Phosphatodraco

''Phosphatodraco'' is a genus of azhdarchid pterosaur that lived during the Late Cretaceous of what is now Morocco. In 2000, a pterosaur specimen consisting of five cervical (neck) vertebrae was discovered in the Ouled Abdoun Phosphatic Basin. The specimen was made the holotype of the new genus and species ''Phosphatodraco mauritanicus'' in 2003; the genus name means " dragon from the phosphates", and the specific name refers to the region of Mauretania. ''Phosphatodraco'' was the first Late Cretaceous pterosaur known from North Africa, and the second pterosaur genus described from Morocco. It is one of the only known azhdarchids preserving a relatively complete neck, and was one of the last known pterosaurs. Additional cervical vertebrae have since been assigned to the genus, and it has been suggested that fossils of the pterosaur '' Tethydraco'' represent wing elements of ''Phosphatodraco''.

Due to the fragmentary nature of the holotype cervical vertebrae, there has been controversy over their order. The describers considered them as cervicals (abbreviated as C) C5–C9 in the series, the first preserved vertebra (C5) being broken in two, but others consider them C3–C8, C3 and C4 being two different vertebrae. The interpretation followed has consequences for how ''Phosphatodraco'' is distinguished from other azhdarchids and how large it is thought to have been; the describers considered it to have had a wingspan of ; the alternate interpretation would lead to a wingspan. The complete neck may have been long. ''Phosphatodraco'' is mainly distinguished by its C8 (or C7) vertebra being very elongated, 50% longer than the C5, and in having a prominent neural spine that is almost as tall as the centrum (the main part of the vertebra), truncated in a square shape at the top, and located far back. As an azhdarchid, it would have had a proportionally long neck, small body, and long limbs, compared to other pterosaurs.

The closest relatives of ''Phosphatodraco'' appear to have been ''Aralazhdarcho'' and ''Eurazhdarcho''. Azhdarchids have historically been considered skim-feeders that caught prey from water in coastal settings, but it has since been suggested that the context in which their fossils are found and their morphologysuch as their long, stiffened necks (informed by for example the neck of ''Phosphatodraco'')is more consistent with them having foraged terrestrially like storks or ground hornbills, but this is still debated. Although pterosaurs were thought to have declined in diversity towards the time of their extinction 66 million years ago, the diversity in taxa, including ''Phosphatodraco'', in the Ouled Abdoun Basin, which dates to the late Maastrichtian, right before the Cretaceous-Paleogene extinction event, indicates their extinction happened abruptly.

Discovery

During the late 1990s, remains of pterosaurs began to be discovered in different fossil localities of Morocco, all dating to the Cretaceous period. In 2000, pterosaur fossils were found by the '' Office Chérifien des Phosphates'' (OCP, located in Casablanca) during paleontological field work in the eastern part of the Ouled Abdoun Phosphatic Basin near the city of Khouribga in central Morocco. They were collected from "site 1" in the Sidi Daoui mine in the northern part of Grand Daoui, an area in which phosphate is quarried. The fossils were found in the upper part of the stratigraphic unit which miners called "couche III". These excavations were part of a collaboration between the OCP, the '' Ministère de L'Energie et des Mines'', Rabat, and the French ''Centre National de la Recherche Scientifique'', which had taken place since 1997.

The pterosaur material, catalogued as specimen OCP DEK/GE 111, consists of five disarticulated but closely associated cervical (neck) vertebrae and an indeterminate bone, most likely belonging to a single individual. The vertebrae are crushed and damaged, and the surface of the bone is missing in some areas, with some infilling of phosphate sediments, and the fossils have therefore not been removed from the matrix. The block containing the bones is long and wide. During mechanical preparation of the specimen fossil remains of other animals were also found in association, including of several types of fish and mosasaurs.

