| Neovenator | |
|---|---|
 | |
| Reconstructed skeleton in Japan | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | †Allosauria |
| Clade: | †Carcharodontosauria |
| Family: | †Neovenatoridae |
| Genus: | †Neovenator Hutt,Martill &Barker, 1996 |
| Species: | †N. salerii |
| Binomial name | |
| †Neovenator salerii Hutt, Martill & Barker, 1996 | |
Neovenator (nˈiːə͡ʊvˌɛne͡ɪtə; "new hunter") is agenus ofcarcharodontosauriantheropoddinosaur. It is known primarily from several skeletons found in theEarly Cretaceous (Hauterivian–Barremian)Wessex Formation on the south coast of theIsle of Wight, southern England. The first remains ofNeovenator were discovered in 1978 alongside those of theornithopodBrighstoneus, after the collapse of part ofGrange Chine. In 1996, Steve Hutt,David Martill and Michael Barker named the genusNeovenator. One species is known: thetype species,N. salerii, after the Salero family who owned the site on which its remains were discovered.
Between the type specimen and multiple referred specimens, roughly seventy per cent ofNeovenator's skeleton is known. While incompletely known, it was likely around 7 metres (23 ft) in length, and probably weighed 1 metric ton (1.1 short tons), though a specimen possibly referrable to the genus indicates a larger body size of 10 metres (33 ft). Its skull is known from bothpremaxillae, parts of the leftmaxilla, rightnasal, rightpalatine, and the front portion of adentary. The snout ofNeovenator is covered inrugosities, similar tocarcharodontosaurids andabelisaurids, which indicates that it either had an extensive blood supply, possibly forthermoregulation, or an extensiveneurovascular system, possibly fortactile purposes. However, this hypothesis has seen scrutiny. Teeth found in association with the type specimen ofNeovenator, while they do possess the characteristicenamel wrinkles of carcharodontosaur teeth, differ in their precise pattern.
The taxonomic position ofNeovenator has been a subject of debate. Prior to its description, its fragmentary remains led to a tentative referral toMegalosaurus. The authors who described the genus suggested that it was a British representative ofAllosauridae, or at least closely related toAllosaurus. A position within the relatedclade Carcharodontosauria was first suggested in 1998, and subsequently saw support using both comparative morphology and phylogenetic analyses. In 2012, it was assigned to a family of its own,Neovenatoridae.
Neovenator is best known from theWessex Formation of the Isle of Wight, although teeth possibly referrable to the genus have been recovered from theAngeac-Charente bone bed in France. Bite marks found on a fossil of theiguanodontidMantellisaurus suggest that it was amongNeovenator's prey base. Like many fossils of the closely relatedAllosaurus, the type specimen ofNeovenator bears numerouspathologies.
The first bones ofNeovenator were discovered in the summer of 1978 when a storm made part of theGrange Chine collapse. Rocks containingfossils fell to the beach ofBrighstone Bay on the southwestern coast of the Isle of Wight.[1] The rocks consisted ofplant debris bed L9 within the variegatedclays andmarls of theWessex Formation,[2] dating from theBarremian stage of theEarly Cretaceous, about 125 million years ago.[2][3] They were first collected by the Henwood family and shortly afterwards geology student David Richards collected additional material. Richards sent the remains to the Museum of Isle of Wight (nowDinosaur Isle) and theBritish Museum of Natural History. In the latter institution,palaeontologistAlan Jack Charig determined that the bones belonged to two kinds of animal:Iguanodon and a theropod. The "Iguanodon", later referred toMantellisaurus and ultimately made the separate genusBrighstoneus,[4] generated the most interest and in the early 1980s a team was sent by the BMNH to secure more material. The theropod material, meanwhile, was fairly incomplete (consisting ofvertebrae and fragments of thepelvis), and a lack of diagnostic features meant that it was ignored until its significance was recognised by Dr. William Blows.[1]
