Avialan alvarezsaurs I: Cranial Characters

In this series of posts examining the evidence for placing alvarezsaurs in various parts of the theropod tree, I decided to break up the bird part due to the huge number of characters suggested.  Various analyses have suggested alvarezsaurids are members of Avialae, here used in the sense of closer to birds than Deinonychus.  In addition, some of these have placed alvarezsaurids closer to modern birds than Archaeopteryx, which means they fall into Ornithurae sensu Gauthier.  This alvarezsaurid-ornithothoracine clade was usually called Metornithes, but that name is valid for the smallest group containing Aves and Mononykus, regardless of where the latter falls.  So if alvarezsaurids are arctometatarsalians, Metornithes is a senior synonym of Maniraptoriformes.  If alvarezsaurids are basal maniraptorans, Metornithes is a subset of Maniraptora.  Thus the term Metornithes will not be used in the discussion below.  One problem with these early phylogenies is that many basal avialans were not yet known, except Archaeopteryx, ornithothoracines and in 1995 and later, confuciusornithids (not actually included in analyses until 2000).  Thus exactly where alvarezsaurids would fall in comparison to taxa like scansoriopterygids, Shenzhouraptor or omnivoropterygids is not obvious.  The following is a list of every cranial character found to support placing alvarezsaurids in Avialae (first 18 characters) and its subgroups (characters 19-26), based on every analysis to support this arrangement.  These were (not including later variations)- Perle et al., 1993; Chiappe et al., 1996; Novas, 1996; Chiappe et al., 1998; Forster et al., 1998; Chatterjee, 1999; Holtz, 2000; Chiappe, 2001; Maryanska et al., 2002.  Most found them to be ornithurines, though the last two found them to be outside the Archaeopteryx+Aves clade.

1. Premaxilla long and pointed with long nasal process (Holtz, 2000). A composite of three characters, none of which are in TWG and only one of which is in Choiniere et al. (premaxilla angled <70 degrees). However, it is composited there with the proportion of the premaxilla which is anterior to the external nares so is difficult to evaluate the codings for. In any case, Shuvuuia has a very short premaxilla which is blunt and has a short nasal process (Sereno, 2001), unlike most birds. It was thus miscoded by Holtz. Haplocheirus is similar.

2. Premaxillary teeth conical (Holtz, 2000). In both TWG and Choiniere et al.. Miscoded- Allosaurus, Giganotosaurus, Incisivosaurus, Troodon, Sinornithosaurus, Saurornitholestes, Tsaagan, Velociraptor, Deinonychus, Utahraptor. Not coded- Daspletosaurus, Sinovenator. Premaxillary teeth are unknown in Shuvuuia (Chiappe et al., 2002), so Holtz miscoded it. It turns out that Haplocheirus has the character though, potentially supporting avialan alvarezsaurs. However, the character is not unique to avialans. Basal troodontids and oviraptorosaurs also have it (the condition in therizinosaurs is unknown), so this could be congruent with a basal maniraptoran placement as well.

3. Nasal shorter than frontal (Maryanska et al., 2002). Not in TWG, but in Choiniere et al. as part of a character coding for rostrum length and composited with maxilla length compared to premaxilla length. Again, the compositing means I won't be evaluating its accuracy, but again this was miscoded in Shuvuuia which has longer nasals than frontals (Sereno, 2001). Haplocheirus is similar.

4. Jugal shallow or rod-like under orbit (Maryanska et al., 2002). Not in TWG, but in Choiniere et al. when specified to define the cross section, not necessarily the depth. Miscoded- Garudimimus, Rahonavis. Not coded- Acrocanthosaurus, Carcharodontosaurus, Dromiceiomimus, Byronosaurus, Saurornithoides, Buitreraptor, Velociraptor, Shenzhouraptor, Sapeornis, Confuciusornis, Yixianornis. Haplocheirus lacks this, so does not support avialan alvarezsaurs.

5. Frontals anteriorly triangular (Forster et al., 1998). In both TWG and Choiniere et al.. This is not true in Shuvuuia, where the nasofrontal suture is W-shaped (Sereno, 2001), or Haplocheirus where it is transverse. It was thus miscoded by Forster et al. and its accuracy in Choiniere et al.'s matrix is not examined here. It is present in the derived Ceratonykus though.

