Alvarezsauria

From EvoWiki

Alvarezsauria (=Alvarezsauridae Bonaparte 1991)

Matched in oddity only by the Therizinosauria, these enigmatic Cretaceous forms have been the center of acrimonious controversy since their initial description by Perle et al (1993).

The phylogenetic status of Alvarezsauria remains highly disputed (Novas & Pol 2002). Initial work by Ostrom (1994), Wellnhofer (1994), Kurochkin (1995), Zhou (1995), Feduccia (1996) and Martin (1997) in which the presence of avian characters were discounted in Alvarezsauria are no longer congruent with subsequent material attributed to this taxon. The most recent phylogenetic analyses of the alvarezsaur material (Karhu & Rautian 1996, Chiappe et al 1996, 1998, Novas 1997, Padian & Chiappe 1998, Longrich 2000, Chiappe et al. 2002) have demonstrated the presence of multiple derived avian characters, particularly in the skull.

The skull has been modified to permit prokinesis, and the orbits are confluent with the ventral aspect of the post-temporal fenestra, as the postorbital bar is incomplete medially. The jugal and quadratojugal are fused into a jugal bar, and the articulation with the quadrate is highly kinetic. The proximal condyle of the quadrate is doubled, and articulates with the braincase, and the mobility of the quadrate is furthered by the lack of descending process of the squamosal. Lastly, the ectopterygoid is absent (Chiappe et al 1996, 1998, Padian & Chiappe 1998, Paul 2002). These are derived avian characters, reflecting a higher level of organization than the Archaeopteryx node as least as regards the cranial osteology.

Further similarities between Aves and Alvarezsauria are found in the pelvic girdle and the femur, which especially in proximal aspect, is astonishingly avian. A robust antitrochanter is present on the acetabular wall and the pubic symphysis is limited to the distal portion of the pubes. The pubes are stronly opisthopubic in orientation, forming a 45 degree angle relative to the postacetabular process of the ilium. A pubic boot is lacking. The femora display confluence of the lesser and greater trochanters. The fibuula is reduced to a splint. A secondary cnemial crest is present in Mononykus, but absent in Patagonykus and Parvicursor. The distal aspect of the crus is completely coosified in all but Alvarezsaurus (Chiappe et al 1996, 1998, Padian & Chiappe 1998, Chiappe et al. 2002).

Paradoxically, however, the alvarezsaurs also display a number of plesiomorphic traits which are difficult to reconcile with the presence of numerous derived, avian characters in both the postcrania and crania. For instance, the presence of some derived avian traits, such as the secondary cnemial crest, is greatly restricted within Alvarezsauria. The tarsometatarsus is arctometatarsalian, which as Karkhu & Rautian (1996) noted is not readily associated with the tarsal architecture observed in Aves. Martin (1997) argued that this condition allied Alvarezsauria with Ornithomimosauria. Novas & Pol (2002) found that alvarezsaurs lack a list of characters synapomorphic of a higher level of organization within Maniraptora, which they labelled "node X", including: Therizinosauria, Oviraptorosauria, Deinonychosauria, and Aves. Such characters missing from alvarezsaurs include:

a) Cervical centra with a kidney-shaped articular surface in cranial aspect

b) Exaggerated proximodorsal lip on the manal unguals

c) Supracetabular crest absent

d) Iliac postacetabular process is subvertical in orientation

e) Medial flange of ilium reduced

f) Pedal unguals of digits III and IV are dorsoventrally deep, and display enlarged flexor tubercles.

Novas & pol (2002) concluded from these data that Alvarezsauria lies outside Aves and thus argued that previously supposed similarities to birds are in fact homoplastic traits. Considering the lack of of other avian synapomorphies in the postcrania and crania of alvarezsaurs, this revised hypothesis looks increasingly plausible.

Sereno (1997, 1999, 2001) has argued that, in addition to the arctometatarsalian pes, alvarezsaurs share at least 19 synapomorphies with the ornithomimosaurs, representing the only explicit phylogenetic hypothesis unifying the two groups. However, Suzuki et al (2002) have convincingly shown that at least 15 of these 19 are dubious at best, representing potentially misidentified cranial elements (Sereno's (1999) characters 139 and 140 depend on the indetification of the ostensibly homologous element as the prefrontal, when it in fact may be a ectethmoid), symplesiomorphies (Sereno's character 146, chevron hight four to five times corresponding neural spine, is a tetanurine condition), or character states absent in basal members of the group (Sereno's character 150, the presense of a dorosmedial turbercle on the proxmial phalanx of manal digit 1).

It must be cautioned, however, that the avian relationship hypothesis, and those posed by Novas & Pol (2002) and some of other authors, in which alvarezsaurs are considered a group of aberrant theropods, are still equally viable. Further data is needed before the phylogenetic status of these extremely peculiar dinosaurs can be resolved.

Chiappe et al. (1996, 1998, 2002) listed the following characters as synapomoprhic of Alvarezsauria:

a)Synsacrals laterally compressed

b)Cranial and caudal articular facets of the centra are concave and convex, respectively

c)Coracoid short, wider than tall, biceps tubercle absent

d)Sternum hypertrophied

e)Humerus bears a prominent tubercle in ventral aspect

f)Olecranon process of ulna hypertrophied

g)Metacarpal I hypertrophied, depressed strongly

h)Manal digit I robust, ungual with paired proximoventral foramina

i)Ischiadic and pubic fusion distally not observed

See also: Ornithomimoidea

JGK