Thalassodromeus

Although T. sethi was named after the god Seth for having a crest similar to his crown, Seth did not wear such a crown; the crest was instead similar to the crown of Amon (left).

The first known specimen of this pterosaur (an extinct order of flying reptiles) was collected in 1983 near the town of Santana do Cariri in the Araripe Basin of northeastern Brazil. Found in outcrops of the Romualdo Formation, it was collected over a long period in several pieces. The specimen (catalogued as DGM 1476-R at the Museu de Ciências da Terra) was preserved in a calcareous nodule, and consists of an almost-complete, three-dimensional skull (pterosaur bones are often flattened compression fossils), missing two segments of the bottom of the skull and mandible and the front of the lower jaw. The left jugal region and right mandibular ramus (half of the mandible) are pushed slightly inward. The skull was first reported in a 1984 Italian book, and preliminarily described and figured in 1990 by palaeontologists Alexander Kellner and Diogenes de Almeida Campos. Although the pieces of skull had been divided between museums in South and North America, they were assembled before 2002.[1][2][3][4][5]

In 2002, Kellner and Campos described and named the new genus and species Thalassodromeus sethi, skull DGM 1476-R being the holotype specimen. The generic name is derived from the Ancient Greek words θάλασσα, thálassa, '"sea"' and δρομεύς, dromeús, '"runner"', meaning "sea runner" in reference to the animal's supposed skim-feeding behaviour. The specific name refers to the Egyptian god Seth.[1][3] The specimen was not fully prepared at the time of this preliminary description.[6] The original describers chose the name sethi because the crest of this pterosaur was supposedly reminiscent of the crown worn by Seth, but the palaeontologists André Jacques Veldmeijer, Marco Signore, and Hanneke J. M. Meijer pointed out in 2005 that the crown (with its two tall plumes) was typically worn by the god Amon (or Amon-Ra) and his manifestations – not by Seth.[7][6]

Diagram of the T. sethi holotype skull (hatched lines represent missing parts), based on its initial interpretation

In 2006, palaeontologists David M. Martill and Darren Naish suggested that Thalassodromeus was a junior synonym of the related genus Tupuxuara, which was named by Kellner and Campos in 1988 based on fossils from the same formation. In the view of Martill and Naish, the differences between these genera (including two species of Tupuxuara, T. longicristatus and T. leonardii) were due to ontogeny (changes during growth) and compression of the fossils; Thalassodromeus was simply an older, larger, and better-preserved individual.[8] This idea was rejected by Kellner and Campos in 2007, who pointed out these species had differences in features other than their crests. They also noted that one specimen of Tupuxuara had a larger skull than Thalassodromeus (measured from the tip of the premaxilla to the back of the squamosal bone), despite Martill and Naish's contention that the latter was an older individual.[9] Kellner and Campos' view has since been accepted by other researchers, including Martill and Naish.[6][10]

Veldmeijer and colleagues assigned the front part of a mandible collected from the same formation to T. sethi in 2005. They concluded that although the two specimens differed in several details, the differences were not significant enough to base a new species on the mandible, and that the new specimen filled in the gap of Kellner and Campos' T. sethi skull reconstruction.[7] Palaeontologists Jaime A. Headden and Herbert B. N. Campos coined the new binomial Banguela oberlii, based on their reinterpretation of the jaw tip as belonging to a toothless member of the family Dsungaripteridae, in 2014. The generic name is Portuguese for "toothless" and the specific name honours private collector Urs Oberli, who had donated the specimen to the Naturmuseum St. Gallen (where it is catalogued as NMSG SAO 25109). Headden and Campos interpreted the tip of T. sethi's lower jaw as downturned; this and other features distinguished it from Banguela.[11] In their 2018 re-description of the further-prepared T. sethi holotype skull, palaeontologists Rodrigo V. Pêgas, Fabiana R. Costa, and Kellner assigned B. oberlii back to Thalassodromeus while recognising it as a distinct species, and thereby created the new combination T. oberlii. Pêgas and colleagues also rejected the theory that the lower jaw of T. sethi was downturned, and reinterpreted the frontmost piece of the lower jaw to have connected directly with the subsequent piece (with no gap).[6]

Reconstruction of the T. sethi skull, showing the possible extent of the undamaged crest and downturned jaws proposed by Headden, but not supported by Pêgas and colleagues