Several amateur paleontologists, among them Keith and Jenny Simmonds, now began to search for additional remains of the predator. Ultimately, the total of secured bones included the snout, teeth, the front of amandible (lower jaw), most of thevertebral column, ribs,gastralia (belly ribs),chevrons, the leftshoulder girdle,pelvic bones and a hindlimb. These were accessioned under numbers BMNH R10001 and MIWG 6348. From 1997–2001, further elements were recovered, in the form of the first caudal (tail) vertebra, a leftfemur, a righttibia, and apedal phalanx (toe bone). In all, the bones recovered equalled approximately 70% of the skeleton. In 1985, excavations undertaken bySteve Hutt of the MIWG revealed two vertebrae of a second individual, specimen MIWG.5470, recovered 500 m (1,600 ft) from the first. In 1987, Jenny Simmonds found a third skeleton, containing vertebra and pelvic bones, specimen MIWG.6352;[5] this specimen was originally believed to be a different taxon.[6] A fourth individual found by Nick Oliver is represented by specimen IWCMS 2002.186,[5] consisting of a lower jaw, parts of thecervical (neck) vertebrae and limb elements. In 1990 the material, then considered a possible new species ofMegalosaurus, was provisionally described by Hutt. Having mistaken theischium of MIWG 6352 for a pubic bone, Hutt suggested this specimen represented a separate species.[7]
In 1996, Steve Hutt,David Martill andMichael Barker named and described thetype speciesNeovenator salerii. The generic nameNeovenator means "new hunter" from the Greekneo~, "new" andLatinvenator, "hunter". Thespecific namesalerii honours the land owners of the site, the Salero family. In view of the large number of individuals involved in the discovery process, it was considered improper to single out one of them as discoverer. Theholotype is the skeleton accessioned as BMNH R10001 and MIWG 6348.[2]
In 1999, Hutt dedicated his (unpublished) master thesis toNeovenator.[8] This thesis would form the foundation for amonograph discussing theosteology of the genus. That monograph was published in 2008, and was authored byStephen L. Brusatte, Roger B. J. Benson and Hutt by thePalaeontographical Society.[1]
In 2012, teeth indistinguishable from those associated with the holotype ofNeovenator were found in theAngeac-Charente bone bed, in France,[9] dating to theBerriasian.[10] They were distinguished from those ofErectopus, a basal allosauroid also known from the Early Cretaceous of France, by differences in the carinae of their teeth.[9]
Neovenator was a mid-sized, lightly built carcharodontosaurid. The holotype specimen measured approximately 7–7.5 metres (23–25 ft) in length, and was fairly lightly built.[2][11] A hip height of about 2 metres (6 ft 7 in) has been suggested.[6]Gregory S. Paul estimated its mass at around 1 t (1.1 short tons).[11] It has been suggested that the holotype may be asubadult, and if this is the case, this may not be reflective of the maximum body size of the genus. However, in 2016, Jeremy Lockwood noted that most of the elements referred toNeovenator are roughly the same size as the holotype, suggesting that adults would have had a similar body size.[12] Specimen MIWG 4199 indicates an individual with a possible length of about 10 metres (33 ft),[1][6] though it consists only of a pedal phalanx and its position inNeovenator is dubious.[1][13] Afootprint (IWCMS:2016.273) is believed to have been left by an animal with a hip height of 2.5 metres (8 ft 2 in), and while the trackmaker is uncertain, it has been suggested that it was left by a matureNeovenator.[12]
The skull ofNeovenator is not known in its entirety, but is instead represented by bothpremaxillae (both complete), a leftmaxilla, a rightnasal, a rightpalatine, and the anterior (front) portion of a leftdentary, part of the mandible. The premaxillae were longer than they were tall, measuring 8.7 cm (3.4 in) in length and 7 cm (2.8 in) in height. This condition is seen in several non-carcharodontosaurid taxa (i.e.Allosaurus andSinraptor), but not in other, more derived carcharodontosaurs, such asAcrocanthosaurus.[1] The premaxillae have an additional connection (referred to in 2001 as a "pen-in-socket connection"), by not seen in other theropods.