Comparative Anatomy of Halszkaraptor

Halszkaraptor possesses a set of aberrant characteristics that together produce a bauplan superficially unlike those of other known paravian theropods32. Because Halszkaraptor was originally interpreted as a dromaeosaurid paravian, its morphology was differentiated from the other members of that lineage, which are all terrestrial and arboreal20,21. However, the anatomy of Halszkaraptor was not extensively compared with non-paravian maniraptorans, which ought to be done given the taxon’s basal phylogenetic position within Dromaeosauridae and Paraves at large32. Below is a comprehensive review of the comparative anatomy of Halszkaraptor, many of which support a critical reassessment of the hypothesis that this taxon was an aquatic piscavore. This reevaluation is based on an extensive review of the literature and firsthand examination of several specimens.

Rostral neurovasculature

One of the features used to support an aquatic lifestyle for Halszkaraptor is the presence of an extensive system of neurovascular canals in the premaxillae of this taxon (Fig. 1A–J)32. This characteristic is present in a variety of semiaquatic and aquatic piscavorous tetrapods, including crocodylians33. As Cau et al.32 noted, extensive neurovascular systems are also found in a variety of terrestrial theropods34,35,36. Although Halszkaraptor was differentiated from other theropods in possessing a rostral neurovascular system not entirely restricted the lateral portions of the premaxillae32, the rostral neurovasculature extends onto the dorsal surface of the body of the premaxilla in basal members of most other maniraptoran clades. In the basal therizinosaur Jianchangosaurus, the premaxillae are covered with neurovascular foramina that are present on both the lateral surface of the premaxillae and the medial portion of the dorsal surface (including the subnarial fossa) of each bone (see Fig. 3b in 18, Fig. 1E)18. In the more derived therizinosaur Erlikosaurus, the same morphology, where the premaxillae harbor neurovascular foramina on both their lateral and mediodorsal surfaces, is clearly present (see Lautenschlager et al.19 for clear scans of the premaxillae of Erlikosaurus; Fig. 1C,D). Basal ornithomimosaurs like Shenzhousaurus also show neurovascular foramina on the lateral, anterior, and subnarial surfaces of their premaxillae13 (Fig. 1F; although note the possible absence of any clear neurovasculature in the anterior skull of Hexing)37, which, as in therizinosaurs for which the premaxillae are known (e.g., Jianchangosaurus, Erlikosaurus), were laterally expanded12,13,37. In the derived deinocheirid ornithomimosaur Deinocheirus17, numerous neurovascular foramina are present towards on the mediodorsal, anterior, and lateral surfaces of the premaxillae (Fig. 2 in 41). In the related Garudimimus, a similar distribution of foramina is present34. This distribution of foramina on the anterior, lateral, and dorsal surfaces of the premaxillae is well-documented for ornithomimid (Gallimimus, Struthiomimus, Ornithomimus, Sinornithomimus, etc.) premaxillae and dentaries, where rhamphotheca are present and probably relate to the development of the neurovasculature14,16,19,35,36. In the basal-most alvarezsaur Haplocheirus, neurovascular foramina are present on the lateral, anterior, and mediodorsal surfaces of the body of the premaxillae, as in other alvarezsaurs and theropods38. The premaxillae of definite basal oviraptorosaurs where the skull is preserved bear similar distributions of foramina. In Incisivosaurus gauthieri and Caudipteryx zoui, numerous foramina are scattered along the lateral surface of the premaxillae, with one larger foramen placed ventral to the anterior end of the naris in the former taxon39,40. The skulls of both these taxa are moderately to entirely crushed, meaning that a precise understanding of the distribution of foramina on the premaxillae is not attainable39,40. This is also the case for the bizarre scansoriopterygids, which may be basal oviraptorosaurs41. In caenagnathoid oviraptorosaurs, numerous neurovascular foramina are present across the whole surface of the premaxilla8. As in ornithomimosaurs5,16, numerous neurovascular foramina are also present on the lateral, anterior, and ventroanterior surfaces of the dentary in some oviraptorosaurs42. Therefore, the presence of neurovascular foramina on the lateral, anterior, and dorsal portions of the exposed surface of the premaxillae is found in basal to derived members of the Ornithomimosauria, Therizinosauria, and Alvarezsauria, and intermediate and derived members of Oviraptorosauria, undermining Cau et al.’s claim that a neurovascular system present on the dorsal, in addition to lateral, surface of the premaxillae distinguishes Halszkaraptor from other maniraptorans. Because this feature is present in basal-intermediate and and derived members of all major clades of non-paravian maniraptorans and maniraptoriforms (Maniraptora + Ornithomimosauria), it is most probably plesiomorphic with respect to Maniraptora and secondarily lost within dromaeosaurids more derived than Halszkaraptor. Furthermore, it is important to note that the presence of extensive neurovasculature is not unique to semi-aquatic forms. The presence of extensive neurovasculature was previously used to support a semiaqautic lifestyle in spinosaurs43,44.

