Thalassodromeus

Genus of azhdarchoid pterosaur from the Early Cretaceous

Thalassodromeus is a genus of pterosaur that lived in what is now Brazil during the Early Cretaceous period, about a hundred million years ago. The original skull, discovered in 1983 in the Araripe Basin of northeastern Brazil, was collected in several pieces. In 2002, the skull was made the holotype specimen of Thalassodromeus sethi by palaeontologists Alexander Kellner and Diogenes de Almeida Campos. The generic name means "sea runner" (in reference to its supposed mode of feeding), and the specific name refers to the Egyptian god Seth due to its crest being supposedly reminiscent of Seth's crown. Other scholars have pointed out that the crest was instead similar to the crown of Amon. A jaw tip was assigned to T. sethi in 2005, became the basis of the new genus Banguela in 2014, and assigned back to Thalassodromeus as the species T. oberlii in 2018. Another species (T. sebesensis) was described in 2015 based on a supposed crest fragment, but this was later shown to be part of a turtle shell. Thalassodromeus had one of the largest known skulls among pterosaurs, around long, with one of the proportionally largest cranial crests of any vertebrate. Though only the skull is known, the animal is estimated to have had a wingspan of The crest was lightly built and ran from the tip of the upper jaw to beyond the back of the skull, ending in a unique V shaped notch. The jaws were toothless, and had sharp upper and lower edges. Its skull had large nasoantorbital fenestrae (opening that combined the antorbital fenestra in front of the eye with the bony nostril), and part of its palate was concave. The lower jaw was blade like, and may have turned slightly upwards. The closest relative of Thalassodromeus was Tupuxuara; both are grouped in a clade that has been placed within either Tapejaridae (as the subfamily Thalassodrominae) or within Neoazhdarchia (as the family Thalassodromidae). Several theories have been suggested to explain the function of Thalassodromeuss crest, including thermoregulation and display, but it likely had more than one function. The crests of thalassodromids appear to have developed late in growth (probably correlated with sexual maturity) and they may have been sexually dimorphic (differing according to sex). As the genus name implies, Thalassodromeus was originally proposed to have fed like a modern skimmer bird, by skimming over the water's surface and dipping its lower jaws to catch prey. This idea was later criticised for lack of evidence; Thalassodromeus has since been found to have had strong jaw musculature, and may have been able to kill and eat relatively large prey on the ground. The limb proportions of related species indicate that it may have adapted to fly in inland settings, and would have been efficient at moving on the ground. Thalassodromeus is known from the Romualdo Formation, where it coexisted with many other types of pterosaurs, dinosaurs and other animals.

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. 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 and δρομεύς, meaning "sea runner" in reference to the animal's supposed skim feeding behaviour. The specific name refers to the Egyptian god Seth. 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 manifestationsnot by Seth. 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. Kellner and Campos' view has since been accepted by other researchers, including Martill and Naish. 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). 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. Grellet Tinnera and Codrea denied the turtle identity suggested by Dyke and colleagues, noting that those researchers had not directly examined the fossil.

