Geosaurus giganteus (Von Sommerring, 1816) Cuvier, 1824 sensu Von Quenstedt, 1852

Young, Mark Thomas & Andrade, Marco Brandalise De, 2009, What is Geosaurus? Redescription of Geosaurus giganteus (Thalattosuchia: Metriorhynchidae) from the Upper Jurassic of Bayern, Germany, Zoological Journal of the Linnean Society 157 (3), pp. 551-585 : 559-565

MORPHOLOGY OF GEOSAURUS GIGANTEUS

GENERAL FEATURES OF THE SKULL

The ornamentation is composed by elliptic pits, rather than the subpolygonal pits and deep grooves usually seen in neosuchians, peirosaurids, and Araripesuchus ( Benton & Clark, 1988; Clark, 1994; Ortega et al., 2000), or the irregular pattern of shallow sulci found in most notosuchians ( Bonaparte, 1991; Carvalho & Bertini, 1999). The pits found in G. giganteus are faintly indistinct, loosely packed, and much shallower when compared with the pits of extant crocodilians. Elliptic pits can be identified in the maxilla, lacrimal, jugal, and postorbital of G. giganteus , but because of the poor preservation it is difficult to truly evaluate the extent of the development of this ornamentation in most elements. However, the pitted pattern is absent from the nasal and frontal, as in several metriorhynchids.

ROSTRUM

Nares and premaxillae

The entire anteriormost section of the rostrum and mandible is not preserved on the holotype ( Fig. 1 ). This contrasts with the description given by Von Sömmerring (1816), in which an isolated portion of the premaxilla is described and illustrated. However, this element is not with the rest of the material in the Sömmerring collection of the NHM, and is lost (interestingly, Cuvier, 1824 does not illustrate it either). As with other metriorhynchids, Von Sömmerring describes the premaxilla as possessing three alveoli, with its overall shape being much more reminiscent of Dakosaurus than the longirostrine geosaurs. The structure of the naris is highly variable in thalattosuchians: Pelagosaurus and S. bollensis have a single anterodorsally oriented naris, as with most metriorhynchids; G. gracilis (NHM R.3948), G. araucanensis (MLP 72-IV-7-1), G. suevicus (SMNS 9808), and Enaliosuchus macrospondylus Koken, 1883 (RNGHP 990201) have bifurcated nares, which are anterodorsally and laterally oriented; Steneosaurus leedsi Andrews, 1909 shows an undivided dorsally oriented naris ( Andrews, 1913; Gasparini & Dellapé, 1976; Pierce & Benton, 2006). Although the actual morphology of the naris for the G. giganteus holotype is unknown, one would assume the presence of a bifurcated naris, based upon the narial morphology of the other geosaurs. However, as the phylogenetic analysis herein demonstrates, this may not be a valid prediction (as Von Sömmerring’s description of the lost premaxilla suggests). This is confirmed in NHM 37020 ( Fig. 4 ). The morphology of the premaxilla is identical to that described and illustrated by Von Sömmerring (1816), although it is more complete.

Many other elements were not preserved (or were obscured by matrix) in the holotype, making their morphology unknown, such as the parietal, quadrate, occipital surface, choanae, pterygoids, ectopterygoids, coronoid process, articular, retroarticular process, and the glenoid fossa.

Maxilla

Only the posterior and middle sections of both maxillae are preserved in NHM R.1229. These sections are long, low, and narrow, as in other metriorhynchids. In the specimen, the maxillae contact at least the nasals, jugals, and lacrimals. At the anteriormost section of the preserved rostrum, the maxillae meet dorsally. The alveolar margin is not differentiated from the remaining surface, and, as in Dakosaurus manselii ( Hulke, 1870) Woodward, 1885 (NHM 40103), there are few neurovascular foramina, which are positioned distant to the alveoli ( Fig. 1 ). In NHM 37020 almost all of the maxilla is exposed. The maxillae meet along the midline of the skull, preventing the nasal from contacting the premaxilla, whereas the tooth row remains straight along its length, so that the anteriormost and posteriormost teeth are not displaced from one another ventrally. This contrasts with D. maximus (SMNS 8203) and Dakosaurus andiniensis Vignaud & Gasparini 1996 (see Gasparini et al., 2006), where the last three maxillary alveoli (along with the maxillary posterior process) are displaced ventrally to the rest of the tooth row. At the contact between the premaxilla and maxilla in NHM 37020, there is a notch for the reception of an enlarged dentary tooth ( Fig. 4 ).

