Lepidus praecisio, Nesbitt & Ezcurra, 2015

Nesbitt, Sterling J. & Ezcurra, Martín D., 2015, The early fossil record of dinosaurs in North America: A new neotheropod from the base of the Upper Triassic Dockum Group of Texas, Acta Palaeontologica Polonica 60 (3), pp. 513-526 : 515-521

Lepidus praecisio sp. nov.

Fig. 2 .

Etymology: From Latin praecisio , fragment or scrap; in reference to the common preservation of early dinosaurs from North America as bony fragments.

Holotype: TMM 41936-1.3, articulated distal ends of the left tibia and fibula and a left astragalocalcaneum ( Fig. 2 ).

Type locality: Dockum Site 7 General (TMM locality 41936), just northeast of the classic Otis Chalk localities, Howard County, Texas (see above) ( Fig. 1 ).

Type horizon: Otis Chalk area, Dockum Group, Upper Triassic

Referred material.—TMM 41936-1, fragment of left femoral shaft ( Fig. 3 ), TMM 41936-1.1, partial left maxilla ( Fig. 4 ).

Diagnosis.— Lepidus praecisio gen. et sp. nov. possesses an autapomorphically well-developed posterior pyramidal process on the astragalus that delimits the posterolateral margin of the tibial facet and the posteromedial portion of the facet of the fibula, and is separated from the proximal surface of the calcaneum by a shallow notch that opens dorsolaterally. Additionally, Le. praecisio shares the following combination of plesiomorphic and synapomorphic character states with Neotheropoda (character enumeration follows that of Nesbitt et al. 2009b and Ezcurra and Brusatte 2011): fused astragalus and calcaneum (283-1); low anterior ascending process of the astragalus (273-1); calcaneum mediolaterally compressed (291-1); proximodistally extended ridge on the posteromedial surface of the tibia (256-1); distinct scar on the anterior surface of the distal end of the tibia (333-1).

Furthermore, Le. praecisio can be differentiated from other Upper Triassic North American theropods by other features beyond the autapomorphies listed above. Lepidus praecisio is differentiated from Camposaurus arizonensis by the lack of an oval depression (= medial fossa of Ezcurra and Brusatte 2011) on the medial surface of the astragalus. Lepidus praecisio is differentiated from Coelophysis bauri , the “Padian Coelophysis ” (UCMP 129618), and Chindesaurus bryansmalli by the less laterally expanded posterolateral process (= lateral malleolus) of the distal end of the tibia. Lepidus praecisio is differentiated from Tawa hallae by the presence of a proximodistally oriented ridge on the posteromedial surface of the tibia.

Description.—TMM 41936-1.3: The articulated tibia, fibula and astragalocalcaneum of Le. praecisio are well preserved with fine details of muscle scars and articulation surfaces Fig. 2 ). The astragalus and the calcaneum are clearly fused together with no sign of any sutural surface or cleft. The fibula is in articulation with the astragalocalcaneum whereas the tibia is in near articulation with the astragalocalcaneum but slightly displaced laterally by about 0.5 millimeters.

