Ornithocheiridae


Ornithocheiridae is a group of pterosaurs within the suborder Pterodactyloidea. These pterosaurs were among the last to possess teeth. Members that belong to this group lived from the Early to Late Cretaceous periods, around 140 to 96 million years ago.
Ornithocheirids are generally infamous for having an enormously controversial and very confusing taxonomy. Although agreements that these animals were related, and therefore similar to istiodactylids and pteranodontians, there is still no virtual consensus over the exact content and interrelationships of this group. Ornithocheirids were the most successful pterosaurs during their reign, they were also the largest pterosaurs before the appearance of the azhdarchids such as Quetzalcoatlus. Ornithocheirids were excellent fish hunters, they used various flight techniques to catch their prey, and they are also capable of flying great distances without flapping constantly. The later existence of the crested pteranodontids made paleontologists think that ornithocheirids were their potential ancestors. This is due to their very similar aspects such as the different flying techniques, the long distance flights, and most of all their diet, which consists mainly on fish.

History

Naming

The family Ornithocheiridae is without a doubt, one of the most well-known pterosaur groups, this is mostly due to their very controversial and convoluted taxonomic history. Most of the ornithocheirid fossil record consists of isolated teeth, as well as fragmentary bones, reaching hundreds or even thousands of remains in some localities. The first uncovered ornithocheirid remains were described in 1861 by British paleontologist Sir Richard Owen; he assigned the fossil remains to a new species of Pterodactylus: P. simus. In 1869, British paleontologist Harry Govier Seeley erected the new generic name Ornithocheirus, and assigned P. simus as its type species, therefore creating Ornithocheirus simus. Later, in 1870, Seeley created the name Ornithocheirae to only contain Ornithocheirus. However, in the same year, this was emended to Ornithocheiridae by Seeley himself following the article 11.7.1.3 of the ICZN.
In 1874, Owen had proposed two new genera for the Cretaceous British pterosaurs: Coloborhynchus and Criorhynchus based on highly distinctive jaw fragments. Owen reassigned P. simus as the type species of Criorhynchus, creating Criorhynchus simus. He referred three species to Coloborhynchus, including the addition of a new species called C. clavirostris; no type species was designated however. In 1876 however, Seeley pointed out that Criorhynchus was a junior synonym of Ornithocheirus, a concept that was followed by paleontologist Richard Lydekker in 1888. In the latter year, Lydekker acknowlegded that Ornithocheirus simus was the type species of Ornithocheirus, he also distiguished O. simus by its tall rostrum, while other species referred to Ornithocheirus had jaw tips. Therefore, to avoid confusion, Lydekker preferred to use the name Criorhynchus for O. simus, and Ornithocheirus for the species with lanceolate jaw tips, this concept was later favored by paleontologist Reginald Walter Hooley in 1914. In his review of Ornithocheirus, he divided the family Ornithocheiridae into two subfamilies: Ornithocheirinae and Criorhynchinae; the former consisted of Ornithocheirus and Lonchodectes, while the latter consisted of Amblydectes and Criorhynchus. In his review, Hooley also considered the species Coloborhynchus clavirostris as a synonym of Criorhynchus simus. In 1967, paleontologist Oskar Kuhn placed Criorhynchus within the family Criorhynchidae, and also recognized Ornithocheirus within the family Ornithocheiridae and subfamily Ornithocheirinae. He also designated the species Coloborhynchus clavirostris as the type species of Coloborhynchus, but agreed with Hooley that it was synonymous with Criorhynchus simus. In 1994 however, Yuong-Nam Lee revalidated the genus Coloborhynchus, and regarded it as distinct from Criorhynchus simus. Later, in 2001, paleontologist David Unwin revised the taxonomic history of the Cambridge Greensand pterosaurs, and divided Ornithocheiridae into three genera: Ornithocheirus, Coloborhynchus and Anhanguera. Unwin also designated Pterodactylus simus as the type and only species of Ornithocheirus.
In 2003, Unwin defined the family Ornithocheiridae as Haopterus gracilis, Ornithocheirus simus, their most recent common ancestor, and all its descendants. He included the genera Anhanguera, Brasileodactylus, Coloborhynchus, Haopterus, Ludodactylus and Ornithocheirus within the family, and also concluded that Araripesaurus, Arthurdactylus and Santanadactylus may belong to this family as well. However, their taxonomic status and precise relationships with other ornithocheirids are uncertain. In 2019, upon the description of the pterosaur Mimodactylus, Haopterus, which was assigned to this family by Unwin, and recovered as a basal eupterodactyloid by Brian Andres and colleagues, was reassigned by Alexander Kellner and colleagues as the sister taxon of the former.
and Tropeognathus
In 2014, Andres and colleagues defined the Ornithocheiridae with a different definition: the most inclusive clade containing Ornithocheirus simus but not Anhanguera blittersdorffi. They placed the genera Coloborhynchus, Ornithocheirus and Tropeognathus within the Ornithocheiridae, while placing Anhanguera'' within the separate family Anhangueridae. However, back in 2001, Unwin considered the name Anhangueridae a junior synonym of Ornithocheiridae, a concept that was later followed by several paleontologists such as Mark Witton. Later phylogenetic analyses however, contradict this name synonymy, with Ornithocheiridae and Anhangueridae classified as different families, meaning that they follow the 2014 analysis by Andres and colleagues.