Nasals

In NHM R.1229 the left nasal was completely preserved, whereas the right nasal lacks the anteriormost tip, and the medial section is broken and distorted. Nasals are paired, unfused elements. They show a triangular outline in dorsal view, as in all thalattosuchians ( Andrews, 1913) and many notosuchians ( Andrade, 2005). Although the specimen is badly crushed and most of the rostrum is missing, it is possible to identify the extent of the nasals over the maxilla, and to recognize that the nasals could not have any contact with the premaxillae, as in almost all thalattosuchians ( Fig. 1 ). Furthermore, the nasals contact the maxillae, lacrimals, prefrontals, and frontal, also taking part in the antorbital fossae and fenestrae (a metriorhynchid apomorphy). In NHM 37020 the nasals are poorly preserved, but are consistent with features shown by the holotype.

Lacrimals

Both the left and right lacrimals are preserved in NHM R.1229. They face laterally, as in other metriorhynchids, with a concave and lightly ornamented surface. There are sutural contacts with the jugal, ventrally, and with the prefrontal, dorsally. At the contact with the prefrontal, the lacrimals form an anteroposteriorly oriented crest or ridge. This lacrimal–prefrontal crest is present on both sides of the specimen, but does not contribute to the antorbital fossae. Anteriorly, the lacrimal is isolated from the maxilla only by the antorbital fossa and the nasal, but not by a jugal–nasal contact ( Fig. 1 ).

Prefrontals

In NHM R.1229 the prefrontals are badly crushed, but present on both sides. On the right side, the prefrontal is partially covered by a periocular element in its posterior section. Prefrontals contact the lacrimal ventrally, the nasals anteriorly, and the frontal medially. Only a small distal piece of the right prefrontal can be identified in NHM R.1230. However, from this fragment it is possible to identify that the prefrontal was well-developed mediolaterally, covering the orbit ( Fig. 3 ), as in all metriorhynchids, except for Teleidosaurus . The mediolateral expansion of the left prefrontal is clear in NHM 37020, with the anteriodorsal third of the orbit overhung by the prefrontal.

SKULL TABLE AND PERIORBITAL STRUCTURES

Frontal

The frontal is well preserved. It appears as a single element, without any signs of the interfrontal suture. The surface is smooth and unornamented, as in Dakosaurus , Enaliosuchus , other Geosaurus , and some species of Metriorhynchus ; there is no indication of a frontal sagittal crest. Anteriorly, the frontal progresses in an acute angle between the nasals, extending beyond the orbits and the nasal–prefrontal contact, and reaching almost as far as the jugal. Posteriorly, the contact with the parietal is not preserved, but it is possible to identify that the element takes part in the supratemporal bar, and also in the supratemporal fossa. The frontal–postorbital contact is preserved on the right side of NHM R.1229 ( Fig. 1 ). A small section of frontal can also be witnessed in NHM R.1230, but the frontal is poorly preserved in NHM 37020.

Postorbitals

Only the right postorbital (at the contact with the frontal and the postorbital bar) can be identified in NHM R.1229. The postorbital–frontal suture of NHM R.1230 ( Fig. 3 ) has the characteristic posteriorly directed ‘V’-shape seen in other metriorhynchids (created by a scarf joint between the elements, with the frontal overlapping the postorbital). The postorbital descending ramus constitutes most of the postorbital bar, although it is not possible to identify clearly its contact with the jugal. The bar is subdermic, laterally flattened, and thin. It shows the same vertical disposition as other metriorhynchids ( Figs 1 , 4 ). In contrast, teleosaurids (e.g. Steneosaurus and Pelagosaurus ) have a dermic bar ( Andrews, 1913; Pierce & Benton, 2006), which is also found in sphenosuchians and protosuchians ( Clark, 1994). Ornamentation is present in the dorsal part of the postorbital, but is completely absent from the descending ramus. The right postorbital is partially preserved in NHM R.1230, but the posterior ramus is also missing. The left postorbital is partially preserved in NHM 37020, with the descending ramus missing, and the overall shape concurs with NHM R.1229.