Tibia: In overall morphology, the tibia resembles that of neotheropods. The anterior surface of the bone has a very well developed tuberosity just proximomedially to the anterior ascending process of the astragalocalcaneum (= anterior diagonal tuberosity of Ezcurra and Brusatte 2011) ( Fig. 2 ). The tuberosity is well pronounced from the anterior surface of the tibia and the external surface consists of striated bone fibers oriented proximolaterally. A similar tuberosity, but comparatively less-developed, is present in the same position in Camposaurus arizonensis ( Ezcurra and Brusatte 2011) , the Hayden Quarry coelophysoid (GR 227), Coelophysis bauri (AMNH FR 30614, 30615) and Coelophysis rhodesiensis (cast of QG 1) but absent in Tawa hallae (Nesbitt et al. 2009) , Eodromaeus murphi ( Martinez et al. 2011) , and Herrerasaurus ischigualastensis (PVSJ 373). The facet for reception of the ascending process of the astragalus of TMM 41936-1.3 is straight and slanted proximomedially at an angle about 15 o to the mediolateral horizontal plane in anterior view. The medial surface of the tibia lacks the diagonal, anteriorly bowed tuberosity present in Ca. arizonensis ( Ezcurra and Brusatte 2011) , the Hayden Quarry coelophysoid (GR 227), and more weakly developed in Co. rhodesiensis (cast of QG 1). The distal end of the tibia possesses a distinct proximodistally oriented ridge on the posteromedial surface ( Fig. 2 ), as occurs in neotheropods ( Langer and Benton 2006) and Eodromaeus murphi (PVSJ 562), but absent in T. hallae (Nesbitt et al. 2009) , Chindesaurus bryansmalli PEFO 33982), and H. ischigualastensis (PVSJ 373). The lateral malleolous of TMM 41936-1.3 is weakly developed beyond the shaft of the tibia and is lobe-shaped in posterior view, contrasting with the more expanded and tabular-shaped process of Zupaysaurus rougieri ( Ezcurra and Novas 2007) , Liliensternus liliensterni (MB R2175) , and Co. bauri ( Colbert 1989: AMNH uncatalogued). The posteromedial corner of the distal end has a distinct notch for reception of a posteromedial process on the astragalus, as occurs in several neotheropods (e.g., Co. bauri, AMNH FR 7239) but not in T. hallae ( Nesbitt et al. 2009b) and H. ischigualastensis (PVSJ 373). The notch in Le. praecisio is considerably deeper in Li. liliensterni (MB R2175) and Zupaysaurus rougieri ( Ezcurra and Novas 2007) . Furthermore, the presence of the notch creates a sigmoidal articulation (ventrally concave on the anterior half and convex posteriorly) surface with the astragalocalcaneum when viewed in medial view ( Fig. 2 ). The lateral surface of the distal end of tibia lacks the longitudinal sharp ridge present in Ca. arizonensis ( Ezcurra and Brusatte

2011). The lateral malleolous appears to contact the fibula in posterior view, but this condition seems to be an artefact as a result of the lateral displacement of the tibia (see above). Therefore, the condition of TMM 41936-1.3 seems to differ from that of Coelophysis rhodesiensis (cast of QG 1) and Co. bauri (CM 11894) in which the fibula and the tibia contact each other proximal to the articulation with the astragalocalcaneum.

Fibula: The distal end of fibula is expanded anteroposteriorly in lateral view ( Fig. 2 ). The distal end of the bone is asymmetric in lateral view, where the anterior portion is more distally expanded than the posterior portion. This is in contrast to the continuously convex and near symmetrical distal end of the fibulae present in Ca. arizonensis (UCMP 34498), Li. liliensterni (MB R2175) , Co. bauri (AMNH FR 30614 Co. rhodesiensis (cast of QG 1), Zupaysaurus rougieri (PULR 076), T. hallae (Nesbitt et al. 2009) , and Dilophosaurus wetherilli (UCMP 37302). The lateral surface of the fibula is smooth and lacks the distinct scar present in T. hallae ( Nesbitt et al. 2009b) . In lateral view, the distal end of the fibula is slightly larger than that of its facet with the astragalocalcaneum. In anterior view, the distal end of fibula expands slightly medially and, as a result, likely slightly overlapped the anterior surface of the ascending process of the astragalus. The shaft of the fibula is mediolaterally- and to a lesser degree, anteroposteriorly compressed relative to the shaft of the tibia.

