements of Santacruzodon hopsoni gen et sp. nov. (in mm)
MCN PV 2768 (Holotype) MCN PV 2752
Rostrum 49.6
Orbital region 24.5
Orbital diameter 22.4
2nd upper incisor length 4.1
4th upper incisor length 3.6
Upper canine length 5.3
Upper postcanine length 29.2
1st upper postcanine length 2.1
1st upper postcanine width 2.2
Last upper postcanine length 3.9
Last upper postcanine width 5.4
Height of the preserved portion of the ascendant ramus of the mandible 28.2 32.1
Mandibular length (incomplete) 69.2
Length from the tip of the mandible to the angle 65.1 60.3
2nd lower incisor length 4 2.9
Lower canine length ?4.4
Lower postcanine length 27.6 25.5
Last lower postcanine length 5.6 5.3
Last lower postcanine width 4.0 3.8
Figure 3. Santacruzodon hopsoni gen et sp. nov. (A) Upper postcanine in occlusal view. Labial, right; anterior top. (B) Lower postcanine in occlusal view. Labial, left; anterior, top. Abbreviations : aalc, anterior accessory labial cusps; ac, anterior cingular crest; cc, central cusp; lac, labial cusp; lic, lingual cusp; mlc, main labial cusp; palc, posterior accessory labial cusps; palic, posterior accessory lingual cusp; pc, posterior cingular crest. Comparison. Santacruzodon hopsoni resembles the Madagascan traversodontid Dadadon isaloi ( Flynn et al., 2000 ) in the rounded suborbital process, and the differentiation between the triangular occlusal outline of the anterior and the more rectangular posterior upper postcanines (a feature also recorded in Gomphodontosuchus ; Hopson, 1985 : fig. 3). The high number of postcanines, the overall structure of the upper posterior postcanines, including a slightly developed shouldering are also seen in Dadadon and the South American Ladinian Massetognathus . Other similarities (i.e. the presence of three cusps in the posterior transverse crest of the upper postcanines, and the close spacing of the central and the lingual cusps) shared by Santacruzodon , Dadadon and Massetognatus are plesiomorphies, also occurring in more basal traversodontids (see Phylogenetic analysis below). A synapomorphy shared by Santacruzodon , Massetognathus and other late traversodontids is the presence of three cusps in the sectorial labial border of the upper postcanines ( Dadadon shows two cusps instead). As in many late traversodontids (i.e. Dadadon and Massetognathus ), Santacruzodon features a remarkably enlarged postero-external upper cusp. The incisors of Santacruzodon , are flattened buccolingually, as in Massetognathus , but overall incisor morphology more closely matches that of Arctotraversodon plemmyridon of Nova Scotia , Canada ( Sues et al. , 1992 ; =? Scalenodontoides plemmyridon of Hopson, 1984 ), also showing cuspules along their edges.