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Sauriurae Haeckel 1886 sensu Hou et al., 1996 "lizard tails"

Sauriurae is an allegedly holophyletic clade, the diagnosis of which was amended from Haeckel's original in 1983, 1985 and 1987 by Martin, and it allies Archaeopterygiformes and Enantiornithes, with the subsequent addition in 1996 of Confuciusornithidae by Hou et al. (1996). Though Sauriurae is intellectually appealing for its convenient compartmentalization of the vast adaptive radiation of archaic birds in the Cretaceous, the validity of this clade is suspect. The following characters have been used to underwrite holophyly of Sauriurae (following Hou et al 1996, Feduccia 1999):

a)Arms of the furcula broad, grooved caudodorsally rendering them laterally incompressible

b)Outer condyle of the femur possesses a dorsal crest

c)Dorsal ischial process

d)Proximal to distal metatarsal fusion

e)Scapula possesses a craniomedial process which articulates with the craniothoracic vertebrae.

Hou et al. (1996) allied Confuciusornis and Enantiornithes to the exclusion of Archaeopteryx on the basis of their shared acquisition of a pygostyle and distinct pleurocoels. These data do seem to suggest a confuciusornithid/enantiornithine relationship, and the former may represent the outgroup to the more derived Enantiornithes. Neverthless, there remains compelling evidence, summarized by Chiappe (2002), for an alternative hypothesis in which Confuciusornithidae represents the sister clade of Ornithothoraces.

As for the holophyly of Sauriurae as a whole, character (a) as Chiappe (2001) observes, is rather ambiguous, and never properly defined. If it is intended to mean a wide interclavicular angle, it is in fact plesiomorphic, present in dromaeosaurs (Norell et al. 1997) and therizinosaurs (Xu et al. 1999). If instead the authors imply a large furcular width, the condition is not observed in Enantiornithes. The lack of lateral compressibility in the furcula would also appear to be a plesiomorphic attribute, and thus of no phylogenetic relevance. Both the urvogel and Enantiornithes lack character (b). Character (c) is an avian symplesiomorphy, and thus of no phylogenetic relevance, and the presence of character (d) has been conclusively documented only in Enantiornithes. Character (e), is apparently not present in any "sauriurine" bird (Chiappe 2002). Given these data, holophyly of Sauriurae seems untenable for now, but further data on these archaic birds may yet prove otherwise, and it is certain that regardless of the phylogenetic details, there exists a deep bifurcation early in avian history (Feduccia 1999, Zhou & Hou 2002). Indeed this is the greatest strength of the sauriurine hypothesis, in that it most readily models this fundamental avian dichotomy, whereas alternative phylogenies (e.g. Chiappe 2002) are unable to accomodate this pattern. It is conceivable that by removing Archaeopteryx from Sauriurae, that taxon might be considered holophyletic. At any rate, additional cladistic analysis including recently discovered archaic birds needs to be undertaken to confirm or reject that of Hou et al. (1996), which is currently the "only game in town".

A principal objection to the holophyly of Sauriurae is that it would require a degree of parallelism that is arguably not parsimonious to assume. And yet this contention is difficult to reconcile with the phylogenetic history of birds, in which we see homoplasy and parallelophyly run rampant. Some of the most astounding cases of convergence in either unrelated lineages or those stemming from a common ancestor are found in Aves, one of the most striking being that of Aenigmavis, a "phorusrhacid" from the Eocene of Europe. The marked convergence between Podicipediformes and Gaviiformes is equally noteworthy. Given these data it seems difficult to reject the sauriurine hypothesis on simple grounds of parsimony, as some have done (e.g., Paul 2002).

For references and a more detailed analysis, see Urvogel Take Two: Confuciusornithidae and the Early Evolution of Birds.

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