Tuesday, December 14, 2010

The Myth of Martinavis

After 28 years, the monograph on Lecho enantiornithines has finally been published (Walker and Dyke, 2009).  This follows the shorter paper by Walker et al. (2007), which assigned many Lecho remains to their new genus Martinavis.  Now in addition to M. cruzyensis from France and M. vincei from Argentina, we also have M. saltariensis, M. minor and M. whetstonei living alongside M. vincei, plus an unnamed specimen from the US.  At first glance, this seems to be an amazing diversity for an enantiornithine genus, but some further digging indicates trouble for Martinavis.

Walker et al.'s diagnosis for Martinavis is sprawling, but most of the characters are just enantiornithine synapomorphies- proximal margin of humerus concave in its central portion, rising both ventrally and dorsally on either side; bicipital crest prominent; ventral surface of bicipital crest bearing a small fossa for muscle attachment; proximally L-shaped humeral head; well-marked depression underneath the proximal head of the humerus; weakly developed distal condyles; flat distal end that is not deflected dorsally.

The pneumotricipital fossa is no wider in vincei, saltariensis or minor than Elbretornis, Enantiornis, Gurilynia, Halimornis or Otogornis, and is narrow in cruzyensis and whetstonei. It is wide in the American specimen KU-NM-37 though.

The lack of a perforated ventral tubercle is plesiomorphic and also present in such taxa as Halimornis and Eoalulavis.

Most enantiornithine deltopectoral crests could be described as "flat and broad", while the stated lack of ventral curvature in the crest is difficult to understand since it projects dorsally in enantiornithines.

The lack of a marked distal angle between the deltopectoral crest and shaft distally is primitive and also seen in such taxa as Eoalulavis, Eocathayornis, Hebeiornis, Otogornis, Pengornis and Cathayornis, but is absent in saltariensis and not determinable in KU-NM-37.

The bicipital crest in cruzyensis and vincei is no smaller than in Gurilynia, while KU-NM-37, minor, saltariensis and whetstonei have large crests.

The bicipital crest is indeed more anteriorly angled in vincei and KU-NM-37 than Elbretornis, Enantiornis, Halimornis or Gurilynia, but is less angled than Elsornis. Supposed Martinavis species minor, saltariensis and whetstonei have less angled crests.  The condition in cruzyensis is not illustrated, though stated to be less than vincei at least. 

"Ventral margin of bicipital crest small" is a confusing statement, and the ventrally placed bicipital fossa is also present in Elbretornis, Gurilynia and Halimornis while those of M. cruzyensis, saltariensis and minor are anteroventrally placed.

The ventral condyle is as poorly developed in Elbretornis, Elsornis, Eocathayornis, Kizylkumavis and Cathayornis. Note neither this nor the next three characters can be evaluated in minor, whetstonei or KU-NM-37. 

Alexornis, Elbretornis, Elsornis, Kizylkumavis and Otogornis lack both scapulotricipital and humerotricipital grooves as well.

The ventral epicondyle is as large and distally projected in Kizylkumavis and probably Alexornis.

A transversely oriented dorsal condyle is present in almost all enantiornithines, even Elbretornis (contra Walker and Dyke)

Walker et al. also included a differential diagnosis, though it repeats some characters of the general diagnosis (anteriorly angled bicipital crest; deltopectoral crest smoothly angled; transversely oriented dorsal condyle) and has another which contradicts the general diagnosis (small entepicondyle). Of the remaining characters, the shaft is actually less gracile than Enantiornis (and most other enantiornithines), not more. A laterally positioned ectepicondyle is present in all enantiornithines.  Walker and Dyke do not add further characters to the diagnosis.

In conclusion, species referred to Martinavis do not share any apomorphies not found in numerous other enantiornithines.  Worse, the type species lacks two of the supposed diagnostic characters, saltariensis lacks four, both minor and whetstoni lack at least three, and the American specimen lacks at least one.  Walker et al. should have really made M. vincei the type species, since it seems most representative.  Walker and Dyke refer numerous additional postcranial elements to Martinavis sp. based on size, but since there's no evidence that genus was present in Argentina, it seems best to keep these as Enantiornithes indet..  The small size of some means they probably belonged to minor or whetstonei, while the larger may have belonged to vincei or saltariensis.

Martinavis - The Cathayornis of the Late Cretaceous. ;)

References- Walker, Buffetaut and Dyke, 2007. Large euenantiornithine birds from the Cretaceous of southern France, North America and Argentina. Geological Magazine. 144(6), 977-986.

Walker and Dyke, 2009. Euenantiornithine birds from the Late Cretaceous of El Brete (Argentina). Irish Journal of Earth Sciences. 27, 15-62.

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