The specimen was made the holotype of the new genus and species ''Phosphatodraco mauritanicus'' by paleontologist Xabier Pereda-Suberbiola and colleagues in 2003. The genus name derives from the words phosphate and the Latin ''draco'', meaning " dragon from the phosphates", and the specific name refers to the region of Mauretania where the fossils were found. The describers gave the etymology of Mauretania as Latin for North Africa; other sources specify it as an area stretching from Algeria to Morocco. ''Phosphatodraco'' was the first Late Cretaceous pterosaur known from North Africa (and thus the first known member of the family Azhdarchidae of this age from the region), and only the second pterosaur genus described from Morocco (the first being '' Siroccopteryx''). At the time it was described, it was one of the only known azhdarchids preserving a relatively complete neck (the others being '' Zhejiangopterus'' and '' Quetzalcoatlus''), and was one of the last known pterosaurs. Complete neck vertebral series are rare for azhdarchids, but such vertebrae are some of the most commonly found and best known remains of the group.

In 2018 paleontologist Nicholas R. Longrich and colleagues reported pterosaur fossils collected from "couche III" in cooperation with the Moroccan fossil industry the previous three years; until that point, only the single specimen of ''Phosphatodraco'' was known from the assemblage. At the time, the collection was the largest and most diverse collection of pterosaurs from the Maastrichtian age of the Late Cretaceous, and included two cervical vertebrae they assigned to ''Phosphatodraco'', based on similarity with the holotype in size and proportions. One of the vertebrae, specimen FSAC-OB 12, was identified as a C5 (though stated in the description to be similar to C6 of the holotype); the other, FSAC-OB 13, was identified only as a cervical vertebra. The cervical ribs (ribs of the neck vertebrae) of FSAC-OB 12 do not appear to have yet fused to the centrum (the main part of the vertebra), so the animal may not have been completely mature. These specimens are housed at '' Faculté des Sciences Aïn Chock'' in Casablanca.

In 2020 paleontologists Claudio Labita and David M. Martill described an articulated (where the bones are connected as in life) pterosaur wing from "couche III" (specimen FSAC CP 251, bought from fossil dealers), which they assigned to '' Tethydraco'', a genus also described by Longrich and colleagues in 2018 based on a humerus (upper arm bone). ''Tethydraco'' was originally considered a pteranodontid, but Labita and Martill concluded it was an azhdarchid, and that it possibly represented the wing elements of ''Phosphatodraco''. They noted that more associated and articulated pterosaur fossils were being collected from these deposits due to improving methods used by fossil diggers, and that azhdarchid fossils were becoming abundant. They also cautioned that the provenance of some of the Moroccan fossils was difficult to establish, due to the commercial nature of their collection.

Interpretations of cervical (neck) vertebra order

Pereda-Suberbiola and colleagues originally interpreted the five preserved cervical vertebrae of ''Phosphatodraco'' as cervicals C5–C9. The frontmost preserved vertebra they interpreted as C5 consisted of two fragments; they found it unlikely that these belonged to two different vertebrae, since they lay in continuity with no sediment in between, and overlapped each other in some areas. They considered the sideways expansion at the front of this vertebra to be due to crushing, and pointed out that such preservation where fragile, yet well-preserved bones are associated with damaged material of the same individual is known from other vertebrate fossils in the same level. They identified the frontmost vertebra as a C5 because this is usually the longest cervical vertebra in pterosaurs, their length decreasing hindward.

In 2007 paleontologist Alexander W. A. Kellner and colleagues noted that ''Phosphatodraco'' was one of the most interesting azhdarchids found in Africa, but used cautious language about the original interpretation of the vertebrae. Kellner suggested in 2010 that interpretation of the holotype specimen had been affected by taphonomy (changes during decay and fossilisation); he instead proposed that the element originally described as a fifth cervical vertebra was actually the third and fourth vertebrae which had been compressed together, giving the impression that they were a single vertebra broken in the middle. This would shift the order of the vertebrae to C3–C8, and though this did not change the validity of the taxon (its distinctness being based on its stratigraphy, geography, and morphology), Kellner noted that the diagnosis (the suite of features that distinguish a taxon) and size estimate had to be reevaluated. The frontmost cervical vertebrae which are missing from ''Phosphatodraco'' in either scheme are the atlas (C1, which connects with the back of the skull) and the axis (C2).

