Phuwiangvenator

Samathi and colleagues performed a phylogenetic analysis in their description of Phuwiangvenator and Vayuraptor based on the data set used by Apesteguía and colleagues in their 2016 description of Gualicho.[2][5] In their analysis, they added the two new taxa from their description as well as the contemporaneous theropod Siamotyrannus and removed the problematic Santanaraptor from their data set. They recovered Phuwiangvenator as a basal member of megaraptora with the following synapomorphies for the group: a ventral groove on the first manual phalanx, a transversely compressed first manual ungual, a prominence on the anterior surface of the distal end of the tibia, a lateral extension of the astragalus, and a flattened sacrum. Notably, they recovered Vayuraptor as a non-megaraptoran coelurosaur, which has not been recovered by subsequent analyses.[2][4] The strict consensus tree from their analysis is shown below.[2]

Coelurosauria	Siamotyrannus Chilantaisaurus Gualicho Vayuraptor Tyrannoraptora Tanycolagreus Compsognathidae Maniraptoriformes Tyrannosauroidea Megaraptora Phuwiangvenator Fukuiraptor Megaraptoridae Megaraptor Orkoraptor Aerosteon Australovenator

The most recent phylogenetic analysis to include Phuwiangvenator was the one which accompanied the description of Maip in 2022. The analysis of the authors, led by Alexis Aranciaga Rolando, included every taxon confidently assigned to megaraptora including an unnamed specimen from the Griman Creek Formation (called the "Lightning Claw" by some publications) and another unnamed specimen from the Bajo Barreal Formation. Their analysis of Megaraptora differs from the initial only in the exact classification of Vayuraptor and the inclusion of more taxa, which the authors state led to a more resolved tree consistent with previous analyses.[4]

Coelurosauria	Maniraptoriformes Compsognathidae Tyrannosauroidea Megaraptora Phuwiangvenator Vayuraptor Fukuiraptor Megaraptoridae Lightning Ridge megaraptoran Australovenator "Clade A" Aoniraptor Bajo Barreal megaraptoran Megaraptor Murusraptor "Clade B" Aerosteon Tratayenia Orkoraptor Maip

The known material of Siamotyrannus, which coexisted with Phuwiangvenator

There are three mid-sized theropods which have been discovered so far in the Sao Khua Formation, all of which are known from relatively incomplete remains. These are: Phuwiangvenator, Vayuraptor, and Siamotyrannus. The authors who described the former two were careful to draw numerous distinctions between all three taxa to ensure that they none of the three are junior synonyms of the others. The distinctions they draw between Phuwiangvenator and Siamotyrannus are as follows: the dorsal vertebra of the former is longer than it is tall, whereas in the latter they are much taller in relation to their length and the rims of the sacral centra of Siamotyrannus are much more convex than in Phuwiangvenator.[2]

The distinctions between Phuwiangvenator and Vayuraptor which they outline are as follows: the cranioproximate process of the astragalus is much more pronounced in Vayuraptor and they note that Vayuraptor had two grooves on the body of the astragalus whereas Phuwiangvenator only had one. The final distinction between the two are the lack of an upward curving process of the lateral condyle in Vayuraptor, which is one of the autapomorphies of Phuwiangvenator. This last trait was also observed in all ontogenetic stages of Allosaurus, which the authors state implies that the absence of this trait in Vayuraptor means that the distinction is probably not the result of ontogeny.[2]

In 2023, Alexis M. Aranciaga Rolando and colleagues published an analysis of the inferred forelimb musculature of megaraptorans. Their reasoning for pursuing this analysis was the uniquely large forearms of megaraptorans that were not seen in any other group of large theropods in the Late Cretaceous. The analysis focuses mostly on the derived members of megaraptoridae, but they comment on the muscular and tendinous attachment sites on the bones of Phuwiangvenator. They outline several key differences between the manual phalanges and unguals of Phuwiangvenator and those of younger megaraptorans like Megaraptor, Maip, and Australovenator. These differences are concentrated in the first finger of the hand, which in derived megaraptorans hosts a single giant claw which is presumed to have served a purpose in hunting or feeding. The first claw of Phuwiangvenator lacks the well-developed furrows which are present on the proximal extension of the claw. In other species, these are thought to have been attachment sites for powerful muscles, enabling greater control and flexibility of the finger movements of these animals.[6]

