Kryptops

Kryptops is a genus of theropod dinosaur from the Early Cretaceous of Niger. It is known from a partial maxilla (upper jaw bone) found at the Gadoufaoua locality in the western Ténéré Desert, in rocks of the Aptian–Albian-age Elrhaz Formation. The fossils were collected in 2000 by a University of Chicago expedition to Niger led by American paleontologist Paul Sereno. Following preparation and research, they were described in 2008 by Sereno and Steve Brusatte, and made the basis of a new genus and species Kryptops palaios. The genus name means 'covered face', in reference to evidence that the face bore a tightly adhering covering of keratin. The type and only known species is K. palaios, which means 'old'. Several postcranial remains were referred to the Kryptops by Sereno and Brusatte, but later studies have shown that these elements belong to a tetanuran theropod, leaving Kryptops to be only known from the incomplete maxilla.

Kryptops Temporal range: Early Cretaceous (Aptian to Albian), 115–110 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
The maxilla (upper jaw bone) of Kryptops.
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Family:	†Abelisauridae
Genus:	†Kryptops Sereno & Brusatte, 2008
Type species
†Kryptops palaios Sereno & Brusatte, 2008

This dinosaur has been classified as an abelisaurid, a family of medium-sized, ceratosaurian theropods that existed during the Cretaceous, and potentially Jurassic. Kryptops is one of the oldest known members of the family, making it vital to the understanding of abelisaurid evolution. Its length was estimated to be around 6–7 m (19.7–23.0 ft), making it smaller than later genera such as Carnotaurus. Kryptops was a large, carnivorous dinosaur which had highly serrated teeth, bearing many denticles, and robust upper jaws. Its maxilla's outer surface features deep striations, grooves, and rugosities, indicating that it was covered in keratin.

Discovery and naming

In 2000, an expedition conducted by the University of Chicago led by American paleontologist Paul Sereno and funded by the National Geographic Society explored fossiliferous sandstone outcrops in a site known as Gadoufaoua on the western edge of the Ténéré Desert, Niger.^[1][2] These layers belong to the Elrhaz Formation, which dates to the Aptian and Albian ages of the Early Cretaceous, around 112 million years ago. During the expedition, a maxilla (main tooth-bearing bone of the upper jaw) of a theropod was collected 15 m (49 ft) away from a set of postcranial elements consisting of three dorsal (back) vertebrae, two ribs, the sacrum, and the pelvis. These remains were then transported to the University of Chicago for study and preparation before being returned to the Musee National du Niger and deposited under the catalog number MNN GAD1. The maxilla received the catalog number MNN GAD1-1, whereas the postcranial fossils were assigned MNN GAD1-2 through MNN GAD1-8.^[3]

Skeletal diagram of Kryptops including the erroneously referred postcranial remains

These theropod remains were believed to belong to the same individual by Paul Sereno, who, with Jeffrey Wilson and Jack Conrad, later mentioned them in a 2004 paper as an undescribed abelisaurid.^[4] Sereno and Steve Brusatte described the fossils as a new genus and species of abelisaurid, named Kryptops palaios. The generic name derives to the Ancient Greek words Ancient Greek: krypto 'covered' and Ancient Greek: ops 'face', both in reference to the unique anatomy of the maxilla. The species name palaios is from the Greek term palaios "old" in reference to the age of the fossils. The maxilla, MNN GAD1-1, was made the holotype (name-bearing) specimen of the species, and MNN GAD1-2 through MNN GAD1-8 were assigned specimens.^[3][5]

Identity of postcranial material

Sereno and Brusatte assigned the postcranial material to the same individual as the maxilla based on their close association and alleged basal abelisaurid features in the vertebrae and pelvis. Kryptops is not the only theropod known from the Elrhaz Formation; other genera include the carcharodontosaurid Eocarcharia, the possible noasaurid Afromimus, and the spinosaurid Suchomimus.^[6] In 2012, Matthew Carrano and colleagues considered Kryptops palaios to be a chimera (specimen composed of multiple species), and state that its postcranial remains, especially a pelvis and sacrum, may actually belong to a carcharodontosaurid, possibly Eocarcharia.^[7] However, there is no overlap with the Eocarcharia holotype, which consists only of skull elements.^[8][3] This hypothesis has been supported by later studies,^[9][10] though researcher Andrea Cau, in 2024, suggested that the assigned postcranial material may instead belong to the family Metriacanthosauridae.^[11] The validity of Kryptops entirely came into question in 2018, when paleontologist Rafael Delcourt argued that due to the occurrence of only one valid autapomorphy (unique feature), Kryptops is a nomen dubium.^[12][13] This viewpoint has been supported by some other authors,^[13] though a detailed reanalysis of the genus has not been conducted.

