Technical post [English text: do not use Google Translator function!] Comparison between Halszkaraptor and Alvarezsauroidea

This is a technical post that aims to reply to those suggestions on a chimerical status of Halszkaraptor based on (not described neither specified) similarities between the skull of H. escuilliei and those of alvarezsauroids.

Note that the whole idea of a composite status for the skeleton of Halszkaraptor is definitively dismissed by careful examination of the specimen using 3D microtomography (scan data available for registered users at ESRF site: http://paleo.esrf.fr/picture.php?/2740/category/2102).

Halszkaraptor shows an unexpected combination of features, otherwise seen in distinct coelurosaur clades. In particular, it shares with the alvarezsaurids a relatively small size, slender jugal and postorbital bars, small and numerous unserrated teeth, and a short forelimb with an asymmetrical hand bearing a hypertrophied finger (although the longest finger is the medialmost in alvarezsaurids, instead of the lateralmost in Halszkaraptor). These similarities, in particular in the skull and cervical vertebrae, might eventually indicate a close relationships between these taxa. This hypothesis was mentioned in some online comments to our paper.

Nevertheless, detailed comparison between Halszkaraptor with the skull and neck of the best known alvarezsauroids (e.g., Haplocheirus, Shuuvuia and Mononykus), reveals several differences in phylogenetically informative features that dismiss a referral to named alvarezsauroids or a close relationships with any of them (features listed below). The referral of the skull and neck of H. escuilliei to Alvarezsauroidea is thus rejected, being it based on vaguely-defined and size-related homoplasies shared by many small-bodied coelurosaurs (including other small paravians).

1. - Premaxilla body relatively larger, U-shaped in dorsal view. In Halszkaraptor, the body of the premaxillary is similar to other pennaraptorans in being relatively enlarged compared to snout size. In H. escuilliei, the premaxilla is longer than tall in lateral view, dorsoventrally flattened (platyrostral), and describes a parabolic outline in dorsal view. In Shuvuuia and Haplocheirus, on the contrary, the premaxillary body is relatively much smaller, not elongate and forms just the tip of a narrower snout apex.
   
2. - External naris posterior to the alveolar border of premaxilla. In Halszkaraptor, the entire margin of the external naris is placed posterior to the alveolar border of premaxilla. In Shuvuuia, the anterior margins of the external naris broadly overlaps the premaxillary oral margin. Haplocheirus external naris overlaps the premaxillary oral margin, as in most theropods. The condition in Halszkaraptor is shared with other paravians (e.g., avialans).
   
3. - Internarial bar dorsally convex. In Halszkaraptor, the internarial bar is mediolaterally compressed and transverally convex dorsally. This condition is shared with dromaeosaurids. In Shuvuuia, and probably also Haplocheirus, the internarial bar is instead dorsoventrally flattened.
   
4. - Elongate preantorbital part of maxilla. In Halszkaraptor, the preantorbital (subcutaneous) part of the maxilla is very elongate and form a significant part of the maxilla. This feature is shared with the majority of dromaeosaurids (e.g., Velociraptor). In alvarezsauroids, e.g., Haplocheirus and Shuvuuia, on the contrary, the preantorbital part of maxilla is very short compared to the rest of the maxilla.
   
5. - Maxilla lacking an anterior process. In Halszkaraptor, the preantorbital process of the maxilla is long and triangular in lateral view, with a straight dorsal margin. This feature is widespread among dromaeosaurids (e.g., Velociraptor, Austroraptor). Accordingly, the maxilla lacks a distinct anterior (subnarial) process that is offset from the remainder of the preantorbital process. In alvarezsauroids (i.e., Haplocheirus, Shuvuuia), the maxilla bears instead a concave dorsal margin ventrally to the external naris, and a short but distinct anterior process.
   
6. - Antorbital fossa relatively shorter. In Halszkaraptor, the antorbital fossa extends for about 1/2 of the preorbital length of the maxilla. A short antorbital fossa is widepread among pennaraptorans. In alvarezsauroids, instead, the antorbital fossa is more elongate, extending for more than 2/3 of maxillary length. Accordingly, the preantorbital (subcutaneous) process of the maxilla in Halszkaraptor is much larger and more elongate than among alvarezsauroids.
   
7. - Smaller maxillary fenestra. The antorbital fossa of Halszkaraptor is perforated by a small fenestra, slightly dorsally displaced relative to antorbital fossa main axis. A small and dorsally placed maxillary fenestra is seen in many dromaeosaurids. A much larger fenestra (probably homologous with the maxillary fenestra of most tetanurans) separated from the antorbital fenestra by a vertical bar, is present in both Haplocheirus and Shuvuuia.
   
8. - Preorbital bar formed exclusively by the lacrimal. The preorbital bar of Halszkaraptor is formed by a single bone. Since this bone borders the antorbital fenestra and reaches the level of the jugal-maxillary bar, it is interpreted as the lacrimal. Accordingly, Halszkaraptor lacks a distinct prefrontal bone. In alvarezsauroids, the preorbital bar is formed by two distinct elements, the lacrimal and the prefrontal, the latter bearing a large orbital flange in Shuvuuia. Therefore, in Halszkaraptor the lacrimal articulates with the frontal, a condition absent among alvarezsauroids. The posterodorsal process of lacrimal of Halszkaraptor recalls that of other paravians (e.g., Zanabazar, Velociraptor) in overhanging the orbit.
   
