| Megalosaurids | |
|---|---|
 | |
| Skeletal mount ofTorvosaurus tanneri,Museum of Ancient Life | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | †Megalosauria |
| Family: | †Megalosauridae Huxley, 1869 |
| Type species | |
| †Megalosaurus bucklandii Mantell, 1827 | |
| Subgroups | |
| Synonyms | |
| |
Megalosauridae is amonophyleticfamily ofcarnivoroustheropod dinosaurs within the groupMegalosauroidea. Appearing in theMiddle Jurassic, megalosaurids were among the first major radiation of large theropod dinosaurs.[1] They were a relatively primitive group of basaltetanurans containing two main subfamilies, Megalosaurinae and Afrovenatorinae, along with the basal genusEustreptospondylus, an unresolved taxon which differs from both subfamilies.[2]
The defining megalosaurid,Megalosaurus bucklandii, was first named and described in 1824 byWilliam Buckland after multiple finds in Stonesfield,Oxfordshire, UK.Megalosaurus was the first formally described dinosaur and was the basis for the establishment of the clade Dinosauria. It is also one of the largest knownMiddle Jurassic carnivorous dinosaurs, with the best-preservedfemur at 805 mm and a proposed body mass of around 943 kg.[3] Megalosauridae has mainly been recognized as a European group of dinosaurs, based on fossils found in France and the UK, but fossils show that the group is also found in North America, Africa, South America and possibly Asia.[4][5][6]
The family Megalosauridae was first defined byThomas Huxley in 1869, yet it has been contested throughout history due to its role as a "waste-basket" for many partially described dinosaurs or unidentified remains.[7] In the early years ofpaleontology, most largetheropods were grouped together and up to 48 species were included in the cladeMegalosauria, the basal clade of Megalosauridae. Over time, most of these taxa were placed in other clades and the parameters of Megalosauridae were narrowed significantly. However, some controversy remains over whether Megalosauridae should be considered its own distinct group, and dinosaurs in this family remain some of the most problematic taxa in all Dinosauria.[5][7] Some paleontologists, such asPaul Sereno in 2005, have disregarded the group due to its shaky foundation and lack of clarified phylogeny. However, recent research by Carrano, Benson, and Sampson has systematically analyzed all basal tetanurans and determined that Megalosauridae should exist as its own family. They have been generally closely related to the familySpinosauridae.
Like other tetanurans, megalosaurids are carnivorous theropods characterized by large size andbipedalism. Specifically, megalosaurids exhibit especially giant size, with some members of the family weighing more than onetonne. Over time, there is evidence of size increase within the family. Basal megalosaurids from theEarly Jurassic had smaller body size than those appearing in the late Middle Jurassic. Due to this size increase over time, Megalosauridae appear to follow a size increase pattern similar to that of other giant sized theropods likeSpinosauridae.[2] This pattern followsCope's rule, the postulation by paleontologistEdward Cope about evolutionary increase in body size.[5][8]
One unambiguoussynapomorphy of Megalosauridae is a lower and longerskull with a length to height ratio of 3:1. In addition, the typicalskull roof tends to be much less ornamented than that of other tetanurans, and crests or horns are either very small or absent entirely. Megalosaurids also havefemoral heads with an orientation 45 degrees between anteromedial and fully medial. Megalosauridae are also defined by the following unique unambiguous synapomorphies:[1][2]
Megalosaurinae (all megalosaurids more closely related toMegalosaurus thanAfrovenator) are characterized by a moderate (0.5–2.0) height/length ratio of the premaxilla below the level of the nares, compared to other megalosaurids which have a lower ratio and thus less tall snout tip.
Afrovenatorinae (all megalosaurids more closely related toAfrovenator thanMegalosaurus) are characterized by a squared anterior margin of theantorbital fossa and the puboischiadic plate being broadly open along the midline.
