Susisuchus anatoceps

Figueiredo, Rodrigo G., Moreira, João K. R., Saraiva, Antônio Á. F. & Kellner, Alexander W. A., 2011, Description of a new specimen of Susisuchus anatoceps (Crocodylomorpha: Mesoeucrocodylia) from the Crato Formation (Santana Group) with comments on Neosuchia, Zoological Journal of the Linnean Society 163 (5), pp. 644-645 : 644-645

SUSISUCHUS ANATOCEPS SALISBURY ET AL., 2003

Holotype: SMNK 3804 PAL – an incomplete partially articulated skeleton, comprising the skull and mandible; cervical, dorsal, sacral, and caudal vertebrae; pectoral girdle including the right and left forelimbs and osteoderms from the nuchal and dorsal shield ( Salisbury et al., 2003). Cast at the Museu Nacional/ UFRJ ( MN 6924 - V), Rio de Janeiro.

Referred specimens: MPSC-R 1136 – partially articulated postcranial remains comprising cervical and dorsal vertebrae; bones of the pectoral girdle including the right forelimb; and osteoderms from the dorsal shield. MPSC-R 1137 – a complete partially articulated right hind limb, including femur, tibia, fibula, and pes ( Figueiredo & Kellner, 2009).

Emended diagnosis: Susisuchus anatoceps differs from other crocodylomorph taxa based on the following combination of osteological features (autapomorphic characters marked with an asterisk): posterior process of the maxillary bone separating the lacrimal from nasal; lacrimal extends anteriorly beyond the anterior limit of the prefrontal; teeth needle-like and homodont; 11 thoracic vertebrae; four lumbar vertebrae; minimum width of the sacral ribs in an anteroposterior direction exceeds the maximum width of any of the diapophyses; postzygapophyses of caudal vertebrae 6–11 (the vertebrae terminal to caudal 11 are not preserved) unite medially to form a flat, horizontally aligned shelf, which extends terminally over the vertebral foramen; *maximum width of the proximal extremity of the ulna equivalent to that of the distal extremity, and about twice the minimum thickness of the ulnar shaft; absence of an anterior tubercle on the proximal extremity of the ulna; *unguals present only in hand digits 1 and 2; *scapular blade has straight anterior margins and concave posterior margins; dorsal shield comprising two rows of paravertebral osteoderms and two left and two right rows of accessory scutes; procoelous cervical vertebrae and platycoelous last cervical vertebra; amphicoelous thoracic, lumbar, and caudal vertebrae.

DESCRIPTION

GENERAL

Specimen MPSC-R1136 comprises postcranial remains of a small neosuchian crocodylomorph preserved on a limestone plate typical of the Crato Formation ( Figs 1 , 2 ). The anterior dorsal vertebrae are preserved followed by the dorsal shield osteoderms. Posterior to the dermal skeleton are the last vertebrae of the dorsal series. Bones of the right forelimb are also present: the ulna, the radius, and the manus. The humerus, the scapula, and the coracoid were removed from the limestone matrix after mechanical and chemical prepared with acid. An almost complete sequence of cervical vertebrae in articulation with the ribs were also prepared and removed from the slab.

No great taphonomic deformation is perceptible on most of the specimen, although the left surface of the cervical vertebrae (preserved facing down on the matrix) is slightly damaged and the ribs are broken. The ulna and radius also show some fractures along their shafts and only the broken coracoid articular surface with scapula is preserved. MPSC-R1136 is housed at the Museu de Paleontologia de Santana do Cariri (MPSC) from Universidade Regional do Cariri (URCA) and a cast has been deposited at the Paleovertebrate Sector collection of Museu Nacional/ UFRJ under number MN 6924-V.

AXIAL SKELETON

Cervical vertebrae

The cervical sequence comprises seven vertebrae from axis to cervical 8 and also a small trapezoidal-shaped bone associated with the axis, which we tentatively regard as the atlas intercentrum ( Fig. 3 ). The axis is elongated and has the centrum longer than higher. The anterior portion of the neural arch and the neural spine are slightly damaged. Axial postzygapophyses are short in comparison with those from the posterior vertebrae. The axial centrum is cylindrical in ventral view and shows neither parapophyses nor hypapophysis. The absence of axial hypapophyses is atypical in neosuchians and eusuchians; they are present in Bernissartia , Theriosuchus , and Isisfordia and usually they are prominent in extant species ( Brochu, 1999; Salisbury et al., 2006). Nevertheless, the lack of diapophyses is much more common in Neosuchia (including eusuchians; exceptions include Bernissartia and Gavialis ) ( Brochu, 1999). Unfortunately the axis is not preserved in the holotype of Susisuchus anatoceps ( Salisbury et al., 2003) .