In 2015 palaeontologists Gerald Grellet Tinner and Vlad A. Codrea named a new species, T. sebesensis, based on what they interpreted as part of a cranial crest in a concretion found near the Sebeș River in Romania. The authors said that this would extend the range in time and space for the genus Thalassodromeus considerably, creating a 42-million-year gap between the older South American species and the younger European species.[12] Palaeontologist Gareth J. Dyke and a large team of colleagues immediately rejected the pterosaurian identification of the T. sebesensis fossil, instead arguing that it was a misidentified part of a plastron (lower shell) of the prehistoric turtle Kallokibotion bajazidi (named in 1923). The idea that the fragment belonged to a turtle had been considered and rejected by Grellet-Tinnera and Codrea in their original description.[13] Grellet-Tinnera and Codrea denied the turtle identity suggested by Dyke and colleagues, noting that those researchers had not directly examined the fossil.[14]

Size of T. sethi (shown from above and the side), compared to a human

The holotype (and only known skull) of Thalassodromeus sethi is one of the largest pterosaur skulls ever discovered. The entire skull is estimated to have been 1.42 m (4 ft 8 in) long; the bones were fused together, indicating adulthood. Based on related pterosaurs, its wingspan was 4.2 to 4.5 m (14 to 15 ft), making Thalassodromeus the largest known member of its clade, Thalassodromidae. Of similar proportions, its skull was more heavily built than that of its relative Tupuxuara.[1][6][3] Although the postcranial skeleton of Thalassodromeus is unknown, relatives had unusually short and blocky neck vertebrae, with well-developed front and hind-limbs that were almost equal in length (excluding the long wing-finger). The hindlimbs were eighty percent that of the forelimb length, a unique ratio among pterodactyloids (short-tailed pterosaurs).[3] As a pterosaur, Thalassodromeus was covered with hair-like pycnofibres and had extensive wing membranes (which were extended by the wing finger).[15]

The skull of T. sethi had a streamlined profile, especially from the tip of the snout to the front edge of the nasoantorbital fenestra (opening which combined the antorbital fenestra in front of the eye with the bony nostril). The most conspicuous feature of the skull was the large crest, which ran along the upper edge from the tip of the snout and beyond the occiput at the back of the skull, almost doubling the length and height of the skull. With the exception of the pterosaur Tupandactylus imperator (whose crest consisted mainly of soft tissue), T. sethi had the proportionally largest cranial crest of any known vertebrate (75 percent of the skull's side surface). The crest was mainly formed by the premaxillae (the frontmost snout bones), frontal bones, parietal bones, and part of the supraoccipital bone. The premaxillae formed most of the crest, extending to its back, and contacted the frontoparietal part of the crest by a straight suture (a distinct feature of this species). The crest varied from 1 to 10.5 mm (0.039 to 0.413 in) in thickness; it thickened at the contact between the premaxillae and the frontoparietal part, and became gradually thinner toward the top and back (except for the lower part behind the occiput, where it had a thick base).[1][6][3]

Cast of the T. sethi holotype skull, Cleveland Museum of Natural History

Despite its size, the crest was lightly built and essentially hollow; some areas indicate signs of skeletal pneumatisation and a well-developed trabecular system uniting the bones. The crest's surface had a system of channels of varying size and thickness, probably the impressions of extensive blood vessels.[1][3] A small, 46 mm (1.8 in) opening was present above the orbit (eye socket), piercing the basal part of the crest; such a feature is unknown in other pterosaurs, and does not appear to be due to damage. The margins of the opening are smooth, and the inner border has fenestration connecting it to the inner structure of the crest.[6][9] The back of the crest ended in a prominent V-shaped notch, a unique feature of this species.[3][1] Although other parts of the crest have V-shaped breaks, the V shape at the end does not appear to have been due to breakage; the margins of the bone can be seen there, still encased by matrix. The crest probably had a keratinous (horny) covering and may have been extended by soft tissue in some areas, but the extent of this is unknown.[1][16]

