Zaraasuchus shepardi, POL & NORELL, 2004

POL, DIEGO & NORELL, MARK A., 2004, A New Gobiosuchid Crocodyliform Taxon from the Cretaceous of Mongolia, American Museum Novitates 3458, pp. 1-32 : 2-17

Zaraasuchus shepardi , new genus,

new species

HOLOTYPE: IGM 100/1321, posterior region of the skull and lower jaws preserved in articulation with cervical vertebrae, osteoderms, and forelimb elements.

ETYMOLOGY: Zaraa, Mongolian for hedgehog in reference to the spiny character of the skull and osteoderms, and shepardi , in reference to Dr. Richard Shepard, a friend of the expedition for many years.

DIAGNOSIS: Small crocodyliform diagnosed by the following combination of characters: sculptured skull bones that are ornamented with thin and continuous ridges; anterior margin of infratemporal fenestra almost completely formed by the postorbital; retroarticular process with a pointed, welldeveloped posterolateral process, the surface of which is ornamented; extremely well­developed keels on dorsal and lateral cervical osteoderms, the heights of which are approximately equal in length to the lateromedial extension of the dorsal osteoderms.

DESCRIPTION

SKULL

The skull of IGM 100/1321 has most of its dorsal and lateral postorbital elements in articulation with the posterior region of the lower jaws ( figs. 1 , 3 ). Zaraasuchus shepardi has large, laterally facing orbits. The supratemporal fenestrae are almost completely obliterated by a lateral expansion of the parietals and frontals and by a medial expansion of the squamosals. These expansions are depressed in respect to the rest of the skull roof, although this could be a preservational artifact. A narrow oblique slit between the squamosal, parietal, and frontal is the only remnant of the supratemporal opening ( figs. 1 , 2 ). This differs from the derived condition seen in Gobiosuchus kielanae , where the supratemporal fenestra is completely closed and the squamosal and parietals are sutured along their entire length (ZPAL MgR­II/68 and 69). This character, however, is probably subject to ontogenetic change. In some living crocodylians ( Osteolaemus tetraspis and Paleosuchus trigonatus ) the supratemporal fenestrae may close during ontogeny. The infratemporal fenestra is reduced due to the wide ascending process of the quadratojugal ( fig. 3 ). This fenestra, however, is not as reduced as in Gobiosuchus kielanae (ZPAL MgR­II/67), since the postorbital process and the infratemporal bar of the jugal are not as wide in Zaraasuchus shepardi as in Gobiosuchus kielanae . The skull table is very broad, being as wide as the infratemporal region, as in Gobiosuchus kielanae ( Osmólska et al., 1997; ZPAL MgR­II/67, 68, and 69) and Sichuanosuchus shuhanensis ( Wu et al., 1997; IVPP V 10594 ). The external surface of the skull is ornamentated with a unique pattern of extremely thin ridges and grooves. These thin ridges are more marked and continuous in comparison with the slightly marked ornamentation preserved in some of the specimens of Gobiosuchus kielanae (ZPAL MgR­II/67 and 69), although this difference could relate to preservational or ontogenetic causes. However the more marked sculpture of the cranium in Zaraasuchus shepardi suggests a more advanced ontogenetic stage than that of specimens of Gobiosuchus kielanae , while the previous character of an open supratemporal fenestra suggests a younger comparable stage.

A large anterior palpebral forms most of the dorsal margin of the orbit of Zaraasuchus shepardi . The palpebral is tabular rather than triangular as in all crocodyliforms. Its anterior contact with the prefrontal is not preserved in IGM 100/1321. The dorsal surface of this element is heavily sculptured with thin ridges and is sutured to the frontal along its medial margin ( fig. 1 ) excluding the frontals from the orbital margin. Its posterior margin is sutured to the the posterior palpebral ( fig. 1 ), extensively overlapping the depressed articular facet of this element ( figs. 2 , 3 ). A similar morphology is present in Gobiosuchus kielanae ( Osmólska et al., 1997; ZPAL MgR­II/68), although in this form the two palpebrals are completely fused and more tigthly sutured to the frontal.

