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| Deltadromeus | |
|---|---|
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| 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 | |
| |
Deltadromeus (meaning "delta runner") is an extinctgenus of controversialtheropoddinosaur that lived in present-day Morocco during the mid-Cretaceousperiod. It was described by AmericanpaleontologistPaul Sereno and colleagues in 1996. The genus contains a singlespecies,D. agilis, named based on an incompletepostcranial skeleton, theholotype (name-bearing) specimen. However, some fossils from theBahariya Formation of Egypt that were formerly referred to the theropodBahariasaurus have been suggested to belong toDeltadromeus. The holotype specimen ofDeltadromeus was unearthed by a joint expedition by theUniversity of Chicago and the Service Géologique du Maroc inErrachidia Province, western Morocco in rock layers coming from theGara Sbaa Formation of theKem Kem Group. This indicates that these fossils date to theCenomanianstage of the Cretaceous period, 100-95 million years ago.
Deltadromeus was around 8 metres (26 ft) in length and 1,050 kilograms (2,310 lb) in weight based on the holotype. However, if the Egyptian remains do belong toDeltadromeus, this would make it among the largest known theropod genera at around 12.2 metres (40 ft), comparable toTyrannosaurus. The forelimbs ofDeltadromeus are relatively long for a theropod, being proportionately longer than those of taxa likeAllosaurus.Deltadromeus also preserves gracile, slender, and relatively longhindlimbs, indicating it wascursorial. As for the vertebrae, itscaudal (tail)vertebrae have broad, quadrangularneural spines (spines emerging from the base of the vertebra) withpleurocoels (cavities) which would store pneumaticair sacs.
The classification ofDeltadromeus has been in flux since its original description. OriginallyDeltadromeus was considered to be a basalcoelurosaur related toOrnitholestes, however later studies have challenged this idea. Some studies have classified it as anavetheropod closely related toGualicho, a similar theropod with debated affinities, or as amegaraptoran outside the familyMegaraptoridae. Other studies have proposed thatDeltadromeus is aceratosaur, however its position within Ceratosauria is debated. Studies have classified it as a basal ceratosaur,noasaurid, or anElaphrosaurus-grade ceratosaur. The idea that it is a ceratosaur of some kind is most mainstream, with support from severalphylogenetic analyses. Additionally, some studies have stated that it may be ajunior synonym ofBahariasaurus, another potential ceratosaur which lived around the same interval asDeltadromeus. This too, however, is under discussion.
Many gigantic theropods are known from North Africa during this period, includingDeltadromeus andBahariasaurus as well as thespinosauridSpinosaurus, thecarcharodontosauridsCarcharodontosaurus andTameryraptor, and several unnamedabelisaurids. IfDeltadromeus is a ceratosaur closely related to theherbivorous/omnivorousLimusaurus andBerthasaura, it would be herbivorous or omnivorous based onphylogenetic bracketing. This would indicate more niche partitioning in theropods in Cretaceous North Africa and suggest thatDeltadromeus filled a role traditionally filled byornithoschians. North Africa at the time was blanketed inmangrove forests andwetlands, creating a hotspot offish,crocodyliforms, andpterosaur diversity.