Astragalocalcaneum: The astragalus and calcaneum of Le. praecisio are fused into an astragalocalcaneum ( Fig. 2 ), as also occurs in hypothesized mature individuals of early neotheropod dinosaurs (e.g., Ca. arizonensis, UCMP 34498; Co. bauri, AMNH FR 30614, 30615; Co. rhodesiensis , cast of QG 1; Zupaysaurus rougieri, PULR 076). The ascending process is dorsoventrally short, about one-third the height of the astragalar body ( Fig. 2 ). The anterior surface of the ascending process of the astragalus lacks the large fossa present in most basal neotheropods (e.g., Co. bauri, AMNH FR 30576; Z. rougieri, PULR 076; D. wetherilli, UCMP 37302). However, there is a subcircular blind pit between the base of the ascending process and the astragalar body that is likely homologous to the larger fossa commonly present in early neotheropods. The anterior surface of the astragalar body possesses a very faint swelling of the horizontal tuberosity present in Ca. arizonensis ( Ezcurra and Brusatte 2011) , the Hayden Quarry coelophysoid (GR 227) and Co. rhodesiensis (QR 1) (= horizontal large tuberosity of Ezcurra and Brusatte 2011). The anteromedial corner of the astragalar body is acutely angled like all dinosauromorphs ( Langer and Benton 2006), but has an angle closer to 90° than the more pointed anteromedial corner present in Ca. arizonensis ( Ezcurra and Brusatte 2011) . The medial surface of the astragalar body is nearly flat with long, paralleled bone fibers decorating the surface. In contrast, in Ca. arizonensis , the Hayden Quarry coelophysoid, Co. rhodesiensis and D. wetherilli the medial surface of the astragalus possesses an oval fossa ( Ezcurra and Brusatte 2011). The astragalus of TMM 41936-1.3 is nearly symmetric in medial view, with similarly distally developed anterior and posterior sides, resembling the condition present in most basal neotheropods (e.g., Co. bauri, AMNH FR 30576, 30614; Co. rhodesiensis , cast of QG 1; Li. liliensterni, MB R2175 ; Z. rougieri, PULR 076; D. wetherilli, UCMP 37302). In contrast, in Ca. arizonensis the anterior edge of the astragalus is much more distally expanded than the posterior side in medial view (UCMP 34498). The posteromedial corner of the astragalus of Le. praecisio possesses a low and blunt dorsally directed process (= posteromedial process of Ezcurra and Nova 2007) ( Fig. 2 ), which contrasts with the more dorsally projected and pyramidal posteromedial process present in Z. rougieri ( Ezcurra and Novas 2007) and Li. liliensterni (MB R2175) . In posterior view, the pyramidal structure that delimits the posterolateral margin of the tibial facet and the posteromedial portion of the facet of the fibula is very well developed ( Fig. 2 ), contrasting with the condition present in other early neotheropods (e.g., Coelophysis bauri, AMNH FR 30576; Co. rhodesiensis, Raath 1977 : pl. 26d; Li. liliensterni, MB R2175 ; Z. rougieri, PULR 076; D. wetherilli, UCMP 37302), and is an autapomorphy of Le. praecisio . The posterior pyramidal process is of similar height to the anterior ascending process. The posterior pyramidal process may connect to the anterior ascending process, but this cannot be determined given that the tibia and fibula were preserved in tight articulation with the astragalocalcaneum. This posterior pyramidal process is separated from the proximal surface of the calcaneum by a shallow notch that opens dorsolaterally into a posterior sulcus ( Fig. 2 ), which is also not present in any other dinosaur observed by us.

Based on the position of the posterior pyramidal process, the calcaneal portion of the astragalocalcaneum should be strongly transversely compressed as in neotheropods and unlike the condition present in Herrerasaurus ischigualastensis PVSJ 373). The lateral margin of the proximal articular surface of the calcaneum is concavo-convex from anterior to posterior. The latter is a result of the asymmetric distal end of fibula, a condition that differs from that observed in most early neotheropods (see above). The lateral surface of the calcaneum portion of the astragalocalcaneum is mostly covered by a shallow concavity, which is subdivided by a low, anteriorly curved ridge as in a theropod specimen (UCMP 152645) from the Upper Triassic Canjilon Quarry (Nesbitt and Stock- er 2008). The ventral surface of the astragalocalcaneum is strongly anteroposteriorly convex. The ventral margin of the astragalocalcaneum is only weakly transversely concave in anterior or posterior views.