Figure 1 Rostral anatomy of select theropods. Rostrum of Halszkaraptor after Cau et al.32 in lateral (A) and dorsal (B) views. Rostrum of Erlikosaurus after Lautenschlager et al.19 in (C) lateral and dorsal (D) views. Rostrum of Jianchangosaurus after Pu et al.18 in (E) lateral view. Rostrum of Harpymimus after Kobayashi and Barsbold46 in (F) lateral view. Rostrum of Tsaagan (cast) in (G) lateral view. Rostrum of Velociraptor in (H) lateral and (I) ventral views. Rostrum of Gorgosaurus (cast) in (J) lateral view nar, naris; pmf, premaxillary foramina; max, maxilla; pmd, premaxillary dentition; md, maxillary dentition. Scale bar = 9 mm (A,B), 100 mm (C,D), 50 mm (E–F), 10 mm (G), 20 mm (H–I). Full size image

Recent research into the neurovasculature of tetanuran theropods like Neovenator45 has shown that many different predatory theropod clades possessed complex and extensive neurovasculature, and that superficial comparisons with the neurovasculature seen in groups like crocodylians is unwarranted. In coelurosaurs, extensive neurovasculature in the premaxilla, maxilla, and dentary may be related to the development of rhamphothecae2,4,5,8,11,14,15,16,17,18,19,35,36,39,41, which in turn is probably related to the acquisition of omnivorous or herbivorous diets11,15,16,17,18. Laterally, the neurovasculature of the premaxillae of Halszkaraptor resembles the condition in other dromaeosaurids and hypercarnivorous theropods like tyrannosauroids, where numerous foramina appear on the surfaces of the premaxillae (Fig. 1A,G–H,J)32,45.

Platyrostral premaxillae

Halszkaraptor differs from other dromaeosaurids in possessing low, laterally expanded (“platyrostral) premaxillae that contain the extensive neurovascular system found in this taxon (Fig. 1A,B,H,I). Cau et al.32 drew comparisons between this feature in H. escuilliei and spinosaurids, the preserved gut contents of which include fish remains44. Cau et al.32 compared the construction of the skull of H. escuilliei to the anterior skulls of modern birds like ducks and geese, with which Halszkaraptor was considered somewhat analogous. However, moderately to strongly (=platyrostral) laterally expanded premaxillae are found in a variety of maniraptorans and maniraptoriforms, including primitive and intermediate ornithomimosaurs like Shenzhousaurus13 and Harpymimus46, derived ornithomimid ornithomimosaurs14,16,19,35,36 and deinocheirids17,34, where they likely acted as an anchorage point for enlarged rhamphotheca4,5,16,17. The morphology of the premaxillae in ornithomimosaurs has often been likened to the beaks of birds4,5,35,36,47 Jianchangosaurus and Erlikosaurus both show laterally divergent premaxillae that also seem to have supported rhamphotheca (Fig. 1C–E)18,19,48. Among these, the premaxillae of Erlikosaurus are the best preserved and are highly reminiscent of the premaxillae of Halszkaraptor in their clear lateral expansion in dorsal view (Fig. 1D)19,48, compared to the mediolaterally thin rostra of other dromaeosaurids (Fig. 1I)20,21. However, basal7 and derived alvarezsaurs like Haplocheirus38 and Shuvuuia49 show the mediolaterally ‘thin’ condition. In even the most basal oviraptorosaurs, the skull is very bizarre compared to other coelurosaurs8,39,40. However, in these and more derived oviraptorids8,50,51 and caenagnathids (based on the mediolaterally widened dentaries)42,52 the premaxillae do not show the heavily mediolaterally compressed condition present in all dromaeosaurids besides Halszkaraptor20,21,32, troodontids53 and alvarezsaurs38,49. However, the premaxillae of oviraptorosaurs are dorsally expanded to form a crest in most taxa8,50,51,52, contrasting with the condition in Halszkaraptor. Because this feature seems to be variously present and absent in basal and derived members of clades along and bracketing Maniraptora, it is premature to consider laterally expanded premaxillae a plesiomorphic state for maniraptoriforms. Nonetheless, the presence of laterally expanded to strongly platyrostral17 premaxillae in a variety of maniraptoriforms with diverse bauplans indicates this feature is not at all suggestive of a semiaquatic lifestyle in Halszkaraptor. The condition of platyrostral premaxillae in Halszkaraptor and some other maniraptorans is vaguely reminiscent of the premaxillae of herbivorous dinosaurs like the rebbachisaurid Nigersaurus53, titanosaurs54,55,56, and hadrosaurids57. Given this evidence, platyrostral premaxillae in theropod, sauropod, and ornithopod dinosaurs is probably related to omnivory or herbivory, which was probably present in most—if not all—of the maniraptoran clades bracketing Paraves5,11,15,16,17.