Fossils definitively assigned toDeltadromeus were first discovered in 1996 by American paleontologistPaul Sereno, who was part of a joint expedition by theUniversity of Chicago and the Service Géologique du Maroc in an outcrop of theKem Kem Beds known as Aferdou N'Chaft inErrachidia Province, western Morocco.[1] This expedition was one of many that explored dinosaur-bearing outcrops inNorth Africa throughout the 1990s and 2000s.[1][2][3][4] The remains found consisted of an incomplete postcranial skeleton, cataloged as UCRC PV11 at the University of Chicago before being transferred to theMinistry of Energy, Mines and Environment inRabat and catalogued as SGM-Din 2, that had been found in a single site.[1][5] The strata of this site is made up ofsandstones which belong to the Gara Sbaa Formation of the Kem Kem Beds, which dates to theCenomanian-Albian ages of themiddle Cretaceous period.[1][5]
The skeleton was found preserved in articulation, one of the few articulated dinosaur skeletons known from the Kem Kem Beds,[5] and consists of: twodorsal ribs, twogastralia ("belly ribs"), severalcaudal (tail)vertebrae, eightchevrons, an incompletescapulocoracoid, incompleteforelimbs, a partialpelvis, two partialhindlimbs, incompletepeses (hindfeet), and several additional fragments.[1] Some of these fragments were later identified as being shafts of thehumerus and fibula. The distal (away from body) end of theischium, a part of the pelvis, was unearthed and misidentified as being the distal part of thepubis,[1][5] however this has been corrected.[6][7] This skeleton was likely buried suddenly in achannel deposit based on thecoarse-grainedmatrix surrounding the bones and the discovery ofrostralteeth of thesawskateOnchopristis andcrocodyliform teeth, two aquatic taxa, near the skeleton.[5] In 1996, Sereno and colleaguesscientifically described the remains and assigned them to a newgenus andspecies oftheropod, which they namedDeltadromeus agilis. The generic nameDeltadromeus is a combination of theGreek wordsdelta meaning "delta", anddromeus meaning "runner". The specific name is the Greek wordagilis meaning "agile".Delta is in reference to thedeltaicfacies where the skeleton is found whereasdromeus andagilis refer to thecursorial proportions ofDeltadromeus' hindlimbs.[1]
The skeleton SGM-Din 2 was selected as theholotype (name-bearing) specimen ofDeltadromeus agilis, however Sereno and colleagues also assigned several Egyptian theropod fossils to the species. These fossils, consisting of a leftcoracoid,pubes, rightfemur,proximal (towards body) end of a righttibia, and a left fibula,[1] had been unearthed in 1911 and 1912 byAustro-Hungarian paleontologistRichard Markgraf and his crews during German paleontological expeditions to the Cenomanian-agedBahariya Formation of northern Egypt.[8][1][6] 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 paleontologistErnst Stromer described as belonging to a new genus and species of theropod,Bahariasaurus ingens, in 1934.[8][9] The proximal right tibia had also been assigned to theallosauroidErectopus,[9] then identified as belonging toBahariasaurus by Stromer, and finallyDeltadromeus by Sereno and colleagues. Unfortunately, all of the fossils known ofBahariasaurus, including those Sereno and colleagues referred toDeltadromeus, were destroyed in theBombing of Munich duringWorld War II, leaving the nature ofBahariasaurus mysterious.[9][10] Sereno and colleagues maintained thatBahariasaurus andDeltadromeus were different genera, however the validity andsynonymy of the two genera is controversial.[5][6][11] Several other Kem Kem fossils have been suggested to belong toDeltadromeus, but the holotype is the only specimen confidently referrable toDeltadromeus.[12] 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. In their description, they identified the femur as belonging to anoasaurid, but stated it may belong to a juvenileDeltadromeus based on some shared characteristics.[13] In 2024, Argentine paleontologist Christophe Hendrickx and colleagues described several theropod teeth from the Kem Kem Beds, one of which they stated is from either a non-abelisauroidceratosaur or amegaraptoran, possiblyDeltadromeus itself. However, the lack of overlap and the controversy surroundingDeltadromeus' affinities made this impossible to prove until more fossils are found.[14]
In 2001, Australian researcher Frank Seebacher estimated the holotype individual to have measured 8 metres (26 ft) in length and weighed around 1,050 kilograms (2,310 lb), slightly more than animperial ton.[15] Assigned specimens from the Bahariya Formation come from a much larger individual, with the femur IPHG 1912 VIII 69 having a length of 1.22 metres (4 ft), compared to the 0.741-metre-long (2.43 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 aTyrannosaurus rex.[1] In 2016, American paleontologistGregory S. Paul gaveDeltadromeus/Bahariasaurus a length estimate of 11 metres (36 ft) and a weight estimate of 4 metric tons (3.9 long tons; 4.4 short tons).[16]