Dental findings are frequently used to differentiate between various theropods and to further inform cladisticphylogeny. Tooth morphology and dental evolutionary markers are prone tohomoplasy and disappear or reappear throughout history. However, megalosaurids have several specific denture conditions that differentiate them from other basal theropods. One dental condition present in Megalosauridae is multiple enamel wrinkles near the carinae, the sharp edge or serration row of the tooth. Ornamented teeth and a well-marked enamel surface also characterize basal megalosaurids. The ornamentation and well-marked surface appears in early megalosaurids but disappears in derived megalosaurids, suggesting that the condition was lost over time as megalosaurids grew in size.[9]
From the family's inception, many specimens found in the field have been wrongly classified as megalosaurids. For example, most large carnivores found for about a century after the naming ofMegalosaurus bucklandii were placed in Megalosauridae.Megalosaurus was the first paleontological finding of its kind when William Buckland discovered a giantfemur and named it in 1824, predating even the term Dinosauria.[1] When initially defined, the speciesM. bucklandii was anatomically based on various dissociated bones found in quarries around the village of Stonesfield, UK. Some of these early findings included a rightdentary with a well-preserved tooth,ribs,pelvic bones, andsacral vertebrae.[10] As earlypaleontologists and researchers found more dinosaur bones in the surrounding area, they attributed them all toM. bucklandii since it was the only named and described dinosaur at this point in history. Therefore, the species was initially described and classified by a mass of possibly unrelated characteristics.[3]
Modernpaleontology first began to approach the problematiccladistic separation of Megalosauridae during the early 20th century.Fredrich von Huene separated carnivorous theropods, which had all been grouped into the broad category of megalosaurids, into two distinct families of larger, more giant sized and smaller, more lightly built theropods. These two groups were namedCoelurosauria andPachypodosauria respectively. Later on,Huene distinguished between carnivorous andherbivorous dinosaurs in Pachypodosauria, placing the meat-eaters in a new groupCarnosauria.[2]
As more information was uncovered about basal theropods and phylogenetic characteristics, modern paleontologists began to question the proper naming for this group. In 2005 paleontologistPaul Sereno rejected the use of the clade Megalosauridae due to its ambiguous early history in favor of the name Torvosauridae.[11] Today, it is accepted that megalosaurids existed at least as a group of basal tetanurans, due to the fact that they have more derived taxa thanceratosaurs[2] and that the name Megalosauridae should represent this group. Megalosauridae also has priority over Torvosauridae under ICZN rules governing family names.[11]
Megalosauridae was first phylogenetically defined in 1869 by Thomas Huxley, yet was used as a ‘waste-basket’ clade for many years. In 2002, Ronan Allain redefined the clade after he discovered a complete megalosaurid skull in northwestern France of speciesPoekilopleuron. Using the characters described in this study, Allain defined Megalosauridae as dinosaurs includingPoekilopleuron valesdunesis, now known asDubreuillosaurus,Torvosaurus,Afrovenator, and all descendants of their common ancestor. Allain also defined two taxa within Megalosauridae: Torvosaurinae was defined as all Megalosauridae more closely related toTorvosaurus than toPoekilopleuron andAfrovenator, and Megalosaurinae was defined as all those that are more closely related toPoekilopleuron.[3] Megalosauridae also falls under the basal clade Megalosauroidea, which also containsSpinosauridae. However, many taxa are still quite unstable and cannot be placed in one clade with absolute certainty. For example,Eustreptospondylus andStreptospondylus, while they are both defined as Megalosauridae, are often excluded to make more stable cladograms since they are not defined to a certain subgroup.[1][2] Thecladogram presented here follows Benson (2010) and Bensonet al. (2010).[12][13]
| Megalosauridae | |
Then, in 2012, Carrano, Benson, and Sampson did a much larger analysis of tetanurans and defined Megalosauria more broadly as the clade containingMegalosaurus,Spinosaurus, and all its descendants. In other words, Megalosauria is the group that contains the two families Megalosauridae and its close relative Spinosauridae. Within this new cladogram, Megalosauridae was given a new subfamily Afrovenatorinae, which included all megalosaurids more closely related toAfrovenator thanMegalosaurus.
