Deltadromeus

Deltadromeus (meaning "delta runner") is an extinct genus of controversial theropod dinosaurs from the Aoufous Formation (Kem Kem Beds) of Morocco. The genus contains a single species, Deltadromeus agilis, known from multiple partial skeletons from the mid-Cretaceous period (mid-Cenomanian age), about 95 million years ago.

Deltadromeus Temporal range: Late Cretaceous (Cenomanian), 99.6–93.5 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Mounted skeleton cast with a speculative skull
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Superfamily:	†Abelisauroidea (?)
Genus:	†Deltadromeus Sereno et al., 1996
Species:	†D. agilis
Binomial name
†Deltadromeus agilis Sereno et al., 1996
Synonyms
Bahariasaurus? Stromer, 1934

The classification of Deltadromeus has been in flux since its original description. In 2016, the South American megaraptoran Gualicho was noted to have many similarities with Deltadromeus. Other studies have considered it a ceratosaurian, potentially as a member of the family Noasauridae. It may be a junior synonym of the contemporary Bahariasaurus.

Discovery

The holotype of Deltadromeus agilis (museum catalogue number SGM-Din2) is a partial skeleton. A number of specimens (catalogued under IPHG 1912 VIII) from the Bahariya Formation of Egypt were originally considered by Ernst Stromer to be conspecific with Bahariasaurus,^[1] but were referred to Deltadromeus by Paul Sereno and colleagues in 1996 based on extreme similarities between them.^[2]

Description

Size of the holotype compared to a human

The holotype is estimated to have measured 8 metres (26 ft) long. The weight of the living animal was estimated to have been around 1,050 kilograms, slightly more than an imperial ton.^[3]

Assigned specimens from the Bahariya Formation come from a much larger individual, with a femur (upper leg bone) length of 1.22 metres (4 ft), compared to the 0.74-metre-long (2.4 ft) femur of the holotype. These referred specimens, if legitimate, would indicate that members of the genus could grow up to 12.2 metres (40 ft) in length, approximately the size of a Tyrannosaurus rex.^[2]

Classification

As an avetheropod

Reconstructed skeleton with earlier version of speculative skull

Speculative life restoration of Deltadromeus as a coelurosaur feeding on a sauropod

The original description of Deltadromeus in 1996 regarded it as a basal coelurosaur, only slightly more derived than the Late Jurassic genus Ornitholestes.^[2]

In 2016, an analysis of Gualicho, a South American theropod considered to belong to the allosauroid family Neovenatoridae, found Deltadromeus to be a possible sister taxon of Gualicho. However, the analysis also noted that Deltadromeus shared many features with ceratosaurs and that if Gualicho was removed from the analysis, Deltadromeus would resolve to a member of Ceratosauria.^[4] In the 2016 description of Aoniraptor, Deltadromeus was speculated to potentially form a clade with Aoniraptor and Bahariasaurus ('Bahariasauridae') within Megaraptora, distinct from the Megaraptoridae.^[5]

The cladogram below follows the 2016 Gualicho analysis by Sebastián Apesteguía, Nathan D. Smith, Rubén Juarez Valieri and Peter J. Makovicky.^[4]

Allosauroidea	Metriacanthosauridae Allosauria Allosauridae Carcharodontosauria Carcharodontosauridae Neovenatoridae Deltadromeus Gualicho Neovenator Chilantaisaurus Megaraptora (?)

As a ceratosaur

Diagrams showing holotype remains (A) and size of a lost femur (B)

Restoration as a ceratosaur

Many studies published since the original description of Deltadromeus have considered it to be a ceratosaur, although different studies disagree on what kind of ceratosaur. A 2003 study suggested it was a member of the Noasauridae,^[6] though others have found it to be more primitive, possibly related to the more ancestral ceratosaurs Elaphrosaurus and Limusaurus.^[7][8] A more comprehensive study of noasaurid relationships published in 2016 effectively agreed with both of these interpretations, with Deltadromeus, Limusaurus and Elaphrosaurus all found to be within the Noasauridae.^[9] A 2017 paper describing ontogenetic changes in Limusaurus and the effect of juvenile taxa on phylogenetic analyses placed Deltadromeus as a noasaurid in every analysis regardless of which Limusaurus specimen was used, although the analyses did not include Gualicho or Aoniraptor. According to the wuthors, resolving the phylogenetic positions of Gualicho, Aoniraptor, Deltadromeus and megaraptorans is a critical issue facing theropod systematics.^[10] Deltadromeus was also considered a noasaurid in a 2020 review of the Kem Kem Group geology and fauna.^[11]

The roughly contemporaneous theropod Bahariasaurus, some of the bones of which have been referred to Deltadromeus,^[2] has been suggested to be synonymous with the latter taxon.^[12] In a 2010 analysis of the Ceratosauria, Carrano and Sampson noted that the differences between Deltadromeus and Bahariasaurus were partily due to misidentified bones in the former, and that other distinctions were subtle and insufficient to distinguish the two.^[13] In 2020, Ibrahim and colleagues acknowledged similarities between the two genera, but considered it unlikely that Deltadromeus represents a specimen of Bahariasaurus due to perceived differences in the pelvic bones. They further regarded Bahariasaurus as a nomen dubium without explanation.^[11]

In 2024, Andrea Cau published a comprehensive theropod phylogenetic framework that could be used to identify immature specimens of other taxa. He included the Bahariasaurus type specimen in his analyses and recovered it within the ceratosaur clade Abelisauroidea in a polytomy including Deltadromeus.^[14] The following year, Cau and Paterna used an updated version of this dataset to reanalyze the relationships of Bahariasaurus, Deltadromeus, and other Cretaceous theropods from Africa. They determined that the variation observed between specimens of Deltadromeus and Bahariasaurus was the result of individual and ontogenetic variation, as the former is known from immature remains. They further reidentified specimen SNSB-BSPG1912VIII82—incorrectly recognized as a indeterminate theropod pubis by Stromer in his 1934 description of Bahariasaurus—as a complete ischium. The authors observed anatomical characters that the bone shares with the less complete ischia of the holotypes of both Bahariasaurus and Deltadromeus, which they used to strengthen their argument. They concluded that Deltadromeus should be regarded as a junior synonym of Bahariasaurus. The results of their phylogenetic analysis are displayed in the cladogram below, with Bahariasaurus (including Deltadromeus) indicated in the so-called "abelisauroid clade 1".^[15]

Abelisauroidea	PRC-NF-1-21 (large Cabao Fm. taxon[16]) Kiyacursor Elaphrosaurus Berberosaurus "abelisauroid clade 1" Ligabueino Berthasaura MNN TIG6 (referred Spinostropheus specimen[17]) Angeac-Charente taxon Limusaurus Bahariasaurus (including Deltadromeus) Vespersaurus Afromimus Velocisaurus Masiakasaurus Abelisauridae

Paleobiology

Pectoral and forelimb

Deltadromeus specimens are known from the same formation as the giant theropods Carcharodontosaurus, Spinosaurus, and Bahariasaurus (which may be synonymous with Deltadromeus). No skull material has been found for either Deltadromeus or Bahariasaurus. However, they may be omnivorous or herbivorous if abelisauroid affinities are supported, implying niche partitioning in the large theropods in these localities.^[15]