Dynamosuchus collisensis, Müller & Baczko & Desojo & Nesbitt, 2020

Müller, Rodrigo T., Baczko, M. Belén Von, Desojo, Julia B. & Nesbitt, Sterling J., 2020, The first ornithosuchid from Brazil and its macroevolutionary and phylogenetic implications for Late Triassic faunas in Gondwana, Acta Palaeontologica Polonica 65 (1), pp. 1-10 : 3-7

Dynamosuchus collisensis sp. nov.

Fig. 2 .

ZooBank LSID: urn:lsid:zoobank.org:act:4B1FFAFB-6287-4448-9E34-B7976D307C80

Etymology: From Latin collis, hill, reflecting the fact that the holotype was collected from the Janner site, which is located at the base of the Agudo Hill (Morro Agudo in Portuguese).

Holotype: CAPPA / UFSM 0248 , a partial skeleton including the premaxillae, left maxilla, frontals, right postfrontal, right parietal, squamosals, quadratojugals, quadrates, parabasisphenoid, both hemimandibles, some cervical, dorsal, sacral and caudal vertebrae, several ribs, gastralia, osteoderms, left scapula, both forelimbs, left ilium, right pubis, left femur, right tibia, and left fibula.

Type locality: Janner site (29°39 ′ 10.89 ″ S, 53°17 ′ 34.20 ″ W), Agudo , Rio Grande do Sul, Brazil ( Fig. 1 ) .

Type horizon: Santa Maria Formation; Candelária Sequence, Paraná Basin. The presence of the genus Hyperodapedon and Exaeretodon at the Janner site ( Langer et al. 2007) supports a Carnian age based on correlation with radioisotope dating of another Brazilian site ( Langer et al. 2018) and Argentine beds from the La Peña Member of the Ischigualasto Formation ( Martínez et al. 2011) with the same taxa.

Diagnosis.— Dynamosuchus collisensis differs from all other known ornithosuchids based on a unique combination of character states (* autapomorphy): dorsal process of the quadratojugal mostly dorsally oriented and with the lateral surface densely covered by rugosities; additional foramen on the medial wall of the quadrate foramen*; parabasisphenoid component of the basal tubera ventrally separated from the basipterygoid process by a slight notch at the midline; main axis of the parabasisphenoid process posteroventrally directed; surangular with a conspicuous lateral shelf and an anteroposteriorly wide surangular foramen; anterior dorsal vertebrae with anterior and posterior centrodiapophyseal laminae and without prezygodiapophyseal and postzygodiapophyseal laminae; dorsal body osteoderms with an external pattern of dense crests and grooves; and iliofibularis tubercle of the fibula located within the proximal half of the bone.

Ornithosuchus woodwardi differs from Dynamosuchus collisensis in that O. woodwardi has (for comparative illustrations see the SOM, Supplementary Online Material available at http://app.pan.pl/SOM/app65-Muller_etal_SOM.pdf): dorsal process of the quadratojugal is more anteriorly projected ( O. woodwardi , 45°; D. collisensis , 90°); no additional foramen on the medial wall of the quadrate foramen; the lateral surface of the angular has ornamentation; less laterally extended surangular shelf; presence of prezygodiapophyseal and postzygodiapophyseal lamina in the anterior dorsal vertebrae; presence of a fossa ventral to the neurocentral suture on the lateral side of the dorsal centra; presence of an accessory process on the anterior margin of the neural arch of the middle caudal vertebrae; and a less expanded distal end of the radius.

Venaticosuchus rusconii differs from D. collisensis in that V. rusconii has: absence of foramina on the lateral surface of the maxilla; no additional foramen on the medial wall of the quadrate foramen; parabasisphenoid component of the basal tubera ventrally separated from the basipterygoid process by a notch that forms an acute angle; basipterygoid processes more transversely separated from each other in comparison to D. collisensis ; basipterygoid processes are ventrally directed with a semicircular shape in lateral view; and absence of the surangular foramen (autapomorphy of V. rusconii ; Baczko et al. 2014).