Squamosals

Only in NHM 37020 is there a preserved squamosal (left) ( Fig. 4 ). It forms the posterior half of the supratemporal arch. The morphology is very similar to that of D. andiniensis ( Gasparini et al., 2006) . The suture with the prefrontal is indistinct, whereas the contact with the parietal is not preserved. In dorsal view, the squamosal is transversely wide, especially in comparison with the longirostrine geosaurs. The height of the squamosal is less than a third of its width, although the distortion of the cranium could exaggerate this.

Periorbital elements

Palpebrals are absent in G. giganteus , as in all thalattosuchians. Nevertheless, a sclerotic ring is clearly recognizable in NHM R.1229, with most of the preserved sclerotic ossicles preserved in their original position ( Fig. 1 ). On the right side there is a large anterodorsal ossicle, close to the contact with the prefrontal, and a smaller ossicle can be seen in the posterior section of the orbit, partially covering the postorbital bar. On the left side, the ring shows a better preservation, including five ossicles. Four of them are in place, and constitute the anterodorsal section of the ring. The fifth ossicle is positioned in the lower part of the orbit, and, because of the deformation of the skull, it lies over an anterior section of the nasopharyngeal duct ( Fig. 1 ). In all cases, the ossicles are flattened but thick elements, slightly variable in their morphology. The sclerotic ring is better preserved in NHM 37020 ( Fig. 4 ). Here, the ring is laid out in its original position, with only the ossicles along the ventral margin being out of place. Twelve ossicles can be clearly distinguished, all flattened, but just as robust as those of the holotype. The sclerotic ring occupies almost all of the orbit. The ventral ossicles partially cover the aperture for the eye itself, obscuring the exact size. The annular sulcus is absent from the sclero–corneal junction, just as with Metriorhynchus superciliosus (Blainville, 1853) (GLAHM V983, GLAHM V985, GLAHM V987, GLAHM V1015, GLAHM V1140, NHM R.2051, and NHM R.2058), G. suevicus (SMNS 9808), and Enaliosuchus schroederi Kühn, 1936 (MMGLV unnumbered). In D. andiniensis ( Gasparini et al., 2006) , the presence or absence of the annular sulcus is unknown. In nonmarine adapted species, the sclerotic ossicles are concave at the sclero–corneal junction, forming an annular sulcus that is fundamental to the process of accommodation ( Walls, 1942). However, like ichthyosaurs ( McGowan, 1972, 1973; Fernández et al., 2005), fish ( Helfman, Collette & Facey, 1997), and Magellanic penguins ( Suburo & Scolaro, 1990), metriorhynchids lack a sulcus, suggesting that the cornea had lost its role in focusing, a key adaptation to aquatic vision ( Walls, 1942). In G. giganteus , the sclerotic ring is reasonably heavy and robust (much more so than the relatively delicate sclerotic ossicles of M. superciliosus , and more robust than those of G. suevicus and D. andiniensis ), occupying most of the orbit, which would have offered good support for the eye, suggesting that this species was able to venture on deep dives, and/or was a faster swimmer than other metriorhynchids ( Motani, Rothschild & Wahl, 1999; Humphries & Ruxton, 2002). The sclerotic ring of E. schroederi is the largest and most robust of any metriorhynchid.

Jugal

Only the anterior ramus of both jugals could be identified in NHM R.1229, as the posterior ramus is not preserved on both sides, and the ascending ramus is indistinct on the right side and missing from the left side. The anterior ramus is slender, ventrally arched, and slightly compressed. As a result of lateral compression, the anterior process has a subcircular to elliptic cross section. However, this compression might be taphonomic, as the entire skull is flattened. In NHM 37020, both the anterior and posterior rami are preserved, whereas the dorsal ramus is not. The rostral extent of the jugal is unclear; however, the anterior ramus is slender, with an elliptic cross section. Once again, taphonomic compression cannot be discounted. The posterior ramus is more circular in cross section, and its suture with the quadratojugal cannot be determined. There is no neurovascular foramen on the surface of the jugal in any of the specimens examined.