Description

Size compared to a human

The holotype individual is large for an abelisaurid, belonging to an adult about 6–7 m (19.7–23.0 ft) in length. As other abelisaurids, Kryptops had a short, deep skull attached to a long, slender body. The maxilla has a preserved tooth row length of 15 cm (5.9 in) and is missing its tip, some of the alveolar margin (the upper edge of the maxilla that supports the tooth sockets), and tooth crowns. The left maxilla preserved 11 alveoli (tooth positions) but likely bore 17 or 18 alveoli in total when complete based on the related genus Rugops. The exterior face of the maxilla is extremely rugose, giving the genus its name. Pits and short vascular grooves adorn the surfaces, a condition similar to that of other abelisaurids and some carcharodontosaurids. This may imply a keratinous integument on the maxillae instead of scales, though the skull of Carnotaurus was scaly.^[14][15] The external texture of the maxilla, with its short linear grooves, is a diagnostic characteristic of Kryptops and distinguishes it from other abelisaurids according to Sereno and Brusatte (2008). However, Delcourt, in 2018, noted that this same condition is observable in Rugops and therefore not unique to Kryptops.^[12] There is a row of neurovascular foramina located above the alveolar margin, a characteristic of abelisauroids.^[3]

Life restoration

Its maxilla arches medially towards the articulation with the premaxilla, resulting in a broad, short skull as in its relatives. The front section of the maxilla is short and deep, even shorter than that of other abelisaurids.^[3] The proximal (part from towards the body) portion of the posterior ramus has sub-parallel dorsal (top) and ventral (bottom) margins, but these are scalloped rather than smooth, a trait unique to the genus and species.^[12] According to Delcourt, this is the only valid autapomorphy of Kryptops.^[12] Dorsal and ventral edges of the promaxillary fenestra are hidden in lateral view by the lateral wall of the antorbital fossa, similar to Majungasaurus, Abelisaurus, and other abelisaurids. The interdental plates are fused, textured with striations similar to the anatomy interdental plates of other abelisaurids. The medial (part facing towards the inside of the mouth), inner surface of the maxilla is broken, which exposes the replacement teeth. There are several complete teeth preserved within the maxilla and exposed along the tooth row. Their crowns are relatively flat and wide, as in other abelisaurids, and have about 15 serrations for every 5 mm (0.20 in). This is comparable to the serration counts of indeterminate abelisaurids from Morocco and Egypt but greater than that of Rugops.^[3]

Classification

The phylogenetic position of Kryptops has been unstable, as the genus is only known from a maxilla, though Kryptops is generally regarded as a basal abelisaurid. Abelisauridae is a group of theropod dinosaurs that existed during the Cretaceous, though potentially Jurassic,^[16][17] up until the Cretaceous-Paleogene extinction event.^[18] Abelisaurids have been recovered as the sister family to the noasaurids within the superfamily Abelisauroidea.^[19] In their 2008 phylogenetic analysis of the relationships of Kryptops, Sereno and Brusatte recovered Kryptops as the most basal genus of Abelisauridae at a similar grade to Rugops and Rajasaurus, far more basal than Majungasaurus and Carnotaurus.^[3] This phylogenetic analysis assumed that the postcranial remains belong to Kryptops, skewing the results due to the presence of tetanuran characters. Later phylogenetic analyses excluded Kryptops due to the lack of material and its instability in phylogenetic analyses,^[12][13] though it has been included in a few published cladograms.^[20][21] When the postcranial material and holotype maxilla are scored separately, the postcranium is found isolated from Ceratosauria and often with Tetanurae, whereas the Kryptops is classified as a basal abelisaurid.^[22] Below is the phylogenetic analysis conducted by paleontologist Leonardo Filippi and colleagues during their description of Viavenator in 2016:^[20]

Abelisauridae	Kryptops Rugops Brachyrostra Xenotarsosaurus Dahalokely Rahiolisaurus Ilokelesia Skorpiovenator Ekrixinatosaurus Furileusauria Majungasaurinae

Kryptops and Rugops are some of the earliest-known abelisaurids. This makes Kryptops critical to understanding the evolution and diversification of Abelisauridae. Sereno and Brusatte noted that the maxilla bore a mix of characteristics seen in other basal abelisaurids as well as derived members from the Late Cretaceous.^[3]

Paleoenvironment

Location of Gadoufaoua in Niger

Kryptops is known from the Elrhaz Formation of the Tegama Group in an area of the Ténéré Desert called Gadoufaoua, located in Niger. It is rare, with only one specimen definitively known. The Elrhaz Formation consists mainly of fluvial sandstones with low relief, much of which is obscured by sand dunes.^[23] The sediments are coarse- to medium-grained, with almost no fine-grained horizons.^[24] Kryptops lived in what is now Niger about 120 to 112 million years ago, during the Aptian and Albian ages of the mid-Cretaceous.^[3] It likely lived in habitats dominated by inland floodplains (a riparian zone).^[24]

Kryptops lived alongside the other theropods Suchomimus, Eocarcharia, and Afromimus. Several megaherbivores like hadrosauriformes Ouranosaurus and Lurdusaurus, dryosaurid Elrhazosaurus, and two sauropods, the rebbachisaurid Nigersaurus and an unnamed titanosaur, have been unearthed from Gadoufaoua. Together, these compose one of the few associations of megaherbivores with a balance of sauropods and large ornithopods. Crocodylomorphs like Sarcosuchus, Anatosuchus, Araripesuchus, and Stolokrosuchus also lived there. In addition, remains of a pterosaur, chelonians, bony fish, a hybodont shark, and freshwater bivalves have been found. The aquatic fauna consists entirely of freshwater inhabitants.^[3][25][24]