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 ofDeltadromeus poorly understood.[7][1][6] Thelimb bones ofDeltadromeus are extremely slender, with a length/diameter ratio comparable to that of the cursorial CretaceousornithomimosaurOrnithomimus. Additionally, a study of the femur/lower leg ratios of theropods found that the lower leg ofDeltadromeus 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 cursorialdeinonychosaurs,compsognathids,tyrannosauroids, and the allosauroidConcavenator.[17] The tibia/femur length/diameter ratio for example is 0.95 inDeltadromeus, whereas it is only 0.76 inAllosaurus and 1.16 inOrnithomimus. This indicates that the hindlimbs are relatively long for a theropod. Additionally, the humerus length/femur length ratio is higher inDeltadromeus than genera likeAllosaurus, indicating that the forelimbs are not as reduced in other genera likeAllosaurus andTyrannosaurus.[1] The humerus is elongate with a relativelyproximodistally (towards-away body) shortdeltopectoral crest that only spans 1/4 of the total length of the humerus. In contrast toGualicho, the total length of the humerus is less than that of the scapula.[7] The lateral side of the deltopectoral crest is rugose with a transversely curved and concave anterior margin, as in ornithomimosaurs and some ceratosaurs.[18]
The coracoid andacromion process (aprocess of theshoulder joint) are broadly expandedanteroposteriorly (front-back), a characteristic believed to be diagnostic ofDeltadromeus[1] that also appears in fossils assigned toBahariasaurus.[6] This acormion process is shallow inDeltadromeus, Gualicho, and somecarcharodontosaurids, but is deeper in some other theropods.[7] Additionally, the coracoid has a shallow notch that is also observed in basal ceratosaurs likeLimusaurus.[19] Overall, the scapulae ofDeltadromeus andGualicho are very similar, with both having relatively short, distally tapering, and narrow scapular blades. This tapering scapula blade is found in taxa likeDeltadromeus, Gualicho,Masiakasaurus andLimusaurus, but these blades are straight blades in manytetanurans.[7] The midshaft of theischium isdorsoventrally (top-bottom) compressed, with triangularobturator 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[12] but to an extent only seen in an ischium assigned toBahariasaurus.[6]
As for the femur, itsfemoral head is elevated dorsomedially, similar to that of an indeterminate Kem Kem noasaurid but different from the femora ofMasiakasaurus and most other theropods.Deltadromeus' femur is very gracile, with a shaft width/total femoral length ratio of 0.074, as in noasaurids.[13] On thedistal (away from body) end of the femur is anaccessory trochanter (protrusion of the femur) that is unique toDeltadromeus, though a femur referred toBahariasaurus has it as well, suggesting that this femur belongs toDeltadromeus.[5] At the distal end of the femur is themedial condyle, which bears a unique extension on itsanterior (front) end. The fibula bears a largefossa (depression) at its anterior end, a characteristic found in many ceratosaurs.[12] Unlike its purported relativeGualicho and some other theropods,Deltadromeus' gastralia are unfused to the pubic boot.[7] As inMasiakasaurus, the medial condyles of themetatarsal IV are reduced inDeltadromeus.[9] The anteriorcaudal vertebrae have broad, elongate, quadrangularneural spines (spines emerging from the base of the vertebra), a trait absent in many other theropods but present in ceratosaurs. These vertebrae also bearpleurocoels,[1][7] large openings on the lateral sides of the centra that would storeair sacs.[20]
The phylogenetic position ofDeltadromeus is complicated by the fragmentary nature of the holotype,[1][6] the destruction ofBahariasaurus' fossils,[9][10] and the lack of overlap between fossils found in the Kem Kem Beds and theDeltadromeus holotype.[14][13] In addition to the mysterious affinities ofDeltadromeus, it may be a synonym ofBahariasaurus, another poorly understood dinosaur.[6]
Sereno and colleagues (1996) regardedDeltadromeus as a basalcoelurosaur, only slightly more derived than theJurassic genusOrnitholestes, based onDeltadromeus' expanded coracoid, reduced femoralfourth trochanter, and the presence of a deep fossa on the proximal end of the fibula. Based on its position as a basal coelurosaur, Sereno and colleagues (1996) hypothesized that coelurosaurs had spread to Africa prior to the end of the Jurassic, giving rise toDeltadromeus and other African coelurosaurs.[1]
