Non-avian dinosaur with streamlined body shows potential adaptations for swimming – Communications Biology

Dinosauria Owen, 1842

Theropod Marsh, 1881

Dromaeosauridae Matthew and Brown, 1922

Halszkaraptorinae Cau et al., 2017

Revised diagnosis

Small dromeosaurids that possess dorsoventrally flattened premaxillae, premaxillary bodies perforated by numerous neurovascular foramina, enlarged and crowded premaxillary teeth that used delayed replacement patterns, reduced anterior maxillary teeth, dorsolateral placement of retracted external nostrils, very elongated cervical vertebrae, anterior cervical vertebrae with lobes formed by the postzygapophyses, horizontal zygapophyses and pronounced zygapophyseal laminae in the anterior caudal vertebrae, ulnae compressed mediolaterally with sharp posterior edges, second and third metacarpals of similar thickness, supratrochanteric process in shelf-like over the ilia, elongated pits lining the posterolateral ridges on the posterodistal surfaces of the femoral diaphyses and third metatarsals in which the proximal halves are unconstricted and convex v forward.

Natovenator polydontus gen. it’s sp. nov.


MPC-D 102/114 (Institute of Palaeontology, Mongolian Academy of Sciences, Ulaanbaatar, Mongolia) is a mostly articulated skeleton with a nearly complete skull (see Supplementary Table 1 for measurements).

Locality and horizon

Baruungoyot Formation (Upper Cretaceous), Hermiin Tsav, Omnogovi Province, Mongolia13 (Supplementary Fig. 5).


NatevenatorLatin nato (swim) and hunter (hunter), in reference to the hypothetical swimming behavior and piscivorous diet of the new taxon; polydontusGreek polys (a lot and odious (tooth) in reference to the unusual number of teeth.


A small halszkaraptorin dromaeosaurid with the following autapomorphies: broad furrow bounded by a pair of ridges on the antero-dorsal surface of the premaxilla, premaxilla with an elongated internarial process that covers the nasal and extends posterior to the external nostrils, 13 premaxillary teeth with large incisor-like crowns, the first three most anterior maxillary teeth are considerably reduced and grouped with the next tooth without any separation by interdental septa, anteroposteriorly long external naris (about 30% of the length of the preorbital skull), paroccipital process with a anteroposterior dorsal surface broad, maxilla elongated process of the palatine extending anteriorly beyond the middle of the antorbital window, pterygoid with a deep pit on the medial aspect of the square ramus, distinct posterolateral oriented projection on the lateral aspect of the ‘atlas, absence of pleuroceles in the ve cervical rtebrae (unconfirmed in missing fifth cervical c entrum), proximal shafts oriented posterolaterally and nearly horizontal in dorsal ribs, metacarpal II hourglass-shaped with distinctly concave medial and lateral surfaces.

The description

The skull of Natevenator is almost complete, although the preorbital region has been affected by compression and is slightly offset from the rest of the skull (Figs. 1c, d, 2a–d and Supplementary Figs. 1, 2). Near the end of the snout, the premaxilla is marked by a wide furrow. The body of the premaxilla is also low dorso-ventrally and is perforated by numerous foramina which lead to a complex network of neurovascular chambers (Supplementary Fig. 1b) as in Halszkaraptor4. Similarly, the external naris is positioned posteriorly and is at the level of the premaxillary-maxillary contact (Fig. 2a,b), although it is slightly posterior to this position in Halszkaraptor4. It is also placed dorsal to those of other non-avian theropods and faces dorsolaterally. The unusually long external nostrils and elongated internarial process as a result of Natevenator (Fig. 2c) are unique among dromaeosaurids but comparable to those of toothed waterfowl14 as well as in therizinosaurs15.16. The frontal is similar to those of other halszkaraptorins4.17 in that it is vaulted to accommodate a large orbit and has little contribution to the supratemporal fossa. A pointed nuchal crest is formed by the parietal and squamosal (Supplementary Fig. 2a–e). The latter also produces a shelf that spans the square head as in other dromaeosaurids18. The paroccipital process curves gently over the occiput and has a broad dorsal surface that tapers laterally (Fig. 2f and Supplementary Fig. 2b,e). Its ventrolateral orientation recalls Mahakala17 but is different from the more horizontal paroccipital process of Halszkaraptor4. The occipital condyle is long and constricted at its base. A shallow dorsal tympanic recess on the side wall of the puzzle is different from the deep one of Mahakala17. The palatine is tetraradiate with a very elongated maxillary process, which extends anteriorly beyond the level of the medio-antorbital window. The pterygoid is missing its anterior part (Fig. 2g and Supplementary Fig. 2a–e). A deep pit on the medial surface of the thin quadratic branch is not seen in any other dromaeosaurid. The mandibles of Natevenator retaining most of the elements, especially those on the left side (Fig. 1a, b, d and Supplementary Figs 1a, 2). Each jaw is characterized by a slender dentary with nearly parallel dorsal and ventral margins, a surangular partially fused with the articular, a distinctive surangular plateau, and a fan-shaped retroarticular process that projects dorso-medially. The upper dentition of Natevenator is heterodont because the premaxillary teeth are morphologically distinct from the maxillary teeth (Fig. 2a,b,e and Supplementary Fig. 1a,c). There is an unusual number of tight premaxillary teeth without any separation of the sockets by bony septa. The roots of the teeth are long, and the crowns are large and incisiform as in Halszkaraptor4. Additionally, the large premaxillary replacement teeth suggest that the replacement of the premaxillary teeth was delayed as in Halszkaraptor4. However, the number of teeth in each premaxilla is 13 Natevenatorwhen he is only 11 Halszkaraptor4. In the maxilla, the three most anterior maxillary teeth are significantly shorter than the premaxillary teeth and the more posterior maxillary teeth. This pattern is also observed in Halszkaraptoralthough the number of shorter maxillary teeth differs as it has two reducedseven. Maxillary and dental teeth have sharp fang-like crowns without serrations. Although the posteriormost parts are poorly preserved, there are at least 23 sockets in each of the maxilla and dentium, suggesting a high number of teeth in both elements.