Riojasuchus tenuisceps differs from D. collisensis in that R. tenuisceps has: absence of foramina on the lateral surface of the maxilla; anterior and ventral margins of the antorbital fossa almost meet with the anterior and ventral edges of the maxilla; lateral surface of the quadratojugal is smooth; dorsal process of the quadratojugal is more anteriorly projected (c. 45°); no additional foramen on the medial wall of the quadrate foramen; ventral process of the squamosal anteroposteriorly larger proportionally; absence of a semilunar depression on the posterolateral surface of the parabasisphenoid; absence of the anterior and posterior centrodiapophyseal laminae of the anterior dorsal vertebrae; osteoderms with smooth ornamentation; internal tuberosity of the humerus is more pronounced; postacetabular process of the ilium is proportionally anteroposteriorly longer; iliac blade with a concave dorsal margin in lateral view; and iliofibularis tubercle of the fibula more distally located.

Description.— The body of the premaxilla of Dynamosuchus collisensis is ventrally deflected at the anterior portion (= downturned) and bears three teeth, as typical of ornithosuchids ( Baczko and Ezcurra 2013), and the anterior tip of the premaxilla extends further anteriorly than the anterior tip of the dentary ( Fig. 2A–C ). The elongated palatal process of the premaxilla of D. collisensis bears a ridge at its ventral margin, like in Venaticosuchus rusconii (PVL 2578; Baczko et al. 2014). There is a lateral edentulous constriction at the posterior process of the premaxilla, where two caniniform teeth from the dentary fit. The lateral surface of the maxilla is pierced by several foramina (see the Supplementary Online Material for additional figures), which are not recognized in V. rusconii (PVL 2578; Baczko et al. 2014) and R. tenuisceps (PVL 3827, PVL 3828; Baczko and Desojo 2016), but present in Ornithosuchus woodwardi (NHMUK PV R 3143; Walker 1964). The anterior and ventral portions of the antorbital fossa do not coincide with the anterior and ventral edges of the maxilla in D. collisensis , whereas in R. tenuisceps (PVL 3827, PVL 3828) these margins almost coincide with each other ( Baczko and Desojo 2016). There are six tooth positions preserved; however, the total number of maxillary teeth of D. collisensis is uncertain, but probably does not exceeds seven or eight teeth based on the preserved length of the maxilla, the length of the hemimandible, and through comparisons with other ornithosuchids.

The right frontal, postfrontal, and anterior portion of the parietal were preserved in articulation showing a markedly constricted interorbital area and delimiting the medial half of the supratemporal fenestra (SOM: fig. S2). A depression for the olfactory bulbs occurs on the ventral surface of the anterior half of the frontal. Like in R. tenuiscep s (PVL 3827), the supratemporal fenestra is anteroposteriorly short, approximately half of the orbit, whereas in Saurosuchus galilei (PVSJ 32; Alcober 2000), Prestosuchus chiniquensis (UFRGS-PV-0629-T; Mastrantonio et al. 2019), Tarjadia ruthae (CRILAR-Pv 495; Ezcurra et al. 2017) and in aetosaurs (e.g. Neoaetosauroides engaeus ; PVL 5698; Desojo and Báez 2007), the supratemporal fenestra is equivalent to the length of the orbit. The ventral process of the squamosal of D. collisensis is elongated and slender ( Fig. 2A ), resembling that of O. woodwardi (NHMUK PV R 2409, NHMUK PV R 3562), whereas in R. tenuisceps (PVL 3827) it is proportionally wider anteroposteriorly. There is a longitudinally oriented sulcus bounded medially by a sharp crest on the dorsal surface of the squamosal of D. collisensis . The general morphology of the quadratojugal of D. collisensis resembles that of V. rusconii (PVL 2578) , where the dorsal process is less anteriorly inclined than in R. tenuisceps (PVL 3827). Like in O. woodwardi (NHMUK PV R 3142), the lateral surface of the quadratojugal of D. collisensis bears a strongly rugose surface ( Fig. 2D ), whereas in other ornithosuchids the homologous surface is smooth ( Baczko et al. 2018). The quadrate foramen lies between the medial margin of the quadratojugal and the lateral margin of the quadrate. In addition, there is a foramen piercing the wall that forms the medial margin of the quadrate foramen in the quadrate ( Fig. 2D ). Although present in some archosauriforms (e.g., Sarmatosuchus otschevi ; PIN 2865/68; Ezcurra 2016), this trait was not reported previously for ornithosuchids.

