This is an accepted version of this page
| Megaraptorans | |
|---|---|
 | |
| Diagram showing the skull and skeleton ofMurusraptor | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | Coelurosauria (?) |
| Clade: | †Megaraptora Benson, Carrano & Brusatte, 2010 |
| Type species | |
| †Megaraptor namunhuaiquii Novas, 1998 | |
| Subgroups | |
Megaraptora is aclade of carnivoroustheropoddinosaurs with controversial relationships to othertetanuran theropods. Itsderived members, theMegaraptoridae are noted for their large hand claws and powerfully-built forelimbs, which are usually reduced in size in other large theropods.
Megaraptorans are incompletely known, and no complete megaraptoran skeleton has been found. However, they still possessed a number of unique features. Their forelimbs were large and strongly built, and theulna bone had a unique shape in members of thefamily Megaraptoridae, a subset of megaraptorans which excludesFukuiraptor andPhuwiangvenator. The first two fingers were elongated, with massive curved claws, while the third finger was small.[2] Megaraptoran skull material is very incomplete, but a juvenileMegaraptor described in 2014 preserved a portion of the snout, which was long and slender.[3] Leg bones referred to megaraptorans were also quite slender and similar to those ofcoelurosaurs adapted for running.[2] Although megaraptorans were thick-bodied theropods, their bones were heavilypneumatized, or filled with air pockets. Thevertebrae,ribs, and theilium bone of the hip were pneumatized to an extent which was very rare among theropods, only seen elsewhere intaxa such asNeovenator.[4] Other characteristic features includeopisthocoelous neck vertebrae andcompsognathid-like teeth.[2]
Megaraptorans were originally placed as basaltetanurans as part of the familyNeovenatoridae within theallosauroid cladeCarcharodontosauria.[4] By the early 2020s, many studies had come to find that megaraptorans instead represented members ofCoelurosauria, with their exact position within this group being uncertain.[5] However, some studies still support an allosauroid classification.[6]
Megaraptorans were most diverse in the earlyLate Cretaceousperiod of South America, particularlyPatagonia. However, they had a widespread distribution.Phuwiangvenator andFukuiraptor, the mostbasal and second most basal known members of the group, lived inThailand andJapan, respectively. Megaraptoran material is also common in Australia,[7] and the largest known predatory dinosaur from the continent,Australovenator, was a megaraptoran.[8]
Megaraptorans were medium to large-sized theropods, ranging fromFukuiraptor, which was about 4.2 meters (14 feet) in length, to the 8.4 meters (28 feet) longOrkoraptor, the 9 to 10 meters (30 to 33 feet) longMaip[5] and the 12.8 meters (42 feet) longBahariasaurus, if it is a member.[9] Most megaraptorans are known from very fragmentary remains, although certain characteristics can be identified in multiple members of the clade. At least some megaraptorans, such asMurusraptor andAerosteon, had extensively pneumatic bones (most noticeably theilia and ribs), which likely housed sinuses connected to the lungs, similar to modern birds.[10] The slender leg bones and longmetatarsals of several species indicate that members of this group likely hadcursorial habits.[4] Most megaraptorans are part of thefamily Megaraptoridae, which was named byFernando Novas and his colleagues in 2013. This family is united by several adaptations of the ulna and claws which are not present in the basal megaraptoranFukuiraptor.[2]
No megaraptoran fossil is known to preserve a complete skull, although skull material is known for several taxa.Aerosteon,Megaraptor,Orkoraptor, andMurusraptor preserve several bones of the rear part of the skull,[10] lower jaws are known fromAustralovenator,[8] and a juvenile specimen ofMegaraptor described in 2014 preserved much of the snout as well asparietal fragments. Teeth have been found in many genera. Collectively, megaraptorans can be reconstructed as having a long, lightly built skull with many relatively small teeth.[3]