The postaxial cervical vertebrae are higher than longer and they slightly increase in stoutness on approaching the posterior end of the neck. These vertebrae show well-developed diapophyses and parapophyses of essentially the same size. These processes are lateroventrally directed and have a gap between them, which is marked by the narrowing of the tuberculum and capitulum of the cervical ribs.

The neural spines are located posteriorly on the neural arch and all are laminar of about the same width, contrary to what was reported for Susisuchus anatoceps by Salisbury et al. (2003). This uniformity in neural spines width is also seen in Goniopholis , Isisfordia , and Gavialis ( Brochu, 1997, 1999; Salisbury et al., 2006). The total height of the neural spines cannot be determined.

The first postaxial vertebra has a weakly developed hypapophysis, which is the general condition for most neosuchian taxa (e.g. Bernissartia , Theriosuchus ) and also for Isisfordia duncani . Extant crocodylomorphs have prominent hypapophyses in first postaxial cervical vertebrae, except Gavialis gangeticus ( Brochu, 1997, 1999).

All hypapophyses of MPSC-R1136 are poorly developed in comparison with living taxa. In the posterior vertebrae, these structures are keel-shaped, without any forking, running anteroposteriorly in the ventral surface of the centrum. There is no information available regarding the cervical hypapophyses of the holotype of Susisuchus ; nevertheless in other neosuchians such as Theriosuchus guimarotae and Pachycheilosuchus trinquei they are knob-like ( Rogers, 2003; Schwarz & Salisbury, 2005).

At least the axis and the third, sixth, and seventh vertebrae have slightly procoelous centra ( Fig. 4 ), which are very similar to Isisfordia vertebrae but rather different from the amphicoelic condition described for the holotype of Susisuchus anatoceps ( Salisbury et al., 2006) . Another taxon that shows an unequal arrangement of the cervical vertebrae articular surfaces is Pachycheilosuchus trinquei from the Glen Rose Formation (Albian) of Texas ( Rogers, 2003). However, this taxon can be distinguished from Susisuchus by the presence of a secondary ossified plug that is present in the vertebrae of the North American species. This seems to be a unique and therefore diagnostic trait for Pachycheilosuchus .

the eighth vertebra is deformed, in posterior view it is possible to observe the slightly flat end of the centrum, indicating platycoely in this specific vertebra. Theriosuchus and Susisuchus (MPSC-R1136) are two of the few neosuchians that have two types of articular posterior ends in cervical vertebrae.

The eighth cervical vertebra is quite different from the other cervicals. The diapophysis and parapophysis are considerably smaller and the thin and sharp keel-like hypapophysis is more developed. Although Dorsal vertebrae

The first thoracic vertebrae are preserved in articulation and except for dorsal 1 all other prothoracic vertebrae are overlapped by osteoderms ( Figs 1 , 2 ). Many features of the first dorsal resemble those from the last cervical vertebra: the vertebral fossa have a subcircular outline; vertebral foramina are wider than dorsoventrally higher; they have poorly developed parapophyses in comparison with those from anterior cervical vertebrae; and their diapophyses are placed just below the zygapophysial plane. The hypapophysis on dorsal 1 is not thin and keel-like but instead it has a strong and rounded knob shape, as in Pachycheilosuchus ( Rogers, 2003) .

There are at least six vertebrae preserved after the dorsal shield, all of them displaced from the anatomical position and overlapping each other, which suggests intense decay processes prior to burial. We tentatively interpret the first of them as the seventh dorsal vertebra. The transverse processes of dorsals 7, 8, and 9 are very wide, laminar, and dorsoventrally low, as in extant crocodylians. In general the zygapophyses of posterior dorsal vertebrae are relatively small and rectangular. The tenth dorsal is preserved with its anterior plane facing up, revealing the subcircular vertebral fossa in the centrum. In thoracic 11 it is possible to observe the neural arch with subcircular prezygapophyses and narrow transverse processes in comparison with dorsals 7, 8, and 9. This vertebra has its anterior face directed to the posterior region of the trunk. The 12th vertebra has a spool-shaped centrum with shallow concave posterior end, and is therefore amphicoelic. This is the plesiomorphic condition for Crocodylomorpha , and thus common for most neosuchians as Theriosuchus , Goniopholis , Bernissartia , and the holotype of Susisuchus anatoceps ( Benton & Clark, 1988; Norell & Clark, 1990; Salisbury et al., 2003, 2006). Its postzygapophyses are slightly rounded and the transverse processes are similar to those on thoracic vertebra 11.