PALATE AND VENTRAL STRUCTURES OF THE SKULL

Despite the strong deformation of the skull, a few elements of the palate can be distinguished in NHM R.1229 ( Fig. 1 ). The right palatine is partially preserved and exposed on the right side of the specimen, although it is displaced from its original position. It is slightly ornamented ventrally, although the lateral surface (inside the suborbital fossa) is absolutely smooth. The palatine is narrow throughout, extending anteriorly, and reaching at least the same relative position as the antorbital fenestra, and ending in a rounded border ( Fig. 1 ). The anterior extension of the palatines reaches beyond the orbits and the suborbital fenestrae, between the ventral rami of the maxillae. This feature is common to all neosuchians, basal mesoeucrocodylians, and a few notosuchians; although only in neosuchians the anterior extension remains narrow throughout. In Theriosuchus , Araripesuchus , Anatosuchus , and Uruguaysuchus , palatines broaden considerably anterior to the suborbital fenestra, taking part in its anterior border ( Clark & Norell, 1992; Andrade, Bertini & Pinheiro, 2006). In eusuchians, the anterior extension of the palatines only widens anterior to these fenestrae, but the palatines still do not take part in their anterior border ( Buffetaut, 1982a; Benton & Clark, 1988; Clark & Norell, 1992; Andrade et al., 2006). A few other elements of the palate were preserved on the left side of NHM R.1229. These can be seen inside the orbit, and are possibly interpreted as fragments of the dorsal part of the nasopharyngeal duct. The posteriormost elements are paired. These are quite possibly the only remaining parts of the pterygoids, suggesting that these elements were not fused, as in S. leedsi and Steneosaurus durobrivensis Andrews, 1909 , which show unsutured pterygoids ( Andrews, 1913; Buffetaut, 1982a).

It is impossible to verify the relationships between the palatine, ectopterygoid, and pterygoid, but it is more likely that in this species there was no palatine bar or palatine–ectopterygoid contact, as these features mostly occur in derived notosuchians ( Andrade et al., 2006; Andrade & Bertini, 2008).

FENESTRAE AND FOSSAE

These structures are poorly recognizable in NHM R.1229 and NHM 37020, whereas they are mostly absent in NHM R.1230 as a result of poor preservation. On both sides of the holotype it is possible to identify the completely preserved antorbital fossae. These are typically elliptic, and are obliquely orientated in metriorhynchids, but are deformed in NHM R.1229. They are shallow, and are much longer than high. In the bottom of each fossae, close to the contact of the maxilla, jugal, lacrimal, and nasal, there is a circular antorbital fenestra. On both sides, these fenestrae are difficult to identify, as they are filled with matrix ( Fig. 1 ) .

In addition to the antorbital fossa, another depressed area is present on the rostral section of G. giganteus . This structure, identifiable on both sides of the type, is considered here as a fossa. The depressed area is located between the antorbital fossa and the orbit, where the shallow fossa is limited within the concave surface of the lacrimal and prefrontal. The lacrimal–prefrontal fossa is limited dorsally by a lateral expansion of the prefrontal, and ventrally by the jugal ( Fig. 1 ). This fossa shallows anteriorly, closer to the nasal and the antorbital fossa, but without clear limits. The lacrimal–prefrontal fossae can also be identified in numerous metriorhynchids, such as G. araucanensis (MLP 72-IV-7-1), Metriorhynchus brachyrhynchus (Eudes-Deslongchamps, 1868) (NHM R.3939), and D. andiniensis ( Gasparini et al., 2006) , but are absent from basal metriorhynchids such as Teleidosaurus calvadosii (Eudes- Deslongchamps, 1866) (NHM R.2681) and Teleidosaurus gaudryi Collot, 1905 (NHM R.3353), and teleosaurids such as Pelagosaurus typus Bronn, 1841 (NHM 19735) and S. leedsi (NHM R.3320). A lacrimal–prefrontal ridge develops at the contact of these elements, almost dividing the lacrimal– prefrontal fossa into upper and lower fossae, which is well preserved in three-dimensionally complete specimens, as in G. araucanensis (MLP 72-IV-7-1) and M. brachyrhynchus (NHM R.3939). This ridge is not present in thalattosuchians without the lacrimal– prefrontal fossae.

The infratemporal (= laterotemporal) fenestra is well preserved in NHM 37020. In comparison with the orbit it is large (approximately three-quarters of the length of the orbit), far larger than that observed in the longirostrine geosaurs (e.g. in G. araucanensis MLP 72-IV-7-1, the fenestra is less than half of the orbit length). The shape is similar to that of D. andiniensis , having a tall triangular shape (with the height of the orbit and infratemporal fenestrae being subequal) with the posterior margin being the most sharply tapering ( Fig. 4 ).