Description

Size

Ornithocheirids were among the largest pterosaurs that ever flew the skies, and perhaps even comparable to the sizes of the largest azhdarchids. The ornithocheirid species Coloborhynchus capito likely measured around, this made it quite large for its time, and it is considered one of the largest toothed pterosaurs. The largest size for an ornithocheirid was recorded in 2013 by Kellner et al., which uncovered and described unusually large specimens of the ornithocheirid Tropeognathus. These specimens consisted of a skeleton with skull, and extensive elements of all body parts except the tail and the lower hindlimbs, this then led to an impressive wingspan estimate measuring, though a maximum estimate was concluded to have reached.
Other smaller genera include the crested Caulkicephalus, meaning "caulk head", in reference of the Isle of Wight; an estimated wingspan for Caulkicephalus measured around. Another similarly sized ornithocheirid is newly named Ferrodraco from the Winton Formation in Australia, a genus closely related to another Australian ornithocheirid called Mythunga. The wingspan of Ferrodraco however, measured at least, which is slightly shorter than the one estimated for Caulkicephalus.

Skull and crests

Most ornithocheirids are known for their prominent "keeled" rounded crest over, and under their snout, which is more precisely on the premaxilla, and the underside of the mandible. This trait made ornithocheirids unique pterosaurs. Ornithocheirus for example, had very similar jaws to Coloborhynchus, but can be differed by its much narrower structure, Tropeognathus on the other hand, had a larger premaxilla, but its snout isn't nearly as narrow. The "keeled" crest on Tropeognathus was also well-developed and more prominent than in Ornithocheirus, an indication that it was respectively larger in size.
The ornithocheirid Coloborhynchus also shares features with Anhanguera, while also having several dissimilarities. The tip of the snout in Coloborhynchus for example, flared out into a wider rosette, in contrast to its narrow posterior premaxilla. These features differed it from Anhanguera, which has a rounded and spoon-shaped snout in comparison to the robust and box-shaped snout of Coloborhynchus. This led to the idea that Coloborhynchus was more closely related to Ornithocheirus rather than Anhanguera, but then again, it had unique features such as the position of its tooth sockets and its jawtip being less curved. The keel-shaped crest on the premaxilla in Coloborhynchus was also thinner than in close relatives such as Tropeognathus.
One of the more specialized ornithocheirids is Caulkicephalus, it didn't only had the convex "keeled" crests on its snout and underside of the mandible, but it also had the similar cranial crest that the later Pteranodon had. Fossil remains of Caulkicephalus also indicated that it possessed several similarities with another cranial crested pterosaur called Ludodactylus.
The recently named Ferrodraco also shares several features with Ornithocheirus and Tropeognathus, and therefore was placed in this family. Ferrodraco possesses an elongated snout with a "keeled" crest similar to that of Ornithocheirus, but its premaxilla has a triangular shape and it's likely more flattened compared to other ornithocheirids. The first tooth pair in the premaxillae is also directed vertically and is slightly set-off from above the jawline. These autapomorphies are unique in Ferrodraco, and are probably related to its more advanced evolutionary changes.

Wings

According to many studies, the wing-membranes of the ornithocheirids were extensive and broad, resulting in a high aspect ratio, analyses also show that most ornithocheirids had low wing loadings, which led to an easier maneuver during flight. Ornithocheirid wings mostly differed from earlier pterosaur wings because of their impressive size, and the wings weren't only capable of soaring high, but also soaring very long distances without flapping. These pterosaurs also bore proportionally enormous forelimbs; around five times stronger than their legs, and much stronger in biomechanical testing.