[6] The left maxilla, as preserved, measures around 30 cm (12 in) in length, and 7.5 cm (3.0 in) in depth. Only a small portion near where thejugal connected is missing. As in many basal tetanurans (and the more derivedCarcharodontosaurus), the maxilla contributed to the posterior (rear) border of the externalnaris (nasal opening). Similar toabelisaurids and other allosauroids (i.e.Giganotosaurus,Mapusaurus andSinraptor), the surface of the maxilla bore prominent rugosities, and contains a high density offoramina, particularly at its anteriormost (very front) portion, ventral to (below) the external naris. It has been suggested thatNeovenator's snout was highlyvascularised orinnervated.[14][15] Unlike many other allosauroids andceratosaurs, the ascendingramus lacked pneumatic excavation (hollow spaces containing air pockets).[1] A largemaxillary fenestra, with a diameter roughly one-sixth of the length of the tooth row, penetrated the maxilla.[2] Similar to the maxilla,Neovenator's nasal was rugose, though the extent of its rugosity is slightly weaker than inCarcharodontosaurus. The dorsal (upper) and lateral (side) surfaces of the nasal were separated by a rugose ridge. While initially regarded as a diagnostic feature ofNeovenator,[6] the same is also known inAllosaurus. As evidenced by the presence of a single large pneumatopore, the nasal was likely hollow and highly pneumatised. Similar conditions are observed in other allosauroids, though the amount of pneumatopores varied, sometimes within the same genus.[1] Posteriorly, the dorsal portion of the nasals would have contributed to a crest similar to that ofAllosaurus,[1][6] though this crest differed in that it showed no lateral overhang.[1]
The holotype ofNeovenator preserved a near-complete left dentary, measuring 23.5 cm (9.3 in) in length and 6.8 cm (2.7 in), though it was lost at the time of the 2008 monograph's publication. What was written in that monograph was therefore based on figures in published works and examinations of acast. Unlike some other carcharodontosaurs (particularly later genera such asCarcharodontosaurus andGiganotosaurus), the anterior (front) tip of the dentary was not squared-off. It was instead rounded, as in most other theropods.[1] Unlike the maxilla and nasal, the dentary ofNeovenator was fairly smooth, lacking rugosities, unlike what is seen in taxa likeCarcharodontosaurus. The largest foramina present were directly ventral to thealveolar margin (the tooth row), set in a longitudinal groove which deflected ventrally at around the fifth alveolus. The third alveolus of the dentary was enlarged, which is unique among allosauroids, save forAcrocanthosaurus. When viewed from above, the alveoli on both upper and lower jaws were arranged in a weakly sinuous curve, though this may be the result of taphonomy.[1]
As preserved,Neovenator's premaxilla has fivealveoli (tooth sockets),[1] as inAllosaurus;[6] its maxilla bears fifteen, though at least one or two more are likely to have been present. No teeth are preserved in the premaxilla or maxilla, and no erupted teeth are known from the dentary, though four replacement teeth are preserved. Additionally, isolated theropod teeth were found around the holotype specimen, tentatively assigned to the genus. Based on these,Neovenator's teeth were fairly typical for theropod teeth, though differ from those of other carcharodontosaurs in details of the enamel wrinkles. The teeth were asymmetrical in cross-section, due to the orientation of the distalcarina.[1]
The majority ofNeovenator'svertebral column is known from the holotype. Six cervical (neck) vertebrae, including theaxis (the second cervical vertebra positionally), are known, as are twelvedorsal (back) vertebrae. Also known are twenty-threecaudal (tail) vertebrae.[1] The front joint surface of the intercentrum of the axis was widened transversely. Theodontoid process of the axis had small openings along the side edge of the front facet, and the neural process of the axis had a single small opening in the side. The more rearward cervical vertebrae were fused with their cervical ribs; while this is known in many other theropod clades,Neovenator is the only allosauroid in which this was the case.[1][16] The neural spines of the cervical vertebrae were narrow anteroposteriorly (from front-to-back), are thick transversely, and were overall rod-like in shape. The seventh was inclined posteriorly, the eighth anteriorly, and the ninth directly vertically.[1]