Femur: The proximal half of the femoral shaft possibly referable to Le. praecisio is preserved in two pieces ( Fig. 3 ). The base of the anterior trochanter is similar to that of ornithosuchids ( Bonaparte 1971), early dinosauromorphs Nesbitt et al. 2009a), silesaurids (ZPAL Ab III/361/23; Dzik 2003), and other early dinosaurs ( Novas 1996). The base of the anterior trochanter is highly rugose and proportionally mediolaterally wider than in other dinosauromorphs (Fig. A, B). The specimen lacks a trochanteric shelf. The medially extending fourth trochanter is symmetrical in anterior view, where the proximal and distal portions have similar angles relative to the shaft, as in Tawa hallae and neotheropods Langer and Benton 2006; Nesbitt et al. 2009b). The well separated fourth trochanter from the shaft differs from that of the low, mound-like structure in some silesaurids ( Silesaurus opolensis ; Dzik 2003). The fourth trochanter begins immediately distal the level of the base of the anterior trochanter. The trochanter is blade-like, being strongly transversely compressed, contrasting with the proportionally thicker and distally expanded fourth trochanters of ornithischians (e.g., Heterodontosaurus tucki , SAM-PK-K1332) and early saurischians (e.g., Saturnalia tupiniquim, MCP 3844-PV, Langer 2003; Herrerasaurus ischigualastensis, PVSJ 373, Novas 1994). The fourth trochanter originates close to the medial margin of the shaft and trends diagonally towards the lateral margin of the shaft. The most distal tip of the fourth trochanter is not preserved, but it seems that it does not reach the lateral margin of the shaft. Immediately medially to the base of the fourth trochanter there is a very well developed muscle scar, which is delimited by a semilunate shelf, resembling the condition present in other saurischians (e.g., S. tupiniquim, MCP 3844-PV; Li. liliensterni, MB R2175 ). The shaft has a convex, slightly developed anteromedial edge, but it clearly contrasts with the sharp keel present in H. ischigualastensis (PVSJ 373).

The histological section of the referred femur was taken at the base of the fourth trochanter, and the entire cross-section of the femur was recorded in two histological sections ( Fig. 3A, B ). The shaft has a quite thin cortex in cross-section; with a cortex (anterior and posterior parts of shaft = 1.75 mm) to diameter (~ 10.75 mm) ratio ~0.167.

Overall, the structure of the original bony tissues is well preserved and little, if any, recrystallization is present ( Fig. 3C–E ). The medullary cavity is free of trabeculae. The cortex is like that of early theropods ( Padian et al. 2001; Ricqlès et al. 2003) and most Triassic dinosauromorphs ( Werning et al. 2011) in terms of overall composition of boney tissues, vascularization, and cortex thickness. The cortex is composed entirely of woven-fibered primary bone tissue without any evidence of remodeling. The majority of vascular canals are longitudinal primary osteons with at least one, but no more than two, lamellae. Rarely, the longitudinal primary canals possess circumferential anastomoses that connect either one or two canals.

The bone is well vascularized and is comparable to the long bones of Co. bauri (AMNH FR unnumbered) and the “Padian Coelophysis ” (UCMP 129618) but does show variation in vascularization densities across the cortex. Vascularization densities decrease in the outer cortex compared to the inner cortex. In comparison with the “Padian Coelophysis ” (UCMP 129618), the femoral tissues of Le. praecisio clearly have less of a plexiform configuration in the inner cortex, but the overall bone tissue orientations are similar. The osteocyte lacunae surround the longitudinal primary canals but do not appear to be arranged circumferentially around the canals. The woven-fibered boney tissue is similar throughout most of the cortex without any interruptions (i.e., lines of arrest- ed growth), indicating that the specimen was not a mature individual at the time of its death. Furthermore, the lack of lines of arrested growth (LAGs) does not allow an age to be estimated. The absence of LAGs indicates that the individual was in its first year of life when it died or grew throughout its life without laying down any LAGs. The absence of LAGs in femora about the size of that of Le. praecisio appears to be rather common among early dinosauriforms (e.g., Coelophysis bauri and Asilisaurus kongwe ; Christopher Griffin and SJN unpublished data). The outermost cortex (i.e., the outer 10% of the radius of the cortex) has a slight transition to more parallel-fibered bone but no external fundamental system is present. No secondary osteons are present in the cortex as in the dinosauriform A. kongwe and the neotheropods Co. bauri (AMNH FR unnumbered) and the “Padian Coelophysis ” (UCMP 129618).