The posterior palpebral is much shorter than the anterior element ( fig. 1 ). This palpebral dorsally covers the posterior part of the orbit and is firmly sutured to the postorbital and the frontal. The large anterior palpebral overlaps a depressed facet of the posterior palpebral. Its medial edge contacts the frontal along a straight and posterolaterally directed suture. The posterior edge of this palpebral overlaps the postorbital and projects a pointed posterolateral process along the lateral edge of the postorbital ( figs. 1 , 3 ). On its ventral surface, the posterior palpebral has a short descending process that anteriorly covers the dorsal end of the descending postorbital bar ( figs. 3 , 6 ).

The frontals are not completely fused as in some specimens of Gobiosuchus kielanae (ZPAL MgR­II/67 and 68) and other basal crocodyliforms (e.g., Protosuchus AMNH 3024; Orthosuchus SAM­K 409). The anterior half of the suture between the frontals is straight, while the posterior third of this suture is interdigitated ( fig. 1 ). Between these two regions, the suture is not visible and the frontals seem to be fused, at least superficially. The interfrontal suture extends along a slightly elevated ridge, similar to that present in one of the specimens of Gobiosuchus kielanae that has fused frontals (ZPAL MgR­ II/69) and several other basal mesoeucrocodylians. The dorsal surface of these elements is ornamented with thin ridges which radiate from the interfrontal suture. The frontals are posteriorly broad at their contact with the parietal and postorbital and they narrow anteriorly markedly, with their straight lateral edges forming an acute angle ( fig. 1 ), resembling the condition of Gobiosuchus kielanae (ZPAL MgR­II/68).

The posterior contact with the parietal is transverse on the medial elevated region of the skull roof. It projects slightly anteriorly along the medial margin of the supratemporal depression and then is directed laterally, entering into the supratemporal depression where it continues to the slitlike supratemporal opening ( fig. 2 ). The frontal forms the anterolateral margin of the supratemporal slit and meets the postorbital at the anteriormost tip of this opening. Anterior to this point, the frontal narrows, laterally contacting the palpebrals through a robust suture excluding the frontal from the orbital margin ( fig. 1 ). The anterior tip of the frontal and its contact with the prefrontal and nasals are not preserved in IGM 100/1321.

The ventral surface of the frontal has an extremely well­developed crista cranii ( fig. 4 ). The space between them, which enclosed the olfactory tract of the forebrain, is very narrow, and it was probably closed ventrally by a ventromedial extension of the cristae cranii or an ossification fused to them ( fig. 4 ). This peculiar condition is also found in Gobiosuchus kielanae (ZPAL MgR­II/68).

The parietals are fused into a single element as in all crocodyliforms ( fig. 1 ). Their dorsal surface is ornamented like the rest of the skull roof and bears a slight longitudinal ridge, which is also found in Gobiosuchus kielanae and several other basal crocodyliforms. The medial region of its dorsal surface is flat, resembling the condition of most crocodyliforms. This area is bordered by two laterally concave ridges which demarcate the flat dorsal surface of the skull table from the supratemporal depressions ( figs. 1 , 2 ).

The parietal is broad at its anterior edge and continuously narrows toward the occipital margin of the skull, as in Gobiosuchus kielanae ( Osmólska et al., 1997; ZPAL MgR­II/68). The anterolateral edge of the parietal forms the posterormedial margin of the supratemporal slit opening and contacts the squamosal at the posteriormost tip of this aperture ( fig. 2 ). Posteriorly, the squamosal­parietal suture is straight and extends posteromedially along the supratemporal depression. This suture continues in the same direction posterior to the supratemporal depression reaching the posterior edge of the skull roof ( fig. 1 ). Only the right side of the posterior margin of the parietal is preserved in IGM 100/1321. In this region, the parietal has a posterolaterally directed crest running from the medial ridge to the squamosal­parietal suture, close to its posterior end ( fig. 1 ). Unfortunately, in this specimen, it cannot be determined if the parietal extended onto the occipital region of the skull.

The dorsal surface of the squamosal is very long and roughly triangular, with its apex directed anteriorly. Its dorsal surface has the same ornamentation pattern as the rest of the skull roof ( fig. 1 ). Anteriorly it forms the lateral part of the supratemporal depression and laterally it overhangs the otic recess as in most crocodyliforms. As described previously, the squamosal contacts the parietal along an anterolaterally directed suture, entering the supratemporal depression near the posterior end of the supratemporal slit. Anterior to this point, the squamosal forms the lateral margin of the reduced supratemporal opening ( fig. 2 ). The squamosal contacts the postorbital at the anterior apex of the supratemporal opening, where this bone overlaps the squamosal. The squamosal facet receiving the postorbital extends ventral to it, reaching the base of the descending process of the postorbital ( fig. 3 ). On the dorsal surface of the skull, the postorbital­squamosal contact is directed posterolaterally within the supratemporal depression toward the lateral edge of the skull roof ( fig. 2 ). This is in contrast to the laterally directed suture of Gobiosuchus kielanae (ZPAL MgR­II/69).