6. Squamosal-quadratojugal contact absent (Chiappe et al., 1996). In both TWG and Choiniere et al.. Miscoded- Garudimimus, Deinonychus, Archaeopteryx. Not coded- Daspletosaurus, Harpymimus, Oviraptor, Conchoraptor, "Ingenia", Mei, Saurornitholestes, Dromaeosaurus, Scansoriopteryx, Epidexipteryx, Sapeornis, Confuciusornis. As no avialans were coded, except Archaeopteryx which was miscoded as having the contact, it couldn't test the homology of Shuvuuia's condition. Absent in Haplocheirus, so does not support avialan alvarezsaurs.

7. Quadratojugal joined to quadrate by ligament (Chiappe et al., 1998). Not in TWG or Choiniere et al.. Not reported in Haplocheirus, so potentially supports avialan alvarezsaurs. However, the distribution of this character is largely uncertain since many maniraptoriforms have at least loosely sutured articulations which make determination difficult or ambiguous.

8. Parietals and laterosphenoids unfused (Forster et al., 1998). Not in TWG or Choiniere et al.. Forster et al. miscoded basically every taxon, as their Ornithurae has fusion and their non-avialan taxa lack it (e.g. tyrannosaurids, Compsognathus, ornithomimids, oviraptorids). The condition in Shuvuuia is unreported otherwise, though even if I trusted Forster et al.'s coding it would be compatible with any topology where they are outside Ornithurae sensu Chiappe (Hesperornis and more derived birds). The condition in Haplocheirus is unknown.

9. Posterior tympanic recess opens in otic recess (Chiappe et al., 1996). In both TWG and Choiniere et al.. Miscoded- Tyrannosaurus, Sinovenator, Byronosaurus, Saurornithoides, Troodon. Not coded- Acrocanthosaurus, Gorgosaurus, Pelecanimimus, Chirostenotes, "Ingenia". The condition in Haplocheirus is unknown, so potentially supports avialan alvarezsaurs. However, it is also present in Pelecanimimus, Falcarius and troodontids, meaning it could be congruent with other hypotheses as well.

10. Occiput directed ventrally (Holtz, 2000). Not in TWG or Choiniere et al.. This seems true in Haplocheirus, so potentially supports avialan alvarezsaurs.

11. Foramen magnum taller than wide (Holtz, 2000). In both TWG and Choiniere et al.. However, Holtz miscoded Alvarezsauridae since Shuvuuia does not have this character (Chiappe et al., 1998). Haplocheirus also lacks it and its accuracy in Choiniere et al.'s matrix is not examined here.

12. Neck of occipital condyle not constricted (Holtz, 2000). In both TWG and Choiniere et al.. Miscoded- Sinraptor, Dilong, Tyrannosaurus, Avimimus, Troodon, Dromaeosaurus. Not coded- Acrocanthosaurus, Carcharodontosaurus, Giganotosaurus, Daspletosaurus, Struthiomimus, "Ingenia", Microraptor. This is true in Haplocheirus, so potentially supports avialan alvarezsaurs. However, it is also present in most maniraptoriforms except some oviraptorosaurs, so was miscoded by Holtz.

13. Triradiate palatine (Chiappe et al., 1998). In both TWG and Choiniere et al.. Miscoded- Shenzhousaurus, Archaeopteryx. Not coded- Neovenator, Gorgosaurus, Daspletosaurus, Juravenator, Chirostenotes, Sinornithosaurus, Dromaeosaurus, Sapeornis. Absent in Haplocheirus, so does not support avialan alvarezsaurs.

14. Splenial not widely exposed laterally (Holtz, 2000). In both TWG and Choiniere et al.. Miscoded- Compsognathus. Not coded- Daspletosaurus, Juravenator, Pelecanimimus, Caudipteryx, Buitreraptor, Saurornitholestes, Shenzhouraptor. This is true in Haplocheirus, so potentially supports avialan alvarezsaurs. Yet this is also found in almost every theropod except most deinonychosaurs so was only an avialan character in Holz's analysis due to his lack of deinonychosaur monophyly.