The first two dorsal vertebrae were morphologically intermediate between the cervical and dorsal vertebrae, being, among other things, short anteroposteriorly when compared to dorsal vertebrae further along the column. The remaining dorsal vertebrae were fairly homogenous in morphology, characterised by their tallcentra andbiconcavearticular surfaces. Each dorsal vertebra hadtransverse processes which are broad anteroposteriorly yet thin dorsoventrally (up-and-down). Their neural spines are subrectangular in lateral view and are thick transversely, at least in comparison to most non-allosauroid tetanurans. Two large fragments of the dorsal ribs are known, one better-preserved and coming from the left side of the torso, and the other coming from the right side. Some disarticulatedgastralia (constituent bones of the gastral basket, which supported the organs and aided in respiration) are known.Sacral vertebrae are known from referred specimens. The second and third sacral vertebrae were fused, unlikeAcrocanthosaurus andSinraptor, but like some specimens ofAllosaurus. The second and fourth sacrals are known exclusively from their centra, so the neural arches of the sacra are poorly known.[1]
The first caudal vertebra of the holotype is poorly preserved due to damage sustained during the cliff fall, thus leaving the second as the best preserved. Though most of the anterior and middle caudal vertebrae are damaged, the distal ones are fairly complete. The anterior caudal centra had suboval articular surfaces, and posterior surfaces smaller than the anterior surface, and this pattern continues until the vertebra tentatively identified as the twenty-second.[1]
The leftscapula andcoracoid ofNeovenator's holotype are nearly complete, though its forelimbs are not known. Around two-thirds of theglenoid fossa's length is taken up by the scapula. While well-preserved, the coracoid is damaged enough that most of the point where it articulated with the scapula is absent.[1] The shoulder joint is wider transversely than anteroposteriorly.[16] On the medial surface of thescapulocoracoid, the glenoid is buttressed by a slight ridge, which fans out posteriorly to unite with the ventral margin of the scapular blade.[1]
Theilia ofNeovenator, including referred elements, are fragmentary.[1] Ventral to the ilium's front blade is a notch, which has a robust shelf on the inner side. The ilium overall is highly pneumatised.[1][16] While a rightpubis is preserved, it is fragmentary, heavily abraded and badly crushed. Distally, it expands into a large pubic boot, similar to that of other carcharodontosaurs and tyrannosaurids. The "feet" of theischia are connected anteriorly but diverge posteriorly;[1] the ischiac feet were expanded, another trait observed in other carcharodontosaurs.[6] Like in other carcharodontosaurs, and in some tyrannosaurids,Neovenator'sfemoral head is angled dorsomedially (upwards and towards the centrum). Thelesser trochanter of thefemur has a robust ridge on its outer side, and is itself extremely robust. Thefourth trochanter has a depression in the form of a thumbprint located laterally on its dorsal border. The distal portion of thetibia shows an oval rough area at the inner side. The top of the outermalleolus of the tibia is pinched from the front to the rear. The outer front bulge of the top surface of the tibia has a spur deflected ventrally. In the foot, the outer side of the secondmetatarsal has a hollow surface to contact the third metatarsal.[1]
In the 1996 paper describingNeovenator, Hutt, Martill and Barker suggested a close relationship withAllosaurus, and thatNeovenator might itself have been a member of Allosauridae.[2] The findings of that paper were based on morphological comparisons, rather thanphylogenetic analysis.[1] The first analysis to includeNeovenator was published in 2000 byThomas R. Holtz Jr., and indeed recovered as the sister taxon toAllosaurus.[17] However, that topology was the result of incorrect scoring, and two of the characters used to unite the two genera (prezygapophysis morphology and the distal expansion of the scapula) are respectively erroneous and uncorrelated with phylogeny.[18] The idea ofNeovenator being a "proto-carcharodontosaurid" was first put forward by Jerald D. Harris in 1998,[19] and was supported by Darren Naish, Steve Hutt, and David Martill in 2001, based on the presence an inflated "boot" to eachischium and ofpleurocoels (pneumatic hollows) throughout the vertebrae, both classically carcharodontosaurian traits.[6]