The skull roof is separated from the lateral edge of the squamosal by a well­developed longitudinal ridge that runs along most of the length of the squamosal ( figs. 1 , 3 ). Lateral to this ridge, the lateral edge of the squamosal has a wide, concave surface exposed laterodorsally ( fig. 3A ). This longitudinal ridge and the wide concave surface ventral to it closely follows the morphology preserved in one of the specimens of Gobiosuchus kielanae (ZPAL MgR­II/69) and resembles the groove for the attachment of the movable dorsal earflap on the squamosal of extant crocodylians. In Zaraasuchus shepardi , this concave surface is smooth, except for the presence of several slightly marked grooves. The ventral margin of the lateral surface of the squamosal bears a narrow and deep groove near its ventral margin which is preserved only on the right side of IGM 100/ 1321 ( fig. 3B ). Posterior to the supratemporal depression, the concave lateral surface of the squamosal opens onto the dorsal surface of the skull due to a medial deflection of the longitudinal ridge that forms its dorsal margin ( figs. 1 , 3A ).

The dorsal surface of the squamosal posterior to the supratemporal depression is bordered anteriorly by a transversal groove. This is not present in any of the Gobiosuchus kielanae specimens, yet it might be accentuated by preservation, although it is present on both squamosals in a symmetrical way ( fig. 1 ). Posterior to this transverse groove, the squamosal bears a long posterolateral process that, as in Gobiosuchus kielanae ( Osmólska et al., 1997; ZPAL MgR­II/68), extends to reach the level of the posterior end of the mandibular ramus. The dorsal surface of this long process is ornamented and bears three well­developed and sinuous crests oriented longitudinally near its base ( fig. 5 ). This region is poorly preserved in all the specimens of Gobiosuchus kielanae ; however, the specimen ZPAL MgR­II/68 preserves the internal mold of the posterolateral processes of the squamosals showing similar, yet not identical, ridges on its dorsal surface. The distal end of the posterolateral process of the squamosal is distinctly outurned, as in the type specimen of Gobiosuchus kielanae (ZPAL MgR­II/67). Despite these similarities, in Zaraasuchus shepardi the posterolateral process of the squamosal is directed horizontally, approximately at the same level as the skull roof, while in Gobiosuchus kielanae (ZPAL MgR­II/68) and more derived crocodyliforms it is ventrally deflected.

Unfortunately, the occipital flange of the squamosal and its descending process have not been preserved in IGM 100/1321.

The dorsal surface of the postorbital is a narrow and curved bar which forms the anterolateral margin of the skull roof ( fig. 1 ). Posteriorly the postorbital overlaps a depressed articular surface of the squamosal. here the dorsal surface of the postorbital is smooth and slightly concave, continuous with the dorsolateral concave surface of the squamosal. Medial to this surface the postorbital surface is ornamented and borders the squamosal near the supratemporal slit opening, of which it forms the anterior apex ( fig. 2 ). In Gobiosuchus kielanae (ZPAL MgR­II/ 68) the postorbital contacts the parietal at its posteromedial corner, while in Zaraasuchus shepardi this contact is not present, probably due to the presence of the vestigial supratemporal opening. Anterior to the supratemporal slit margin, the medial area of the postorbital dorsal surface contacts the frontal overlapping this element ( figs. 1 , 2 ). The anterolateral margin of the dorsal surface of the postorbital is convex and contacts the posteror palpebral along most of its anterior margin.