Cervical ribs

Axial ribs are distinct from other cervical ribs: they have a Y shape, the proximal articular head is much wider than the flat shaft, and the tuberculum and capitulum have rounded articular surfaces. The wide tuberculum of the axial ribs is also found in the two Susisuchus specimens (holotype and MPSC-R1136) and is shared with Theriosuchus and Gavialis ( Salisbury et al., 2003, 2006). Ribs 3–7 are very similar to each other, consisting of a stout horizontal shaft with the tubercular and the capitular processes extending perpendicularly from it to meet the vertebrae parapophyses and diapophyses. MPSC-R1136 has a conspicuous flattening in the shaft of ribs 3–7, as described by Salisbury et al. (2003) for the first cervical ribs of the holotype of Susisuchus anatoceps . The last cervical rib differs from others in shape: it is long, straight, and flat with triangular distal extremity. The tuberculum and the capitulum are about the same size but the former is thicker. There is a slight constriction in the base of the articular processes, regarded as the anterior fossa. It is bordered by a small process located medially in the rib shaft. This rib resembles the posterior cervical ribs of extant taxa but it is less arched and more robust ( Mook, 1921).

Thoracic ribs

Few thoracic ribs are fully preserved, which includes the first two from the right side and two other posterior ribs. Ribs 1 and 2 are similar to each other and to the last cervical rib. The first rib has a welldeveloped laminar anterior process. Major differences can be seen in the second thoracic rib, which is larger and has a well-marked convex anteroventral margin. Its anterior fossa is deep. These first ribs are much less curved than those of extant crocodylians and their tuberculum and capitulum are less developed ( Mook, 1921). Fragments of the proximal head of ribs 3 and 4 reveal the lack of the tuberculum, which in Crocodylus is absent only posterior to dorsal rib 3 ( Mook, 1921). The posterior thoracic ribs are long and greatly curved, similar to the ribs of extant crocodylians, which lack the tuberculum and have distinct capitular and tubercular facets. These ribs have a conservative morphology and are very similar to those of the holotype of Susisuchus anatoceps ( Salisbury et al., 2003) .

APPENDICULAR SKELETON

Scapula and coracoid

The scapular blade is not symmetrical: the posterior margin is slightly concave, while the anterior edge is very thin and straight ( Fig. 5 ). This trait can be seen in the holotype of Susisuchus anatoceps ( Salisbury et al., 2003: fig. 2) and is a diagnostic feature of the species. The sharp dorsal border is anteroposteriorly wide with concave outline in lateral view. These features are also present in the holotype of Susisuchus ( Salisbury et al., 2003) . Anteriorly to the glenoid fossa the scapulocoracoid facet is broad and tapers toward the anterior edge, similarly to Alligator , Caiman, and Crocodylus ( Brochu, 1997, 1999); however, this feature is unknown in the holotype of Susisuchus and closely related species such as Isisfordia and Bernissartia ( Salisbury et al., 2003, 2006). On the lateral surface of the scapular blade there is a ridge that borders the anteroventral margin, extending dorsally through the middle of the shaft.

The coracoid shaft is not fully preserved, remaining only its proximal articulation. Nevertheless, this fragment corresponds to approximately 56% of the scapular length. This proportion suggests that these bones could be subequal in length. The glenoid articular facet is larger in coracoids than in the scapula and consists of a rounded posteriorly directed process. The coracoid foramen is large and oval and is visible in medial and lateral views. It is located near the dorsal edge, slightly posteriorly directed.

Humerus

The humerus has typical crocodylomorphan morphology: it is a relatively stout bone with concave medial outline and almost straight lateral margin. In lateral view it is possible to observe that the proximal head is offset by about 20° from the long axis ( Fig. 6 ). The proximal and distal articulations have similar width, about twice as thicker than the midshaft, as known in the holotype of Susisuchus anatoceps ( Salisbury et al., 2003) . The articular facet for the radius is more developed in MPSC-R1136 than in Pachycheilosuchus ( Rogers, 2003) .

The deltoid crest is highly developed, located next to the proximal articulation, although in some neosuchians (e.g. Pachycheilosuchus ) the gap between the deltoid crest and the proximal articular surface is smaller ( Rogers, 2003). It emerges gradually from the shaft, tapering anteriorly and is slightly deflected to the medial line – this feature is also observed in the holotype of Susisuchus ( Salisbury et al., 2003) . In recent species, however, the deltapectoral typically emerges abruptly, which is characteristic of more derived eusuchians, with the exception of Gavialis ( Brochu, 1997, 1999). There is a pronounced ridge with a rugose area bordering the deltoid crest, located lateroposteriorly on the humerus. This is regarded as an insertion scar of the common tendon of both the teres major muscle and latissimus dorsi muscle, in the same way as observed in extant crocodylomorphs ( Meers, 2003).