Most of the remaining fenestrae have only poor preservation. However, the anteriormost border of the supratemporal fossa can be identified on the left side of NHM R.1229, on the posterior surface of the frontal, showing an acute medial anterior angle formed by the intertemporal flange. The right suborbital fenestra is partially preserved, in its medial border, formed by the palatine, and can be identified on the right side of NHM R.1229, just below the jugal ( Fig. 1 ). The suborbital fenestra is elongated, but apparently does not extend anterior to the orbit.

Other fenestrae are not preserved. Nevertheless, it can be stressed that: (1) maxillo–palatine fenestrae (= palatine fenestrae) are found only in notosuchids and Eutretauranosuchus ( Woodward, 1896; Buffetaut, 1982a; Vasconcellos & Carvalho, 2005; Andrade et al., 2006); (2) most thalattosuchians either lack or have a highly reduced naso-oral fenestra (= incisive foramen, foramen incisivum); and (3) metriorhynchids more derived than T. calvadosii lack an external mandibular fenestra ( Andrews, 1913; Mercier, 1933).

MANDIBLE

The preservation of the mandible is extremely poor in the holotype, with no remnants of the symphysis, coronoid process, articular and the retroarticular process on the holotype, whereas NHM R.1230 possesses no mandibular elements. Given the section of the mandible preserved in NHM R.1229, the symphysis would not have extended as far posteriorly as the antorbital fossa ( Fig. 1 ). The symphysis is preserved in NHM 37020, which confirms its short length ( Fig. 4 ). This means that the symphysis of G. giganteus is clearly shorter than in other metriorhynchids in which the cranium and mandible are well preserved. In Metriorhynchus cultridens Andrews, 1913 (NHM R.3804), the symphysis reaches the same relative position as the antorbital fenestra, and the same can be observed in G. araucanensis and other longirostrine forms.

A lateral mandibular groove is present on the surface of the dentary and surangular, on the right side of the type ( Fig. 1 ). It reaches the same relative position as the anterior border of the antorbital fenestra, on the dentary, and extends posteriorly as far as the posterior border of the orbit. In most other thalattosuchians bearing such a structure, the groove appears almost like a carving on the bony surface, as its limits are so abrupt and sharply defined. Although the groove can be easily recognized in the holotype, its limits are not sharply defined. In fact, the border seems to be smoother and rounder through the entire extension of the groove, like a simple depressed area in the bony surface. In the genus Dakosaurus , a large foramen can be observed at both ends of this groove (NHM 40103; SMNS 8203; Gasparini et al., 2006). Contra Gasparini et al., (2006), this shallow groove is present in well-preserved specimens of Steneosaurus and Metriorhynchus (e.g. NHM R.3804). Extant crocodilians exhibit a similar groove; however, it is both shorter and less deeply excavated. This groove is associated with an external cutaneous branch of the mandibular nerve (see Holliday & Witmer, 2007; S. Salisbury, pers. com. 2008, apud Buchy).

Dentary

Only small sections of the dentary are preserved on both sides of NHM R.1229 ( Fig. 1 ). It is flattened, and its sutures with the surangular and angular can be easily identified. The posterior end of the dentary– surangular contact is located at the same relative position as the anterior border of the orbit, as in most thalattosuchians. The development of the dentary, however, varies. In most crocodilians, including extant forms and Pelagosaurus , the dentary is longer, reaching or surpassing the same relative position as the posterior orbital border (postorbital bar). In all longirostrine species currently assigned to Geosaurus , the posterior end of the dentary is positioned far in advance of the anterior border of the orbit. The dentary of NHM 37020 is largely complete, concurring with the description given for the holotype. Here, the suture with the splenial can be observed along the ventral surface of the mandible (as a result of the distortion of the skull).

Splenials

Only a slit of the right splenial seems to be preserved in the holotype, which can be seen below the dentary on the right side of the type specimen. It extends from the anterior border of the antorbital fossa to the anterior border of the orbit. However, because of the state of preservation, this extension is likely to be biased, and probably represents only a minor part of the total length of the element. On NHM 37020, the splenial is much better preserved. As with other thalattosuchians, the splenial is well-developed rostrally, extending almost as far as the premaxilla, to the anterior margin of the orbit. In ventral view it significantly contributes to the symphysis ( Fig. 4 ) .