The descending process of the postorbital is covered anteriorly by the descending process of the posterior palpebral ( fig. 6 ). Ventrally, the descending process of the postorbital is a flat and smooth lamina of bone that extends medially to the ascending process of the jugal, reaching almost to the base of the postorbital bar. This thin and laminar postorbital bar is not exposed laterally as in most basal crocodyliforms, but projects posteriorly (thus facing posterolaterally). A similar condition is also present in Gobiosuchus kielanae ( Osmólska et al., 1997; ZPAL MgR­II/ 67). The postorbital forms most of the anterior margin of the infratemporal fenestra, a condition that differs from the interpretation of Osmólska et al. (1997) concerning the condition in Gobiosuchus kielanae (although this region is poorly preserved in all the ZPAL specimens of this taxon). The posterior edge of the descending process of the postorbital extensively contacts the quadratojugal, reaching the posterodorsal margin of the infratemporal opening. The broad participation of the postorbital in the posterodorsal margin of the infratemporal fenestra in Zaraasuchus shepardi is very different from the condition in Gobiosuchus kielanae ( Osmólska et al., 1997) .

The jugal is preserved anterior to the postorbital bar and closely resembles the morphology of Gobiosuchus kielanae . The lateral surface of this region is exposed lateroventrally, resembling Gobiosuchus kielanae ( Osmólska et al., 1997; ZPAL MgR­II/67). This surface is distinctly ornamented with slightly marked, thin grooves, except for its ventral margin, which is smooth ( fig. 3A, B ).

The ascending process of the jugal is, like the descending process of the postorbital, a flat lamina facing posterolaterally. Its anterolateral edge is sharp and superficial, while its posterior edge is inset medially from the lateral ridge of the jugal ( fig. 3 ). This laminar process is compressed anteromedial–posterolaterally, but is wide along its posteromedial– anterolateral axis. This peculiar morphology is identical to that of Gobiosuchus kielanae ( Osmólska et al., 1997; ZPAL MgR­II/67, 69). The base of the postorbital process of the jugal is ornamented along its anterolateral edge, as in basal crocodyliforms, but it is distinctly smooth on its posteromedial region ( fig. 3 ). The ornamentation of this region cannot be determined in any specimen of Gobiosuchus kielanae due to poor preservation.

Below the postorbital ascending process, the external surface of the jugal bears a welldefined longitudinal ridge that divides the jugal into a dorsal surface facing dorsomedially and a ventral surface facing ventrolaterally ( fig. 3 ). The latter is continuous with the external surface of the suborbital process of the jugal and bears the same ornamented pattern bordered ventrally by a smooth margin. The dorsal surface is less ornamented, although there are some slightly marked ridges and grooves on its anterior end. The dorsal edge of this region of the jugal forms the entire ventral margin of the infratemporal fenestra and contacts the quadratojugal at the posterior corner of this opening ( fig. 3 ). Posterior to the infratemporal fenestra, the extensive jugal­quadratojugal projects posteroventrally, probably approaching the quadrate condyles (not preserved in IGM 100/ 1321).

This region of the jugal is also identical to that of Gobiosuchus kielanae ( Osmólska et al., 1997; ZPAL MgR­II/68 and 69), except for the ornamentation pattern that is barely present in this taxon (possibly due to preservational causes). A similar longitudinal ridge on the infratemporal region of the jugal is also present in Sichuanosuchus shuhanensis (IVPP V 10594 ), but this taxon lacks the other similarities of the jugals of Zaraasuchus shepardi and Gobiosuchus kielanae .

The quadratojugal is preserved on the right side of IGM 100/1321. The quadratojugal’s posteroventral region is thickened and sculpted at its contact with the jugal ( fig. 5B ). The ascending process is broad and directed anterodorsally as in basal crocodyliforms. This region forms most of the posterior edge of the infratemporal fenestra and its surface is smooth. It contacts the postorbital extensively dorsally to the infratemporal fenestra ( fig. 5B ). Unfortunately, its posterior contact with the quadrate has not been preserved.

MANDIBLE

Only the posterior regions of the mandibular rami of Zaraasuchus shepardi are preserved in IGM 100/1321. Most of the lateral and ventral surfaces are ornamented with a similar pattern to the skull bones ( fig. 3 ). The mandibular ramus is dorsoventrally high and the external mandibular fenestra is completely closed, a condition only present in Gobiosuchus kielanae ( Osmólska et al., 1997; ZPAL MgR­II/69) among basal crocodyliforms, but also present in some derived mesoeucrocodylians (e.g., atoposaurids, some goniopholids, Bernissartia ).