Retracted, elongate nares

It is unclear how Cau et al.32 observed retracted nares in Halszkaraptor, as the anterior nasals are not preserved in that taxon. Their reconstruction of the skull of H. escuilliei restores the nares as elongate fenestrae extending posteriorly into the first fifth of the nasals. However, the anterior margins of the nares on the premaxillae are comparable in curvature to those of other dromaeosaurids20,21, where the ratio between the semi-major and semi-minor axes is smaller than in the restoration of the complete skull by Cau et al.32 (Fig. 1A,G,H). In this way, Cau et al.’s32 reconstruction of this portion of the skull in Halszkaraptor is slightly inaccurate, given that the preserved portion of each naris in this taxon does not differ extensively from the corresponding portion in other dromaeosaurids. Slightly retracted nares are also present in some other paravians, including the troodontid Mei long58. In the therizinosaurs Erlikosaurus and Jianchangosaurus, elongate nares extend posteriorly above a third or more of the maxillae (Fig. 1C–E)18,19. Elongate nares extending over at least a third of the maxillae are also clearly present in the basal oviraptorosaurs Incisivosaurus39 and Caudipteryx40, the giant ornithomimosaur Deinocheirus17, and non-maniraptoriform coelurosaurs, like the basal tyrannosaurs Guanlong59 and Proceratosaurus60, and the possibly coelurosaurian megaraptorans61. More derived tyrannosaurs also show elongate nares, although not to the extent seen in more basal taxa (Fig. 1J). Basal troodontids like Mei long and the recently described Liaoningvenator curriei also show this elongate condition of the nares58,62,63 Cau et al.32 also rightly note in the supplementary text of their paper that several early avialians also show nares somewhat similar to that of Halszkaraptor. Given the presence of this characteristic in basal members of the majority of major coelurosaurian clades (Tyrannosauroidea, Therizinosauria, Oviraptorosauria, Troodontidae, Aves), it may be that elongate nares are plesiomorphic with respect to Coelurosauria60,61, with the absence of elongate nares in basal ornithomimosaurs explainable due to the extreme elongation of the maxillae in the most basal members of that clade12,13. Basal alvarezsaurs also display moderate elongation of the maxillae, which may also explain the absence of elongate nares in their skulls7,38. Notably, the bizarre possible maniraptoran Fukuivenator also displays an enlarged naris that might have extended over a third of the maxilla, although this feature remains to be verified64. Despite the support for it found here, if the presence of elongate nares is not found as the plesiomorphic state for coelurosaurs in future analyses, the presence of them in a variety of theropods that do not show any features for a semiaqautic lifestyle provides evidence against the argument of Cau et al.32, who argued this feature was indicative of such an ecology. The ‘dorsally-oriented’ nature of the nares of Halszkaraptor as described by Cau et al.32 also does not substantially differ from the condition in some other coelurosaurs, such as therizinosaurs18,19. Furthermore, the comparison drawn between Halszkaraptor and spinosaurs by Cau et al. is misleading, as in spinosaurines the naris is relatively small and sits closer to the center or posterior end than the anterior end of the skull44,65. Rather, the condition in Halszkaraptor is more closely comparable to that in baryonychine spinosaurs like Suchomimus66 and Baryonyx43, which do not seem to have been adapted for an aquatic lifestyle like some spinosaurines44. The anterior portion of the skull of Halszkaraptor is clearly not as flattened as those of crocodylians, such as alligators33. Therefore, there is little evidence to suggest the nares of Halszkaraptor were different from those of basal members of other coelurosaurian clades in a way that might indicate a novel ecology for this taxon.

Dentition

As in some herbivorous maniraptorans and paravians, the teeth of Halszkaraptor lack serrations entirely15,21. As in almost all other other dromaeosaurids, many troodontids, avians, tyrannosaurs, basal oviraptorosaurs, and Fukuivenator20,21,39,40,53,58,62,63,64, the dentition of Halszkaraptor is slightly heterodont, with the premaxillary and maxillary teeth showing slightly different morphologies as in other dromaeosaurids20,21,32. One interesting feature of the premaxillary teeth of Halszkaraptor described by Cau et al.32 was their delayed replacement rate. A large amount of research into the loss of teeth in some maniraptoran dinosaurs has found a delayed replacement rate to be linked to tooth loss in several clades, including therizinosaurs and ornithomimosaurs11,15. As in basal members of the Ornithomimosauria like Nqwebasaurus67 and Pelecanimimus12, Halszkaraptor possesses a large number of premaxillary teeth32 On the whole, the skull of Halszkaraptor also shares many similarities with basal troodontids, including an increased number of maxillary teeth, tightly packed teeth, and recurved, ziphodont, unserrated crowns15,53,58,62,63. Therizinosaurs, such as Erlikosaurus and the basal taxa Jianchangosaurus, Falcarius, and Beipiaosaurus, also possesses an increased number of maxillary teeth (Fig. 1C,E)3,18,19,67, as do basal alvarezsaurs like Haplocheirus7,38, basal ornithomimosaurs like Pelecanimimus12, and the basal tyrannosaur Proceratosaurus60. Members of basal clades in the Dromaeosauridae, including microraptorans and unenlagiines, also possess a large number (20+) of teeth in their maxillae20,21,26,27,28,29,30. Basal oviraptorosaurs seem to represent the exception, possessing very few crowns39,40. In the phylogenetic analysis conducted, the presence of serrations on teeth is coded for by character 81, whereas the number of maxillary teeth was coded for using character 8241,68. Cau et al.32 drew comparisons the dentition of Halszkaraptor and that of marine reptiles like plesiosaurs and possibly ichthyophagous dinosaurs like spinosaurs based on features like the unserrated nature of the crowns, the large number of crowns in both the premaxilla and maxilla, and the delayed replacement rate of the premaxillary crowns. However, as I note and as Cau et al.32 noted, unserrated tooth crowns are distributed in a variety of paravians, including basal members of Aves69, Troodontidae58,62,63,69, and even Dromaeosauridae21,22,23,26,29. As I have noted, a the presence of 20 or more maxillary teeth is widespread among the basal members of almost all clades of maniraptorans and maniraptoriforms, and is also present in basal members of some non-maniraptoran coelurosaur clades. Therefore, it is likely that unserrated teeth are plesiomorphic with respect to Paraves, whereas a large number of maxillary teeth are plesiomorphic with respect to Maniraptoriformes, and derived eudromaeosaurian dromaeosaurids like Velociraptor simply regained serrations on their teeth and reduced their number of maxillary crowns21. Given that unserrated teeth are found in virtually all toothed paravians besides eudromaeosaurian dromaeosaurids and derived troodontids and a large number of maxillary teeth are found in all tooth maniraptorans besides eudromaeosaurs, there is absolutely no evidence that the presence of these features in the teeth of Halszkaraptor are anything but plesiomorphic features, much less adaptations to ichthyophagy as hypothesized by Cau et al.32 Furthermore, a delayed replacement rate in the premaxillary crowns of Halszkaraptor is shared with a variety of more basal maniraptoran taxa, suggesting this feature might be plesiomorphic as well and diminishing the apparent similarity between the teeth of H. escuilliei and marine reptiles like plesiosaurs remarked upon by Cau et al.32. However, further study of tooth replacement in maniraptorans will have to be performed before delayed tooth replacement is able to be tested for being a synapomorphy of Maniraptora or a more inclusive clade.