TMM 41936-1.1: The left partial maxilla ( Fig. 4 ) consists of the main body of the bone missing the distal tip of the anterior process, posterior half of the posterior (= horizontal) process, and the distal end of the dorsal (= ascending) process. In lateral view, the anterior portion of the maxilla is triangular with a low, sloping anterodorsal straight margin, a straight and horizontal ventral margin, and a posteriorly tapering anterior portion of the posterior process. The lateral surface of the maxilla bears a distinct antorbital fossa ( Fig. 4 ) separated from the dorsal process by a thin ridge that curls posteriorly, creating a slight pocket. The extent of the lateral exposure of the antorbital fossa in TMM 41936-1.1 differs from the minute lateral exposure of the antorbital fossae of Tawa hallae (GR 241), Herrerasaurus ischigualastensis (PVSJ 407), and Daemonosaurus chauliodus (CM 76821). This ridge continues ventrally, becomes less pronounced and turns posteriorly about half the dorsoventral distance from the ventral edge. More posteriorly, the antorbital fossa is only separated from the rest of the body of the maxilla by a low ridge, thus lacking the alveolar ridge present in several early saurischians (e.g., Eoraptor lunensis, PVSJ 512; Zupaysaurus rougieri, PULR 076; Li. liliensterni, MB R2175 ; Co. bauri, AMNH FR 7224; Co. rhodesiensis, QG 1; Eodromaeus murphi, PVSJ 561). Overall, the shape of the ridge demarcating the antorbital fossa is “squared-off” as in the putative sauropodomorph Eoraptor lunensis ( Sereno et al. 2013) , and some basal neotheropods (e.g., Z. rougieri, Ezcurra 2007 ; “ Syntarsus ” kayentakatae, MNA V2623; Co. rhodesiensis , cast of QG1; Rauhut 2003). The promaxillary foramen observed in some neotheropods ( Rauhut 2003) and small fossae within the antorbital fossa are clearly absent in TMM 41936- 1.1. The straight dorsal margin of the posterior process of the maxilla forms the ventral margin of the antorbital fenestra. Although incomplete because of the loss of part of the dorsal process of the maxilla, the preserved anterior extent of the antorbital fenestra suggests that the angle of the anterior portion of the antorbital fenestra was acute unlike the condition in T. hallae (GR 241), Eodromaeus murphi (PVSJ 560), Eoraptor lunensis ( Sereno et al. 2013) , Co. bauri (CM 31374),

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and H. ischigualastensis (PVSJ 407). The anterior portion of the maxilla tapers anteroventrally, but the shape of its anteriormost portion is not known because it is slightly broken. The preserved portion of the anterior process is nearly flat laterally as in Co. bauri (CM 31374), but with no indication that there was any ventral (as in Dilophosaurus wetherilli ; Welles 1984) or lateral (as in Protosuchus richardsoni ; Colbert and Mook 1951) notch. Anteriorly, the angle between the ventral margin and the anterodorsal margin of the anterior process is about 35° relative to the ventral margin. This angle is comparable to that of Co. bauri (CM 31374) and Co. rhodesiensis (cast of QG1), and contrasts with the lower angle present in “ S. ” kayentakatae (ca. 20°; MNA V2623) and the higher one present in Z. rougieri (ca. 45°; PULR 076). The thin anterodorsal margin is nearly straight and it is not clear if the maxilla participated in the external naris as in most early neotheropods ( Tykoski and Rowe 2004), but there is no facet for reception of the posterior (= maxillary) process of the premaxilla so it is conceivable that the maxilla (TMM 41936-1.1) could have participated in the external naris. The straight anterodorsal border of the dorsal process resembles that of Co. bauri (CM 31374). Two rows of nutrient foramina are present near the ventral edge of the maxilla, one row 1–2 millimeters above the ventral margin, and another paralleling and just ventral to the low ridge separating the antorbital fossa from the rest of the posterior process of the maxilla.

The medial surface of the maxilla is well preserved. The main body of the medial surface is smooth with few distinguishing features, without an antrum anterior to the anterior margin of the antorbital fenestra. A distinct step paralleling the ventral margin of the bone separates the medially inflated main body from the interdental plates and tooth-bearing margin. Each interdental plate is polygonal with a ventrally directed vertex, bears some small irregular striations, is dorsoventrally low, and there is no evidence of fusion across each plate ( Fig. 4 ). The distinct palatal process of the maxilla is located at the anterior margin of the medial side and the lateral edge of the process is clearly separated from the medial side of the anteriormost portion of the maxilla (a bit of matrix was left in this area to stabilize the anteriormost portion of the maxilla) ( Fig. 4 ). Medially, the articulation surface of the palatal process has two deep longitudinal grooves separated from each other by a very thin ridge. The entire long axis of the palatal process projects anteroventrally with an angle of 10° relative to the horizontal ventral margin of the maxilla as in T. hallae (GR 241); most other early dinosaur maxillae are either broken in this area or the area is covered by other cranial elements (e.g., Co. bauri ). This anteroventral deflection of the process suggests that the premaxilla may have been downturned, resembling the condition present in coelophysids ( Colbert 1989), Z. rougieri (Ezcurra 2007) , and D. wetherilli (UCMP 37303).