Only the posterior end of the right dentary is preserved in IGM 100/1321. Its lateral surface is smooth and bears a single, small neurovascular foramen. The dorsal region of the lateral surface of the dentary below the orbits is projected medially, being exposed laterodorsally rather than laterally ( figs. 3 , 4 ). This would produce an inset mandibular toothrow, similar to the condition of most non­neosuchian crocodyliforms. Posteriorly, the dentaries contact the surangular to the level of the postorbital bar and they seem to extensively overlap the angular, although this contact is not well preserved.

The angular is heavily ornamented and forms most of the ventral half of the lateral surface of the mandibular ramus ( fig. 3 ). Dorsally, the angular is bordered by the surangular, and the suture between them extends posteriorly and is deflected slightly ventrally near the posterior end of the mandibular ramus ( fig. 3 ). The ventral edge of the angular bears a sharp and well­defined longitudinal ridge dividing the lateral surface from a ventromedially facing surface of the angular ( fig. 7B ). This morphology is not known in other crocodyliforms except for Gobiosuchus kielanae (ZPAL MgR­II/68), although Shantungosuchus hangjinensis seems to have a similar, but less developed condition ( Wu et al., 1994a). This ridge is directed posteriorly along a horizontal plane ( fig. 3 ), in contrast to the condition in derived crocodyliforms where the angular is deflected dorsally. The posterior end of the angular reaches the level of the quadrate­mandibular articulation.

The surangular anteriorly contacts the posterodorsal region of the dentary near the dorsal margin of the lower jaw at the level of the postorbital bar. Posterior to this point the surangular bows slightly to form the dorsal margin of the mandible. The lateral surface of the surangular is heavily ornamented and has a sharp, prominent ridge that extends posteroventrally from its dorsal margin (at the level of the posterior edge of the infratemporal fenestra) up to the level of the cranio­mandibular articulation ( figs. 3 , 7A ). Posterior to this point, the ridge ventrally deflects abruptly, forming the lateral border of the reduced retroarticular process ( figs. 3 , 7A ). A similar feature is also present in Gobiosuchus kielanae ( Osmólska et al., 1997; ZPAL MgR­II/68) and Sichuanosuchus shuanensis (IVPP V 10594 ). Medial to this ridge, the surangular is exposed laterodorsally and is slightly ornamented ( fig. 7A ). Its dorsomedial edge is smooth and forms the lateral wall of the articular surface for the quadrate condyles. Posterior to this point, this smooth surface of the surangular deflects ventrally, contacting the retroarticular process medially and the surangular ridge laterally ( fig. 7A ).

The dorsal surface of the articular is exposed posteriorly to the articular facets for the quadrate. The lateromedial extension of the articular surface is reduced although its medial extent is poorly preserved. Posteriorly, a reduced, flat, and triangular­shaped retroarticular process extends ventrally, resembling the condition seen in most basal crocodyliforms. The distal (posteroventral) end of the retroarticular process of Zaraasuchus shepardi , however, is uniquely autapomorphic in that it has a laterally curved conical process ornamented with slight ridges and grooves and four well­developed apically converging ridges. This condition is absent in Gobiosuchus kielanae ( fig. 7 ).

The ventral surface of the articular has a dorsomedially directed shelf forming the support for the articular facet with the quadrate. This shelf is subtriangular and is slightly concave on its ventral surface. A similar process is present in Gobiosuchus kielanae (medial process of Osmólska et al., 1997); however, it cannot be determined if the dorsomedial tip of this process contacted the basisphenoid in Zaraasuchus shepardi . Unlike in Gobiosuchus kielanae , the entire retroarticular region of the lower jaw of Zaraasuchus shepardi projects medially and is not ‘‘squared­off’’ ( fig. 7B ).

POSTCRANIUM

Several postcranial elements were found in association with the skull of IGM 100/1321. Some fragmentary cervical vertebral elements were crushed underneath the posterior end of the skull roof, including partially preserved neural arches and cervical ribs. A series of cervical osteoderms and a posterior cervical were found in articulation with the skull. Posterior to these elements, the left humerus and ulna were also preserved in association with appendicular osteoderms.

The best preserved postcranial elements are eight transverse rows of cervical osteoderms found in life articulation with the skull of IGM 100/1321. The first two rows of osteoderms seem to be composed exclusively of two dorsal osteoderms as in most basal crocodyliforms. These are heavily sutured to each other and have rounded lateral and anterior edges ( fig. 8A ). The dorsal surface of these osteoderms is ornamented with shallow and sinous grooves and have, near their posterior edge, a well­developed medial keel ( fig. 8A, B ). Five ridges radiate from this keel (two laterally, one anterolaterally, one medially, and one anteromedially). The posterior edge of the osteoderms imbricates with a thin smooth area of the posterior osteoderms.