There is a shallow semilunar depression on the posterolateral surface of the parabasisphenoid of D. collisensis ( Fig. 2E ), a trait shared with V. rusconii (PVL 2578; Baczko et al. 2014). Like in R. tenuisceps (PVL 3827), in lateral view, the parabasisphenoid component of the basal tubera of D. collisensis is ventrally separated from the basipterygoid process by a gentle notch, as in most of the pseudosuchians ( Fig. 2E ). On the other hand, in V. rusconii (PVL 2578) there is a more acute angle (about to 40°) formed by the main axis of the basal tubera and the basipterygoid process in lateral view, whereas in D. collisensis the angle is wider (about to 60°). The basipterygoid processes of V.rusconii (PVL 2578) are more transversely separated from each other when compared to D. collisensis . Moreover, in V. rusconii (PVL 2578) the basipterygoid processes are ventrally directed with a semicircular shape in lateral view ( Baczko et al. 2014), whereas in D. collisensis , the processes lie posteroventrally, which results in a more elongated ventral margin in comparison. The basipterygoid recess of D. collisensis is well-excavated differing from the shallower recess seen in R. tenuisceps (PVL 3827) and in aetosaurs (e.g., Neoaetosauroides engaeus PVL 5698, Desmatosuchus spurensis TTUP 9024).

The lower jaws of D. collisensis are anteroposteriorly shorter than the skull ( Fig. 2A–C ). The symphysis is approximately 35% of the mandible length (synapomorphic character state of ornithosuchids; Sereno 1991) and the external mandibular fenestra is also anteroposteriorly longer than tall in comparison with other ornithosuchids. The dorsal margin of the anterior portion of the dentary is elevated relative to the more posterior portion. The splenial forms most of the medial portion of the lower jaw and lacks any foramen. The dorsoventrally slender angular is anteroposteriorly elongated, and delimits the posterior ¾ of the ventral margin of the external mandibular fenestra. Posteriorly, the angular meets the surangular, which delimits the posterodorsal and dorsal margin of the fenestra. There is a wide surangular foramen dorsal to the contact with the angular ( Fig. 2A ). Such structure occurs in the other ornithosuchids, except in V. rusconii (PVL 2578; Baczko et al. 2014). Dynamosuchus collisensis bears a well-developed surangular shelf, which resembles the sharp shelf of R. tenuisceps (PVL 3827; Baczko and Desojo 2016) and V. rusconii (PVL 2578; Baczko 2018), whereas in O. woodwardi (NHMUK PV R 2409) the shelf is less pronounced. This shelf is also present in proterochampsids, erpetosuchids, and gracilisuchids ( Ezcurra 2016; Ezcurra et al. 2017).