The metatarsals of the holotype, with the fracture visible (B5)

One of the metatarsals of the holotype of Phuwiangvenator was found to have signs of a greenstick fracture when analyzed by Samathi and colleagues in 2022. They inferred that this fracture is not taphonomic because of the texture of the bone, which is smooth on the medial side, but is very calloused on the lateral side. These are interpreted as signs of the fracture being healed, which implies that the animal survived this injury. The bone itself is also twisted, which would not be the normal condition because the second metatarsal would have been in the center of the foot in life. Samathi and colleagues compare this fracture to other known pathologies on theropod bones, including those seen in specimens Allosaurus and Yangchuanosaurus. From these comparisons, they conclude that the injury was likely caused by an external blunt force. Other features of the bone determined by computer tomography led the authors to conclude that the animal was not yet fully grown when it died.[3]

Samathi and colleagues suggest several possible causes of the fracture. They first suggest a blow from the tail club of an ankylosaur, but they discard this hypothesis because no ankylosaur remains are known from the Early Cretaceous of Thailand. Possible competition or combat with another large theropod is also suggested, because there are several large theropods known from nearby localities, but they state that there is no direct evidence for this and it is impossible to determine without additional evidence. They conclude that the most likely explanation is that the animal hit its foot into a hard object such as a rock or possibly fell over to cause the type of incomplete fracture which is observed.[3]

Evidence for severe pathologies being healed has been used by some authors to infer herding behavior in extinct animals. There are known associations of large theropods (i.e. Albertosaurus and Mapusaurus), so the authors state that it is possible that Phuwiangvenator exhibited similar behaviors. However, they also suggest that the animal may have simply survived for a short time after the injury and lived with a limp.[3]

None of the skull of Phuwiangvenator has been preserved, but the animal is presumed to have been carnivorous. However, it does not have the same specialized arm musculature to aid in feeding as other megaraptorans.[6]

A life reconstruction of some animals which coexisted with Phuwiangvenator

The Sao Khua Formation has thus far only yielded continental fossils, with no evidence of marine fossils or marine sediments.[7] There have historically been reports of ichthyosaur and plesiosaur teeth from the Sao Khua Formation, but these have now been identified as belonging to crocodyliformes, which are mostly non-marine during the Cretaceous.[8][9] The sediments of the Sao Khua Formation are composed of red clays, mudstones, sandstones, siltstones, and conglomerate rocks, which indicate a fluvial environment which also possessed lakes, floodplains, and braided channels.[7][10]

Of all the Mesozoic formations in northeastern Thailand, the Sao Khua has the most abundant and diverse fossils.[11] In addition to Phuwiangvenator and Vayuraptor,[2] the region was host to numerous other theropods including the spinosaurid Siamosaurus,[11] the large theropod Siamotyrannus, an unnamed spinosaurid, and an unnamed carcharodontosaurid.[8][12] The area was also home to numerous small theropods including the ornithomimosaur Kinnareemimus and an unnamed compsognathid.[13] Numerous sauropods are also known from the area including Phuwiangosaurus and several unnamed forms. However, there have yet to be any ornithischians discovered from the formation.[8]

Freshwater life is abundant in the Sao Khua Formation. These include the neosuchians Siamosuchus, Sunosuchus, and Theriosuchus, numerous species of turtles, amiiform fish, several hybodontiform sharks, and freshwqater bivalves.[10] There is also an indeterminate pterosaur known from the formation.[14]