Number of cervicals and elongation of cervical vertebrae

Cau et al.32 noted the comparatively long neck of Halszkaraptor, which, unlike other paravians, composes at least 50% of the snout-to-sacrum length in this taxon. However, given its basal position in Paraves in their combinable components topology of Coelurosauria32, it is unclear why Cau et al. allied this feature to elongate necks in derived semiaquatic avians (e.g., Cygnus) rather than the many long-necked (approximately 50% of snout-sacrum length) non-paravian maniraptorans and coelurosaurs. Despite the fact that Cau et al.32 claimed the neck of Halszkaraptor composed the greatest percentage of snout-to-sacrum length among non-avian coelurosaurs, a large number of clades include taxa that approach, reach, or possibly even exceed that threshold. These include ornithomimosaurs4,12,13,14,17,34,35,36,37, therizinosaurs2,3,11,18 and oviraptorosaurs (Fig. 2)8,40,50,51,52. The possible maniraptoran theropod Fukuivenator possessed a notably elongate neck with up to 11 cervical vertebrae64, one more than in Halszkaraptor. However, Fukuivenator seems to have possessed a longer caudal series than Halszkaraptor32,64. The basal therizinosaur Jianchangosaurus possessed 10 cervical vertebrae that produced a moderately elongate neck approximately the length of the thorax of this taxon18. Beipiaosaurus possessed 9 elongate cervical vertebrae70,71 and a thoracic morphology similar to other therizinosaurs, suggesting the neck made up approximately 50% of the length from the snout to the sacrum of this taxon. The precise number of cervical vertebrae in Falcarius cannot be determined, but the cervicals of this taxon were elongate3, suggesting the neck of Falcarius composed a similar percentage of the snout-to-sacrum length seen in other basal ornithomimosaurs (Fig. 2B, but see the reconstruction in Kirkland et al.72). More derived therizinosaurids possessed highly elongate, sometimes massively built necks that easily composed more than 50% of the length between the tip of the premaxillae and the sacrum2,11,70,71. The basal alvarezsaur Haplocheirus possessed 10 relatively elongate cervical vertebrae that form 40+% of the snout to sacrum length73, and the slightly more derived Bannykus and Xiyunykus seem to have possessed similar counts7. Among basal oviraptorosaurs, Caudipteryx preserves an elongate neck consisting of 12 cervical vertebrae that form approximately half of its pre-caudal length40. Among oviraptorids, the cervical count varies between 9 and up to 13 cervicals, with 22 to 23 presacral vertebrae usually present (closely comparable to the 22 known for Halszkaraptor)8. Oviraptorosaurs possessed a deep, shortened thorax, and the necks of some even surpassed 50% of the snout to sacrum length (see Corythoraptor for an extreme example)8,50,51,52,73. The cervical vertebrae of basal ornithomimosaurs, such as Hexing and Pelecanimimus, are elongate, as in more derived forms4,12,14,35,36,37. In ornithomimosaurs, there are 10 elongate cervical vertebrae (Fig. 2C) and 13 dorsal vertebrae, for a total of 23 presacral vertebrae4. The well-preserved nature of many ornithomimosaur specimens shows the cervical series clearly formed at least 50% of the snout-to-sacrum length in these taxa4,12,14,35,36,37. Therefore, the cervical—indeed presacral—count in Halszkaraptor is closely similar to that found in basal members of every single non-paravian maniraptoriform clade, and, as in all of these taxa, the cervical series is elongate and forms a large percentage (40+% of the snout-to-sacrum length).

Figure 2 Comparative anatomy of the cervical series in selected theropods. Cervical series of Halszkaraptor (A) after Cau et al.32, Falcarius (B) after Zanno3, and Struthiomimus (C) after Osborn104. Full size image

The length of the neck of Halszkaraptor and the elongate nature of the cervical vertebrae in that taxon are what would be expected for the basal-most paravian and simply represent the probably plesiomorphic condition of a neck formed by elongate cervicals. Furthermore, among dromaeosaurids, the cervical and presacral counts of Halszkaraptor (11, 22) are only slightly greater or less than that of Velociraptor (9, 21)20, Buitreraptor (~10+, ~22–23)31 or Linheraptor (10,?)74. Furthermore, the cervicals in some of these taxa are also rather elongate and comparable to Halszkaraptor31,32. Therefore, neither the cervical count of Halszkaraptor are especially aberrant among dromaeosaurids, and certainly not among basal and derived members of clades bracketing Paraves. Based on the available evidence, it seems most reasonable to conclude elongate cervical vertebrae are plesiomorphic among maniraptorans, and derived dromaeosaurids like Velociraptor show a secondary reduction of this feature. In any case, the presence of elongate cervicals in many other maniraptorans strongly suggests the elongate neck of Halszkaraptor is not unique nor a clear indicator of a switch towards an ichthyophagous, semi-aquatic lifestyle. Comparisons with long-necked marine groups like plesiosaurs are therefore entirely unjustified.