The upper jaw of T. sethi was primarily composed of premaxillae and maxillae; the suture which formed the border between these bones is not visible. As in all members of its clade, the jaws were edentulous (toothless). The rostrum (snout) was 650 mm (26 in) long from the tip of the premaxilla to the joint where the quadrate bone of the skull connected with the articular bone of the lower jaw. The front of the premaxillae had sharp upper and lower edges, unique to this species. As in related genera, the nasoantorbital fenestra was comparatively large; it was 650 mm (26 in) long and 200 mm (7.9 in) high, which was 71 percent of the skull length (excluding the crest). The lacrimal bone, which separated the orbit from the nasoantorbital fenestra, was vertically elongated and higher than the upper surface of the orbit (in contrast to the condition seen in pterodactyloids with smaller nasoantorbital fenestrae). The orbit was slender and compressed from front to back compared to Tupuxuara and tapejarids, but similar to some of them in being more than half the height of the nasoantorbital fenestra. The orbit was positioned lower than the upper margin of the nasoantorbital fenestra, and therefore very low on the skull. Although the bones bordering the lower temporal fenestra (an opening behind the orbit) were incomplete, it appears to have been elongated and slit-like (as in Tupuxuara and Tapejara).[6][3]

Life restoration of a T. sethi pair in flight

The palatal area at the tip of T. sethi's snout was a sharp ridge, similar to the keel seen on the upper surface of the mandibular symphysis where the two halves of the lower jaw connected. Small slit-like foramina (openings) on the lower side edges of the ridge indicate that it had a horny covering in life, similar to Tupandactylus. The lower edge of the area was somewhat curved, which probably created a small gap when the jaws were closed. Further back, immediately in front of the nasoantorbital fenestra, the palatal ridge became a strong, blunt, convex keel. This convexity fit into the symphyseal shelf at the front end of the lower jaw, and they would have tightly interlocked when the jaws were closed. The palatal ridge ended in a strongly concave area unique to this species. The postpalatine fenestrae (openings behind the palatine bone) were oval and very small, differing from those of related species. The ectopterygoid (bone on the side of the palate) had large, plate-like sides, and was well-developed compared to related species. The supraoccipital bone, which formed the hindmost base of the cranial crest, had muscle scars at its upper end (probably corresponding to the attachment of neck muscles).[6]

Although the lower jaw of T. sethi is incomplete, its total length is estimated at 670 mm (26 in) – 47 percent of which was occupied by the mandibular symphysis. The tip of the mandible is missing, but its front surface indicates that it might have been turned slightly upwards as in T. oberlii (the possible second species of Thalassodromeus, or possibly a different genus (Banguela) which is known only from a jaw tip). The symphyseal shelf, the upper surface of the symphysis, extended for 170 mm (6.7 in) and had a flat surface. Seen from above, the side edges of this area were tall and formed a sharp margin. Near the front end of the symphysis, the edges which formed the margins became broader towards the front of the shelf until they met and fused. The upper and lower surfaces of the jaw at the front of the shelf were keeled (the upper keel more robust and starting before the lower), which gave the symphysis a blade-like shape. The lower keel became deeper towards the front of the jaw, giving the impression that the jaw deflected downwards; it was actually straight, except for the (perhaps) upturned tip. The mandibular fossae (depressions) at the back of the upper jaw were deeper and broader than usual in pterodactyloids, creating large surfaces for the lower jaw to articulate with. The possible species T. oberlii differed from T. sethi and other relatives by the upper surface of its mandibular symphysis being slightly shorter than the lower surface, and was further distinguished from T. sethi by the upper edge of the symphysis being much sharper than the lower. The two species shared features such as the compression of the symphysis sideways and from top to bottom, the sharp keel at the upper front of the symphysis, and the small groove running along the upper surface of the shelf.[6]

Reconstructed skeleton of the related genus Tupuxuara in the American Museum of Natural History

The classification of Thalassodromeus and its closest relatives is one of the most contentious issues regarding their group.[3] Kellner and Campos originally assigned Thalassodromeus to the family Tapejaridae, based on its large crest and large nasoantorbital fenestra. Within this clade, they found that it differed from the short-faced genus Tapejara but shared a keel on the palate with Tupuxuara.[1] Kellner elaborated on the relationships within Tapejaridae in 2004, and pointed out that Thalassodromeus and Tupuxuara also shared a crest consisting primarily of bone; the crest had a large component of soft tissue in other members of the group.[16]

