Elmisaurus elegans no more? Leptorhynchos

Jaime Headden, Lord and Master of All Oviraptorosaurian, just wrote a detailed commentary on this work, but my commentary is limited to the phylogenetic analysis and Elmisaurus monophyly.  Just so everyone's up to speed, Longrich et al. (2013) describe some new caenagnathid remains from Texas, revise the taxonomy of the whole clade, and present a phylogenetic analysis of oviraptorosaurs.  Among their actions is to create the genus Leptorhynchos for Elmisaurus elegans and their new species gaddisi. 

The analysis in the paper is represented as an Adams consensus that leaves out two included oviraptorosaurs- Epichirostenotes and Nomingia.  For those of you who don't know, Adams consensus' can be misleading.  What they do is move unstable taxa to the base of the smallest clade they definitely belong to.  So if e.g. segnosaurs were most parsimoniously either plateosaurs like Sereno thought or oviraptorosaurs like Russell and Dong thought, an Adams consensus would move them to the base of Saurischia where no one was advocating they go.  Which is fine in an incertae sedis sort of way, except that PAUP doesn't tell you when any taxon actually is basal in a clade or is just being displayed basally because it can occupy multiple more derived positions.  Also, Longrich et al. excluded Nomingia and Epichirostenotes a priori instead of a posteriori, which meant none of their information affected the topology.

Analyzing every taxon, but only leaving those in which preserve mandibles results in-
(Microv(Gigant(Caenagnathasia(elegans,gaddisi(pergracilis(collinsi,sp.))))))

The Adams consensus places Elmisaurus rarus out by Caenagnathasia, making it seem like the analysis supports creating a new genus for elegans.  But that's just an illusion caused by E. rarus not preserving mandibles, so it can go many places in Caenagnathidae and is placed basally due to the Adams consensus artifact noted above.  Indeed, if we delete taxa which don't preserve feet a posteriori, we get-

(Microv(Gigant(elegans,pergracilis,rarus)))

So it seems rarus isn't separated from elegans. Trees constrained to place elegans in Elmisaurus are equal length to trees which don't.  How many Elmisaurus apomorphies were included?

1. tarsometatarsal fusion- included, but E. rarus miscoded as lacking metatarsal fusion.

2. posterior surface of metatarsus deeply concave- not included.

3. metatarsals II and IV subequal in length to III (>93%)- not included.

4. distal end of metatarsal II curves anteriorly away from the longitudinal axis of the metatarsus- not included.

5. proximolateral process on metatarsal IV- not included.

6. more extensive proximodistal contact between metatarsals II and IV- not included.

Longrich et al. do claim elegans is more similar to Chirostenotes in one way though.  They state "Longrich (2008a) tentatively placed [elegans] in Chirostenotes, because the third metatarsal has an anteroposteriorly flattened shaft that is concave and broadly exposed on the posterior of the metatarsus (Currie 1989). This is a derived feature found in Chirostenotes (Currie and Russell 1988) but not Elmisaurus (Osmólska 1981)."  This seems related to their new character 205- "Metatarsal III with an ovoid or subtriangular cross section (0) or anteroposteriorly flattened, with a concave posterior surface (1). Primitively in theropods the third metatarsal has an ovoid cross section, or a triangular cross section in arctometatarsalian forms. This condition is retained in most oviraptorosaurs, including the basal caenagnathid Elmisaurus rarus. In Caenagnathinae, the third metatarsal is anteroposteriorly compressed."

Posterior view of proximal left caenagnathid tarsometatarsi, metatarsal III colored blue.  Left- Elmisaurus/Leptorhynchos elegans holotype ROM 781 (after Currie, 1989); Middle- Elmisaurus rarus holotype ZPAL MgD-I/172 (after Osmolska, 1981); Right- Elmisaurus/Leptorhynchos elegans referred RTMP 92.39.4 (after Currie, 1989).

Yet the posterior transverse exposure of proximal metatarsal III proximally seems intermediate in E. rarus' holotype compared to the two specimens of elegans (see figure above).  More distally, Currie's (1989) figure 2P section indicates the posterior exposure is narrow as in E. rarus (see lower left section below).  Currie's (1989) figure 2Q shows E. rarus has a concave posterior metatarsal III surface as well (see middle left section below, outlined in blue). 

Left- cross sections of caenagnathid metatarsi after Currie (1989), anterior/extensor to top, anterior and posterior surfaces lined with red and blue respectively; Top- Elmisaurus/Leptorhynchos elegans referred RTMP 92.39.4; Middle- Elmisaurus rarus holotype ZPAL MgD-I/172; Bottom- Elmisaurus/Leptorhynchos elegans holotype ROM 781.  Green lines indicate region of sectioning.  Right- Holotype of Elmisaurus rarus in anterior (left) and posterior (right) views (after Olsmolska, 1981).