15. Coronoid absent (Chiappe et al., 1998). In both TWG and Choiniere et al.. Miscoded- Shuvuuia, Citipati, Scansoriopteryx. Not coded- Acrocanthosaurus, Daspletosaurus, Compsognathus, Segnosaurus, Caudipteryx, Conchoraptor, "Ingenia", Shenzhouraptor, Sapeornis. Since Shuvuuia was miscoded, this character could not function to support avialan alvarezsaurs. The condition in Haplocheirus is unknown, so potentially supports this relationship. However, ornithomimosaurs and therizinosaurs show this as well.

16. Elongate and tapering retroarticular process (Holtz, 2000). In TWG, but not Choiniere et al.. Both Shuvuuia and Haplocheirus have this, but Holtz miscoded Archaeopteryx as having it as well. In actuality, almost all avialans lack it, so it does not support this relationship.

17. Unserrated teeth (Chiappe et al., 1996). In both TWG and Choiniere et al.. Miscoded- Compsognathus, Avimimus, Sinovenator, Microvenator, Adasaurus. Not coded- Daspletosaurus, Alxasaurus, IGM 100/44, Scansoriopteryx, Shenzhouraptor. Oddly, only some toothless taxa are coded as inapplicable. Absent in Haplocheirus, so does not support avialan alvarezsaurs.

18. Teeth uncompressed and unrecurved (Forster et al., 1998). Not in TGW, but in Choiniere as the same composite of characters, with an additional state added compositing crown elongation as well. Miscoded- Shenzhousaurus, Harpymimus, Anserimimus, Buitreraptor, Sinornithosaurus, Microraptor, Archaeopteryx. Not coded- Acrocanthosaurus, Carcharodontosaurus, Giganotosaurus, Mapusaurus, Beipiaosaurus, IGM 100/44, Sinornithoides, Adasaurus, Utahraptor, Scansoriopteryx, Yixianornis. Absent in Haplocheirus, so does not support avialan alvarezsaurs.

Skull of Haplocheirus (modified from Choiniere et al., 2010) showing features which agree (green) and disagree (red) with placing alvarezsaurs as birds in various analyses.  Numbers match character numbers in this post.

19. Maxillary fenestra absent (Perle et al., 1993). In both TWG and Choiniere et al.. This was based on the partial maxilla of Mononykus, found before complete skulls of Shuvuuia showed a fenestra in a different position. Thus Mononykus may have had one as well, and Choiniere et al.'s codings are not evaluated for accuracy, though a glance shows taxa left uncoded like Allosaurus and Archaeopteryx, which obviously have the fenestra. Haplocheirus has one as well.

20. Slot in ventral process of lacrimal for jugal absent (Holtz, 2000). Not in TWG or Choiniere et al.. This has never been explicitly described in Shuvuuia, though Sereno (1999) agreed with Holtz's coding and Holtz et al. (2004) later switched Alvarezsauridae's coding. Thus the coding for Alvarezsauridae is controversial. In any case, examination shows basically all maniraptoriforms lack it, contra Holtz's original coding, but agreeing with the 2004 coding. Haplocheirus' condition is unknown, but the character is ignored as it has no affect on alvarezsaurs' position given current data.

21. Postorbital-jugal contact absent (Chiappe et al., 1996). Not in TWG or Choiniere et al.. Absent in Haplocheirus, so does not support ornithurine alvarezsaurs.

22. Dorsal jugal process absent (Forster et al., 1998). Not in TWG or Choiniere et al.. Absent in Haplocheirus, so does not support ornithurine alvarezsaurs.

23. Quadrate articulates with braincase (Novas, 1996). Not in TWG or Choiniere et al. except as a corollary of a character coding for a divided quadrate head. Unknown in Haplocheirus so potentially supports ornithurine alvarezsaurs. This character is more widespread than usually given credit though as even most tyrannosaurids have a surface extending distally from the quadrate head that articulates with the braincase. Even ignoring this primitive condition, at least oviraptorosaurs, troodontids and Mahakala also have the condition among maniraptoriforms, though eudromaeosaurs lack it.