The axial neural spine is anteroposteriorly longer than the neural spine of the other preserved post-axial cervical vertebrae. At the ventral surface of the postzygapophysis of the cervical vertebrae there is a posterolaterally facing fossa ( Fig. 2F ), a trait unusual in pseudosuchians, except by Gracilisuchus stipanicicorum (PULR 08; Ezcurra 2016). The cervical neural spines are tall relative to the neural arch, straight, and possess transverse expansions at their dorsal end, forming “spine tables”. The cervical centra have a strongly developed ventral keel that extends longitudinally and project ventral to the margin of the centra ( Fig. 2G ). This condition is uncommon within pseudosuchians being present only in R. tenuisceps (PVL 3827), Erpetosuchus granti NHMUK PV 3139), Stagonolepis robertsoni (NHMUK PV 4784), and Batrachotomus kupferzellensis (SMNS 80284). The anterior dorsal vertebrae ( Fig. 2I ) of D. collisensis have an anterior and posterior centrodiapophyseal lamina, absent in R. tenuisceps (PVL 3827) and unknown in V. rusconii , but lack prezygodiapophyseal and postzygodiapophyseal lamina. The dorsal vertebrae of D. collisensis lack hyposphene-hypantrum accessory intervertebral articulation similar to R. tenuisceps (PVL 3827). The condition is uncertain in other ornithosuchids. The centra of D. collisensis lack a fossa just ventral to the neurocentral suture like in R. tenuisceps (PVL 3827), whereas a poorly rimmed fossa occurs in O. woodwardi (NHMUK PV R 3916; Walker 1964). The preserved dorsal vertebrae D. collisensis do not preserve the distal tip of the neural spine. The neural spine of the caudal vertebra is posterodorsally directed and bears a pointed posterior projection. Dynamosuchus collisensis lacks an accessory laminar process on the anterior margin of the caudal vertebrae, differing from O. woodwardi NHMUK PV R 3561).

The preserved paramedian osteoderms are square in dorsal view and strongly ornamented by radial ridges and elongated grooves ( Fig. 2H ), like in O. woodwardi (NHMUK PV R 3916; Walker 1964) whereas the same surface of the osteoderms of R. tenuisceps (PVL 3827) bear a gentle radial pattern ( Baczko et al. 2020). Nevertheless, D. collisensis shares with the other ornithosuchids the presence of a prominence on the dorsolateral surface of the osteoderms. In D. collisensis , this dorsolateral prominence is marked by a rugose surface and, in some elements, it has a pointed dorsal tip. The dorsolateral prominence becomes less developed progressively in posterior osteoderms of the cervical and dorsal series. A longitudinally oriented ridge is absent on the dorsal surface of the osteoderms, such as in other ornithosuchids. The anterior margin of the osteoderms is straight and lacks an anterior process or an unornamented anterior articular lamina. The posterior margin is concave in dorsal view ( Fig. 2H ). The ventral surface is smooth and ventrally convex in anterior view. The length of the osteoderms indicate that there is a pair of osteoderms per vertebra, forming two paramedial rows along the dorsal surface of the body (unknown for the tail).

The acromion process of the scapula gently rises from the anterior margin of the scapular blade at an angle of approximately 120°, resembling the condition of O. woodwardi (NHMUK PV R 3916). In contrast, D. collisensis has a well-developed acromion process, which is nearly flat in O. woodwardi (NHMUK PV R 3916); the condition of D. collisensis resembles that of R. tenuisceps (PVL 3828). The humerus ( Fig. 2L ) is robust, about 0.6 times the total length of the femur (0.65–0.75 in R. tenuisceps ), similar to the ration in some aetosaurs and Ticinosuchus ferox (PIMUZ 2817) , among pseudosuchians ( Ezcurra 2016). The proximal end of the humerus is widely expanded with a moderately developed internal tuberosity, which is less pronounced in D. collisensis than in R. tenuisceps (PVL 3826). The deltopectoral crest of D. collisensis is well-developed and restricted to the proximal third of the bone. Dynamosuchus collisensis lacks an ectepicondylar flange or it is poorly developed. The ulna ( Fig. 2M ) bears a moderately developed olecranon process and tappers towards its distal end, whereas the radius expands distally. Metacarpal I is the widest, whereas the metacarpal III is the longest of the metacarpals preserved ( Fig. 2N ).