Posterior to this first pair of osteoderms, six transverse rows are composed of two dorsal and two lateral osteoderms that are strongly sutured to each other. The dorsal pair of osteoderms has the same ornamentation pattern as the anterior ones, although the medial keel is much more developed and is directed dorsolaterally ( fig. 8A, B ). Additionally, an accessory ridge is present on the posterior surface of the medial keel ( fig. 8A ). The lateral osteoderms also have a well­developed lateral keel. This keel, however, is located extremely close to the sutures with the dorsal osteoderms, and the radiating ridges are not as developed and seem to be absent in some of the lateral osteoderms ( fig. 8B ).

Several ventral osteoderms are preserved in the cervical region, although slightly disarticulated. The ventral osteoderms are notably different from the dorsal elements. The ventral surface lacks well­developed keels and is ornamented with slightly marked grooves, except for the anteriormost region which is smooth and imbricates with the preceding element. A slight ridge is present on its ventral surface extending obliquely to the longitudinal axis of the osteoderm, which projects anterolaterally. These osteoderms were probably contacted to a corresponding pair along their medial margins, as in Gobiosuchus kielanae (ZPAL MgR­II/71).

The cervical dermal armor of Zaraasuchus shepardi is unique among crocodyliforms; however, it shares with Gobiosuchus kielanae several derived characters ( fig. 8 ). First, the derived presence of lateral cervical osteoderms that are strongly sutured to a pair of dorsal osteoderms, and the presence of five radiating ridges on the dorsal surface of the osteoderms ( fig. 8 ). The three anterior ridges were described as displaying a ‘‘fleur de lys’’ pattern by Osmólska et al. (1997). Other crocodyliforms have multiple ridges on their osteoderm dorsal surface (e.g., Pristichampsus ), but their similarities with the osteoderms of Zaraasuchus shepardi and Gobiosuchus kielanae are only superficial.

Despite these similarities, the cervical dermal armor of Zaraasuchus shepardi is distinguished from that of Gobiosuchus kielanae by the presence of extremely elevated keels on the posterior edge of each osteoderm rather than a low ridge (ZPAL MgR­II/68, 71), the presence of a thin ridge directed parasagittally on the posterior surface of the osteoderm, and the ornamentation pattern of the dorsal surface of osteoderms composed by shallow grooves rather than by discrete pits (ZPAL MgR­II/71). Additionally, the first pair of osteoderms preserved in Zaraasuchus shepardi differs in being notably narrower and in lacking a lateral spur, although this pair of osteoderms might not be actually the first pair of Zaraasuchus shepardi (i.e., the ‘‘nuchal’’ osteoderms sensu Osmólska et al., 1997).

In Gobiosuchus kielanae , four smooth ventral osteoderms close the cervical region ventrally ( Osmólska et al., 1997; ZPAL MgR­II/68), while in Zaraasuchus shepardi these elements are ornamented. This difference, however, is subject to ontogenetic variation and it is not possible to determine how many ventral osteoderms composed each transverse row in Zaraasuchus shepardi due to its poor preservation. It is important to note, however, that some features of Gobiosuchus kielanae are ontogenetically more advanced (e.g., complete closure of the supratemporal fenestrae), than in Zaraasuchus shepardi .

In addition to the dorsal osteoderms, several small and subrectangular osteoderms were found on the dorsal surface of the humeral and ulnar shafts ( fig. 9 ). The preserved humeral osteoderms are located near the proximal end of the humeral shaft. These are slightly ornamented with shallow grooves and a slightly marked ridge. The proximal osteoderm overlaps the element distal to it, although this might not be its natural position ( fig. 9 ). Two osteoderms were preserved on the distal section of the ulnar shaft. These elements are less ornamented and more elongate than the humeral osteoderms ( fig. 9 ). Gobiosuchus kielanae has appendicular osteoderms along the hindlimbs (ZPAL MgR­II/67 and 68); however, as noted by Osmólska et al. (1997), both limbs were probably covered by osteoderms, as suggested by the postition of similar disarticulated appendicular elements. The distribution of appendicular osteoderms among crocodyliforms has not been extensively studied, although it has been reported in some taxa, ranging from basal forms (CUP 2083; Wu, personal comun.) to goniopholids ( Sunosuchus ; Wu et al., 1996) and alligatorids ( Cong et al., 1998).

