Kryptops palaios, Sereno & Brusatte, 2008

Sereno, Paul C. & Brusatte, Stephen L., 2008, Basal abelisaurid and carcharodontosaurid theropods from the Lower Cretaceous Elrhaz Formation of Niger, Acta Palaeontologica Polonica 53 (1), pp. 15-46 : 17-25

Kryptops palaios sp. nov.

Figs. 1A , 2, 3, 4A , 5–7, Table 2.

Derivation of the name: From Greek palaios , old; in reference to its Early Cretaceous age.

Holotype: MNN GAD1 , partial skeleton including a left maxilla ( MNN GAD1−1 ; Figs. 1B , 2, 3, 4A , 5), several partial vertebrae and ribs ( MNN GAD1−3 to GAD1−8 ; Figs. 1B , 6 ), and an articulated pelvic girdle and sacrum ( MNN GAD1−2 ; Fig. 7 ).

Type locality: “Gadoufaoua” on the western edge of the Ténéré Desert ( Fig. 1A ), coordinates N 16 ° 26’, E 9 ° 7’.

Type horizon: Elrhaz Formation (Aptian–Albian, ca. 112 Ma).

Diagnosis.—Abelisaurid theropod characterized by the following two autapomorphies: (1) a deep secondary wall in the anteroventral corner of the antorbital fossa that completely

Maxilla

Length of preserved tooth row 15.0 Depth at level of sixth alveolus 6.8 Length of base of posterodorsal ramus 9.5 Sacrum

Sacral 4, centrum length 11.0 Sacral 4, centrum width 4.8 Sacral 5, maximum depth of vertebra 44.7 Sacral 5, centrum length 11.0 Sacral 5, centrum depth 9.4 Sacral 5, centrum width 9.8 Acetabulum

Acetabulum, maximum anteroposterior length 18.3 Acetabulum, maximum dorsoventral length 16.4 Ilium

Blade, length 65.0 Blade, depth above acetabulum 29.4 Preacetabular process, maximum length 17.0 Preacetabular process, maximum depth (36.5) Postacetabular process, maximum length 20.8 Postacetabular process, maximum depth 21.5 Pubic peduncle, maximum length 16.2 Ischial peduncle, maximum length 4.0 Ischium

Shaft, maximum length from acetabulum (58.0) Shaft at mid length, maximum dorsoventral diameter 2.5 Shaft at mid length, maximum transverse diameter 2.5 Pubis

Maximum length from acetabulum (62.0) Boot, maximum length (28.0) Boot, width near posterior end 3.0

obscures the antorbital fossa and that has a scalloped and fluted dorsal margin and (2) external texture on the maxilla, which is composed of short linear grooves.

It differs most obviously from other abelisaurids and nearly all other theropods in the marked development of a secondary wall on the maxilla that completely obscures the antorbital fenestra in lateral view ( Fig. 2). In addition, the derived abelisaurid articular trough for the nasal on the maxilla is narrower and less developed in K. palaios than in other abelisaurids ( Fig. 4 ), a primitive condition. Finally, the texturing of the external surface of the maxilla is composed of shorter grooves than typical of similar ornamentation on other abelisaurids. The sacrum and ilium are also more primitive than in Majungasaurus and Carnotaurus ( Bonaparte et al. 1990; Carrano 2007); the sacrum is composed of only five vertebrae, and the ilium has a relatively deeper preacetabular process.

Description

Maxilla.—The left maxilla is missing the distal portion of the posterior ramus and some of the alveolar margin and crown tips ( Figs. 2, 3). The preserved portion of the tooth row is 15 cm long and contains 11 alveoli. Compared to the similar−sized maxilla of Rugops primus (MNN IGU1), there are probably six to seven posterior alveoli that are missing for a similar total of 17 or 18 maxillary teeth. In medial view, most of the medial lamina that encloses the maxillary antrum is broken away along with the distal portion of the anteromedial process ( Fig. 3).