In a 2008 phylogenetic analysis of Allosauroidea, which slightly predated theNeovenator monograph, Steve Brusatte andPaul Sereno recoveredNeovenator as the most basal member of Carcharodontosauridae.[1][18] In 2010, Brusatte, along with Roger B. J. Benson and Matt Carrano, recoveredNeovenator in a similar position. However, their database included megaraptorans, which were recovered in a clade withNeovenator. Therefore, Benson, Carrano and Brusatte erected the familyNeovenatoridae, to encompass both taxa.[16]
Thecladogram below follows the 2010 analysis by Benson, Carrano and Brusatte.[16]
| Neovenatoridae |
| ||||||
In the years since that paper, however, an increasing number of phylogenies have recovered megaraptorans in Coelurosauria, as opposed to being sisters toNeovenator.[20][21][22] If this is the case, Neovenatoridae is likelymonotypic. Fernano Novas et al. (2013) recoveredNeovenator at the base of Carcharodontosauridae, in apolytomy withEocarcharia andConcavenator.[23]
Cladogram after Novas et al., 2013[23]
Chris Barker and colleagues suggested thatNeovenator may have possessed integumentary sensory organs on its snout, much as modern waterfowl and crocodilians use to find food in muddy water, based on neurovascular structures found on the skull. AsNeovenator is believed to be completely terrestrial, unlike the modern species, it is assumed that these sensory organs were used for other purposes, such as sensitivity to pressure and temperature, controlling jaw pressure and precision feeding. In support of this, the tooth wear forNeovenator seems to indicate that it avoided eating or biting into bone while it fed. Additionally,Neovenator might have used these integumentary sensory organs in courtship and sensing nest conditions, a technique seen today in most species of crocodilians and megapode birds. Though such structures are known for another theropod, thetyrannosauridDaspletosaurus horneri,Neovenator's neurovascular structures that likely supported these organs are the best preserved and most complete in any known theropod yet discovered.[15][24] However, a more recent study reviewing the evolution of the trigeminal canals among sauropsids notes that a much denser network of neurovascular canals in the snout and lower jaw is more commonly encountered in aquatic or semiaquatic taxa (e.g.,Spinosaurus,Halszkaraptor,Plesiosaurus), and taxa that developed a rhamphotheca (e.g.,Caenagnathasia), while terrestrial taxa such as tyrannosaurids andNeovenator may have had average facial sensitivity for non-edentulous terrestrial theropods, although further research is needed.[25]
As with many specimens ofAllosaurus, the holotype ofNeovenator salerii was highlypathological.[1][26] The pathologies listed in the 1996 paper and the 2008 monograph: are the fusion of two caudal vertebrae and their associated chevron into a single mass (also seen in certainAllosaurus and likely the result ofneoplastic ankylosis);[1] healed fractures to the gastralia;[26] a fracture to the scapula; one gastralium which appears to exhibit regrowth across its entire length,[1][26] andosteophytes on some of thepedal (foot)phalanges. One of the left pedal phalanges has been almost completely covered by exostotic bone growth and small lesions, suggesting osteomyelitis, similar to one specimen ofAllosaurus.[1]
The only definite remains ofNeovenator are known from theBarremian-ageWessex Formation of the Isle of Wight, though teeth identical to the genus are known from the Berriasian-age[10]Angeac-Charente bone bed in France.[9]
Sedimentological data suggests that the depositional environment of the Wessex Formation was afloodplain intersected byfluvial (river) andlacustrine (lake) deposits. Water levels likely varied throughout the year,[27] due to there being more evaporation than precipitation, though precipitation was regardless quite high. The Wessex seems to have regularly experienced extreme storms[28] and periodic flood events, resulting in debris flows which would have deposited dead organisms in ponds.[29] Burned plant and insect material andfusain suggests that the environment experienced frequentwildfires, stifling for the most part the dense growth ofgymnosperms.[27][29] Much of the flora of the formed of low ground cover, consisting primarily ofpteridophytes, with occasional stands ofconifers,cycads and the tree fernTempskya.[27] Most vertebrate material from the Wessex Formation originates from plant debris beds, resulting from the aforementioned flooding events.[29]