Angular and surangular

The anterior section of these elements can be seen in both sides of NHM R.1229. They are sutured alongside, by an almost horizontal contact. The anterior border of the surangular meets the dentary next to the same relative position as the anterior border of the orbit, just below the anteriormost elements of the sclerotic ring. The dentary–surangular suture is a diagonally-oriented truncated line, with the ventral border anterior relative to the dorsal border. As the medial side of the mandible is not exposed, it was not possible to verify the actual extension of the medial ramus of the surangular, but it is likely to be smaller than the lateral one. The angular is reasonably longer than the surangular (lateral ramus), and reaches at least the same relative position as the anterior border of the antorbital fossa. Although these elements are better preserved in NHM 37020, preservation at the jaw joint is poor, as it is posterior to this. There does not appear to be a deepening of the mandibular ramus along its length, in contrast with all other metriorhynchids. As seen in NHM R.1229 and NHM 37020, there is no external mandibular fenestra ( Figs 1 , 4 ).

Coronoid

On the right side of the holotype, the coronoid is positioned between the maxilla and the dentary, next to teeth and the palatine ( Fig. 1 ). It is thin and tall, as with other metriorhynchids, and the position suggests that it is displaced from its original location, in the medial face of the right hemimandible.

Hyoid

The right hyoid can be clearly seen on NHM 37020 ( Fig. 4 ). The element is long, with a subcircular cross section, and with a pronounced curvature dorsally .

DENTITION

Preservation

The dentition is preserved only in the holotype and NHM 37020 ( Figs 1 , 4 ). In both specimens, all crowns show extensive nontaphonomic lateromedial compression that is identical to that observed in M. brachyrhynchus and M. cultridens . On both sides of the holotype there are 14 preserved teeth: eight in the maxilla and six in the dentary. However, on the right side it is possible to see that the last (preserved) tooth is overlying another tooth, which was displaced from its original position, and has its root exposed ( Fig. 1 ). On the corresponding position on the left side, no element is preserved. This displaced tooth seen on the right side is considered here as being the missing tooth from the left side, possibly resulting from the extent of the deformation and compression that the skull has undergone. On the left side, the fourth upper tooth, with only the crown preserved, also seems displaced from its original position, and might have been part of the lower dentition. Although most teeth are complete, well exposed, and in their original position, a few are only represented by the crown apex, and by the impression left by the base of the crown on the remaining matrix. This can be seen on the first and the second preserved teeth on the left dentary, as well as the first preserved maxillary and dentary teeth from the right side. Considering the elements preserved, it is most likely that the maxillary dentition extended posteriorly up to the area below the orbit, whereas the dentary dentition extended only up to the same relative position as the contact between the dentary, surangular, and angular. Furthermore, we consider that the maxillary alveoli between the anteriormost border of the nasal and the antorbital fenestra probably held six teeth. In NHM 37020, 22 teeth are preserved: three premaxillary teeth, 12 in the maxilla, and seven in the dentary ( Fig. 4 ). As part of the rostrum is missing, and as matrix covers some dentary teeth, it is likely that both the maxillary and dentary tooth counts are underestimated. This specimen displays an enlarged dentary tooth opposite the premaxilla–maxilla suture, which is unique throughout the Metriorhynchidae .

Type of dentition and occlusion

The dentition can be considered as homodont, but not isodont, as the dentary tooth opposite the premaxilla– maxilla suture is greatly enlarged relative to the other teeth ( Fig. 4 ). Furthermore, the posteriormost elements of the maxilla, below the lacrimal and the orbit, are smaller than the remaining teeth. The teeth are not procumbent, but the implantation of teeth in the premaxilla of NHM 37020 does appear to be procumbent. However, this may be a preservational bias. On the right side of the holotype and NHM 37020, the teeth show better preservation ( Figs 1 , 4 ). During occlusion, each maxillary tooth should have occluded laterally with a dentary tooth, with the upper element in a more lateral and mesial position relative to the lower tooth, which is settled in a more posterior and medial position. Consequently, the distal border of the upper tooth and the mesial border of the lower one would be able to slide past each other during occlusion, acting as efficient scissors, contrasting with the common overbite pattern shown by most terrestrial or freshwater ziphodont and falseziphodont crocodilians, and also with the interlocking arrangement found in most extant crocodilians and thalattosuchians. There is, however, no evidence of wear facets on the crowns, indicating that this dentition was not used to process food. No other thalattosuchian presents a similar feeding mechanism, as G. araucanensis , Pelagosaurus , Steneosaurus , and longirostrine Metriorhynchus species had procumbent, slightly lateromedially compressed piercing teeth. Dakosaurus also had serrated teeth, but the morphology of the carinae is quite distinct, and the teeth, although lateromedially compressed, are robust, and not blade-like. A somewhat similar mechanism was precluded by Price (1950) for the terrestrial notosuchian Sphagesaurus . Here, the middle and posterior molariform teeth were also arranged as opposing blades, acting like scissors. However, the crowns have extensive wear surfaces ( Price, 1950; Pol, 2003).

