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Re: What's cookin', Doc?
Robert J. Asher, Meng Jin, John R. Wible, Malcolm C. McKenna, Guillermo W.
Rougier, Demberlyn [sic] Dashzeveg & Michael J. Novacek: Stem Lagomorpha and
the Antiquity of Glires, Science 307, 1091 -- 1094 (18 February 2005)
"We describe several fossils referable to *Gomphos elkema* from deposits
close to the Paleocene-Eocene boundary at Tsagan Khushu, Mongolia. *Gomphos*
shares a suite of cranioskeletal characters with extant rabbits, hares, and
pikas but retains a primitive dentition and jaw compared to its modern
relatives. Phylogenetic analysis supports the position of *Gomphos* as a
stem lagomorph and excludes Cretaceous taxa from the crown radiation of
placental mammals. Our results support the hypothesis that rodents and
lagomorphs radiated during the Cenozoic and diverged from other placental
mammals close to the Cretaceous-Tertiary boundary."
"*G. elkema* was first described as a paramyid-like rodent on the basis of
lower-cheek teeth from deposits overlying Paleocene rocks at Gashato,
Sort of like *Carcharodontosaurus*.
"It differs from extant rodents and lagomorphs in having two pairs of
mandibular incisors but is similar to extant lagomorphs in having two pairs
of upper incisors [...]"
Cladistic analysis: 68 taxa, 225 morphological characters, five genes!
"The most parsimonious trees (MPTs) we recovered from the combined data set
(Fig. 3) show Gomphos on the stem leading toward Lagomorpha and
zalambdalestids outside the crown radiation of Placentalia. This signal was
stable despite changes in character ordering (fig. S1), assumptions
regarding anterior incisor homology (fig. S2), exclusion of characters (fig.
S3) and taxa (fig. S4) with less than 75% representation in our anatomical
data set, and treatment of third positions in our DNA data set (fig. S5). In
addition, analysis of the morphological data set constrained to agree with a
recent molecular topology of Glires [...] also supported Gomphos on the stem
to Lagomorpha, as well as Cretaceous eutherians outside of Placentalia (fig.
Everything that starts with S is in the supplementary information.
"Placing zalambdalestids as the sister taxon to Glires requires an
additional 27 steps in the equally weighted NONA analysis; a
zalambdalestid-duplicidentate clade requires 54 additional steps. As
measured by Wilcoxon rank sum tests (Table 1), both alternatives can be
rejected with a high level of significance."
The supp. inf. lists 18 unambiguous autapomorphies for Placentalia excluding
*Zalambdalestes* and friends.
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