Martill and Naish considered Tapejaridae a paraphyletic (unnatural) group in 2006, and found Tupuxuara (which included Thalassodromeus in their analysis) to be the sister taxon to the family Azhdarchidae. This clade (Tupuxuara and Azhdarchidae) had been named Neoazhdarchia by palaeontologist David Unwin in 2003, an arrangement Martill and Naish concurred with. According to Martill, features uniting members of Neoazhdarchia included the presence of a notarium (fused vertebrae in the shoulder region), the loss of contact between the first and third metacarpals (bones in the hand), and very long snouts (more than 88% of the skull length).[8][17] Kellner and Campos defended the validity of Tapejaridae in 2007, dividing it into two clades: Tapejarinae and Thalassodrominae, the latter containing Thalassodromeus (the type genus) and Tupuxuara. They distinguished thalassodromines by their high nasoantorbital fenestrae and the bony part of their crests beginning at the front of the skull and continuing further back than in other pterosaurs.[9]

The interrelationship of these clades within the larger clade Azhdarchoidea remained disputed, and the clade containing Thalassodromeus and Tupuxuara had received different names from different researchers (Thalassodrominae and Tupuxuaridae). Palaeontologist Mark Witton attempted to resolve the naming issue in 2009, noting that the name "Tupuxuaridae" (first used in the vernacular form "tupuxuarids" by palaeontologist Lü Junchang and colleagues in 2006) had never been validly established and Thalassodrominae should be the proper name (although it was bestowed a year later). Witton further converted the subfamily name Thalassodrominae into the family name Thalassodromidae, and considered the clade part of Neoazhdarchia.[10]

A 2011 analysis by palaeontologist Felipe Pinheiro and colleagues upheld the grouping of the clades Tapejarinae and Thalassodrominae in the family Tapejaridae, joined by the Chaoyangopterinae.[18] A 2014 study by palaeontologist Brian Andres and colleagues instead found thalassodromines to group with dsungaripterids, forming the clade Dsungaripteromorpha within Neoazhdarchia (defined as the most inclusive clade containing Dsungaripterus weii but not Quetzalcoatlus northropi).[19]

Pêgas and colleagues kept Tapejarinae and Thalassodrominae as part of Tapejaridae in 2018, but acknowledged that the subject was still controversial.[6]

Irritator mounted as attacking an anhanguerid, both from the Romualdo Formation, National Museum of Brazil

Thalassodromeus is known from the Romualdo Formation, which dates to the Albian stage of the Early Cretaceous period (about 110 million years ago). The formation is part of the Santana Group and, at the time Thalassodromeus was described, was thought to be a member of what was then considered the Santana Formation. The Romualdo Formation is a Lagerstätte (a sedimentary deposit that preserves fossils in excellent condition) consisting of lagoonal limestone concretions embedded in shales, and overlies the Crato Formation. It is well known for preserving fossils three-dimensionally in calcareous concretions, including many pterosaur fossils. As well as muscle fibres of pterosaurs and dinosaurs, fish preserving gills, digestive tracts, and hearts have been found there.[1][6] The formation's tropical climate largely corresponded to today's Brazilian climate.[28] Most of its flora were xerophytic (adapted to dry environments). The most widespread plants were Cycadales and the conifer Brachyphyllum.[29]

Other pterosaurs from the Romualdo Formation include Anhanguera, Araripedactylus, Araripesaurus, Brasileodactylus, Cearadactylus, Coloborhynchus, Santanadactylus, Tapejara, Tupuxuara,[30] Barbosania,[31] Maaradactylus,[32] Tropeognathus,[33] and Unwindia.[34] Thalassodromines are known only from this formation, and though well-preserved postcranial remains from there have been assigned to the group, they cannot be assigned to genus due to their lack of skulls.[35][36] Dinosaur fauna includes theropods like Irritator, Santanaraptor, Mirischia, and an indeterminate unenlagiine dromaeosaur.[29] The crocodyliforms Araripesuchus and Caririsuchus,[37] as well as the turtles Brasilemys,[38] Cearachelys,[39] Araripemys, Euraxemys,[40] and Santanachelys, are known from the deposits.[41] There were also clam shrimps, sea urchins, ostracods, and molluscs.[28] Well-preserved fish fossils record the presence of hybodont sharks, guitarfish, gars, amiids, ophiopsids, oshuniids, pycnodontids, aspidorhynchids, cladocyclids, bonefishes, chanids, mawsoniids and some uncertain forms.[42] Pêgas and colleagues noted that pterosaur taxa from the Romualdo Formation had several species: two of Thalassodromeus, two of Tupuxuara, and up to six species of Anhanguera. It is possible that not all species in each taxon coexisted in time (as has been proposed for the pteranodontids of the Niobrara Formation), but there is not enough stratigraphic data for the Romualdo Formation to test this.[6]

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