Based onMegaraptor, thepremaxillary bone at the tip of the snout is small, with a long and rod-like branch of bone which extends above theexternal nares (nostril holes). The nares themselves were very large and elongated, akin to some early tyrannosauroids (Dilong,Proceratosaurus, etc.). The snout also had some similarities to carcharodontosaurids, namely the straight upper edge of the maxilla and rectangular nasal bones. The parietal bones at the top of the skull, behind the eyes, had a strongly developedsagittal crest, as in tyrannosauroids.[3] Otherwise, the rear part of the skull is rather simple, without any pronounced crests or bosses, although thelacrimal andpostorbital bones did haverugose patches in some genera.Aerosteon andMurusraptor possessed a pneumaticquadrate, as in a few allosauroids (Sinraptor,Mapusaurus) and tyrannosauroids.[10] Thedentary, which is only known inAustralovenator, is long and graceful, with the first tooth smaller than the rest (as in tyrannosauroids). Themandible as a whole has only a single meckelian foramen, as in carcharodontosaurians, tyrannosaurids, and ornithomimids.[2] However, the rear part of the mandible (as seen inMurusraptor) was significantly more lightly built than that of tyrannosauroids. Preservedbraincase material has similarities to both carcharodontosaurians and tyrannosauroids.[10]
The premaxillary teeth ofMegaraptor were variably similar to those of tyrannosauroids, being small, incisiform (chisel-like) and D-shaped in cross section.[3] However,Murusraptor's premaxillary teeth were fang-like, as in non-tyrannosauroid theropods. Megaraptoran maxillary teeth show much variety betweengenera, although they were generally small compared to the snout with minimal enamel ornamentation. Some megaraptorans, such asOrkoraptor, Australovenator, andMegaraptor, had teeth which were 8-shaped in cross section and completely unserrated from the front (similar todromaeosaurids andcompsognathids),[3] whileMurusraptor had anterior serrations only at the tip of its teeth.[10]Fukuiraptor had very laterally compressed and blade-like teeth (similar tocarcharodontosaurs) with both anterior and posterior serrations.[10]
Thecervical (neck)vertebrae of megaraptorans were nearly unique among theropods in the fact that they were stronglyopisthocoelous.[2] This means that they were convex from the front and concave from behind. Opisthocelous vertebrae are also characteristic ofAllosaurus[11] andsauropods, and they may facilitate high flexibility without sacrificing defense againstshear forces.[12] Otherwise, the cervicals were similar to those of carcharodontosaurians, with shortneural spines,transverse processes (projecting rib facets) located around mid-length on thecentra, and a pair of large lateral pits known aspleurocoels. In fact, one or more pleurocoels were present in most megaraptoran vertebrae, and they connected to a complex system of numerous small air pockets within the vertebrae. This web-like internal structure of megaraptoran vertebrae (and that of a few other theropods) has been described as "camellate".[4][2]
The proximal caudals (vertebrae at the base of the tail) had a longitudinal ridge running along their lower surface, similar to the case inNeovenator but unlike tyrannosauroids.[4] They also had a pair of lateral ridges which stretched downwards from the transverse processes to the centra. These ridges, known as centrodiapophyseallaminae, defined a large depression (infradiapophyseal fossa) under the transverse processes. Although these ridges were also present in dorsal (back) vertebrae and have been found in other theropods, megaraptorans were practically unique in the fact that their centrodiapophyseal laminae were well-developed at the base of the tail, sometimes even more so than the dorsal vertebrae. Only spinosaurids share this feature.[10] The strong development of these ridges may indicate that the tail was deep and muscular.[3]
The dorsalribs were thick and curved yet hollow and pierced by a hole near their connection to the vertebrae. Thegastralia (belly ribs) were wide and strongly built paddle-shaped structures, with the left and right sides fused at the midline of the chest. These features signified that megaraptorans were wide-bodied theropods, akin to the condition in tyrannosaurids.[3]
Megaraptorans have a sigmoid (S-shaped)humerus (upper arm bone), similar to that of both basal allosauroids and basal coelurosaurs. Most megaraptorans had large, robust humeri akin to those ofAllosaurus, but the basal-most memberFukuiraptor has a much more slender humerus.[4] The distal part of the humerus (near the elbow) has a well-developed system ofcondyles and grooves similar to that of coelurosaurs, particularly the dromaeosaurids.[13]