In lateral view the external surface of the maxilla is rugose and textured with small pits and short vascular grooves that course in several directions ( Fig. 2). This ornamentation is similar to that in other abelisaurids and some carcharodontosaurids ( Sereno et al. 1996; Sampson et al. 1998; Sereno et al. 2004; Sampson and Krause 2007) and may indicate that much of the face that was underlain by bone had more of a keratinous, than scaled, integument ( Goodwin et al. 2006). The grooves in Kryptops are relatively short compared to those in Rugops ( Fig. 4 ). A larger ventral row of neurovascular foramina, a few of which are preserved ( Fig. 2), are located immediately above the alveolar margin, an abelisauroid synapomorphy ( Wilson et al. 2003; Sereno et al. 2004). In carcharodontosaurids and most other theropods, this row of foramina is separated farther from the ventral alveolar edge, the intervening margin of which is usually smooth. This suggests that the fleshy edge or labial scales at the margin of the mouth was narrower in abelisaurids than in most other theropods.

The maxilla arches medially toward the premaxillary articulation, which is beveled at about 45 ° and fully exposed in medial view ( Fig. 3). In most theropods including carcharodontosaurids, the premaxillary articulation faces more anteriorly than medially (e.g., Allosaurus ; Madsen 1976). The inward curve of the maxilla and beveled premaxillary articulation suggest that the snout in Kryptops was quite broad, one of the unusual structural features of the abelisaurid cranium ( Bonaparte et al. 1990; Sampson et al. 1998; Sampson and Witmer 2007). The articular surface is rugose and dorsally may preserve portions of pneumatic diverticulae, as occur in several other abelisaurids ( Wilson et al. 2003).

The anterior ramus is particularly short with a length to depth ratio of about 0.33. The ramus is also shorter in length than depth in other abelisaurids, Allosaurus and carcharodontosaurids, in contrast to many basal tetanurans (e.g., Torvosaurus , Afrovenator ; Britt 1991; Sereno et al. 1994). The posterodorsal ramus is particularly short and narrow in lateral view ( Fig. 2). The principal reason for its narrow proportions is the very narrow lamina bordering the antorbital fossa. In most other theropods, including other abelisaurids, the antorbital fossa forms a broad band along the trailing edge of the posterodorsal ramus.

The well preserved nasal articulation is exposed in lateral view, a derived condition shared with other abelisaurids ( Sereno et al. 2004). In Kryptops , the articulation is developed as a narrow slot with a tapered ventral end ( Fig. 4A ). Other abelisaurids show a more derived condition, in which the slot broadens in width distally and terminates in a concave socket as in Rugops ( Fig. 4B ).

50 mm 50 mm

The proximal portion of the posterior ramus of the maxilla has subparallel dorsal and ventral margins as in other abelisaurids and the carcharodontosaurid Giganotosaurus ( Coria and Salgado 1995) . The unusual feature in Kryptops is that the dorsal margin is scalloped rather than smooth. The raised and fluted margin forms a secondary lateral wall enclosing the antorbital fossa ( Figs. 2, 3). The absence of the posterior portion of the ramus precludes determining if Kryptops also had the derived, laterally−facing jugal articulation as in other abelisaurids ( Wilson et al. 2003; Calvo et al. 2004; Sereno et al. 2004).

The openings into the antorbital sinus system are incomplete, because much of the medial lamina is broken away. Dorsal and ventral margins of a transversely narrow oval fenestra, nevertheless, are discernable opening anteriorly into the maxillary antrum. This fenestra is hidden in lateral view by the secondary wall of the antorbital fossa ( Fig. 3). A very similar configuration is present in the more completely preserved maxillae of Rugops , Ekrixinatosaurus , Abelisaurus , Majungasaurus , and Carnotaurus ( Bonaparte and Novas 1985; Bonaparte et al. 1990; Calvo et al. 2004; Tykoski and Rowe 2004; Sampson and Witmer 2007). Given its location, shape and direction, this opening has been identified as the promaxillary fenestra ( Witmer 1997). There is no trace of any other external fenestrae in this region of the antorbital fossa, nor is there any available fossa margin for a maxillary fenestra in the more common posterolateral location. The aforementioned abelisaurids also lack a maxillary fenestra.

In medial view, the deep interdental plates are fused and textured with subtle striations coursing in different directions as in other abelisaurids ( Rauhut 2004; Novas et al. 2004; Sampson and Witmer 2007). These striations appear to shift from a predominantly subvertical orientation anteriorly to one angled at about 45 ° in the middle of the tooth row. The groove for the dental lamina is invaginated and associated with a row of replacement foramina ( Fig. 3). Some breakage of the medial wall shows that the entire body of the maxilla is packed with replacement teeth. A strong maxillary shelf projects medially, its posterior end located just above a marked attachment scar for the palatine ( Fig. 3). Dorsal to this ridge, the antorbital fossa is well exposed, walled laterally by the secondary crest. The medial shelf continues anteriorly to join the posteromedial margin of the maxillary antrum, which is fully exposed due to the loss of its medial wall.