Ulna

The ulna shaft is curved, with a caudally convex surface and uniform width down to the distal end of the bone ( Fig. 7 ). In lateral view the proximal articulation is much wider than the axis, being more than twice as broad in dorsoventral direction. The proportions of proximal and distal ulnar articulations are less discrepant in lateromedial width; the distal extremity of the ulna is about 81% of the proximal end. The small difference in width between the proximal and distal ends of the ulna is only known in Susisuchus anatoceps (regarded as a diagnostic feature), Theriosuchus pusillus , Borealosuchus formidabilis , and Isisfordia duncani ( Salisbury et al., 2003, 2006). The total length of the ulna is about 77% that of the humerus. The proximal head has a roundedtriangle shape but the shaft becomes more ellipsoidal in mid-session due a dorsoventral compression. Distally the ulna gradually becomes more triangular again.

Radius

The radius is a slender bone and has general shape, size, and proportions that are very similar to that found on the holotype of Susisuchus anatoceps ( Fig. 7 ). The near straight shaft has total length about 86% of the ulna and 66% of the humerus and the minimum thickness of the axis is about 10% of the radius proximodistally length. The proximal articulations in both specimens are about twice as wide as the minimum thickness of the radius ( Salisbury et al., 2003). As in the ulna, the radius axis has a dorsoventrally compression that gives it an elliptical shape in cross-section, which is more rounded in the holotype of Susisuchus ( Salisbury et al., 2003) . The radius head is subcircular in shape in proximal view while its distal end is more triangular. The shaft is smooth and there are no scars, grooves or ridges that can be properly distinguished in this bone; nevertheless some striations can be observed on both the proximal and the distal ends.

Manus

The bones of the manus are preserved close to their original anatomical positions, although digits 4 and 5 are missing ( Fig. 7 ). The carpal radiale, ulnare, and pisiforme bones are present and can be observed in dorsal view along with metacarpals and phalanges of digits 1, 2, and 3. The radiale is the largest and most robust of the carpal bones. It is longer than wider and has asymmetrical articulations: the proximal one projects itself medially more acutely. The ulnare is an hourglass-shaped bone that is approximately 65.5% of the radial carpal length. The metacarpal on the first digit is about half the length of metacarpals 2 and 3 but is much stouter in build. The phalangeal formula pits and crossed grooves can be observed. There are large square and smaller ellipsoidal osteoderms, resembling the morphology of those found on the dorsal shield.

The presence of two paravertebral rows of osteoderms in Susisuchus anatoceps (reinterpreted based on MPSC-R1136) is consistent with other advanced neosuchians such as the ‘Las Hoyas neosuchian’, which also have lateral accessory osteoderms ( Salisbury & Frey, 2001). Recent eusuchians normally have six to eight rows of osteoderms, although Gavialis shows only four ( Brochu, 1999).

(without the unguals) is 1:2:3:?:? from the first to fifth digits. Claws are present only on digits 1 and 2 and both of them are relatively long and slightly curved; this is a unique trait of Susisuchus anatoceps ( Salisbury et al., 2003) .

Figures

Figure 1. Dorsal vertebrae, dorsal shield, and forelimb (zeugapodial elements and manus) of Susisuchus anatoceps (MPSC-R1136). Abbreviations: dg1–3, digits 1–3; dr1–4, dorsal ribs 1–4; dv7–10, dorsal vertebrae 7–10; lac, lateral accessory osteoderm; mac, medial accessory osteoderm; pav, paravertebral osteoderm; rad, radius; uln, ulna; vos, ventral osteoderms. Scale bar = 30 mm.

Figure 2. Drawing of the dorsal vertebrae, dorsal shield, and forelimb (zeugapodial elements and manus) of Susisuchus anatoceps (MPSC-R1136). Scale bar = 30 mm.

Figure 3. Cervical vertebrae of Susisuchus anatoceps (MPSC-R1136). A, ventral view showing the poorly developed hypapophyses. B, lateral view showing the neural spines. Abbreviations: ax, axis; c3–8, cervical vertebrae 3–8. White arrows indicate the neural spines. Scale bars = 10 mm.

Figure 4. Detail of the sixth cervical vertebrae of Susisuchus anatoceps (MPSC-R1136). The arrow indicates the procoelic end of the vertebra centrum. Scale bar = 5 mm.

Figure 5. Shoulder girdle of Susisuchus anatoceps (MPSC-R1136). A, lateral view of the right scapula. B, medial view of the right scapula. C, lateral view of the right coracoid. D, medial view of the right coracoid. Scale bars = 10 mm.

Figure 6. Right humerus of Susisuchus anatoceps (MPSC-R1136). A, anterior view; B, posterior view; C, lateral view; D, medial view. Scale bars = 10 mm.

Figure 7. Detail of the hand bones (zeugapodial elements and manus) of Susisuchus anatoceps (MPSC-R1136). Abbreviations: dg1–3, digits 1–3; rad, radius; uln, ulna. Scale bar = 20 mm.