The elbow Natevenator, as preserved, is twisted and includes ten elongated cervical vertebrae, although most of the 5th cervical is missing (Figs. 1, 3a–d). This lengthening of the cervicals results in a neck that is noticeably longer than those of most dromaeosaurids and is estimated to be longer than the dorsal series. It is, however, proportionately shorter than that of Halszkaraptorwhich has a neck as long as its dorsal and sacral vertebrae combined4. Another peculiarity of the Col du Natevenator is a pronounced projection extending posterolaterally over the neurapophysis of the atlas (Fig. 3a and Supplementary Fig. 2b,c,e). The postzygapophyses of each anterior cervical are fused into a single lobe-like process as in Halszkaraptor4. The pleurocoels are absent from the cervical vertebrae. In contrast, Halszkaraptor has pleuroceles on his 7th to 9th cervicals4. A total of 12 thoracic vertebrae are preserved (Figs. 1a, b, 3e, 4a and Supplementary Figs. 3a–d). They are all devoid of pleuroceles and their parapophyses on the anterior and medial parts of the dorsals are placed high on the antero-dorsal end of each centrum. Interestingly, the positions of the parapophyses are similar to those of hesperornithiforms19,20,21 rather than other dromaeosaurids such as Deinonychus22 Where Velociraptor23. The preserved dorsal ribs, articulated with the second to seventh dorsals, are flattened and pointing backwards (Figs. 1, 3e, 4a–d). The proximal rods are also nearly horizontal, indicating a dorso-ventrally compressed rib cage. Each proximal caudal vertebra has a long centrum and horizontal zygapophyses with enlarged laminae (Fig. 3f and Supplementary Fig. 3e–i), all of which are characters shared with other halszkaraptorins4.17. The forelimb elements are partially exposed (Figs. 1a, b, 2a–d, 3e, g). The almost complete right humerus is proportionally short and distally flattened like that of Halszkaraptor4. The shaft of the ulna is compressed mediolaterally to produce a sharp posterior margin as in Halszkaraptor4 and Mahakala17. Metacarpal III is robust and only slightly longer than metacarpal II. Likewise, metacarpal III is almost as thick and long as the other second metacarpals of the other halszkaraptorins4.17. The femur has a long ridge on its posterior surface, which is another feature shared by halszkaraptorins4. Typically for a dromaeosaurid, metatarsals II and III have distal ginglymoid articular surfaces (Fig. 3h and Supplementary Fig. 4f,h). The ventral surface of metatarsal III is overgrown with a ridge near the distal end, unlike other halszkaraptorins (Fig. 3h)4,5,17,24.

Phylogenetic analysis

Phylogenetic analysis found over 99,999 most parsimonious trees (CI=0.23, RI=0.55) with 6574 steps. Deinonychosaurian monophyly is not supported by the strict consensus tree (Supplementary Fig. 6). Instead, Dromaeosauridae was recovered as a sister clade to a monophyletic clade formed by Troodontidae and Avialae, which is consistent with the results of Cau et al.4 and Causeven. Halszkaraptorinae is positioned at the base of Dromaeosauridae as in Cau et al.4although there are claims that dromaeosaurid affinities with halszkaraptorins are not well substantiated25. Nine synapomorphies (seven ambiguous and two unambiguous) support the inclusion of Halszkaraptorinae in the Dromaeosauridae. The two unambiguous synapomorphies are the anterior tympanic recess at the same level as the basipterygoid process and the presence of a ventral collarette on the paroccipital process. A total of 20 synapomorphies (including one unambiguous synapomorphy) unite the four halszkaraptorins, including Natevenator (Supplementary Fig. 7). In Halszkaraptorinae, Halszkaraptor is the first taxon to branch, and the three remaining taxa form an unresolved clade supported by three ambiguous synapomorphies (characters 121/1, 569/0, and 1153/1). Two of these synapomorphies are related to the paroccipital process (characters 121 and 569), which is not conserved in Hulsanpes5.24. The other is the presence of an expansion on the medial edge of the distal half of metatarsal III, which is not fully preserved in the Natevenator. When rated 0 for this character, Natevenator bifurcates from the unresolved clade. This suggests that the medial expansion of the dorsal surface of metatarsal III may be a derived character in halszkaraptorins.

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