Deltadromeus (meaning "river delta or change runner") is an extinct genus of controversial theropod dinosaur that lived in present-day Morocco during the mid-Cretaceous period. It was described by American paleontologist Paul Sereno and colleagues in 1996. The genus contains a single species, D. agilis, named based on an incomplete postcranial skeleton, the holotype specimen. However, some fossils from the Bahariya Formation of Egypt that were formerly referred to the theropod Bahariasaurus have been suggested to belong to Deltadromeus. The holotype specimen of Deltadromeus was unearthed by a joint expedition by the University of Chicago and the Service Géologique du Maroc in Errachidia Province, eastern Morocco in rock layers coming from the Gara Sbaa Formation of the Kem Kem Beds. This indicates that these fossils date to the Cenomanian stage of the Cretaceous period, 100-95 million years ago.
Deltadromeus was around in length and in weight based on the holotype. If the Egyptian remains do belong to Deltadromeus, this would make it among the longest known theropod genera at around , comparable to Tyrannosaurus. The forelimbs of Deltadromeus are relatively long for a theropod, being proportionately longer than those of taxa like Allosaurus. Deltadromeus also preserves gracile, slender, and relatively long hindlimbs, indicating it was cursorial. Its tail vertebrae have broad, quadrangular (spines emerging from the base of the vertebra) with (cavities) which would store pneumatic air sacs.
The classification of Deltadromeus has been in flux since its original description. Originally Deltadromeus was considered to be a basal coelurosaur related to Ornitholestes; however, later studies have challenged this idea. Some studies have classified it as an avetheropod closely related to Gualicho or as a megaraptoran outside the family Megaraptoridae. Other studies have proposed that Deltadromeus is a ceratosaur; however, its position within Ceratosauria is debated. Studies have classified it as a basal ceratosaur, noasaurid, or an Elaphrosaurus-grade ceratosaur. Many studies find that it is a ceratosaur of some kind, supported by several phylogenetic analyses. Additionally, some studies have suggested that it may be a junior synonym of Bahariasaurus, another potential ceratosaur which lived around the same time and region as Deltadromeus.
Many gigantic theropods are known from North Africa during this period, including Deltadromeus and Bahariasaurus as well as the spinosaurid Spinosaurus, the carcharodontosaurids Carcharodontosaurus and Tameryraptor, and several unnamed abelisaurids. If Deltadromeus is a ceratosaur closely related to the herbivorous/omnivorous Limusaurus and Berthasaura, it possibly would be herbivorous or omnivorous. This would indicate more niche partitioning in theropods in Cretaceous North Africa and suggest that Deltadromeus filled a role traditionally filled by ornithischians. North Africa at the time was blanketed in mangrove forests and wetlands, creating a hotspot of fish, crocodyliforms, and pterosaur diversity.
Discovery and naming
Fossils definitively assigned to Deltadromeus were first discovered in 1996 by American paleontologist Paul Sereno, who was part of a joint expedition by the University of Chicago and the Service Géologique du Maroc in an outcrop of the Kem Kem Beds known as Aferdou N'Chaft in Errachidia Province, eastern Morocco. This expedition was one of many that explored dinosaur-bearing outcrops in North Africa throughout the 1990s and 2000s. The remains found consisted of an incomplete postcranial skeleton, cataloged as UCRC PV11 at the University of Chicago before being transferred to the Ministry of Energy, Mines and Environment in Rabat and catalogued as SGM-Din 2, that had been found in a single site. The strata of this site is made up of sandstones which belong to the Gara Sbaa Formation of the Kem Kem Beds, which dates to the Cenomanian-Albian ages of the middle Cretaceous period.
The skeleton was found preserved in articulation, one of the few articulated dinosaur skeletons known from the Kem Kem Beds, and consists of: two dorsal ribs, two gastralia ("belly ribs"), several (tail) vertebrae, eight chevrons, an incomplete , incomplete forelimbs, a partial pelvis, two partial hindlimbs, incomplete pedes (hindfeet), and several additional fragments. Some of these fragments were later identified as being shafts of the humerus and fibula. A bone initially described by Sereno and colleagues as the end of a was later reidentified as the end of an , another pelvic bone, by later studies. This skeleton was likely buried suddenly in a channel deposit based on the coarse-grained matrix surrounding the bones and the discovery of rostral teeth of the sawskate Onchopristis and crocodyliform teeth, two aquatic taxa, near the skeleton. In 1996, Sereno and colleagues scientifically described the remains and assigned them to a new genus and species of theropod, which they named Deltadromeus agilis. The generic name Deltadromeus is a combination of the Greek words delta meaning "change", and dromeus meaning "runner". The specific name is the Greek word agilis meaning "agile". Delta is also a reference to the deltaic facies where the skeleton is found whereas dromeus and agilis refer to the cursorial proportions of Deltadromeus' hindlimbs. The skeleton SGM-Din 2 was selected as the holotype (name-bearing) specimen of Deltadromeus agilis.