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 long; the bones were fused together, indicating adulthood. Based on related pterosaurs, its wingspan was , 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. 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). 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. 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. 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). Cladogram based on Pinheiro and colleagues, 2011: In 2013, Witton agreed that the substantially larger crests of adult thalassodromids indicated that they were more important for behavioural activities than for physiology. He found the idea that the crests were used for thermoregulation problematic, since they did not grow regularly with body size; they grew at a fast pace in near adults, quicker than what would be predicted for the growth of a thermoregulatory structure. According to Witton, the large, highly vascular wing membranes of pterosaurs would provide the surface area needed for thermoregulation, meaning the crests were not needed for that function. He concluded that the crest's blood vessel patterns did not differ much from those seen on bones under the beaks of birds, which are used for transporting nutrients to the bone and soft tissues rather than for thermoregulation. Witton noted that although bird beaks lose heat quickly, that is not what they were developed for; the crests of pterosaurs might also have had an effect on thermoregulation, without this being their primary function. Pêgas and colleagues noted that sexual dimorphism in crest size and shape has been proposed for some pterosaurs; the crest shape seen in the T. sethi holotype may correlate with one sex and may have been the result of sexual selection. They suggested that both sexes could have had similar crests due to mutual sexual selection, but interpretation of exaggerated features was challenging due to the small sample size; more T. sethi specimens would have to be found to evaluate these theories. They did not think that thermoregulation correlated with crest growth relative to body size, since the bills of toucans (the largest of any modern birds) grow drastically out of proportion to body size and function as thermoregulatory structures, as well as facilitating feeding and social behaviour. Pêgas and colleagues found the vascular structure of toucan bills comparable to that in the crest of T. sethi, concluding that the crest also had multiple functions. In 2004, palaeontologist Sankar Chatterjee and engineer R. Jack Templin said that smaller pterosaurs may have been able to skim feed. They doubted that this was possible for larger ones, due to their lesser manoeuvrability and flying capability while resisting water. Chatterjee and Templin noted that skimmers have blunter beaks than pterosaurs like Thalassodromeus, to direct water from the jaw while skimming. In 2007, biophysicist Stuart Humphries and colleagues questioned whether any pterosaurs would have commonly fed by skimming and said that such conclusions had been based on anatomical comparisons rather than biomechanical data. The drag experienced by bird bills and pterosaur jaws was hydrodynamically and aerodynamically tested by creating model bills of the black skimmer, Thalassodromeus, and the (presumably) non skimming Tupuxuara and towing them along a water filled trough at varying speeds. The researchers found that skimming used more energy for skimmers than previously thought, and would have been impossible for a pterosaur weighing more than due to the metabolic power required. They found that even smaller pterosaurs, like Rhamphorhynchus, were not adapted for skimming. The aluminium rigging of the Thalassodromeus model was destroyed during the experiment, due to the high and unstable forces exerted on it while skimming at high speed, casting further doubt on this feeding method. The authors used the jaw tip of T. oberlii to model the performance of Thalassodromeus, since it was assigned to T. sethi at the time. Witton said in 2013 that although skim feeding had been suggested for many pterosaur groups, the idea was criticised in recent years; pterosaurs lacked virtually all adaptations for skim feeding, making it unlikely that they fed this way. Thalassodromeus (unlike skimmers) did not have a particularly wide or robust skull or especially large jaw muscle attachment sites, and its mandible was comparatively short and stubby. Witton agreed with Unwin and Martill that thalassodromids, with their equal limb proportions and elongated jaws, were suited to roaming terrestrially and feeding opportunistically; their shorter, more flexible necks indicated a different manner of feeding than azhdarchids, which had longer, stiffer necks. He suggested that thalassodromids may have had more generalised feeding habits, and azhdarchids may have been more restricted; Thalassodromeus may have been better at handling relatively large, struggling prey than its relative, Tupuxuara, which had a more lightly built skull. Witton stressed that more studies of functional morphology would have to be done to illuminate the subject and speculated that Thalassodromeus might have been a raptorial predator, using its jaws to subdue prey with strong bites; its concave palate could help it swallow large prey. Pêgas and Kellner presented a reconstruction of the mandibular muscles of T. sethi at a conference in 2015. They found that its well developed jaw muscles differed from those of the possible dip feeder Anhanguera and the terrestrially stalking azhdarchids, indicating that T. sethi had a strong bite force. In 2018, Pêgas and colleagues agreed that Thalassodromeus blade like, robust jaws indicated that it could have used them to strike and kill prey, but they thought that biomechanical work was needed to substantiate the idea. They found (unlike Witton) that Thalassodromeus had a reinforced jaw joint and robust jaw muscles, but more work was needed to determine its dietary habits. According to Pêgas and colleagues, the articulation between T. sethi articular and quadrate bones (where the lower jaw connected with the skull) indicates a maximum gape of 50degreessimilar to the 52 degree gape inferred for Quetzalcoatlus. Witton also expressed hope for further analysis of thalassodromid locomotion. He noted that since their limb proportions were similar to those of the better studied azhdarchids, the shape of their wings and style of flight might have been similar. Thalassodromids might also have been adapted for inland flight; their wings were short and broad (unlike the long, narrow wings of marine soarers), and were more manoeuvrable and less likely to snag on obstacles. Their lower shoulder muscles appear to have been enlarged, which would have helped with powerful (or frequent) wing downstrokes and takeoff ability. Although it may have had to compensate for its large crest during flight, its development late in growth indicates that it did not develop primarily for aerodynamics. Witton suggested that the proportional similarity between the limbs of thalassodromids and azhdarchids also indicates that their terrestrial abilities would have been comparable. Their limbs would have been capable of long strides, and their short, compact feet would have made these mechanics efficient. The enlarged shoulder muscles may have allowed them to accelerate quickly when running, and they may have been as adapted for movement on the ground as has been suggested for azhdarchids; Witton cautioned that more analysis of thalassodromids was needed to determine this.

Thalassodromeus is known from the Romualdo Formation, which dates to the Albian stage of the Early Cretaceous period (about 110million 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. Most of its flora were xerophytic (adapted to dry environments). The most widespread plants were Cycadales and the conifer Brachyphyllum. Other pterosaurs from the Romualdo Formation include Anhanguera, Araripedactylus, Araripesaurus, Brasileodactylus, Cearadactylus, Coloborhynchus, Santanadactylus, Tapejara, Tupuxuara, Barbosania, Maaradactylus, Tropeognathus, and Unwindia. 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. Dinosaur fauna includes theropods like Irritator, Santanaraptor, Mirischia, and an indeterminate unenlagiine dromaeosaur. as well as the turtles Brasilemys, Cearachelys, Araripemys, Euraxemys, and Santanachelys, are known from the deposits. There were also clam shrimps, sea urchins, ostracods, and molluscs. 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.