In ventral view, portions of 11 eroded alveoli are visible. As is characteristic of abelisaurids, these are subrectangular rather than elliptical, as in noasaurids and most other theropods ( Carrano et al. 2002; Wilson et al. 2003; Sereno et al. 2004; Sampson and Witmer 2007). The roots of the teeth reflect this alveolar shape and are subrectangular in cross−section.

Although all fully erupted maxillary teeth are broken, several complete teeth are preserved within the alveoli. We exposed two replacement teeth within the eighth alveolus, the crowns of which are exposed in medial view ( Fig. 5). As mentioned above, there were likely 17 or 18 teeth in a complete maxillary series, so these crowns are located at mid length along the tooth row. The crowns are relatively flat, such that the serrations of both mesial and distal carinae are visible in lateral view ( Fig. 5). Broken crowns have an average basal length of 10 mm and basal width of 6 mm, resulting in a length−to−width ratio similar to that in other abelisaurid teeth ( Chatterjee and Rudra 1996; Lamanna et al. 2002; Bittencourt and Kellner 2002; Smith and Dodson 2003).

The posterior margin, which is only slightly concave, has more prominent serrations that are separated by noticeable interserrational sulci ( Fig. 5B, C). Each wedge−shaped serration appears to expand toward its straight outer edge. The distal corner of the serration is prominent, forming a short hooklike projection, which points toward the apex of the tooth. Hooked serrations of similar form are present in Rugops . At mid length along the crown in Kryptops , there are about 15 serrations every 5 mm. This serration count is similar to that in teeth from poorly known Moroccan and Egyptian abelisaurids ( Mahler 2005; Smith and Lamanna 2006), whereas Rugops , the younger abelisaurid from Niger, has only about 10 serrations every 5 mm.

The body of the maxilla is packed with replacement teeth, three to a column as seen in the eighth alveolus; very small replacement crowns are present near the root of near−full size replacement crowns in the sixth and eighth alveoli. As preserved it is not possible to discern a particular replacement pattern for the tooth row.

Axial skeleton.—The axial skeleton is represented by one fragmentary anterior dorsal vertebra (MNN GAD1−3), two partial mid dorsal vertebrae (MNN GAD1−4, 5), an articulated sacrum (MNN GAD1−2), and two ribs (MNN GAD1−6, 7; Figs. 1B , 6 ). Only the sacrum and ribs are complete; the dorsal vertebrae preserve only a portion of the centrum and lack transverse processes and complete zygapophyses. Enough of these vertebrae are preserved, nevertheless, to demonstrate the less modified condition of the axial column compared to later abelisaurids.

The spool−shaped anterior dorsal centrum is proportionately short. Its anteroposterior length of approximately 7 cm is less than the height or width of the posterior centrum face (9 cm, 11 cm, respectively). An oval pleurocoel is centrally located on the side of the centrum below the neurocentral suture, its exact shape and internal passages obscured by erosion. The vertical neural spine is relatively narrow and tall, its width (6 cm) less than one−third its preserved height (18 cm). A rugose ligament process projects from each side fore and aft, and a spinodiapophyseal lamina extends as a web of bone from mid height on the lateral aspect of the spine to the base of each transverse process. The taller proportions of the centrum and neural spine differ substantially from the squat, low−spined anterior dorsal vertebrae of Carnotaurus and Majungasaurus , which also do not have noticeable development of a spinodiapophyseal lamina ( Bonaparte et al. 1990; O’Connor 2007).