24. Separate otic head of quadrate (Forster et al., 1998). In both TWG and Choiniere et al.. Miscoded- Deinonychus, Yixianornis. Not coded- Acrocanthosaurus, Giganotosaurus, Mapusaurus, Gorgosaurus, Daspletosaurus, Compsognathus, Harpymimus, Garudimimus, IGM 100/44, Scansoriopteryx, Sapeornis, Apsaravis. Unknown in Haplocheirus so potentially supports ornithurine alvarezsaurs. However, among birds only Confuciusornis, Enaliornis and some neoavians have well separated heads like Shuvuuia, so convergence is most likely by far.

25. Parietal sagittal crest absent (Forster et al., 1998). In both TWG and Choiniere et al.. However, Choiniere et al. have two sagittal crest characters, one coding for height and one for axial length. This unfairly weights the presence of a crest (since both characters have a state for 'crest absent'). To mitigate this, all taxa coded as lacking a crest in one of the characters are changed to unknown in my recoding. Miscoded- Dilong, Ornitholestes, Citipati, Troodon, Tsaagan. Not coded- Allosaurus, Acrocanthosaurus, Carcharodontosaurus, Giganotosaurus, Gorgosaurus, Daspletosaurus, Juravenator, Pelecanimimus, Shenzhousaurus, Harpymimus, Garudimimus (oddly coded as inapplicable), Dromiceiomimus, Struthiomimus (also coded inappicable), Erlikosaurus, Incisivosaurus, Caudipteryx, Avimimus, Oviraptor, Conchoraptor, "Ingenia", Sinovenator, Saurornitholestes, Velociraptor, Deinonychus, Scansoriopteryx, Archaeopteryx, Sapeornis, Confuciusornis. Unknown in Haplocheirus so potentially supports ornithurine alvarezsaurs. Yet this is also found in ornithomimosaurs, Erlikosaurus and basal troodontids among maniraptoriforms.

26. Dentary teeth set in groove (Chiappe et al., 1996). In both TWG and Choiniere et al.. I already covered this character in the ornithomimosaur post. Unknown in Haplocheirus so potentially supports ornithurine alvarezsaurs. However, it is also present in Pelecanimimus and troodontids, and more importantly is only found in a few derived ornithuromorphs among birds.

As before, a summary of the codings for relevent taxa.  They are divided into supposed avialan (1-18) and supposed ornithurine (19-26) characters.

                          111111111 12222222

                 123456789012345678 90123456


Haplocheirus     010000???10101?100 0?00????
Tugrik           ????????????1????? ????????
Shuvuuia         0?01011?1101111111 0?111111
Ceratonykus      ???111???1???1?1?? ??1111??
Mononykus        ????????1???????11 ????1???

Ornithomimosaur  00001001p001011010 0100?01p

Scansoriopteryx  ????01???????11011 ??00?01?
Epidexipteryx    010??1?????????11p ??10??0?
Archaeopteryx    11011111111101101p 00001010
Shenzhouraptor   ???1?????????1101? 0?00???0
Dalianraptor     0????????????1???? ????????
Jixiangornis     0???????????????1? ???0???-
Yandangornis     1-??????????????-- ???????-
Zhongjianornis   1-01????????????-- ???1???-
Sapeornis        010001??????11101? 0?0010?-
Confuciusornith  1-11011????1?1?0-- 0?00111-
Ornithothoraces  110p011011p1p1p010 0-p010p0

Of the 26 suggested cranial characters, only nine hold up as even potentially supporting alvarezsaurids as birds.  Of these, several (2, 12, 14, 23) are present basally in Maniraptora.  Of the others, several are also found in ornithomimosaurs (9, 15, 25), therizinosaurs (9, 15, 25) or troodontids (9, 25).  Of the remaining, character 7 is hard to unambiguously score for most maniraptoriforms as noted above, leading only character 10 (occiput directed ventrally) as strong evidence for the relationship.  Of the nine potentially valid characters, Choiniere et al.'s analysis includes six.  Yet they did not code alvarezsaurids for one of these, making the total functionally five.  The effect of recoding in Choiniere et al.'s matrix will be examined at the end of this series.