Diodorus scytobrachion, Kammerer & Nesbitt & Shubin, 2012

Kammerer, Christian F., Nesbitt, Sterling J. & Shubin, Neil H., 2012, The first silesaurid dinosauriform from the Late Triassic of Morocco, Acta Palaeontologica Polonica 57 (2), pp. 277-284 : 278-281

Diodorus scytobrachion sp. nov.

Figs. 1–3 .

Etymology: From ancient Greek scytobrachion , leathery arm, a reference both to a possible integument for this taxon and the classical mythographer Dionysius Scytobrachion, who chronicled the mythical history of North Africa.

Holotype: MHNM−ARG 30 , a partial right dentary.

Type locality: Northeastern Argana Basin , 2.9 km east of Imziln , Morocco. Specific coordinate information on file at the Museum of Comparative Zoology , MA, USA, and University of Chicago, IL, USA and available on request. The holotype and all referred specimens were collected in a single quarry as part of a layer of disarticulated skeletal material that also includes phytosaur, “prolacertiform”, fish, and temnospondyl elements .

Type horizon: Base of the Irohalene Mudstone Member (t5), Timezgadiouine Formation (?Carnian–Norian, Triassic; see discussion).

Referred material.—MHNM−ARG 31, 32, and 33, isolated teeth; MHNM−ARG 34 and 35, two humeri; MHNM−ARG 36, a metatarsal; and MHNM−ARG 37, a femur. Although these elements are unassociated and probably represent different individuals, here they are all referred to Diodorus scytobrachion based either on direct comparison with the holotype (the isolated teeth), or on the identification of diagnostic silesaurid (femur, humeri) or dinosauriform (metatarsus) character states. At present, we are operating under the assumption that only a single silesaurid taxon is present in the basal t5 member of the Timezgadiouine Formation, as is the case for other silesaurid−bearing localities (the two nominal silesaurids from Los Chañares, Lewisuchus and Pseudolagosuchus , are probably synonymous [ Nesbitt et al. 2010: supplementary information]).

Diagnosis.—Small silesaurid with triangular, denticulated teeth, cingula absent, and a marked decrease in size anteriorly in the dentary. All teeth preserved in place in the dentary are anteriorly directed at an angle of ~20 ° from the root. Meckelian groove restricted to ventral edge of dentary but expands in dorsoventral height posteriorly, reaching 40% of dentary height by the fourth tooth position. Dentary ventrally bowed. Lateral ridge present near and trending parallel to alveolar margin of the dentary.

Differential diagnosis.—Distinguished from all other archosaurs except silesaurids by the presence of a distinct notch below the femoral head (CS 207[1] in the phylogenetic analysis [see below]) and teeth rooted but firmly fused to their sockets (CS 104[0]) (termed ankylothecodont by some workers [e.g., Chatterjee 1974]). Can be distinguished from all silesaurids except Sacisaurus and Silesaurus by a straight edge to the anteromedial face of the femoral head (CS 206[1]). Can be distinguished from all silesaurids other than Sacisaurus by dental morphology. Both Diodorus and Sacisaurus exhibit a decrease in tooth size anteriorly (CS 291[1]) and possess narrow, anteriorly−directed anteriormost teeth in the dentary (CS 292[1]). Diodorus can be differentiated from Sacisaurus by a Meckelian groove that that does not extend to the anterior edge of the dentary, greater dorsoventral expansion of the Meckelian groove, lack of cingula on the teeth, greater expansion of the tooth crown at base, anterior angulation of at least the first six dentary teeth, and the presence of a lateral ridge on the dentary running parallel with the alveolar margin.

Description.—The holotype of Diodorus scytobrachion (MHNM−ARG 30; Fig. 1A ) is the anterior portion of a right dentary, missing the anteriormost tip. This fragment preserves six tooth positions with four teeth in place (positions 1, 2, 4, and 6), and three with crowns intact (1, 2, and 4). A ridge is present slightly above mid−height on the lateral surface of the dentary ( Fig. 1A 1 ). This ridge is well developed at the posterior end of the fragment, at the level of tooth position 6, weakening anteriorly until it disappears entirely under tooth position 2. This character is absent in all silesaurids for which the dentary is known and is here considered an autapomorphy of Diodorus . No dentary material can be referred with certainty to Pseudolagosuchus or Eucoelophysis , although it is probable that the former is synonymous with Lewisuchus and the latter is identical to the “Hayden Quarry silesaur”, both of which have dentaries preserved ( Irmis et al. 2007a; Nesbitt et al. 2010). A row of nutrient foramina is present between the alveolar margin of the dentary and the lateral ridge. The Meckelian groove is located at the ventral edge of the medial dentary surface ( Fig. 1A 2 ), as in all silesaurids except Asilisaurus . The Meckelian groove is relatively tall in Diodorus compared with the extremely narrow grooves of Sacisaurus and Silesaurus . Although the medial surface of the dentary is poorly preserved in the holotype of D. scytobrachion , the Meckelian groove clearly does not extend anterior to tooth position 2, unlike the condition in Sacisaurus and Silesaurus in which the groove extends anteriorly through the dentary symphysis.