Two vertebrae are identified as mid dorsals, based on their relatively large size, presence of a hyposphene−hypantrum articulation, absence of a parapophysis on either the centrum or ventral portion of the neural arch, and absence of a strong ventral keel and chevron facets ( Fig. 6E, F ). The centrum is hollowed, although it is not possible to determine if a pleurocoel was present. The anterior centrum face is gently concave ( Fig. 6E, F ). The associated neural spine, like that of the anterior dorsal, is anteroposteriorly narrow and tall, measuring 8 cm and 24 cm, respectively ( Fig. 6A–D ). Both spines curve away from the midline ( Fig. 6C, D ), reminiscent of natural spinal variation present in the taller−spined Acrocanthosaurus ( Harris 1998) , Suchomimus ( Sereno et al. 1998) , and Ceratosaurus ( Madsen and Welles 2000) . Unlike Acrocanthosaurus , the ligament processes and edges of the spine are not invaded by pneumatic diverticulae. The bases of robust prezygapophyses are preserved that seem to indicate the presence of hypantral articular surfaces medially. Several other ceratosaurs such as Ceratosaurus , Spinostropheus , and Carnotaurus have a pneumatic fossa below each prezygapophysis, but there is no development of such a depression in dorsal vertebrae in Kryptops . The relatively large size of the neural canal and prezygapophyses and tall proportions of the neural spine differ strongly from that in Carnotaurus and Majungasaurus ; Kryptops had much taller erect neural spines along the dorsal series.

A complete sacrum, composed of a coossified series of five vertebrae, narrows in width and disappears between the blades of opposing ilia ( Fig. 7 ). The reduction in the width of the central portion of the series also characterizes Carnotaurus and several other ceratosaurs (e.g., Ceratosaurus, Gilmore 1920 ; O’Connor 2007). The ventral margin of the sacral series may also be slightly arched, because the middle sacrals are not visible through the acetabulum. This margin, however, is not nearly as arched as in Carnotaurus ( Bonaparte et al. 1990) . Sacral 5, the best exposed of the series, has a spool−shaped centrum 11 cm in length with a nearly circular posterior articular face (10.5 cm wide, 9.5 cm deep). Although the junction between sacrals 4 and 5 is distinct, the centra appear to be coossified, in contrast to the free posteriormost sacral articulation in the Indian abelisaurid Rajasaurus ( Wilson et al. 2003) . A small pleurocoel may have been present in sacral 5, but the side of the centrum is poorly preserved. A low median crest marks the ventral side of the centra of sacrals 4 and 5. Given the degree of coossification present in the sacral series, it is unlikely that there were any further sacral vertebrae. In Carnotaurus , in contrast, a dorsosacral is incorporated into the sacrum ( Bonaparte et al. 1990; O’Connor 2007).

The sacral neural spines, like those in the dorsal series, are tall. In the sacral series, however, they are coossified into a single unit. The smooth, rounded borders of a large D−shaped fenestra separate a section of the neural spines of sacrals 4 and 5 ( Fig. 7 ). Pneumatic foramina open into the neural spines along the anterior and posterior margins of the fenestra, which may have housed a paramedian pneumatic diverticulum. Pneumaticity of the neural arches in general and of the sacral series in particular is common among abelisauroids, such as Masiakasaurus ( Carrano et al. 2002) , Carnotaurus ( Bonaparte et al. 1990; Bonaparte 1991; Tykoski and Rowe 2004), and Majungasaurus ( Sampson et al. 1998; O’Connor 2007). The postzygapophyses of sacral 5 have a well developed hyposphene, stabilizing the articulation with the first caudal vertebra.

The preserved ribs are similar in form and compare most closely to the third dorsal rib in Allosaurus ( Madsen 1976) . They are slender, solid and lack any pneumatic invasion. Their length is between 50 and 60 cm. A web of bone bridges the gap between the capitulum and head and would have approached the ventral edge of the transverse process.

An articulated pelvic girdle is preserved, the more complete right side of which was facing downward ( Fig. 7 ; Table 2). Pelvic remains are poorly known for most abelisauroids. The pelvic girdle and sacrum were preserved as a unit most likely because the bones of the pelvic girdle are coossified, although sutural traces remain between the ilium and pubis. Coossification of the pelvic girdle is common at maturity among coelophysoids and ceratosaurs. Both peduncles of the free ilium of Majungatholus have well developed articular pegs for a secure, and potentially fused, attachment to the ischium and pubis ( Carrano 2007). The pelvic girdle of an unidentified abelisaurid from Argentina shows fusion of both iliopubic and puboischiadic articulations (Coria et al. 2006); probably the articulations of the pelvic girdle in abelisaurids coossify with maturity.