Carrano, Benson, and Sampson also included various megalosaurids that had previously been excluded from cladograms in their 2012 study, such asDuriavenator andWiehenvenator in Megalosaurinae andMagnosaurus,Leshansaurus, andPiveteausaurus in Afrovenatorinae.[2]
Scuirumimus albersodoerferi, a small theropod described in 2012 which preservedprotofeathers, was initially believed to be a juvenile megalosauroid. This led to the belief that megalosaurids may have had feathers.[14] However, subsequent analyses have placedSciurumimus as a basalcoelurosaur,[15] and several supposed megalosauroid synapomorphies reported in the original description are shared with basal coelurosaurs.[16]
In 2016,Wiehenvenator was found by phylogenetic analysis to be in the Megalosauridae as asister taxon toTorvosaurus. The following is a cladogram based on the phylogenetic analysis conducted by Rauhut et al., showing the relationships ofWiehenvenator.[17]
| Megalosauridae | |
In 2019, Rauhut and Pol describedAsfaltovenator vialidadi, a basal allosauroid displaying a mosaic of primitive and derived features seen withinTetanurae. Their phylogenetic analysis found traditional Megalosauroidea to represent a basalgrade ofcarnosaurs,paraphyletic with respect toAllosauroidea. This would render Megalosauridae a family of carnosaurs.[18]
Megalosaurids have been suggested to be predators or scavengers inhabiting coastal environments. Middle Jurassic-eratracks believed to have left by megalosaurids have been found at Vale de Meios in Portugal. During the middle Jurassic, this site would have been a tidal flat exposed at low tide on the edge of a lagoon. Unlike most coastal tracks, which are parallel to the coastline and probably left by migrating animals, the Vale de Meios tracks were perpendicular to the coast, with the vast majority oriented towards the lagoon. This indicates that the megalosaurids which would have left these tracks approached the tidal flat once the tide retreated.[19]
This indicates that megalosaurids could have scavenged for the carcasses of marine creatures left by the receding tides. Another possibility is that megalosaurids werepiscivorous, approaching the coast to hunt for fish. Spinosaurids, which were close relatives of megalosaurids, had numerous adaptations for piscivory and semiaquatic life, so such a lifestyle is supported by phylogenetic data. Shark teeth,cartilage fragments, andgastroliths have been documented as stomach contents inPoekilopleuron. Both this genus andDubreuillosaurus were discovered in sediments also preserving mangrove roots, providing further evidence for a coastal habitat. Nevertheless, this does not rule out the possibility that megalosaurids also fed on terrestrial prey.[20]
Species included in Megalosauridae have been found on every moderncontinent, split relatively equally between sites on theGondwana andLaurasiasupercontinents.Paleogeography findings show that Megalosauridae was mainly restricted to the Middle toLate Jurassic, suggesting they went extinct at theJurassic-Cretaceous boundary 145 million years ago.[10] Teeth from theBerriasian-Valanginian agedBajada Colorada Formation in Argentina suggest that the group may have persisted into the Early Cretaceous in South America.[21]
The global radiation of these carnivoroustheropods occurred in two steps. First, radiation occurred during Pangaea's breakup during theEarly Jurassic, about 200 million years ago. When theTethys Sea emerged between the supercontinent, megalosauroids radiated to the two-halves of Pangaea. The second step of radiation occurred during the Middle and Late Jurassic, 174 to 145 million years ago, inallosauroids andcoelurosaurs. Megalosauridae appears to have gone extinct at the end of this time period.[3]
Megalosaurid remains have been found in various areas of the world throughout history. For example, Megalosauridae contains the most primitive theropodembryo ever found, fromEarly TithonianPortugal 152 million years ago (mya). In addition, various megalosaurid fossil discoveries have been dated toBajocian-Callovian England and France 168 to 163 mya, Middle Jurassic Africa about 170 mya, Late Jurassic China 163 to 145 mya, andTithonian North America about 150 mya.[10] Most recently, megalosaurids have been found in theTiourarén Formation in Niger, proving again that these basal tetanurans have experienced global radiation.[5] Teeth from the Late Jurassic agedTacuarembó Formation of Uruguay and theTendaguru Formation of Tanzania indicate the presence of a large megalosaurine, likelyTorvosaurus.[6]
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