Subsequent articles from 2011 and 2015 with Kellner among the co-authorship have concurred with Kellner's interpretation. Paleontologist Alexander Averianov disagreed with Kellner's reinterpretation of the cervical vertebrae in 2014, and considered the original description accurate. A 2015 article by paleontologist Mátyás Vremir and colleagues called the issue "controversial" and considered the specimen too crushed for proper comparison, and Martill and Markus Moser concurred with this in 2018. Paleontologists Darren Naish and Mark P. Witton (the co-authors of Vremir's article) followed Kellner's interpretation in 2017. Paleontologist Rodrigo V. Pêgas and colleagues also followed Kellner's order in 2021. Though the palaeontologist Alexandru A. Solomon and colleagues noted the suggested change in interpretation of the holotype order in 2019, they stated that even if the reinterpretation was correct, the specimen was too damaged for comparison with the single known cervical vertebra of their new genus '' Albadraco''.

Description

In their 2003 description, Pereda-Suberbiola and colleagues estimated ''Phosphatodraco'' to have had a wingspan close to , based on comparison with other azhdarchids with preserved cervical vertebrae, and referred to it as a "large azhdarchid pterosaur". This is larger than azhdarchids such as ''Zhejiangopterus'' and ''Montanazhdarcho'', and comparable to the smaller species of ''Quetzalcoatlus'', ''Q. lawsoni''; the larger ''Q. northropi'' is thought to have reached , thereby being the largest known flying animal. Witton grouped ''Phosphatodraco'' with "midsized" azhdarchids based on this size estimate in 2013. In 2010, Kellner suggested this size estimate too large, based on his reinterpretation of the neck vertebra order. Naish and Witton, who followed Kellner's interpretation, listed a neck-length of for ''Phosphatodraco'' in 2017, and a wingspan of .

There were two main types of azhdarchid skulls; very long, low skulls that were up to ten times longer than wide, and some that were much shorter than that, closer to those of other pterosaurs. Some had crests and some did not. Azhdarchids had necks that were proportionally longer than those of other pterosaurs, and their vertebral column and much of the rest of the skeleton was pneumatized (filled with air-sacs that lightened it). The body skeleton of azhdarchids was small but robust, and their upper arm bones were solidly built. Their wing-metacarpals (the hand bones that connect with the fingers) were relatively the largest among pterosaurs, and the longest bones in the wings; their wing-fingers (which supported the wing-membranes) were relatively short. The pelvises were relatively robust, and the hindlimbs long. Combined, the long-wing metacarpals and legs made azhdarchids relatively taller when standing than other pterosaurs, though their feet were narrow and short. As a pterosaur, ''Phosphatodraco'' was covered with hair-like pycnofibers.

Cervical vertebrae

The suite of features that distinguish a taxon from other related taxa is called a diagnosis, and in the case of ''Phosphatodraco'', these features are all found in the cervical vertebrae. Since Pereda-Suberbiola and colleagues considered the preserved vertebrae to be C5–C9 of the series in their description, that is the diagnosis and description followed here. Note that if Kellner's suggestion that the series actually represents vertebrae C3–C8 is correct, the diagnostic features listed by Pereda-Suberbiola and colleagues may possibly be inaccurate, and the following description would refer to different vertebrae in the series. Pereda-Suberbiola and colleagues found ''Phosphatodraco'' to differ from other azhdarchids in that the hindmost vertebra, C8 in their order, is very elongated, 50% longer than C5, and has a prominent neural spine (the spine that projects upwards from the vertebra) that is almost as tall as the centrum, truncated in a square shape at the top, and located far back. ''Phosphatodraco'' is also distinct in that the ratio between the maximum length of the vertebrae and the front width between prezygapophyses ( processes at the sides of the centrum which connected with the postzygapophyses of the previous vertebrae) of the middle cervical vertebrae is about 4.3 in C5 and 4.1 in C6.

The five preserved cervical vertebrae have hollow centra and their cortical bone (the outer, thick layer) is about thick. The vertebrae vary in length, the longest being the frontmost of those preserved, a C5 broken in two according to Pereda-Suberbiola and colleagues (C3–C4 according to Kellner), which they estimated to have been about long when complete. The first of these fragments is , the second is about . When viewed from the side, the hind end of this vertebra shows a developed left postzygapophysis, and the convex articular condyle (the condyle

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