Aside fromNeovenator, the dinosaur fauna of the Isle of Wight include the theropodsAristosuchus,Calamospondylus,Ceratosuchops,Eotyrannus,Ornithodesmus,Riparovenator andThecocoelurus, the sauropodsChondrosteosaurus,Eucamerotus andOrnithopsis,[30][31] the thyreophoransPolacanthus[31] andVectipelta,[32] and the ornithopodsBrighstoneus,[4]Comptonatus,[33]Hypsilophodon,Iguanodon,[31]Mantellisaurus,[34]Valdosaurus[30][31] andVectidromeus.[35] Thepterosaur fauna of the Wessex Formation consists ofColoborhynchus,Caulkicephalus,Istiodactylus,[36]Vectidraco,[37] andWightia;[38] multiple unnamed pterosaur taxa, including actenochasmatid, are also known.[36]Neosuchiancrocodyliforms includeBernissartia,Koumpiodontosuchus[39] andVectisuchus.[40] Limited evidence exists ofelasmosaurids andleptocleididplesiosaurs.[41] The mammal fauna of the Wessex Formation includes themultituberculateEobataar[42] and thespalacotheriidYaverlestes.[43]Albanerpetontid amphibians are represented byWesserpeton.[3] The fish fauna of the Wessex Formation, bothbony andcartilaginous, is extensive, includinghybodontiform and modernsharks (Selachii),pycnodontiforms,Lepidotes andScheenstia.[44] Invertebrates are represented by an assortment ofnon-biting midges,[45]hymenopterans (wasps) including multipleparasitoid taxa,[46]coleopterans (beetles), theavicularoid spiderCretamygale,[47] and theostracodCypridea.[48]
IfNeovenator was indeed present in the older Angeac-Charente bone bed, it would have been part of an entirely different, less well-understoodfaunal assemblage. The depositional environment of the Angeac-Charente was likely a tropical or subtropical floodplain combined with a poorly oxygenated swamp, with a flora dominated bycheirolepidiacean conifers. Most of the vertebrate fauna of the locality appear to have inhabited and been preserved in the swamp, making it among the few swamp bone beds.[49] The most well-known dinosaur fossils from the Angeac-Charente are an indeterminatebasalornithomimosaur, whose fossils are known in great numbers.[10][50] Other theropod clades are represented by teeth, including large teeth which have been tentatively assigned tomegalosaurids. Remains of adacentrurine stegosaurian, similar toDacentrurus, are also known. Some ornithischians are known, includingcamptosaurs,hypsilophodontids, an indeterminateankylosaur, andEchinodon sp. Two indeterminatepterodactyloid pterosaurs have been identified.[10]
Neovenator was likely similar to other allosauroids in terms of feeding and hunting behaviour, likely targeting smaller species (or juveniles of larger species). There is evidence in the form ofbite marks thatNeovenator likely preyed onMantellisaurus.[15] Darren Naish, Steve Hutt and David Martill noted in 2001 that allosauroids in general had limb proportions poorly suited for great speeds, and thatNeovenator likely would have hunted throughambush. They speculated that, like what has been suggested for other allosauroids,Neovenator may have ganged up on large prey in a similar fashion to certaincrocodilians andKomodo dragons. Sinceontogenetic niche partitioning, where juveniles occupy differentniches to the adults, has been suggested for certain non-avian dinosaurs, Naish, Hutt, and Martill hypothesised that the same might have applied toNeovenator.[6]
The holotype ofNeovenator was recovered from a plant debris bed, underlain by sandstone and mudstone, and overlain by further mudstone. Its remains were found in association with multiple fossil logs,bivalves of the familyUnionidae,[1] and a partial skeleton of the iguanodontidBrighstoneus.[1][4] Despite being discovered close to one another, it is likely that the two specimens were swept together by one of two sedimentary pulses (one indicative of a small flood, the other indicative of a far larger flood) which formed the locality.[1][8]
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