Modifications to cervical vertebrae

Several features of the cervical vertebrae of Halszkaraptor may be comparable to characteristics of the cervicals of some chelonians and semiaquatic birds. These are (1) neural spines reduced and ridge-like, (2) neural arches elongate, (3) postzygapophyses that are merged together, (4) cervical ribs and vertebrae fused together, and (5) zygapophyseal facets positioned horizontally. Rather heavily reduced to nearly absent neural spines are probably a plesiomorphic character state among maniraptorans, because basal and derived oviraptorosaurs8,51,52,53 basal (Falcarius, Jianchangosaurus)(Fig. 1B)3,18 and derived2,3,11 therizinosaurs, various troodontids (including Mei long)53,62 and basal and derived ornithomimosaurs (Fig. 1C)(Hexing, Nqwebasaurus, Struthiomimus, Ornithomimus, Archaeornithomimus, etc.)4,17,35,36,37,75. That the neural spines of the cervicals of a very basal paravian like Halszkaraptor are reduced is therefore unsurprising. Elongate neural arches are also regularly found in basal and derived ornithomimosaurs (Fig. 1C)37, and are present in basal therizinosaurs (e.g., Falcarius)3. Notably, neural spines are absent in the cervical vertebrae of the unenlagiine Austroraptor27. The complete connection of the postzygapophyses by bone surface is present in the basal-most ornithomimosaur Nqwebasaurus75 and the basal-most therizinosaur Falcarius3, and is present to a lesser extent in basal alvarezsaurs like Aorun and Haplocheirus7,76, the basal ornithomimosaur Pelecanimimus76, and the basal tyrannosauroid Guanlong76. The fusion of the cervical ribs to the cervical vertebrae is a feature that is also widely distributed among maniraptorans, including basal and derived therizinosaurs2,3, adults of some troodontids53, and basal37 ornithomimosaurs. Given that most of the features on the cervical vertebrae of Halszkaraptor that Cau et al.32 likened to adaptations in some aquatic tetrapods are in fact present on various other maniraptorans, maniraptoriforms, and coelurosaurs (including the possibly plesiomorphic feature of reduced neural spines), the presence of these features on Halszkaraptor is not especially aberrant and provides no unambiguous evidence for a semiaquatic ecology.

Flattened forelimb bones and long bone cross-sectional anatomy

One of the main arguments given by Cau et al.32 to support the hypothesis that Halszkaraptor was biomechanically allied with semi-aquatic tetrapods relied on the somewhat strange anatomy of the forelimb of this dromaeosaurid. Cau et al.32 presented cross-sections of the long bones of the forelimb (the humerus, radius and ulna) and suggested the morphology of these cross sections was allied with the flattened state seen in the forelimbs of marine reptiles and diving birds32. However, this comparison is not precise or well-justified, as the distal humerus, radius and ulna of Halszkaraptor are clearly ellipsoid in cross-section (Fig. 1e–h in32) and clearly similar to the ellipsoid cross-sections of the upper forelimb bones of other paravians, such as Archaeopteryx77 and Deinonychus78. In contrast, the cross-sections of the forelimb bones of long-necked marine reptiles, including nothosaurs79 and plesiosaurs80, are far more flattened and do not show a clearly elliptical cross-section (Fig. 3 in 79; Fig. 2a in 80). In penguins, the cross-section of the humerus is elliptical, but far more flattened than the cross-section of the humerus of Halszkaraptor (compared Fig. 3 in81 with Fig. 1e in 32)81.

The morphology of the cross-sections of the long bones of Halszkaraptor presented by Cau et al.32 also show an additional flaw in the hypothesis that this taxon was semiaqautic. The bones of Halszkaraptor are clearly internally hollow to a similar extent as other paravian dinosaurs77,78.

However, in tetrapods adapted for a semi-aquatic or entirely aquatic lifestyle, such as marine reptiles like plesiosaurs, marine mammals, marine birds, and even spinosaurid dinosaurs, pachyostosis, the extreme thickening of cortical bone, occurs in the limbs79,80,81,82,83,84. Given that pachyostosis is present in the limb bones of both avian and non-avian theropods that took to the water84, the absence of such thickening in Halszkaraptor, which Cau et al.32 posit was well-adapted for a semi-aquatic ecology, would be very surprising from a biomechanical standpoint.

The absence of this feature, then, is rather telling that this taxon was probably not biomechanically suited to live in water, as its skeleton, like other paravians, would have probably been too light to keep the animal submerged. Therefore, the cross-sectional limb morphology of Halszkaraptor provides among the strongest evidence against a partially marine ecology in H. escuilliei.