Pelvic structure

Studies of the pelvis of several ornithocheirids show that they were mostly of moderate size compared to the body as a whole, which is similar to the structures in other ornithocheiroids. The pelvic bones, which in total were three, were fused, this is seen in many genera such as Anhanguera. The ilium was long and low, and its front and rear blades projected horizontally beyond the edges of the lower pelvic bones. Given the structure length, the processes of these rod-like forms indicate that the hindlimb muscles attached to them with limited strength. An ischiopubic blade is seen in most ornithocheirids, where the pubic bone is fused with the broad ischium, resulting in a narrow build. The front of the pubic bones were also articulated with a unique structure, resulting in a pair of prepubic bones within, this formed a cusp covering the rear belly of the pterosaur, and was located between the pelvis and the belly ribs. This element suggests of a vertical mobility, and therefore had a function in breathing, with the chest cavity compensating in a relative rigidity. Normally, the blades of ornithocheirids, most notably in the larger genera, were fused in both sides, closing the pelvis from below and forming the pelvic canal. Ornithocheirids didn't have the hip joint perforated, but directed obliquely upwards, preventing a perfectly vertical position of the leg, and therefore allowed considerable mobility for the hindlimbs.

Vertebrae

Vertebral columns and vertebrae of ornithocheirids are only known from scarce remains, and it is hard to determine how their posture were. In some genera such as Tropeognathus, the first five dorsal vertebrae are fused into a notarium, with five sacral vertebrae fused into a synsacrum, and the third and fourth sacral vertebrae are keeled within. Ornithocheirids also had a relatively long and straight neck. The more advanced species typically had their neck longer than their torso.

Classification

Ornithocheirids were impressively large compared to other types of pterosaurs from the Early Cretaceous, and they were also one of the most successful of all pterosaurs. Most ornithocheirids were also long considered to only have a piscivorous diet, while this is mostly true, some remains uncovered were suggested to belong to an ornithocheirid with a non-piscivorous diet.
Other evolutionary changes of ornithocheirids include their territorial range, they were even present in the southern continent of Oceania, represented by two genera: Ferrodraco and Mythunga. Most of the species discovered in Europe and South America belonged to Coloborhynchus and Ornithocheirus, with some currently being synonyms of Tropeognathus and Anhanguera.

Phylogeny

The genus Cimoliopterus, originally Pterodactylus cuvieri, once assigned to Ornithocheirus as O. cuvieri, and formerly classified within this family, was reclassified as a targaryendraconian in 2019. Paleontologists found it to be closer to the newly named Targaryendraco, but as the sister taxon of Aetodactylus and Camposipterus, which were also once classified within the Ornithocheiridae.
Synapomorphies found unique in Coloborhynchus made authors assign it to the family Ornithocheiridae. However, some authors placed Coloborhynchus within the family Anhangueridae instead, and with the later discovery of Uktenadactylus, which shared very similar features with Coloborhynchus, it was suggested that it would be its sister taxon. The phylogenetic placement of the family Ornithocheiridae was conducted by Andres and Myers in 2013, where they placed the family as the sister taxon of Anhangueridae, and within the more inclusive group Ornithocheirae. In 2019, a later analysis by Pentland et al. had included the subfamily Ornithocheirinae within the family, with the genera Ferrodraco, Mythunga, Ornithocheirus, and Coloborhynchus as the members. In the analysis, they also placed the genus Tropeognathus within the family, but in a basal position.
Topology 1: Andres & Myers.
Topology 2: Pentland et al..

Paleobiology

Feeding and diet

Unlike the related istiodactylids, ornithocheirids were mainly fish hunters, consisting on a piscivorous diet that varied depending on the species. Paleontologists suggest that the feeding habits of the earlier istiodactylids changed through the process of evolution, resulting in a more aquatic-based lifestyle. The studies and analysis of ornithocheirids made paleontologists conclude how they hunted or catched fish successfully. The first basis is their long snouts, which they used for catching the fish near the surface of the water, paleontologists also suggest that they flew with moderated speed, giving them an overall advantage of a surprise attack during the hunt. Others suggest that ornithocheirids, similar to pteranodontids, just glided down directly to the water when catching prey, this technique can also be seen in modern-day soaring birds such as albatrosses.