The posterior cervical vertebra was found underneath the posteriormost preserved cervical osteoderms and was removed during preparation ( fig. 10 ). The neural spine is dorsoventrally short and located posteriorly on the neural arch, although the anterior edge is poorly preserved ( fig. 10A ). The prezygapophyses are not preserved. The postzygapophyses are robust, short, and barely curved laterally ( fig. 10A ). Their articular facets are elevated and facing lateroventrally. A welldeveloped ridge extends anteriorly from the postzygapophyseal articular facets on the dorsal region of the lateral surface of the neural arch ( fig. 10B ). The neural canal is large in comparison with most crocodyliforms. The neurocentral suture is visible on the lateral surfaces of this cervical vertebra. Diapophyses are not well preserved, but a long ridge extends posteriorly from them, ventrally on the lateral surface of the neural arch ( fig. 10C ). Ventral to the diapophyseal ridges, a narrow concavity extends anteriorly on the lateral surface of the centrum between them and the parapophyses. The parapophyses are extremely well developed and projected laterovetrally ( fig. 10B, D ). A long ridge extends posteriorly to these, reaching the anteroposterior midpoint of the lateral surface of the centrum. The centrum of this vertebra is unusually long for a crocodyliform, being approximately three times longer than high. Its anterior end is notably more expanded than its posteiror end, mainly due to the well­developed parapophyses ( fig. 10D ). Its ventral surface is constricted at its midpoint and bears a small keel anteriorly, between the parapophyses ( fig. 10D ). None of the specimens of Gobiosuchus kielanae has well­exposed cervical vertebrae; however, as noted by Osmólska et al. (1997), the length of the neck in specimen ZPAL MgR­ II/68 is remarkalbly long and composed by seven postaxial vertebrae. This suggests that at least some of the cervical vertebrae of Gobiosuchus kielanae must be unusually long, as in Zaraasuchus shepardi .

Unfortunately, only the shafts of the humerus and the ulna are preserved in IGM 100/1321. As in Gobiosuchus kielanae , these forelimb elements are extremely long and slender ( fig. 1 ), even longer than the elongate forelimbs of most basal crocodyliforms.

PHYLOGENETIC ANALYSIS

The phylogenetic relationships of Zaraasuchus shepardi were analyzed using a modification of a recently published dataset ( Pol and Norell, 2004), which was based on the addition of several new characters to previously published matrices ( Clark, 1994; Wu and Sues, 1996; Gomani, 1997; Wu et al., 1997; Buckley et al., 2000; Ortega et al., 2000). Twelve new characters were added to this dataset, resulting in a matrix of 192 characters scored across 45 taxa. As in our previous study, the taxon­sampling regime is focused on non­neosuchian crocodyliforms. In our analysis characters had equal weights using Nona ( Goloboff, 1993). A heuristic tree search was performed consisting of 1000 replicates of RAS + TBR with a final round of TBR (mult*1000; max*;), holding 20 trees per replication (hold/20;). Zero­length branches were collapsed using the strictest criterion (i.e., when any possible states are shared between the ancestor and descendant node; amb­).

Two most parsimonious trees of 633 steps (CI = 0.37, CI inf = 0.36, RI = 0.67) were found in 797 of 1000 replications. Further searches employing 10,000 iterations of the Parsimony Ratchet ( Nixon, 1999) implemented in Nona resulted in the same set of topologies (hitting the best length 6190 times).