Modified forelimb and elongate third finger

Morphometric analyses performed by Cau et al.32 on the manual digits of select tetrapods purportedly further evinced the morphological aberrancy of Halszkaraptor, which plotted within the convex hull formed by “long-necked marine reptiles” (plesiosaurs, some pliosaurs, some chelonians, nothosaurs, pistosaurs) in an analysis of the ratios of digits I–III and in the hull formed by wing propelled diving birds in a principle components analysis of several features of the forelimb. These results were used to support a semiaquatic ecological mode in the taxon, with the forelimb acting as a propulsion device. However, the inferences made by Cau et al.32 from the morphometric analyses are flawed, as the forelimb of Halszkaraptor looks strikingly unlike the paddles formed by the forelimb bones of plesiosaurs (Fig. 3A,B). In Halszkaraptor, there are three distinct manual digits tipped by recurved unguals, as in virtually all other maniraptorans and all other dromaeosaurids20,21. Besides showing the condition of the third finger being the longest of the manual digits (also present in scansoriopterygids)32, nothing about the manus of Halszkaraptor is aberrant relative to other dromaeosaurids, coelurosaurs, or even tetanuran theropods (Fig. 3A,E–G). Cau et al.32 also remarked that, apart from the elongation of the third manual digit and metacarpal III being slightly more robust than metacarpal I, the morphology of the manus of Halszkaraptor and the related Mahakala are similar to other dromaeosaurids, showing a lack of fusion, no additional phalanges, and three elongate digits tipped with recurved unguals20,21. The radius, ulna, and humerus of Halszkaraptor also present elongate shafts, as in other dromaeosaurids, paravians, and coelurosaurs20,21,32,41,53. In contrast, the forelimbs of marine reptiles, such as mosasaurs85, plesiosaurs86, and ichthyosaurs87, consist of a massive number of flattened, heavily modified phalanges that form a distinctive paddle shape entirely distinct from the theropod manus (Fig. 3B). The striking morphological differences between the forelimb of Halszkaraptor and those of tanystropheids88 and chelonians like Araripemys89, both of which possess more digits and phalanges than H. escuilliei and other theropods and the latter of which includes highly modified, elongate manual phalanges that help form a paddle (Fig. 3C,D), also stand in contrast to this inference by Cau et al.32 Halszkaraptor lacks the ‘paddle’ in plesiosaurs, Araripemys89, and other aquatic vertebrates like ichthyosaurs, wherein the hand contains many closely appressed phalanges (Fig. 2). Furthermore, recent work has indicated that plesiosaurs possessed a distinctive, four-flipper-powered swim stroke that differed from that seen in forelimb-propelled diving birds90, casting doubt on the locomotory style Cau et al.32 implied Halszkaraptor might have possessed.

Figure 3 Comparative anatomy of the forearm of Halszkaraptor and selected tetrapods. Forelimb of Halszkaraptor (A), forelimb of Muraenosaurus (B) after Andrews105, manus of Tanystropheus (C) after Nosotti88, manus of Araripemys (D) after Meylan89, generalized manus of a therizinosauroid (E) after2,3, and (F) manus of Deinocheirus (pers. obs. of Deinocheirus cast at AMNH). hum, humerus; r&u, radius and ulna; ru, rounded unguals/ultimate phalanges; pm, paddle-like morphology; recu, recurved unguals. Full size image

Another issue with this morphometric analysis was the number of paravians included. Cau et al.32 only included three definite dromaeosaurids besides Halszkaraptor (Velociraptor, Deinonychus, Microraptor), all of which are derived members of the Eudromaeosauria and Microraptoria and would not be expected to be exactly similar to Halszkaraptor in manual morphology (although see Fig. 3, which shows the manus of Halszkaraptor is clearly more similar to Deinonychus than to plesiosaurs and other aquatic reptiles). Microraptor was an arboreal glider21, and thus its manual proportions may have been modified for that purpose. Velociraptor and Deinonychus, in contrast, were terrestrial hypercarnivores, with heavily modified, enlarged unguals on their manual and pedal digits and distinct hands meant for grasping20,21. The absence of any troodontids or anchiornithids in the dataset of Cau et al.32 is also very strange and represents a clear under-sampling of paravians in this morphometric dataset. Similarly, the second principle components analysis of Cau et al.32 does not include any non-avian theropods besides Halszkaraptor, and so the data has not been adequately polarized with data points that could represent the control for what group the forelimb of Halszkaraptor is allied with. Therefore, Cau et al.’s32 hypothesis that the forelimb proportions of Halszkaraptor represent adaptations to an aquatic lifestyle are not at all supported by morphological and biomechanical data. Their resultant reconstruction of the glenoid facing laterally in H. escuilliei is therefore also unsubstantiated, and so the morphology of this bone in Halszkaraptor remains entirely unknown.

Supratrochanteric process of the ilium

A prominent, shelf-like supratrochanteric process was considered a synapomorphy of Halszkaraptorinae by Cau et al.32. However, this feature is widespread in maniraptoran coelurosaurs, including basal dromaeosaurids like Unenlagia and Rahonavis21,25, anchiornithids91,92, and some early avians (Fig. 4A,B)91,92,93. Cau et al.32 noted that this feature in Halszkaraptor had developed into a broadened shelf, as in Mahakala and Buitreraptor but not Rahonavis32. Although it is clear that the prominence of the supratrochanteric process in Halszkaraptor is greater than in these unenlagiines28,32, the supratrochanteric process in many anchiornithids is similarly developed91,92. Given its presence in other paravians and even other basal dromaeosaurids, this feature cannot be used to unite Halszkaraptorinae as an exclusive clade. Because this feature is present in various basal members of the three major paravian clades, Dromaeosauridae, Troodontidae, and Avialae, it is likely that the presence of a supratrochanteric process on the ilium is plesiomorphic for Paraves itself (Fig. 4A–D). Further discussion of the plesiomorphic nature of this feature can be found in the section discussing the results of the phylogenetic analysis conducted on Coelurosauria. Notably, a prominent supratrochanteric process is present in a variety of herbivorous theropods, including some therizinosauroids2,11 and the bizarre Jurassic herbivorous theropod Chilesaurus94.