9. - Lacrimal anterodorsal process short. The anterodorsal corner of the lacrimal is blunt, and the anterior ramus is shorter than the ventral process of lacrimal. In alvarezsauroids, the anterodorsal process of lacrimal is more elongate, being longer than the ventral process of lacrimal.
   
10. - Absence of quadrate-postorbital contact. In Shuvuuia, the postorbital contacts the quadrate, constricting the infratemporal fenestra dorsally. In Halszkaraptor, this contact is absent.
    
11. - Dorsoventrally lower infratemporal fenestra. In Halszkaraptor the infratemporal fenestra is proportionally shorter than the orbit, a feature widespread among paravians. In alvarezsauroids, the infratemporal fenestra is comparable to the orbit in dorsoventral extent.
    
12. - Fronto-parietal suture straight, not notched due to triangular anterior projection of parietals. In Halszkaraptor, the fronto-parietal suture is straight and transversally oriented in dorsal view. This morphology contrasts with that in Shuvuuia, where the posterior margin of the frontals diverges posterolaterally, resulting in a notched profile where the triangular anteromedial processes of the parietal insert.
    
13. - Fronto-parietal roof more vaulted and convex dorsally. The frontoparietal roof in Halszkaraptor is markedly bird-like, being vaulted and convex dorsally, as in all small-bodied paravians. This condition is absent in immature and adult specimens of Shuvuuia, and in the only known specimen of Haplocheirus, all showing the plesiomorphic flatter fronto-parietal roof.
    
14. - Narrowest dorsal point of the parietal wider than narrowest interorbital space of frontal. The parietal of Haplocheirus is also bird-like in being mediolaterally expanded across the supratemporal fenestrae, resulting wider than the minimum width of the skull roof across the orbital margin of frontals. This condition is the opposite of the one present in Shuvuuia and, probably, also Haplocheirus.
    
15. - Ventral process of postorbital straight, not anteriorly concave. In Halszkaraptor, the ventral process of the postorbital is straight in lateral view. In both Haplocheirus and Shuvuuia, the ventral process of the postorbital is curved anteroventrally, with a broadly concave orbital margin.
    
16. - Postorbital contacts jugal. In Halszkaraptor, the ventral process of the postorbital contacts the jugal, separating the orbit from the infratemporal fenestra. This differs from Shuvuuia, where the postorbital-jugal contact is absent.
    
17. - Dentary with subparallel dorsal and ventral margins. The dentary of Halszkaraptor is subrectangular in lateral view, as in all dromaeosaurids. In alvarezsauroids, the dentary is subtriangular in lateral view.
    
18. - Dentary and maxillary alveoli distinct, not set in a groove. In Halszkaraptor, the alveoli are separated by distinct septa. In Shuvuuia, the interalveolar septa are absent, and the alveoli are housed along longitudinal sulci.
    
19. - Maxillary teeth proportionally larger, less numerous and lacking basal constriction. Halszkaraptor differs from Shuvuuia in having relatively larger and less numerous teeth (no more than 25 teeth in H. escuilliei vs about 30 in Shuvuuia deserti). The dentition in H. escuilliei is heterodont, differing from the homogeneous shape and size of the teeth in S. deserti.
    
20. - Maxillary dentition reaches the level of the lacrimal bar. In Halszkaraptor, the maxillary alveoli are extended along the whole ventral ramus of maxilla and reach the level of the lacrimal bar. On the contrary, in Shuvuuia, the maxillary dentition is restricted to the anterior ramus of maxilla, and does not extend below the antorbital fenestra.
    
21. - Absence of epipophyses in all cervicals. In Mononykus and Shuvuuia, the anterior cervical neural arches bear small epipophyses. Halszkaraptor lacks epipophyses on all cervical vertebrae.
    
22. - Cervical centra not opisthocoelous. Based on the exposed ventrolateral surfaces of the cervical centra, the neck vertebrae of Halszkaraptor are amphiplatan-amphicoelous. This differs from the distinct opisthocoelous condition present in the cervical centra of Mononykus, Shuvuuia and other parvicursorines.
    
23. - Ventral surface of cervical centra lacks longitudinal sulcus bordered by ridges. The ventral surface of the mid-cervical centra of Halszkaraptor is keeled. In Mononykus, the ventral surface of the mid-cervical centra bears a mid-line sulcus bordered by lateral crests.
    
24. - Cervical postzygapophyses completely joined by a posteriorly convex interpostzygapophyseal lamina. In Halszkaraptor, the anterior cervical postzygapophyses are joined medially, forming a single lobate process. Although an interpostzygapophyseal lamina is present in Shuvuuia and other maniraptoriforms (e.g., Gallimimus, Falcarius), it does not extend posteriorly to the level of the postzygapophyseal facets, which are distinct in dorsal view by a median concavity. In the posterior cervical vertebrae of Shuvuuia, the postzygapophyses diverge posterolaterally, whereas in Halszkaraptor they are more closely appressed medially.
    
25. -Cervical ribs fused to centra. In Halszkaraptor, the cervical ribs are fused to both diapophyses and parapophyses. In Mononykus and Shuvuuia, the cervical ribs are unfused to the vertebrae.