Ilium.—The ilium is strikingly primitive in shape compared to that in the more derived abelisaurids Ekrixinatosaurus , Majungasaurus , and Carnotaurus ( Bonaparte et al. 1990; Calvo et al. 2004; Carrano 2007). The preacetabular process is more than twice as deep as the postacetabular process in lateral view ( Fig. 7 ), the anterior margin of the preacetabular process is nearly vertical, the posterior margin of the postacetabular process is subrectangular or convex, the supraacetabular crest and the prominent lateral margin of the brevis shelf are not joined as a unified shelf overhanging the ischial peduncle, and the pubic peduncle is massive and significantly longer than the ischial peduncle ( Fig. 7 ). In more derived abelisauroids, the preacetabular process is only moderately deeper than the postacetabular process, the anterior margin of the preacetabular process is angled posteroventrally at about 45 ° from the more prominent anterodorsal corner, the posterior margin of the postacetabular process is concave, the supraacetabular crest and lateral margin of the postacetabular process join to form a single prominent ridge, and the pubic peduncle is extremely short with a distal margin that is near vertical in orientation (Coria et al. 2006; Carrano 2007).

A robust supraacetabular crest overhangs the nearly circular acetabulum. The rim probably would have obscured more of the acetabulum in lateral view were it not for some dorsal crushing of the pelvic girdle that has displaced the right side dorsal to the left ( Fig. 7 ). The pubic peduncle is massive with a broad acetabular margin visible in lateral view and near horizontal distal margin. Its anterior margin does not show any development of a fossa ventral to the preacetabular process (cuppedicus fossa), as occurs in allosauroids and most tetanurans ( Hutchinson 2001). The ischial peduncle, which is completely fused with the ischium, is separated from the remainder of the ilium by a notch. The brevis fossa is transversely broad but does not flare distally as occurs in coelophysoids (Rauhut 2003). Much of the fossa is exposed in lateral view, which may have been enhanced somewhat by upward displacement of the right ilium. Lateral exposure of the brevis fossa seems to vary among abelisauroids.

Pubis.—In lateral view the pubis has a shaft that is vertical in orientation and gently concave anteriorly. In cross−section of the shaft, the anterior surface is flat and a posterior fossa is present throughout most of its length. A substantial distal foot is present, although not as well developed as in allosauroids ( Fig. 7 ). The shaft and foot are straighter and relatively smaller, respectively, in Carnotaurus ( Bonaparte et al. 1990) , Aucasaurus (Coria et al. 2002) , Pycnonemosaurus ( Kellner and Campos 2002) , Masiakasaurus ( Carrano et al. 2002) , and an unnamed abelisaurid from India ( Chatterjee and Rudra 1996). The pubis in noasaurids also has a more limited distal expansion ( Masiakasaurus , unnamed Niger noasaurid; Carrano et al. 2002; Sereno et al. 2004). In Kryptops the foot is expanded equally anteriorly and posteriorly in lateral view, and is transversely broader anteriorly than posteriorly in ventral view. In the region of the foot, the symphysis in anterior view appears continuous with no median fenestra. A foramen is present, in contrast, between the pubes in distal view ( Fig. 7B ).

The anterior border of a large obturator foramen is preserved, which unlike the condition in tetanurans was probably completely enclosed by bone as in Carnotaurus and an unnamed Argentine abelisaurid ( Bonaparte et al. 1990; Coria et al. 2006). The bone tapers posteriorly to a thin lamina as it extends toward the ischium. On the pubic shaft nearby is a raised area, the ambiens process, which likely represents the attachment area for a muscle by that name ( Romer 1923; Hutchinson 2001).

Ischium.—The iliac peduncle of the ischium is coossified with the ilium ( Fig. 7 ). The broader pubic peduncle thins to a plate ventrally where it meets its opposite in the midline. Although some of this ventral border is broken away, there is no indication that there existed a discrete obturator process that characterizes many tetanurans (e.g., Allosaurus , Sinraptor ; Madsen 1976; Currie and Zhao 1993). The ischial border of the acetabulum is divided into a dorsal portion that forms a raised, rounded articular rim and a ventral portion that is non−articular. The articular rim is subtle and is not developed as a raised platform as in Allosaurus or prominent trochanter as in coelophysoids ( Madsen 1976; Raath 1977; Tykoski and Rowe 2004; Munter and Clark 2006). An attachment scar with a nearby foramen is present on the posterior margin of the base of the ischium.