Figure 4 Comparative anatomy of the ilium in selected tetrapods. Ilium of Halszkaraptor (A) after Cau et al.32, ilium of Anchiornis (B) after Xu et al.91, ilium of Tyrannosaurus (C), and ilium of Deinonychus (D). ace, acetabulum; sac, supraacetabular crest; stp, supratrochanteric process; pp, posterior process. Full size image

Shortened caudal series

Halszkaraptor possesses a highly modified caudal series, a feature that Cau et al.32 used to support a modified posture in this taxon analogous to some birds. However, this feature (defined here as a caudal series shorter than or equal to the snout-sacrum length) is shared with a variety of basal taxa along the maniraptoran stem, including the basal therizinosaur Beipiaosaurus, which possesses a pygostyle-like structure95, a number of derived therizinosaurs2,11,15, basal oviraptorosaurs like Caudipteryx and other caudipterygids8,40, many derived oviraptorosaurs8,50,51,52,73, some basal troodontids53,63, anchiornithids91,92,93, and early avialans69,93. Therefore, there is little reason to believe the posture of Halszkaraptor was especially aberrant in any way from many other maniraptoran and paravian dinosaurs, despite the fact that other dromaeosaurids have a more elongate tail and probably took up a different posture from H. escuilliei20,21. Instead, many basal members of maniraptoran clades display a shortening of the caudal series. Given that a short tail is present in many basal members of paravian and non-paravian maniraptoran clades, this feature may also be plesiomorphic with respect to maniraptorans. Further discussion of this possibility follows in the discussion.

Metatarsus and pedal digits

Most of the features shared by Halszkaraptor and other halszkaraptorines are in their metatarsals32. Among halszkaraptorines, Mahakala has the longest metatarsus23, but the metatarsals of all three members of this clade are more elongate than in more derived dromaeosaurids and lack adaptations for a cursorial lifestyle21,32. One notable feature is the unconstrained nature of the proximal end of metatarsal III (Fig. 5A)32. In many coelurosaurs, including derived tyrannosauroids, ornithomimids, deinocheirids, troodontids, and alvarezsaurs, the metatarsals are closely appressed together and interlock proximally to form a single unfused unit. In derived dromaeosaurids, the subarctometatarsalian condition, where metatarsal III is mediolaterally constrained by II and IV but still visible anteriorly, is present (Fig. 5C,D,F,G)20,21,32. However, the morphology of metatarsal III in Halszkaraptor and Mahakala is expected, given that the basal-most alvarezsaur Haplocheirus6, the basal-most ornithomimosaur Nqwebasaurus75, the basal-most therizinosaur Falcarius3, and the basal oviraptorosaur Caudipteryx40 all possess elongate metatarsals and a dorsally visible and convex metatarsal III. Therefore, the morphology of the metatarsus in Halszkaraptor and other halszkaraptorines is expected given their basal phylogenetic position and aligns with the plesiomorphic nature of this feature among maniraptorans and other theropods (Fig. 5A,B)3,6,40,75. Similarly, the morphology of the pedal digits of Halszkaraptor align with its basal phylogenetic position among dromaeosaurids. Cau et al.32 noted that the ‘sickle’ claw on pedal digit II is heavily reduced in Halszkaraptor compared to other dromaeosaurids (Fig. 2A,E)20,21. Given that basal members of other paravian clades display a reduced sickle claw20,21,53,58,62,63,91,92,93 and the presence of any hypertrophied pedal ungual on digit II seems to be a synapomorphy of paravians20,21,53 (absent in other maniraptorans and theropods, Fig. 2B–D,F,G), the presence of a poorly hypertrophied sickle claw in very basal dromaeosaurids like Halszkaraptor is expected and probably represents the transitional condition.

Figure 5 Comparative pedal anatomy of Halszkaraptor. (A) Left and right pes of Halszkaraptor after Cau et al.32. (B) Right pes of Allosaurus. (C) metatarsus of Struthiomimus in lateral view, (D) pes of Deinonychus in medial view. Generalized tyrannosaur metatarsus in (E) dorsolateral and (F) medial views. mt II, metatarsal II; mt III, metatarsal III; mt IV, metatarsal IV; pd II-3, pedal ungual II-3. Full size image

Amended diagnosis of Halszkaraptorinae

The reevaluation of Halszkaraptor above found several features used to diagnose Halszkaraptorinae to either be dubious or to be found in many other dromaeosaurids and paravians. I therefore offer the amended diagnosis of Halszkaraptorinae: basal dromaeosaurids with the combination of: necks composing 50% of snout-to-sacrum length (possible maniraptoran plesiomorphy), proximal caudal vertebrae with horizontally oriented zygapophyses and prominent zygodiapophyseal laminae, metacarpal III shaft transversely as thick as than of metacarpal I, posterodistal surface of shaft of femur possesses an elongate fossa bounded by a crest; proximal metatarsal III unconstrained and anteriorly convex (maniraptoran plesiomorphy).

