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Megalosauridae is a monophyletic family of Carnivore theropod dinosaurs within the group Megalosauroidea. Appearing in the Middle Jurassic, megalosaurids were among the first major radiation of large theropod dinosaurs. They were a relatively primitive group of basal Tetanurae containing two main subfamilies, Megalosaurinae and Afrovenatorinae, along with the basal genus Eustreptospondylus, an unresolved taxon which differs from both subfamilies.
The defining megalosaurid, Megalosaurus bucklandii, was first named and described in 1824 by William 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 known Middle Jurassic carnivorous dinosaurs, with the best-preserved femur at 805 mm and a proposed body mass of around 943 kg. 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.
The family Megalosauridae was first defined by Thomas 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. In the early years of paleontology, most large Theropoda were grouped together and up to 48 species were included in the clade Megalosauroidea, 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. Some paleontologists, such as Paul 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 family Spinosauridae.
Megalosaurinae (all megalosaurids more closely related to Megalosaurus than Afrovenator) 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 to Afrovenator than Megalosaurus) are characterized by a squared anterior margin of the antorbital fossa and the puboischiadic plate being broadly open along the midline.
Modern paleontology first began to approach the problematic Cladistics 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 named Coelurosauria and Pachypodosauria respectively. Later on, Huene distinguished between carnivorous and Herbivore dinosaurs in Pachypodosauria, placing the meat-eaters in a new group Carnosauria.
As more information was uncovered about basal theropods and phylogenetic characteristics, modern paleontologists began to question the proper naming for this group. In 2005 paleontologist Paul Sereno rejected the use of the clade Megalosauridae due to its ambiguous early history in favor of the name Torvosauridae. 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 than Ceratosauria and that the name Megalosauridae should represent this group. Megalosauridae also has priority over Torvosauridae under ICZN rules governing family names.
Then, in 2012, Carrano, Benson, and Sampson did a much larger analysis of tetanurans and defined Megalosauria more broadly as the clade containing Megalosaurus, 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 to Afrovenator than Megalosaurus.
Carrano, Benson, and Sampson also included various megalosaurids that had previously been excluded from cladograms in their 2012 study, such as Duriavenator and Wiehenvenator in Megalosaurinae and Magnosaurus, Leshansaurus, and Piveteausaurus in Afrovenatorinae.
Sciurumimus, a small theropod described in 2012 which preserved protofeathers, was initially believed to be a juvenile megalosauroid. This led to the belief that megalosaurids may have had feathers. However, subsequent analyses have placed Sciurumimus as a basal Coelurosauria, and several supposed megalosauroid synapomorphies reported in the original description are shared with basal coelurosaurs.
In 2016, Wiehenvenator was found by phylogenetic analysis to be in the Megalosauridae as a sister group to Torvosaurus. The following is a cladogram based on the phylogenetic analysis conducted by Rauhut et al., showing the relationships of Wiehenvenator.Rauhut, Oliver W.M., Hübner, Tom R., and Lanser, Klaus-Peter, 2016, "A new megalosaurid theropod dinosaur from the late Middle Jurassic (Callovian) of north-western Germany: Implications for theropod evolution and faunal turnover in the Jurassic", Palaeontologia Electronica 19.2.26A: 1–65
In 2019, Rauhut and Pol described Asfaltovenator vialidadi, a basal allosauroid displaying a mosaic of primitive and derived features seen within Tetanurae. Their phylogenetic analysis found traditional Megalosauroidea to represent a basal grade of Carnosauria, paraphyly with respect to Allosauroidea. This would render Megalosauridae a family of carnosaurs.
This indicates that megalosaurids could have scavenged for the carcasses of marine creatures left by the receding tides. Another possibility is that megalosaurids were Piscivore, 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, and have been documented as stomach contents in Poekilopleuron. Both this genus and Dubreuillosaurus 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.
The global radiation of these carnivorous Theropoda occurred in two steps. First, radiation occurred during Pangaea's breakup during the Early Jurassic, about 200 million years ago. When the Tethys Ocean 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, in Allosauroidea and Coelurosauria. Megalosauridae appears to have gone extinct at the end of this time period.
Megalosaurid remains have been found in various areas of the world throughout history. For example, Megalosauridae contains the most primitive theropod embryo ever found, from Tithonian Portugal 152 million years ago (mya). In addition, various megalosaurid fossil discoveries have been dated to Bajocian-Callovian England and France 168 to 163 mya, Middle Jurassic Africa about 170 mya, Late Jurassic China 163 to 145 mya, and Tithonian North America about 150 mya. Most recently, megalosaurids have been found in the Tiourarén Formation in Niger, proving again that these basal tetanurans have experienced global radiation. Teeth from the Late Jurassic aged Tacuarembó Formation of Uruguay and the Tendaguru Formation of Tanzania indicate the presence of a large megalosaurine, likely Torvosaurus.
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