Theulna of megaraptorids is characteristic in several regards. Theolecranon process is well-developed, though it is thin, blade-like, and extends as a crest longitudinally down the shaft of the ulna. In addition, megaraptorids have acquired another long, crest-like structure on the ulna called the lateral tuberosity, which is perpendicular to the blade of the olecranon. As a result, the ulna of megaraptorids is T-shaped in cross section, with three prongs formed by the forward-projection anterior process, the outwards-projecting lateral tuberosity, and the backwards-projecting olecranon process. These adaptations are absent in the most basal megaraptoran,Fukuiraptor. Theradius is not unusual compared to other theropods.[2][13]
Megaraptorans also had very characteristic hands.[4][2] The first two fingers were large and slender, but the third one was small. These relative differences in finger length are somewhat similar to the case in tyrannosauroids and various other basal coelurosaurs, but the megaraptoran trend of forearm and finger enlargement is opposite to the trend of forearm diminishment which characterizes the lineage towards more advanced tyrannosauroids.Megaraptor retained a vestigial fourthmetacarpal, the hand bone that would have connected to the fourth finger in early dinosaurs. This was a primitive feature lost by most other tetanurans. The first two fingers had absurdly large unguals (claws); inMegaraptor the first claw was larger than the entire ulna. Unlike the large unguals of many other theropods (megalosauroids, for example), megaraptoran claws were thin and oval-shaped in cross-section. These claws also had asymmetrically positioned grooves on their flat faces and a sharp ridge on their lower edge in megaraptorids (non-Fukuiraptor megaraptorans). Thecarpus (wrist) of megaraptorans incorporated a semilunate (crescent-shaped) carpal similar to that ofmaniraptorans.[13]
Examinations of the forelimbs of megaraptorans by Rolando, Novas, and Porfiriet al., that were published in January 2023 show that the megaraptorans' forelimb bones are remarkably well-developed; powered by strong pectoral and front limb muscle that were functionally significant and important to the paleobiology of this group of theropods. Their data also suggests these muscle attachments became increasingly pronounced through megaraptoran evolutionary history, being substantially better developed in derived taxa such asAustralovenator and especiallyMegaraptor itself than in earlier genera such asFukuiraptor. Their results further suggest that the highly specialized forelimbs were capable of highly complex movements, such as great extension and flexion, particularly in the highly derived hands, as well as enhanced humeral protraction; attributes that likely aided in prey capture.[14]
Thefemur (thigh bone) of megaraptorans is only known inAustralovenator andFukuiraptor, but it is similar to that of coelurosaurs in several respects. For example, thegreater trochanter is well-developed and offset from the femoral shaft by a deep concavity. The size of the greater trochanter has the added effect of making the portion of the femur near the hip socket rectangular, when seen from above. In non-coelurosaur theropods, the greater trochanter is small, making the femur teardrop-shaped when seen from above. The femoral head is slightly upturned as in carcharodontosaurians (particularly carcharodontosaurids) and some coelurosaurs. In megaraptorans, the portion of the femur near the knee is asymmetrical when seen from the front due to the lateral condyle projecting further distally than the medial condyle.[2]
Thetibia was also similar to that of coelurosaurs. It was a long and thin bone. The front of the lateral condyle of the tibia hooks downwards, similar to the condition inNeovenator,Tanycolagreus, and some tyrannosauroids. The medial and lateral malleoli are expanded and project away from each other, as in advanced tyrannosauroids (both) and carcharodontosaurians (medial malleolus only). The front surface of the distal tip of the tibia (near the ankle) had the form of a flattened facet for the reception of theastragalus bone of the ankle, similar to the case in coelurosaurs. The inner edge of this facet was defined by a ridge, a feature unique to megaraptorids. The upper edge of the facet lacked a well-defined supra-astragalar buttress, unlike allosauroids. The ascending process of the astragalus, which lays on the facet, is expanded into a large trapezoidal plate of bone, similar to coelurosaurs but unlike the small, triangular ascending process of allosauroids.[2]Fukuiraptor,Australovenator, andAerosteon have a distinct forward-pointing prong on the outer edge of the astragalus, andFukuiraptor andAustralovenator have an additional prong that projects backwards.[4]