A prominent crescent−shaped flange is present on the ischial shaft at mid length on the left side ( Fig. 7 ). The right ischial shaft is broken at mid length with the upper end twisted posteriorly. The natural ventral curve of the ischial shaft is preserved on the left side. The shafts broaden toward their distal ends to about twice their mid shaft width and terminate in a modest foot with a flat, partially coossified symphysis.

Maturity and body size.— The maturity of the holotype and only known specimen of Kryptops palaios is indicated by the coossification of all neural arches and respective centra, sacral centra, and bones of the pelvic girdle. The maxilla and postcranial bones of Kryptops palaios have an absolute size comparable to those of Majungasaurus (Sampson and Krause Maxilla

Maximum length 52.8 Maximum depth 25.0 Anterior extremity to antorbital fenestra, length 19.8 Antorbital fossa margin, anterior end, depth from fenestra 6.2 Antorbital fossa margin, posterior end, depth 2.6 Frontal

Maximum length 10.2 Maximum width 7.1 Prefrontal

Maximum length in dorsal view 5.5 Maximum width 3.8 Maximum Depth 3.0 Postorbital

Maximum depth 16.3 Brow rugosity, maximum length 6.2 Anterior process, length 6.8 Ventral process, length 11.4 Posterior process, length 2.0

2007), suggesting a comparable body length of roughly 6 to 7 meters. The best known abelisaurids appear to have proportionately short skulls, with skull/femur ratios less than 1.00 as estimated in Majungatholus (0.88; Krause et al. 2007: fig. 1) and Carnotaurus (0.58; Calvo et al. 2004). Calvo et al. (2004) calculated a higher ratio for Ekrixinatosaurus (1.08), but this was based on more fragmentary remains. Skull/femur ratios for allosauroids are generally greater than 1.00 (e.g., 1.20 for Acrocanthosaurus ; Currie and Carpenter 2000), although particular taxa have relatively smaller skulls such as Allosaurus (0.76–1.00; Currie and Carpenter 2000). In Majungatholus and Carnotaurus , maxilla length is close to 50% skull length ( Bonaparte et al. 1990; Sampson and Witmer 2007). The maxilla in Kryptops palaios is estimated to be about 25 cm in length, from which we infer an approximate skull length of skull of 50 cm. Judging from the length of the pubis (approximately 62 cm), femur length in Kryptops would have been at least 65 cm, which generates an estimated skull/femur ratio of 0.77. Skull length in Kryptops palaios thus was likely significantly shorter than femur length as in better known abelisaurids.

Figures

Fig. 1. A. The region of central Niger known as Gadoufaoua (black) and the sites (white dots) where the holotypic specimens of Kryptops palaios gen. et sp. nov. (site G88) and Eocarcharia dinops gen. et sp. nov. (site G138) were discovered. B. Site map for Kryptops palaios (MNN GAD1) gen. et sp. nov. with cranial elements shown at twice natural size. The maxilla, located 15 m from the other bones, was not in place and probably was originally closer to the other bones.

Fig. 4. Abelisaurid theropods from the Cretaceous of Niger. Comparison of posterodorsal ramus of the left maxilla in lateral view. A. Kryptops palaios gen. et sp. nov. (MNN GAD1−1) from the Lower Cretaceous Elrhaz Formation. B. Rugops primus (MNN IGU1) from the Upper Cretaceous Echkar Formation.

Fig. 6. Abelisaurid theropod Kryptops palaios gen. et sp. nov. MNN GAD1−5 from the Lower Cretaceous Elrhaz Formation of Niger. Mid dorsal vertebra, spine in left lateral (A, B) and posterior (C, D) views, and centrum in anterior view (E, F). Photographs (A, C, E) and line drawings (B, D, F). Cross−hatching indicates broken bone.

Fig. 7. Abelisaurid theropod Kryptops palaios gen. et sp. nov. MNN GAD1−2 from the Lower Cretaceous Elrhaz Formation of Niger. Pelvic girdle in right lateral view; photograph (A) and line drawing with two cross−sections and ventral view of the pubic foot (B). Cross−hatching indicates broken bone; dashed lines indicate estimated edge; grey tone on specimen indicates matrix.