In their description ofGualicho, Apesteguía and colleagues (2016) foundGualicho andDeltadromeus to be possible sister taxa in the allosauroid familyNeovenatoridae. However, the analysis also noted thatDeltadromeus shared many features with ceratosaurs and that ifGualicho was removed from the analysis,Deltadromeus would resolve to a member of Ceratosauria.[7] Since then,Gualicho has repeatedly been found to be a member of Coelurosauria,[21] whereas most phylogenetic analyses findDeltadromeus in Ceratosauria.[6] Motta and colleagues (2016) suggested thatAoniraptor, a megaraptoran from Argentina, formed a clade withDeltadromeus andBahariasaurus ('Bahariasauridae') that was distinct fromMegaraptoridae within Megaraptora.[22]
The cladogram below follows the 2016Gualicho analysis by Sebastián Apesteguía, Nathan D. Smith, Rubén Juarez Valieri and Peter J. Makovicky.[7]
In a paper analyzing the relationships ofMirischia andSantanaraptor, two coelurosaurs from the Cretaceous of Brazil, Brazilian paleontologist Rafael Delcourt and colleagues foundDeltadromeus andGualicho to be early-branching ornithomimosaurs in their phylogenetic analysis. Delcourt and colleagues (2025) noted thatDeltadromeus' femur lacks a crest seen in many other theropods, a condition observed in ornithomimids likeStruthiomimus andGallimimus. Additionally, the medialcondyle of the tibia ofDeltadromeus bears a large process found in the tibiae of ornithomimosaurs likeMirischia. Although several African theropods from Gondwana have been theorized to be ornithomimosaurs, many of these taxa have since been reclassified as noasaurids, which too have edentulousness, gracile limbs, and elongatedcervical (neck) vertebrae like the ornithomimosaurs. However, Delcourt and colleagues (2025) as well as an unpublished masters thesis[23][24] recoveredDeltadromeus andGualicho within Ornithomimosauria in their phylogenetic analyses, with the former paper stating that the classification of two needs further study.[25]The cladogram below follows the 2025 analysis by Delcourt and colleagues.[25]
Many studies published since the original description ofDeltadromeus 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,[26] though others have found it to be more primitive, possibly related to the more ancestral ceratosaursElaphrosaurus andLimusaurus.[9][27] A more comprehensive study of noasaurid relationships published in 2016 effectively agreed with both of these interpretations, withDeltadromeus,Limusaurus andElaphrosaurus all found to be within the Noasauridae.[28] A 2017 paper describing ontogenetic changes inLimusaurus and the effect of juvenile taxa on phylogenetic analyses placedDeltadromeus as a noasaurid in every analysis regardless of whichLimusaurus specimen was used, although the analyses did not includeGualicho orAoniraptor. According to the authors, resolving the phylogenetic positions ofGualicho, Aoniraptor, Deltadromeus and megaraptorans is a critical issue facing theropod systematics.[29]Deltadromeus was also considered a noasaurid in a 2020 review of the Kem Kem Group geology and fauna.[5]
The roughly contemporaneous theropodBahariasaurus, some of the bones of which have been referred toDeltadromeus,[1] has been suggested to be synonymous with the latter taxon.[30] In a 2010 analysis of the Ceratosauria, Carrano and Sampson noted that the differences betweenDeltadromeus andBahariasaurus were partily due to misidentified bones in the former, and that other distinctions were subtle and insufficient to distinguish the two.[31] In 2020, Ibrahim and colleagues acknowledged similarities between the two genera, but considered it unlikely thatDeltadromeus represents a specimen ofBahariasaurus due to perceived differences in the pelvic bones. They further regardedBahariasaurus as anomen dubium without explanation.[5]
In 2024,Andrea Cau published a comprehensive theropodphylogenetic framework that could be used to identify immature specimens of other taxa. He included theBahariasaurus type specimen in his analyses and recovered it within the ceratosaur cladeAbelisauroidea in apolytomy includingDeltadromeus.[11] The following year, Cau and Paterna used an updated version of this dataset to reanalyze the relationships ofBahariasaurus,Deltadromeus, and other Cretaceous theropods from Africa. They determined that the variation observed between specimens ofDeltadromeus andBahariasaurus was the result of individual andontogenetic 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 ofBahariasaurus—as a complete ischium. The authors observed anatomical characters that the bone shares with the less complete ischia of the holotypes of bothBahariasaurus andDeltadromeus, which they used to strengthen their argument. They concluded thatDeltadromeus should be regarded as ajunior synonym ofBahariasaurus. The results of their phylogenetic analysis are displayed in thecladogram below, withBahariasaurus (includingDeltadromeus) indicated in the so-called "abelisauroid clade 1".[6]