Teeth morphology

Each tooth shows an overall caniniform morphology, and a nontaphonomic lateral compression can be recognized. Crowns are curved posteriorly, but not lingually. The crown is not curved throughout, but only at its base. Middle and apical sections of the crown are mostly straight, with a discrete bending anteriorly. In most well-preserved teeth, the surface has three basi-apical facets, which are more evident in the middle and basal sections of the crown ( Fig. 2 ). The first facet faces mesially and laterally, the second faces laterally, and the third faces laterally and distally. There is no evident limit between the facets, and the transition from one facet to the other is smooth. These facets can be observed through close examination of the second and fourth preserved maxillary crowns of the holotype ( Fig. 2 ), as well as the first and fourth dentary teeth, on the right side. On the left side, facets can be seen in the second and third maxillary and the second dentary preserved crowns. Other teeth are incomplete or crushed, preventing the observation of this feature. Most of the teeth on NHM 37020 display the characteristic tri-faceting, most especially the enlarged dentary tooth ( Fig. 4 ). The facets are only visible on the labial surface of the teeth, as the lingual surface is not exposed on any tooth. However, the isolated tooth NHM 37016, although still partially encased in matrix, lacks the distinctive tri-faceting on one side. Assuming this to be the lingual surface, it suggests the faceting was only on the labial surface, but confirmation must await the discovery of new specimens. Only C. grandis ( Wagner, 1858) presents similar facets on teeth crowns. Interestingly, the exceptional lateromedial compression and triangular blade-like shape of the teeth are identical to those observed in the Oxford Clay brevirostrine species M. brachyrhynchus and M. cultridens . However, whether or not the dentition of these specimens were arranged as opposing blades is unknown, as complete upper and lower dentition on the same specimen is lacking.

Ornamentation, carinae, and wear

Ornamentation is light, composed of microscopic ridges in an arranged an anastomosed pattern, creating a fabric of crests over the surface. Given the small size of these ridges, the overall appearance of the tooth surface is reasonably smooth. There are mesial and distal serrated carinae, with microscopic denticles. It was not possible to evaluate the teeth of the holotype with scanning electronic microscopy, thereby preventing the reliable indentification of these as true denticles (as in Dakosaurus and Sebecus ), or as mere extensions of the enamel ornamentation (= false-ziphodont dentition; Prasad & Broin, 2002), as in Pristichampsus . However, close examination of the dentition of G. grandis (= C. grandis ), which is very similar in cranial and dental morphology to G. giganteus , confirms the presence of true denticles.

Figures

Figure 1. Lateral views and line drawings of the type of Geosaurus giganteus (Von Sömmerring, 1816), showing the right (top) and the left (bottom) sides of the specimen (NHM R.1229). Line drawings show the interpretation of sutures and the main features of the specimen; the grey shading represents fossae. Scale bars: 20 mm.

Figure 2. Dental facets and their occurrence in Geosaurus giganteus. The close-up on the dentition of NHM R.1229 (left), and the corresponding line drawing (centre), shows the gross morphology of the teeth, with their distinctly triangular shape. On the right, a cross section of one tooth is shown, with the three labial facets indicated by arrow heads. On the lower right: an occlusal view of an isolated tooth of Geosaurus (SMNS 81834) displaying the facets and their limits, at the labial face of the crown. Scale bars: 20 mm.

Figure 3. The second specimen of Geosaurus giganteus, NHM R.1230, preserving remnants of the skull in dorsal (top left) and ventral (top right) views, as well as postcranial elements (bottom). The white arrows indicate the impressions of nonpreserved vertebrae on the matrix. Scale bar: 20 mm.

Figure 4. Lateral view of the third, and forgotten, specimen of Geosaurus giganteus: NHM 37020. A, lateral view of the entire cranium. B, close-up of the tooth row, with the arrows highlighting the enlarged dentary tooth and the notch for its reception, whereas the box shows the occlusion of the maxillary/dentary teeth. Scale bars = 20 mm.