Sereno and colleagues also assigned several Egyptian theropod fossils to the species. These fossils, consisting of a left , pubes, right femur, an incomplete right tibia, and a left fibula, had been unearthed in 1911 and 1912 by Austro-Hungarian paleontologist Richard Markgraf and his crews during German paleontological expeditions to the Cenomanian-aged Bahariya Formation of northern Egypt.Stromer, E. (1934). "Ergebnisse der Forschungsreisen Prof. E. Stromers in den Wüsten Ägyptens. II. Wirbeltier-Reste der Baharije-Stufe (unterstes Cenoman)." 13. Dinosauria. Abhandlungen der Bayerischen Akademie der Wissenschaften, Mathematisch-naturwissenschaftliche Abteilung n.f., 22: 1–79. These expeditions recovered many theropod fossils, including an individual found in 1922 known from several vertebrae, a rib fragment, and an incomplete pelvis that German paleontologist Ernst Stromer described as belonging to a new genus and species of theropod, Bahariasaurus ingens, in 1934. The proximal right tibia had also been assigned to the allosauroid Erectopus, then identified as belonging to Bahariasaurus by Stromer, and finally Deltadromeus by Sereno and colleagues. All of the fossils described by Stromer, including those referred to both Bahariasaurus and Deltadromeus, were destroyed in the Bombing of Munich during World War II.
Sereno and colleagues maintained that Bahariasaurus and Deltadromeus were different genera, but the validity and synonymy of the two genera is controversial. Several other Kem Kem fossils have been suggested to belong to Deltadromeus, but the holotype is the only specimen that unambiguously belongs to Deltadromeus. In 2015, Canadian paleontologist David Evans and colleagues described a small, nearly complete femur of a theropod that had been found in the Kem Kem Beds. Their description identified the femur as belonging to a noasaurid, but stated it may belong to a juvenile Deltadromeus based on some shared characteristics. In 2024, Belgian paleontologist Christophe Hendrickx and colleagues described several theropod teeth from the Kem Kem Beds, one of which they stated is from either a non-abelisauroid ceratosaur or a megaraptoran, possibly Deltadromeus itself. However, the lack of overlap and the controversy surrounding Deltadromeus affinities made this impossible to prove until more fossils are found.
Description
The holotype individual was estimated to have measured in length and weighed around , slightly more than an imperial ton,by Australian researcher Frank Seebacher in 2001. Assigned specimens from the Bahariya Formation come from a much larger individual, with the femur IPHG 1912 VIII 69 having a length of , compared to the femur of the holotype. These referred specimens, if the referral to Deltadromeus is validated, would indicate that members of the genus could grow up to in length, approximately the length of a Tyrannosaurus rex. In 2016, American paleontologist Gregory S. Paul gave Deltadromeus/Bahariasaurus a length estimate of and a weight estimate of .
Anatomy Deltadromeus is very poorly known, only being represented by an incomplete postcranial skeleton which is missing much of the vertebrae, ribs, and forelimbs. In addition to its complicated taxonomy, this makes the anatomy of Deltadromeus poorly understood. The limb bones of Deltadromeus are extremely slender, with a length/diameter ratio comparable to that of the cursorial Cretaceous ornithomimosaur Ornithomimus. Additionally, a study of the femur/lower leg ratios of theropods found that the lower leg of Deltadromeus is 7.5% longer than the "average" theropod its size, supporting the conclusion that it was cursorial. These proportions are similar to those of the cursorial deinonychosaurs, compsognathids, tyrannosauroids, and the allosauroid Concavenator. The tibia/femur length/diameter ratio for example is 0.95 in Deltadromeus, whereas it is only 0.76 in Allosaurus and 1.16 in Ornithomimus. This indicates that the hindlimbs are relatively long for a theropod. Additionally, the humerus length/femur length ratio is higher in Deltadromeus than genera like Allosaurus, indicating that the forelimbs are not as reduced in that genus and Tyrannosaurus. The humerus is elongate with a relatively short deltopectoral crest that only spans 1/4 of the total length of the humerus. In contrast to Gualicho, a possible relative of Deltadromeus, the total length of the humerus is less than that of the scapula. The lateral side of the deltopectoral crest is rugose with a transversely curved and concave anterior margin, as in ornithomimosaurs and some ceratosaurs.
The coracoid and acromion processes are broadly expanded anteroposteriorly (front-back), a characteristic believed to be diagnostic of Deltadromeus that also appears in fossils assigned to Bahariasaurus. This acromion process is shallow in Deltadromeus, Gualicho, and some carcharodontosaurids, but is deeper in some other theropods. Additionally, the coracoid has a shallow notch that is also observed in basal ceratosaurs like Limusaurus. The scapulae of Deltadromeus and Gualicho are very similar, with both having relatively short, distally tapering, and narrow scapular blades. This tapering scapula blade is found in taxa like Deltadromeus, Gualicho, Masiakasaurus and Limusaurus, unlike in many tetanurans. The midshaft of the ischium is dorsoventrally (top-bottom) compressed, with triangular obturator flanges on it and the pubis. At the distal end of the ischium, the ischial "foot" is greatly expanded, similar to those of basal abelisauroids but to an extent only seen in an ischium assigned to Bahariasaurus.