In all most parsimonious hypotheses, Zaraasuchus shepardi is depicted as the sister taxon of Gobiosuchus kielanae . Both of these gracile armored corcodyliforms are from the Late Cretaceous of Mongolia ( fig. 11 ). This is a basal clade within Crocodyliformes and is diagnosed by 14 unambiguous synapomorphies (parietal without broad occipital portion [character 32]; absence of external mandibular fenestra [character 75]; more than two parallel rows of dorsal osteoderms [character 97]; cranial table as wide as ventral portion of skull [character 174; paralleled in Sichuanosuchus ]; palpebrals sutured to each other and the frontal, excluding it from the orbital margin [character 181]; external surface of ascending process of jugal exposed posterolaterally [character 182]; longitudinal ridge on lateral surface of jugal below infratemporal fenestra [character 183; paralleled in Sichuanosuchus ]; dorsal surface of posterolateral process of squamosal ornamented with three longitudinal ridges [character 184]; presence of a sharp ridge along ventral surface of angular [character 186; paralleled in Shantungosuchus ]; surangular with a longitudinal ridge on its dorsolateral surface [character 187; paralleled in Sichuanosuchus ]; dorsal surface of osteoderms ornamented with anterolaterally and anteromedially directed ridges [character 188]; cervical region surrounded by lateral and ventral osteoderms sutured to the dorsal elements [character 189]; presence of appendicular osteoderms [character 190; also present in other crocodyliforms, see above]; closed, or incipiently close, supratemporal fenestra [character 191]). Despite the large number of synapomorphies, support for this clade is low (Bremer support = 2). Most other nodes of these hypotheses also have minimal Bremer support values.

The phylogeny indicates that this clade is more closely related to mesoeucrocodylians than to Protosuchus and its allies ( Protosuchidae sensu Clark, 1986 ) due to the presence of five synapomorphic characters (posterolateral process of squamosal elongated, posterolaterally directed, and ventrally deflected [character 36]; squamosal contacts quadrate and otoccipital lateral to cranioquadrate passage [character 49]; maxilla and premaxilla with ventral region facing laterally and dorsal region facing dorsolaterally [character 139]; quadratojugal ornamented at its base [character 145]; thick, pneumatic pterygoid flanges [character 166]).

Our results conflict with those of Wu et al. (1997), where Gobiosuchus was the sister taxon of two taxa from the Early Cretaceous of China (i.e., Sichuanosuchus and Shantungosuchus ). Here, the clade composed by Zosuchus , Sichuanosuchus , and Shantungosuchus is depicted as more closely related to derived mesoeucrocodylians than to Gobiosuchus kielanae and protosuchids. This position is supported by six synapormorphies (fused frontals [character 20]; palatine shelves that extend below narial passage [character 37]; choana opens posteriorly into a midline depression (choanal groove) [character 39]; fusion of pterygoids posterior to choana [character 41]; presence of one enlarged maxillary tooth [character 79]; presence of a well­developed posterodorsal process of the premaxilla [character 125]). In our dataset, forcing a monophyletic group composed by this clade, Gobiosuchus kielanae , and Zaraasuchus shepardi requires five extra steps. Despite these signs of support for the derived position of this clade, there are a considerable number of shared derived similarities between Gobiosuchus , Zaraasuchus shepardi , and these taxa. In particular, Sichuanosuchus shares 3 of the 14 synapomorphies of the Gobiosuchus kielanae + Zaraasuchus shepardi clade (interpreted here as convergences, see above).

The monophyly of the clade traditionally referred as Protosuchia (i.e., including protosuchids, gobiosuchids, and the Shantungosuchus clade) is rejected in this analysis, although this clade is present in trees only two steps longer than the most parsimonious trees.

Figures

Fig. 1. Holotype of Zaraasuchus shepardi IGM 100/1321 in dorsal view.

Fig. 3. Skull of the holotype of Zaraasuchus shepardi IGM 100/1321 in (A) left lateral view; (B) right lateral view.

Fig. 2. Right supratemporal region of Zaraasuchus shepardi IGM 100/1321 in dorsal view.

Fig. 6. Ventral surface of the postorbital of Zaraasuchus shepardi IGM 100/1321.

Fig. 4. Ventral surface of frontals of Zaraasuchus shepardi IGM 100/1321.

Fig. 5. Right posterolateral process of the squamosal of Zaraasuchus shepardi IGM 100/1321 in dorsal view.

Fig. 7. Posterior region of the mandibular ramus of Zaraasuchus shepardi IGM 100/1321 in (A) posterolateral view; (B) ventral view.

Fig. 8. Cervical osteoderms of Zaraasuchus shepardi IGM 100/1321 in (A) dorsal view; (B) lateral view

Fig. 9. Appendicular osteoderms of Zaraasuchus shepardi IGM 100/1321.

Fig. 10. Posterior cervical vertebra of Zaraasuchus shepardi IGM 100/1321 in (A) ventral view; (B) right lateral view; (C) left lateral view; (D) dorsal view.

Fig. 11. Strict consensus of the two most parsimonious hypotheses obtained with Nona.