The preserved portion of the maxilla contains seven alveoli, where the anteriormost alveolus is only partially preserved ( Fig. 4B ). The alveoli are oval with the long axis oriented anteroposteriorly. The size of the alveoli increase through the first four positions and then each alveolus posterior to the third position remain similar in size. Alveoli five and six preserve the root in situ, but the crowns are completely missing ( Fig. 4 ). An unerupted tooth crown is preserved within the fourth alveolus ( Fig. 4D ), and the tip of a replacement tooth is visible between interdental plates medial to the broken tooth in the sixth alveolus. The mesial edge of the crown of the unerupted tooth is convex whereas the distal edge is concave, resulting in a recurved crown. It is also labiolingually compressed. Fine serrations extend along both mesial and distal carina, and there are four serrations per millimeter in the visible portions of the crown, matching serration densities documented in Lophostropheus airelensis ( Ezcurra and Cuny 2007) and D. wetherilli ( Welles 1984) , but much coarser than those of Co. bauri ( Colbert 1989) or T. hallae (Nesbitt et al. 2009) .

The formal attribution of this maxilla to Le. praecisio (TMM 41936-1.3) is nearly impossible given that the maxilla does not preserve any unambiguous synapomorphies or unique character combinations with neotheropods or even with dinosauromorphs. Nevertheless, the morphology of TMM 41936-1.1 is not present in any known Otis Chalk taxa that preserve skulls, or those taxa otherwise found in the Dockum Group or Chinle Formation. The presence of an antorbital fenestra and fossa on the posterior portion of the maxilla clearly places the specimen within Archosauria ( Nesbitt 2011). Furthermore, the character states described above are consistent with, but not exclusive to, neotheropods. Thus, our very tentative referral of the maxilla (TMM 41936-1.1) to the same taxon as TMM 41936-1.3 can only be tested with the discovery of new material from the older Upper Triassic deposits of the Otis Chalk area or strata of a similar age.

Geographic and stratigraphic range.— Type locality and horizon only.

Figures

Fig. 2. Comparisons of almost naturally articulated ankle complexes of neotheropods.A. Lepidus praecisio gen. et sp. nov., Otis Chalk area, Dockum Group, Late Triassic (TMM 41936-1.3). B. Camposaurus arizonensis Hunt, Lucas, Heckert, Sullivan, and Lockley, 1998, Placerias Quarry, Chinle Formation, Late Triassic UCMP 34498), reversed. C. Coelophysis bauri Cope, 1887, Coelophysis Quarry, Chinle Formation, Late Triassic (AMNH FARB 30615). D. Zupaysaurus rougieri Arcucci and Coria, 2003, Colorados Formation, Late Triassic (PULR 076), reversed. Left (A, C) and right (B, D) tibia, fibula, and astragalocalcaneum in anterior (A 1–D1), medial (A2–D2), posterior (A3–D3), lateral (A4–D4), and ventral (A5–D5) views, arrows indicate anterior direction.

Fig. 1. Map of the type locality of Lepidus praecisio gen. et sp. nov. near Signal Peak, southeast of Big Spring, Texas. The dotted circle is the approximate area that holotype came from and its relationship to the famous Otis Chalk localities.

Fig. 3. The referred left femur of neotheropod Lepidus praecisio gen. et sp. nov., Otis Chalk area, Dockum Group, Late Triassic (TMM 41936-1.3) in anterior (A) and posterior (B) views and the bone tissues of the femur (D) through the entire cortex on the anterolateral side (C) and the middle and outer cortex on the posterolateral side (D). Arrows indicate the direction of the external surface of the femur.

Fig. 4. Partial left maxilla of an archosaur (TMM 41936-1.1), Otis Chalk area, Dockum Group, Late Triassic, found with and possibly referable to neotheropod Lepidus praecisio gen. et sp. nov., in lateral (A), ventral (B), and medial (C) views, arrows indicate anterior direction. D. A replacement tooth in labial view within the fourth alveolus in anterolateral view (D 2), close up (D ).