| Abelisauroidea |
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Fossils ofDeltadromeus are known from the Kem Kem Beds, one of many dinosaur-bearing Cretaceous-aged fossil formations in North Africa. Others include the Bahariya Formation of Egypt and theElrhaz andEchkar Formations of Niger.[6][2] If the material assigned toDeltadromeus by Sereno and colleagues (1995) is fromDeltadromeus,[1] its range would include Bahariya Formation. This would indicate that the distribution ofDeltadromeus is similar to that ofSpinosaurus,[34][35] thoughSpinosaurus' taxonomy and distribution too is debated.[36][19][37] North Africa during this period bordered theTethys Sea, which transformed the region into amangrove-dominated coastal environment filled with vasttidal flats andwaterways.[38][39]
The composition of the dinosaur fauna of North Africa at this time is an anomaly, as there are fewerherbivorous dinosaur species relative to carnivorous dinosaur species than in most fossil sites.[6][12][40] This abundance of theropods compared to that of non-theropods was dubbed "Stromer's Riddle", which despite suggestions that this is due to ecological, preservation, or other biases,[34][41] is supported by the fossil record.[6] This over prevalence of theropods indicates that there wasniche partitioning between the different theropod clades, withspinosaurids consuming fish while other groups hunted herbivorous dinosaurs.[42]Isotopic evidence supports this, which found greater quantities of sizable, terrestrial animals in the diets of carcharodontosaurids and ceratosaurs from both the Kem Kem Beds and Elrhaz Formation.[43] North Africa was dominated by a triumvirate of Abelisauroidea, Spinosauridae, and Carcharodontosauridae during the mid-Cretaceous, with all of these groups present in the Kem Kem Beds, Echkar, Elrhaz, and Bahariya Formations.[6][40]
The abelisauroid affinities ofDeltadromeus/Bahariasaurus suggest that they may beomnivorous or herbivorous. This is based on the herbivorous/omnivorous nature ofLimusaurus andBerthasaura,[44][45][46] which were recovered as a relative ofDeltadromeus/Bahariasaurus in phylogenetic analyses. This would indicate niche partitioning in the large theropods in these localities, withDeltadromeus/Bahariasaurus filling a niche typically filled byornithischians. However, this cannot be confirmed no skull material has been found for eitherDeltadromeus orBahariasaurus.[6]
The Kem Kem Beds is composed of three geologic formations: the Gara Sbaa Formation, the Douria Formation, and the Izefouane Formation.[5] Isotopes fromCarcharodontosaurus andSpinosaurus fossils suggest that the Kem Kem Beds witnessed a temporarymonsoon season rather than constant rainfall, similar to modern conditions present insub-tropical andtropical environments inSoutheast Asia andSub-Saharan Africa.[47] This river system wasfreshwater based on the presence oflungfishes and other typically freshwater vertebrates. This indicates that the Kem Kem Beds had a wide variety of features, including river channels, river banks, sandbars, and more.[48][49][5] Fossils of giantfishes have been found in the Kem Kem Beds, including the sawskateOnchopristis,coelacanthAxelrodichthys, andbichirBawitius.[50][5] The Kem Kem Beds also preserves an abundance of crocodyliformes like thestomatosuchidLaganosuchus, thepeirosauridHamadasuchus, and thepholidosauridElosuchus.[51][5] This region also bore an abundance ofpterosaurs like the toothedanhangueridsSiroccopteryx andNicorhynchus and theedentulousazhdarchoidsAlanqa andLeptostomia.[52][49]
The Kem Kem Beds preserve many dinosaur fossils.Sauropod dinosaurs are known by therebbachisaurid sauropodRebbachisaurus,[53] an indeterminatesomphospondyliantitanosauriform, and an indeterminatetitanosaur,[54] one comparable in size to the giantParalititan.[55] Ornithischian fossils are extremely rare, only being represented from an isolatedthyreophoran tooth[5] and footprints of anornithopod, possibly similar toIguanodon.[56] As for theropods, many are known, including one or two distinct indeterminate abelisaurids, the carcharodontosauridsCarcharodontosaurus[1] andSauroniops, indeterminate noasaurids, and the spinosauridsSpinosaurus[5] andSigilmassasaurus, if it is distinct.[6][37][36] However, many of these dinosaurs are known from isolated or incomplete remains, have complicated taxonomies, or are under study.[6][57]
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