Thefibula is also long and strongly tapers away from the knee, as in coelurosaurs. It connects to a small facet on the outer edge of the astragalus (as in coelurosaurs) rather than a large facet on the upper edge (as in allosauroids). Near the knee and facing the tibia, the fibula has a wide groove or depression known as a proximomedial fossa.Metatarsal III, the foot bone which connected to the middle toe, was very long and slender in all megaraptorans, as in coelurosaurs. The joint for the middle toe is tall and pulley-shaped, with a deep and crescent-shaped depression visible from below.[2]
Theilium (upper plate of the hip) was a heavily pneumatized bone, filled with air pockets and perforated by pits. The only other large theropod known to possess a pneumatic ilium isNeovenator.[4] In some megaraptorans, the preacetabular blade has a notch along its front edge, as in tyrannosauroids but also inNeovenator. A stronger concavity was present a bit lower, between the preacetabular blade and pubic peduncle. This concavity, known as the cuppedicus (or preacetabular) fossa, was rimmed by a prominent shelf on the inner face of the ilium. This trait is also known in various coelurosaurs,Chilantaisaurus, and probablyNeovenator. The postacetabular blade, on the other hand, lacks a large concavity. In non-coelurosaurian tetanurans, this portion of the ilium has a large depression known as a brevis fossa, which is visible from the outer face of the ilium. However, coelurosaurs and megaraptorans have a much smaller brevis fossa which occupies only a portion of the rear edge of the ilium, and it is mostly hidden from outside observers.[2]
Theischium (rear lower plate of the hip) is only known inMurusraptor. It is slightly expanded, similar to that of carcharodontosaurids.[10] Thepubis (front lower plate of the hip) has a much more pronounced scythe-like expansion at its tip, which is over 60% as long as the main shaft of the bone. This adaptation, known as a pubic boot, is also known in carcharodontosaurians and tyrannosaurids. The pubis is also expanded near its contact with the ilium. The left and right pubic bones are not entirely fused to each other, they are separated along their midline by an oval-shaped hole.[2]
A palaeobiogeographic assessment was conducted by Phil Bell, Steve Salisbury et al., which accompanied the description of an unnamed megaraptorid (referred to by the public media as "Lightning Claw," and possibly synonymous withRapator) from opal fields southwest of Lightning Ridge, Australia. This supports an Asian origin of Megaraptora in the latest Jurassic (150–135 Ma), an Early Cretaceous (130–121 Ma) divergence of theGondwanan lineage leading to Megaraptoridae, and an Australian root for the megaraptorid radiation of Late Cretaceous Gondwana. The specimen also allowed for alternative phylogenetic testing as to the placement of megaraptorans as either tyrannosauroids or carcharodontosaurids.[7][15] This was expanded upon by Lamannaet al. (2020) who hypothesized that the megaraptorid dispersal from Australia to South America (probably via Antarctica) came with an increase in body size, and that megaraptorids kept their large body size until the K-Pg extinction.[16] The authors also noted that while their phylogenetic analysis didn't support it, Australian megaraptorids likely formed a paraphyletic grade leading to South American forms.
The genera which make up Megaraptora had been placed in a number of different theropod groups before the formation of the clade in 2010.Megaraptor andFukuiraptor were independently considered to be giantdromaeosaurids when they were first discovered in the 1990s due to the large hand claws being misidentified as foot claws. However, these mistakes were rectified after closer inspection of the holotype (in the case ofFukuiraptor) or the discovery of new specimens (in the case ofMegaraptor). By the mid-to-late 2000s, they were considered to be basal tetanurans, usually members of Allosauroidea. Smithet al. (2008) reportedMegaraptor-like ulnae from Australia, and found evidence thatMegaraptor was aspinosauroid. The same year,Orkoraptor was described as an unusual giant coelurosaurian with some similarities with the much smaller compsognathids.Aerosteon was considered a relative ofAllosaurus in its description less than a year later, whileAustralovenator was considered to be the sister taxon to Carcharodontosauridae.