Teeth enamels

Some discoveries, found that several ornithocheirids weren't completely fish hunters or aquatic consumers. This is confirmed with a 2017 analysis of carbon isotopes found on teeth enamels of ornithocheirids. In the analysis, they compared the enamel values of ornithocheirids with the aquatic and terrestrial consumers near the fossil site. The ornithocheirid enamels found in the Twin Mountains Formation were shown with lower enamel values compared to the aquatic consumers that lived nearby, suggesting a more terrestrial diet, similar to the carnivorous theropods found in the site, such as Acrocanthosaurus. Enamels found in the Paw Paw Formation however, were described with higher values, in comparison to the nearby terrestrial consumers. These enamel remains were analyzed, and paleontologists then found out that the values seen were more similar to aquatic consumer enamels rather the terrestrial ones. These analyses are considered very useful reference, since there isn't much data on several ornithocheirid genera. The earlier species that partially fed on terrestrial prey, such as Coloborhynchus, were also analyzed, and paleontologists conclude that there can be a specific evolutionary change between the earlier and later formations, due to the age difference. This can be demostrated with the enamel remains found in the Twin Mountains and Paw Paw formations. The ones found in the Twin Mountains Formation dated back to the earlier Aptian stage, while the remains found in the Paw Paw Formation dated back to the later Albian stage.

Locomotion and flight

Similar to modern-day albatrosses, most ornithocheirids used a flight technique called "dynamic soaring", which consists of travelling long distances without flapping using the vertical gradient of wind speed near the ocean surface as an advantage, but the flight speed is moderate. Several studies showed that most ornithocheirids sprawled their limbs to a large degree, similar to crocodiles, while other studies however, conclude that ornithocheirids were generally quadrupedal. Yet other studies concluded that ornithocheirids held their limbs more or less vertically extended, similar to an avian- or mammal-like configuration. Some studies in the later genera, show that ornithocheirids spend lots of their time sea fishing, and as a result, they perhaps influenced the later pteranodontids with the same piscivorous diet, as well as their well developed flight techniques. Analyses of limb proportions in the genus Anhanguera however, show that some ornithocheirids were consistent on hopping, but the later genera were suggested that they most likely walked on four limbs, which consists on their wing-fingers as the front limbs, and using their hind limbs to balance.

Paleoecology

Ornithocheirids were a widespreaded type of pterosaurs, with many fossil remains found across the world. The first true ornithocheirid specimens were uncovered in the Cambridge Greensand of England, these belong to the infamous genus Ornithocheirus, and dated back to the Albian stage of the Early Cretaceous. Within the fossil site, several other pterosaurs were also found, these include the ornithocheirids Amblydectes and Coloborhynchus, the targaryendraconian Camposipterus, the lonchodectid Lonchodraco, and the azhdarchoid Ornithostoma. The ornithischians Anoplosaurus, Acanthopholis, and the dubious Eucercosaurus and Trachodon were also found within the formation. Fossil remains of the sauropod Macrurosaurus were also present. The bird Enaliornis, as well as the ichthyosaurs Cetarthrosaurus, Platypterygius and Sisteronia were also found alongside the remains of ornithocheirids.
of Brazil, with the extent of the Santana Group shown in dark blue
A Lagerstätte called the Santana Group in northeastern Brazil was found to contain a large number of pterosaur genera. The most diverse formation of the group is the Romualdo Formation, known for its wide variety of pterosaur remains. The formation dates back 111 to 108 million years ago, also during the Albian stage of the Early Cretaceous. The Romualdo Formation is found to contain a variety of ornithocheirids, including Tropeognathus, Coloborhynchus and Araripesaurus, the targaryendraconian Barbosania, and the anhanguerids Anhanguera and Maaradactylus were also found alongside. The related Araripedactylus, Brasileodactylus, Cearadactylus, Santanadactylus and Unwindia were also present within the fossil site. Many other pterosaur were found within, these include the tapejarid Tapejara, as well as the thalassodromids Thalassodromeus and Tupuxuara. Other animals such the theropods Irritator, Mirischia and Santanaraptor, as well as the crocodylomorph Araripesuchus were also found. Several turtle remains were found within the formation, with some specimens referring to the genera Santanachelys, Cearachelys and Araripemys, and along with these, many fish remains were also found, these were assigned to the genera Brannerion, Rhinobatos, Rhacolepis, Tharrhias and Tribodus.
Ornithocheirids were also partially distributed in North America, and several specimens that were found are thought to belong to the genus Uktenadactylus. This pterosaur was uncovered in the Paw Paw Formation of Texas, United States, which dated back to the Albian and Cenomanian stages. The formation includes several ankylosaurian dinosaurs such as Pawpawsaurus, Texasetes, and an indetermine nodosaurid. Within the fossil site, several specimens of ammonoids were thought to belong to the genera Turrilites and Scaphites, and along with these, shark remains of Leptostyrax were also found.