Most of the preacetabular process of the ilium is not preserved ( Fig. 2J ). Nevertheless, it is evident that the ilium of D. collisensis lacks a lateral crest dorsal to the supraacetabular crest, as in other ornithosuchids and many pseudosuchians (e.g., G. stipanicicorum PVL 4597; T. ruthae CRILAR-Pv 478; Ezcurra et al. 2017). As in O. woodwardi (NHMUK PV R 3561), the dorsal margin of the iliac blade of D. collisensis is straight to convex, whereas the homologous area in R. tenuisceps (PVL 3828) is concave. The supraacetabular crest merges into the main body of the bone approximately at the mid-length of the acetabulum in D. collisensis . The postacetabular process is proportionally shorter than in R. tenuisceps (PVL 3828) and lacks any evidence of a brevis fossa on the ventral surface. There are three articular surfaces for the sacral ribs on the medial surface of the ilium, as in other ornithosuchids and poposauroids, differing from the rest of pseudosuchians, which typically possess two sacral vertebrae. Although the distal end of the pubis is not preserved, the length of the pubis is estimated to be markedly elongated (~0.68 times the total length of the femur) compared to other pseudosuchians ( Fig. 2O ). The pubis is anteroventrally projected, but gently arched ventrally along its length. It has a large obturator foramen and a well-developed pubic apron present along the shaft. The distal end is not preserved and it is not possible to determine whether it has a pubic boot or any expansion.

The hindlimb is estimated to be about to 1.6 times longer than the forelimb. The femur ( Fig. 2P ) is sigmoidal with both ends expanded relative to the shaft. The proximal end preserves a well-developed posteromedial tuber, however, the anterolateral and anteromedial tubers as well as the greater trochanter have been eroded. There is a proximodistally oriented anterior trochanter at the proximal portion of the bone and a mound-like fourth trochanter more ventrally located at the opposite surface of the shaft. The presence of an anterior trochanter is a condition only registered in the ornithosuchids R. tenuisceps (PVL 3828) and O. woodwardi (NHMUK PV R 2410, NHMUK PV R 3561, NHMUK PV R 3916) within pseudosuchians (unknown in V. rusconii ). The fourth trochanter of D. collisensis is less pronounced than in R. tenuisceps (PVL 3828), resembling the condition of O. woodwardi (NHMUK PV R 2410) . The anterior surface of the distal end of D. collisensis is gently concave and the popliteal fossa, at the opposite surface, is proximodistally short. The distal condyles were eroded. The tibia is about 0.7 times of the length of the femur. The cnemial crest is poorly expanded from the shaft and has a straight anterior surface. The posterior edge of the proximal end is strongly projected. The fibula ( Fig. 2Q ) has an elliptical proximal end with an expanded posterior portion. There is a well-developed tubercle for the attachment of the iliofibularis muscle at the proximal half of the bone. In R. tenuisceps (PVL 3827), this tubercle is more distally located ( Baczko and Ezcurra 2013). The specimen preserves slender gastralia (diameter 1.7 mm).

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

Figures

→ Fig. 2. Representative skeletal elements of ornithosuchid archosaur Dynamosuchus collisensis gen. et sp. nov. (CAPPA/UFSM 0248) from Janner outcrop, Carnian, Late Triassic. A. Selected skull bones in left lateral view. B. Reconstruction of the skull. C. Skull in ventral view. D. Left quadrate and quadratojugal in posterodorsal view. E. Parabasisphenoid in left lateral view. F. Neural arch of an anterior cervical vertebra in anterior view. G. Centrum of a cervical vertebra in left lateral view. H. Right osteoderm in dorsal view. I. Neural arch of an anterior dorsal vertebra in left lateral view. J. Left ilium in lateral view. L. Right humerus in anterior view. M. Right forearm in medial view. N. Left manus in dorsal view. O. Right (reversed) pubis in lateral view. P. Left femur in anterior view. Q. Left fibula in lateral view. Some unpreserved portions are modified from Baczko et al. in press, for the reconstruction of the skeleton of CAPPA/UFSM 0248 (preserved elements indicated in orange) (K). Scale bars 20 mm.

Fig. 1. A. Location map of the Janner site and the surface distribution of the geologic units in the area (modified from Müller et al. 2017). B. Stratigraphic column of the Janner site (modified from Da-Rosa 2015) depicting its fossiliferous content: 1, Exaeretodon; 2, Hyperodapedon; 3, Pampadromaeus; 4, Trucidocynodon; 5, Bagualosaurus; and 6, the new ornithosuchid.