This influx of new data in the late 2000s led to several major reanalyses of basal tetanuran phylogenetics, with interesting implications for these taxa. A study by Roger Benson, Matt Carrano &Steve Brusatte in 2010 found thatAllosauroidea (orCarnosauria, as it was sometimes called) included a major subdivision known as Carcharodontosauria, which was split into theCarcharodontosauridae and a newly named family:Neovenatoridae. Neovenatorids, as formulated by these authors, containedNeovenator,Chilantaisaurus, and a newly named clade: Megaraptora. Megaraptora containedMegaraptor,Fukuiraptor,Orkoraptor,Aerosteon, andAustralovenator. These genera were allied with the other neovenatorids on the basis of several features spread out throughout the skeleton, particularly the large amount of pneumatization present. The pneumatic ilium ofAerosteon was particularly notable, asNeovenator was the only other taxon known to have that trait at the time. Neovenatorids were envisioned as the latest-surviving allosauroids, which were able to persist well into the Late Cretaceous due to their low profile and coelurosaur-like adaptations.[4] Later studies supported this hypothesis, such as Carrano, Benson & Sampson large study of tetanuran relationships in 2012,[17] and Zanno & Makovicky description of the newly discovered theropodSiats in 2013, which they placed within Megaraptora.Fukuiraptor andAustralovenator were consistently found to be close relatives of each other; this was also the case forAerosteon andMegaraptor;Orkoraptor was a"wildcard" taxon difficult to place with certainty.
Thecladogram below illustrates the most recent revision of the Benson, Carrano, & Brusatte (2010) hypothesis that megaraptorans were allosauroids within the family Neovenatoridae.[4] The cladogram follows Coria & Currie (2016), who addedMurusraptor to the study and utilized the family Megaraptoridae, which was originally named by Novaset al. (2013).[10][18]
However, an alternative hypothesis was forming, first published as anAmeghiniana abstract byFernando Novaset al. (2012).[19] Novas and his colleagues argued that the features used to linkNeovenator to Megaraptora were more widespread than the 2010 paper implied, and that the proposed coelurosaurian convergences may have signified a legitimate connection between Megaraptora and Coelurosauria. In addition, they noted that Benson, Carrano, & Brusatte only sampled three coelurosaurs in their analysis. Novaset al.'s arguments were formulated and published in a 2013 review ofpatagonian theropods, which removed Megaraptora from the Carcharodontosauria and instead placed the group within Coelurosauria. More specifically, megaraptorans were found to be deep within theTyrannosauroidea, a radiation of basal coelurosaurs including the famedtyrannosaurids. As Novaset al. (2013) removed Megaraptora from Neovenatoridae, they named a new family, Megaraptoridae, which contained all Megaraptorans apart from the basal ("primitive") taxonFukuiraptor. They found little evidence thatChilantaisaurus,Neovenator, orSiats were megaraptorans, but they did place the tyrannosauroidEotyrannus within Megaraptora. Despite the hypothesized close relation between megaraptorans and tyrannosaurids, Novaset al. noted that the megaraptoran lineage had a functional morphology which diverged in a direction opposite to the tyrannosaurids. While tyrannosaurids had small arms and large, powerful heads, megaraptorans had large arms, giant claws, and relatively weak jaws.[2] The skull of a newly discovered juvenile specimen ofMegaraptor, published in 2014, supported this hypothesis due to its similarities to the skull of basal tyrannosauroids such asDilong. Nevertheless, megaraptorans still retained many similarities to carcharodontosaurians such asNeovenator, so the uncertainty behind their classification was not fully resolved.[3]
The cladogram below illustrates the results of a study which supports the Novaset al. (2013) hypothesis that megaraptorans are derived tyrannosauroids. This study was Porfiriet al. (2014), which described the juvenileMegaraptor specimen.Gualicho,Murusraptor, andTratayenia were not yet described when this study was undertaken.[3]