The four preserved teeth in the holotype have roots that are firmly fused to their sockets, as in Proterosuchus (based on NMQR 1484), non−archosauriform archosauromorphs, and all silesaurids except possibly Lewisuchus . In the three teeth for which crowns are preserved, the crowns are triangular, denticulated along the mesiodistal edges, and anteriorly canted. Within silesaurids, triangular, denticulated teeth are present in all species except Lewisuchus (which possesses the primitive archosaurian condition of blade−like, recurved teeth) and Asilisaurus (in which the teeth are peg−like). The forward cant to all the anteriormost dentary teeth is an autapomorphy of Diodorus , but the first dentary tooth of Sacisaurus (based on MCN PV10043 and MCN PV10061) is similarly angled. The three preserved crowns decrease in size anteriorly (the crown height at tooth position 1 is ~66% that of tooth position 2, which is ~60% that of tooth position 4), as is also the condition in Sacisaurus (based on MCN PV10043; Ferigolo and Langer 2007). In addition to the poorly preserved three crowns present in the holotype, several very well preserved, isolated tooth crowns from the type locality matching the holotype’s dental morphotype can be referred to Diodorus (MHNM−ARG 31, 32, and 33; Fig. 1B ). These crowns are very similar in morphology to the crown in tooth position 4 in the holotype, but are more bulbous at the base and larger in absolute size, indicating either a more posterior position in the jaw or that they come from a larger individual than the holotype. The teeth of Diodorus are more coarsely denticulated (4–5 denticles per 5 mm in Diodorus versus 6–7 denticles per 5 mm in Silesaurus [ Dzik 2003]) and broader (wider crown base relative to height) than those of Silesaurus (based on ZPAL Ab III/361/26), and can also be distinguished from Silesaurus by the lack of longitudinal striations. The crown proportions of Diodorus are generally similar to those in Technosaurus (TTUP P9021), Sacisaurus , and the “Hayden Quarry silesaur” (probably Eucoelophysis [ Irmis et al. 2007a]; GR 224). However, Diodorus teeth can be distinguished from those of Technosaurus by the lack of an accessory cusp, from Sacisaurus by the absence of a cingulum and a more abrupt expansion of the crown at base (resulting in a more “spade−shaped” tooth in Diodorus ), and from the “Hayden Quarry silesaur” by being relatively taller and less bulbous.

Two isolated humeri (MHNM−ARG 34 and 35; Fig. 2A ) are here referred to Diodorus . As in other silesaurids, the humerus is elongate and largely featureless other than the distinct ect− and entepicondyles separated by a prominent furrow distally. The shaft of the humerus is “ramrod”−straight and the long axes of both the proximal and distal ends are in the same plane. The head of the humerus is very poorly developed and asymmetrical, with the medial portion expanded distally. The proximal and distal ends are poorly expanded relative to the shaft, a character state shared with Silesaurus (ZPAL Ab III/362) among avian−line archosaurs. The delto−

http://dx.doi.org/10.4202/app.2011.0015

pectoral crest extends for one−third the length of the humerus, but the apex of the deltopectoral crest is situated at the proximal tip of the humerus, similar to the condition in Silesaurus (ZPAL Ab III/362). This is in contrast with the condition in Dinosauria , where the apex of the crest is situated around 30% down the shaft of the humerus ( Langer and Benton 2006; Nesbitt et al. 2010).