As for shape, the main issue seems to be we're dealing with different proximodistal points along the bone.  Currie and Russell (1988) state in Chirostenotes pergracilis "The proximal end, viewed dorsally, is diamond shaped, tapering both anteriorly (between the contact of metatarsals 11 and IV) and posteriorly. Its major horizontal axis, 17.5 mm long, is anteroposterior in orientation and thins backwards."  Sternberg (1932) also states the Macrophalangia holotype (possibly Caenagnathus collinsi according to Longrich et al.) has a transversely compressed proximal end.  The proximal ends of E. rarus' and elegans' metatarsal III are fused too well with surrounding bones to compare.  Once C. pergracilis' anterior surface is exposed, "the bone twists until the medial surface is facing anteriorly" and "the anterior edge has broadened out to 7.5 mm to separate the adjacent metatarsals and is triangular in section" (Currie and Russell, 1988).  elegans' holotype is broken at about this same point and also shows a triangular section (upper section left above).  Note a triangular section is what Longrich et al. are claiming caenagnathines don't have, though it exists in both pergracilis and elegans.  Currie (1989) was also wrong in comparing the proximal diamond shape of Chirostenotes pergracilis with the more distal triangular shape of elegans, and none of these areas have been described in E. rarus (thus I need to fix that on the Database).  At two-thirds down in Chirostenotes, "In section, a shallow concave surface faces posteriorly at this level, while slightly concave surfaces face posteromedially and posterolaterally for contact with the adjacent metatarsals."  This appears similar to E. rarus from what the anterior and posterior views suggest (right above), and matches how Snively (2000) described an RTMP "Elmisaurus sp." metatarsus that is probably elegans.  It also matches the cross section of elegans illustrated by Currie (1989- fig. 2P; lower left section above).  While Currie's figure 2Q of Elmisaurus rarus (middle left section above) would suggest a slightly different shape where the articular surfaces are smaller and that for metatarsal IV doesn't angle posteromedially, the narrow anterior exposure of metatarsal III means it must have been taken more proximally, probably about halfway down considering the ratio between anterior and posterior exposure of metatarsal III.  This leaves anteroposterior compression, which varies throughout the bone in elegans at least.  Distally it's transversely compressed but proximally it's anteroposteriorly compressed.  The E. rarus section which is probably intermediate in position is also intermediate in compression, being slightly transversely compressed.  If Chirostenotes/Caenagnathus specimens are any indication, the bone switches back to transversely compressed at its proximal tip.  So there are actually no valid described differences in metatarsal III sectional shape between Chirostenotes and any Elmisaurus species in the primary literature.

Correcting the coding for Elmisaurus rarus having fused metatarsals and adding the 5 other characters (one as an additional ordered state of the arctometatarsaly character) results in elegans and gaddisi being in a trichotomy with E. rarus.  This elmisaur clade is sister to a caenagnath clade containing Chirostenotes, Hagryphus, Caenagnathus collinsi and C. sp. from Hell Creek.  Nomingia and Epichirostenotes both have uncertain positions in the elmisaur-caenagnath clade, which is sister to Caenagnathasia, then Gigantoraptor as in Longrich et al.'s trees.

Given this result, I wonder why Longrich et al. didn't include the Elmisaurus characters suggested by Currie.  Also, making elegans the type of Leptorhynchos puts us in a confusing situation, as the only elements described in all three species are distal metatarsal III and proximal metatarsal IV.  Neither of these seem particularly distinctive in any of the species, and rarus' limited illustrations and description makes them difficult to compare in depth.  Thus there's no obvious reason to refer gaddisi to either Elmisaurus or Leptorhynchos, so it would have been easier and more accurate to just keep elegans in Elmisaurus and not name a new genus.  Longrich et al.'s diagnoses for Leptorhynchos and each of its species are purely mandibular, so we can't even evaluate Elmisaurus rarus or the elegans holotype for them.  But if elegans' diagnosis is based only on characters not observable in the holotype metatarsus, then doesn't that make the species indeterminate?  If the holotype can't actually be distinguished from the two metatarsal fragments referred to gaddisi, we have a problem.  Luckily, it seems Longrich et al. never assigned a type species to Leptorhynchos, making it unavailable (thanks to Cay on Jaime's blog).  I really think it should just be dropped and gaddisi assigned to Elmisaurus, though even then it might only be distinguishable from referred specimens of E. elegans, and perhaps not at all from E. rarus.  Ugh, caenagnathids.

References- Osmolska, 1981. Coossified tarsometatarsi in theropod dinosaurs and their bearing on the problem of bird origins. Palaeontologia Polonica. 42, 79-95.

Currie and Russell, 1988. Osteology and relationships of Chirostenotes pergracilis (Saurischia, Theropoda) from the Judith River (Oldman) Formation of Alberta, Canada. Canadian Journal of Earth Sciences. 25, 972-986.

Currie, 1989. The first records of Elmisaurus (Saurischia, Theropoda) from North America. Canadian Journal of Earth Sciences. 26, 1319-1324.

Snively, 2000. Functional morphology of the tyrannosaund arctometatarsus. Unpublished Masters Thesis. 273 pp.

Longrich, Barnes, Clark and Millar, 2013. Caenagnathidae from the Upper Campanian Aguja Formation of west Texas, and a revision of the Caenagnathinae. Bulletin of the Peabody Museum of Natural History. 54(1), 23-49.