In 2016, Novas and his colleagues published a study of megaraptoran hand anatomy, in an attempt to help settle the question of their classification. They found that megaraptorans lacked most of the key features in the hands of derived coelurosaurs includingGuanlong andDeinonychus. Instead, their hands retain a number of primitive characteristics seen in basal tetanurans such asAllosaurus. Nevertheless, there are still a number of traits that support megaraptorans as members of the Coelurosauria.[13] A 2016 study of the "lightning ridge megaraptoran" by Bellet al. supported the idea that megaraptorans were tyrannosauroids based on the fact that Porfiriet al. (2014) incorporated skull data fromMegaraptor and a wider variety of coelurosaurians compared to Benson, Carrano, & Brusatte (2010).[7] Mottaet al. (2016) agreed, and proposed that a new fragmentary patagonian theropod,Aoniraptor, was a non-megaraptorid megaraptoran. Their study also noted the similaritires betweenAoniraptor, the enigmatic theropodDeltadromeus, andBahariasaurus, a giant African theropod with remains destroyed by World War II bombings. Therefore, they suggested thatBahariasaurus andDeltadromeus were also basal megaraptorans, and thatAoniraptor,Bahariasaurus, andDeltadromeus could have formed a distinct family, the Bahariasauridae.[9] A 2019 redescription ofMurusraptor by Rolando, Novas, & Agnolín continued to find Megaraptora in a polytomy at the base of Tyrannosauroidea, based on the dataset of Apesteguiaet al. (2016).[20]
A 2022 study byNaish andCau, in contrast, classifiedEotyrannus as an intermediate gracile tyrannosauroid outside of Megaraptora. Their research supported a tyrannosauroid position for megaraptorans, even thoughEotyrannus itself was not a megaraptoran. They recovered Megaraptora as radiation of derived tyrannosauroids close to Tyrannosauridae, similar to that found by Porfiri et al. (2014).[21]
In 2016, a third hypothesis for megaraptoran relations was derived from Porfiriet al.'s revision to the Novaset al. dataset in 2014. That year,Sebastian Apesteguía and his colleagues described an unusual new theropod,Gualicho. The addition ofGualicho,Deltadromeus, and several corrections within the Novaset al. dataset led to an interesting result. Megaraptorans were far removed from the position deep within Tyrannosauroidea which the Novaset al. dataset had originally supported. Allosauroidea was rendered aparaphyleticgrade, with carcharodontosaurids,Neovenator, a clade formed byChilantaisaurus andGualicho, and finally Megaraptora progressively closer to traditional coelurosaurs.[22]
Another study, Porfifiet al. (2018), expanded on the dataset of Apesteguíaet al. (2016) by adding two additional megaraptorids.[23] Although the results are different, the methodology analysis was practically identical to that of Apesteguiaet al.. (2016), only differing in the fact that it incorporated two megaraptorans not sampled in the analysis of Apesteguiaet al..[22] One of these wasMurusraptor, which was described in 2016 around the same time asGualicho.[10] The second was a new megaraptorid,Tratayenia. Porfiriet al. (2018) placedTratayenia andMurusraptor as megaraptorids, withFukuiraptor as the basalmost megaraptoran as found by all previous revisions to the Novaset al. dataset. However, Megaraptora was in apolytomy at the very base of Coelurosauria, along withChilantaisaurus,Gualicho, and Tyrannoraptora ("traditional " coelurosaurs). Non-coelurosaurian avetheropods were also subjected to a large polytomy owing to the unstable position ofNeovenator. Porfiriet al. (2018) also commented on Mottaet al.'s erection of Bahariasauridae in 2016, and noted thatGualicho may be a bahariasaurid in light of its similarities withDeltadromeus. If this was the case, then megaraptorans experienced much more diversity in their forelimbs than previously considered;Gualicho had very small, tyrannosaurid-like forelimbs.[23]
In late 2018, Delcourt & Grillo published a study focusing on tyrannosauroids. They reused the 2018 analysis from Porfiriet al.', though corrected some scores and added data from recent studies. The study returnedNeovenator to a monophyletic Allosauroidea, and placed megaraptorans as basal non-tyrannosauroid coelurosaurs close toChilantaisaurus andGualicho.Murusraptor was also placed as the second-most basal megaraptoran, ahead ofFukuiraptor.[24]
Thecladogram below follows the results of the phylogenetic analysis of Delcourt & Grillo (2018).
In the 2022 description ofMaip, Rolandoet al. suggested that Megaraptora was deeply nested within Coelurosauria, with Megaraptora classified as thesister taxon to Tyrannosauroidea. This is in line with previous studies, which have likewise suggested a close relationship between tyrannosauroids and megaraptorans. Rolandoet al.. also noted the presence of two distinct clades within Megaraptora: a more inclusive clade, comprising all megaraptorids exceptFukuiraptor andAustralovenator, (shown below as "Clade A"), and a more exclusive clade of larger, entirely South American megaraptorids (shown below as "Clade B"). The cladogram below displays the coelurosaurian results of the phylogenetic analyses by Rolandoet al.[5]
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