Hindlimb material is represented by an anteroposteriorly crushed femur (MHNM−ARG 37; Fig. 3 ) and a metatarsal (MHNM−ARG 36; Fig. 2B ). The femoral head is triangular in proximal view, with a 5.5: 1.8: 6.3 ratio of anterior: medial: posterior edge lengths. These edges are essentially straight, as in Sacisaurus (based on MCN PV10019) and Silesaurus (based on ZPAL Ab III/361/23), rather than rounded as in most archosaurs. It is unlikely that the straightness of these edges arose from crushing of this specimen, considering that such deformation would distort the anterior/posterior and medial edges in opposite ways. The posteromedial tuber of the proximal portion of the femur is absent in Diodorus . A straight mediolateral groove bisects the femoral head in proximal view. The anterior trochanter is a small, dorsally pointing spike but the proximal tip is broken off. There is no evidence for a trochanteric shelf attached to the anterior trochanter. A distinct, blade−shaped dorsolateral trochanter (sensu Langer and Benton 2006) is present lateral to the anterior trochanter. It is narrow, elongate, and less visible in proximal view than in Eucoelophysis and PEFO 34347. The combination of a “finger−shaped” anterior trochanter, the absence of a trochanteric shelf, and the presence of a blade−like dorsolateral trochanter on the femur of Diodorus is also found in Sacisaurus (based on MCN PV10019), smaller specimens of Silesaurus (e.g., ZPAL Ab III/460/1), and the “gracile” morph of Coelophysis rhodesiensis ( Raath 1990) . The fourth trochanter is located more distally on the femur than in Sacisaurus (based on MCN PV10019) and is similar in position to that of Silesaurus (based on ZPAL Ab III/361/23), albeit more weakly developed than in either of those two taxa. The fourth trochanter is crescent−shaped with a sharp rim, proximodistally symmetrical, and with a shallow depression to its anteromedial side. The fourth trochanter is much less expanded in Diodorus than in Silesaurus (based on ZPAL Ab III/361/23). The distal end is only slightly more expanded (in all views) than the shaft. The crista tibiofibularis and the medial and lateral condyles are rounded on the posterior side. A rounded depression occupies the distal surface. The lateral side of the lateral condyle is rounded like that of other dinosauriforms ( Parker and Irmis 2005). The ridges dorsally extending from the crista tibiofibularis and the medial condyle extend up the shaft of the femur for more than 1/4 the length of the femur. This also occurs in Sacisaurus (based on MCN PV10019), Silesaurus (based on ZPAL Ab III/362), and Asilisaurus ( Nesbitt et al. 2010) .

The isolated metatarsal is a problematic element. It is elongate, with a robust rim for extensor attachment, as in Silesaurus , but the digit identity of MHNM−ARG 36 is unclear. Although compression in this specimen renders interpretation of the proportions difficult, the rectangular distal profile and mediolateral symmetry of the metatarsus suggest that it most likely represents metatarsal III.

Geographic and stratigraphic range.—Argana Basin of Morocco (Timezgadiouine Formation, Late Triassic).

Figures

Fig. 1. Mandibular and dental material of dinosauriform Diodorus scytobrachion gen. et sp. nov., Timezgadiouine Formation, Late Triassic. A. MNHM−ARG 30, holotype right dentary, in lateral (A1, A3) and medial (A2, A4) views. Photographs (A1, A2), explanatory drawings (A3, A4). B. MNHM−ARG 31, referred isolated tooth. B not to scale.

Fig. 2. Limb elements referred to Diodorus scytobrachion gen. et sp. nov., Timezgadiouine Formation, Late Triassic. A. MNHM−ARG 34, isolated right humerus, in anterior (A1), proximal (A2), posterior (A3), and distal (A4) views. B. MNHM−ARG 36, isolated metatarsal, in anterior (B1) and distal (B2) views.

Fig. 3. Isolated left femur referred to Diodorus scytobrachion gen. et sp. nov., Timezgadiouine Formation, Late Triassic, MNHM−ARG 37 in anterior (A), lateral (B), medial (stereopair) (C), proximal (D), and distal (E) views. Phylogenetically important character states visible on this element include: 1, distal condyles of femur divided posteriorly between 1/4 and 1/3 the length of the shaft (CS 223[1], synapomorphy of all silesaurids except Pseudolagosuchus); 2, notch ventral to the proximal head of the femur (CS 207[1], synapomorphy of Silesauridae); 3, posteromedial tuber absent on the proximal portion of the femur (CS 204[2], synapomorphy of all silesaurids except Pseudolagosuchus and Asilisaurus); 4, flat medial articular surface of the femur head in dorsal view (CS 206[1], synapomorphy of Silesaurus, Sacisaurus, and Diodorus).