Phylogenetic results

In light of this anatomical reassessment of Halszkaraptor, I reevaluated the phylogenetic position of this taxon using the matrix of Cau et al.68 and conducting a phylogenetic analysis on the modified dataset. The resulting phylogenetic analysis produced >99,999 most parsimonious topologies, each of a branch length of 3306 steps. The strict consensus topology is in Fig. 6.

Figure 6 Phylogenetic relationships of Halszkaraptor and rostral coverings in maniraptorans. Strict consensus topology (A) recovered from the phylogenetic analysis of Coelurosauria. Clade diets follow Zanno and Makovicky15 (red = inferred carnivory; green = inferred herbivory). Tree length = 3306; Consistency index = 0.329; Retention index = 0.762. Silhouettes by the author. Dentaries of (A) Deinonychus, (B) Dromaeosaurus, (C) Halszkaraptor after Cau et al.32, (D) Velociraptor, (E) “Bambiraptor,” (F) Tyrannosaurus, and (G) Struthiomimus in lateral view. Arrows point to downturned ‘chins’ at the dentary symphysis. dr, lateral dentary ridge scale bar = 20 mm (B–F). Full size image

I could not recover the topology found in Cau et al.32 Instead Halszkaraptor and Mahakala (the two halszkaraptorines included in the dataset) form the sister clade to Unenlagiinae, a group of peculiar paravians from the southern continents20,21,22,23,24,25,26,27,28,29,30,31. This clade is united by five characters: 27 (0, maxillary fenestra situated at anterior border of antorbital fossa), 107 (1, Sacral vertebrae number is six), 193 (1, ascending process of astragalus short and slender), 580 (0, sagittal crest of parietal comprised of two parallel crests), and 828 (0, Meckelian groove centered).

Nesting as a relatively basal dromaeosaurid (Fig. 4), Halszkaraptor would expectedly show several plesiomorphic traits. Several of the features discussed in this paper, including unserrated teeth, a large number of maxillary teeth, elongate nares that extend over 1/3 of the maxillae, medially expanded neurovasculature, a neck consisting of ~10 elongate cervicals that makes up ~50% of snout-sacrum length, reduced to nearly absent neural spines on the cervical vertebrae, the presence of a supratrochanteric process on the ilium, a shortened caudal series, a non-hypertrophied ungual on pedal digit II, and a metatarsus where metatarsal III is clearly visible and convex in dorsal view are considered here to be probable plesiomorphic states for Paraves, Maniraptora, or Maniraptoriformes in this text. The phylogenetic analysis allowed for the possible plesiomorphic nature of several of these features to be tested. The results of the phylogenetic analysis conducted provides support for the recognition of several features relevant to the body plan of Halszkaraptor as plesiomorphies of Paraves or larger clades. These include the posterior extent of the nares (char. 23 [1–>0] found as a synapomorphy of Dromaeosauridae, indicating nare size reduction and supporting Halszkaraptor as the transitional form), unserrated teeth (char. 81, found as a synapomorphy of Eudromaeosaurs [2 or 1–>0], reflecting the complete re-emergence of serrations on all teeth, and found as a synapomorphy of derived troodontids [2–>0], reflecting the partial re-emergence of denticles on some crowns; these data indicate unserrated crowns (state 2) are plesiomorphic to Paraves), a large number of maxillary teeth (char. 82, distributed throughout Coelurosauria, Pelecanimimus may represent an increase in Ornithomimosauria [0–>1], although the maxillary tooth count of Nqwebasaurus is not known), cervical vertebrae number (char. 90, found to change states [0–>1] in only therizinosaurs (increased to 12 or more cervicals), suggesting ~10 cervicals are plesiomorphic to coelurosaurs; note that the cervical count of Falcarius is not precisely known), caudal vertebrae number (char. 119 [0–>2], found as a synapomorphy of Maniraptora and showing reduction of caudal number from 40+ to 25–35 vertebrae), neural spine height (char. 660, shared among the coelurosaurs sampled), a non-arctometatarsalian metatarsus where metatarsal III is fully visible dorsally (char. 200, [0–>1] in derived enantiornithines, [0–>3] in derived alvarezsaurs, [1–>2] in derived ornithomimosaurs, [0–>1] in derived unenlagiines), and a non-hypertrophied ungual on pedal digit II (char. 201, [0–>1] in Dromaeosauridae). Character 826, which documents the anteroposterior length of the premaxilla compared to the maxilla (a feature clearly relevant to the discussion of the premaxilla herein), was found to be reduced [0–>1] in alvarezsaurs, suggesting the plesiomorphic condition for maniraptorans is an elongate premaxilla. Given that the premaxillae of Halszkaraptor form a u-shaped outline (a feature possibly relevant to their lateral expansion), I also assessed changes in character 24 (outline of premaxillae in ventral view) for Coelurosauria. In tyrannosaurs, this character changes from being v-shaped to u-shaped [0–>1], and derived tyrannosauroids have a more pronounced version of state 1 [1–>2]. Otherwise, this feature is distributed among various coelurosaurs, with little discernible pattern. Therefore, it is likely many of the features of Halszkaraptor simply represent the plesiomorphic states for Paraves, Maniraptora, and Coelurosauria and were secondarily changed in more derived dromaeosaurids.