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Re: Bird extinctions at K-Pg boundary



_Palintropus_ and _Apsaravis_ are united into a new clade, the
Palintropiformes, sister clade to the Hesperornithes+crown clade.


Speaking of Hesperornithes...  Two hesperornithiforms (A and B, with A
possibly the same as _Potamornis_) are identified by Longrich &c; but
despite their late Maastrichtian (Lancian) age they appear to be more
primitive than _Baptornis_ and _Hesperornis_ (based on their
metatarsal morphologies).  Hesperornithiforms A and B show the derived
metatarsal IV morphology characteristic of the group, associated with
the dominance of the fourth toe; but the metatarsus is short and wide,
and lacks the torsion seen in the shafts of derived
Hesperornithiformes.  Also, the facet on metatarsal II for metatarsal
I is very large and deep, in contrast to the small, weak facet of
derived Hesperornithiformes.  This indicates that, although they were
foot-propelled divers, these Lancian hesperornithiforms might have had
a different lifestyle to _Baptornis_ and _Hesperornis_ that entailed
better grasping abilities of the foot.  If these Lancian
Hesperornithiforms were perchers, then they were therefore also
fliers.  "Hesperornithiform A" had an estimated body weight of 3.6 kg,
and "Hesperornithiform B" only about 1.2 kg.  So both had body masses
consistent with being volant.  (By comparison, the Lancian
enantiornithean _Avisaurus archibaldi_ had an estimated body weight of
around 5 kg, and coracoids provisionally referred to this species are
consistent with flight ability).




On Thu, Sep 15, 2011 at 10:59 AM, bh480@scn.org <bh480@scn.org> wrote:
> From: Ben Creisler
> bh480@scn.org
>
> I don't recall seeing this paper mentioned yet:
>
> Nicholas R. Longrich, Tim Tokaryk, and Daniel J. Field (2011)
> Mass extinction of birds at the Cretaceous–Paleogene (K–Pg) boundary.
> Proceedings of the National Academy of Sciences 108(37): 15253-15257
> doi: 10.1073/pnas.1110395108
> http://www.pnas.org/content/108/37/15253.abstract
>
> The effect of the Cretaceous-Paleogene (K-Pg) (formerly
> Cretaceous–Tertiary, K-T) mass extinction on avian evolution is debated,
> primarily because of the poor fossil record of Late Cretaceous birds. In
> particular, it remains unclear whether archaic birds became extinct
> gradually over the course of the Cretaceous or whether they remained
> diverse up to the end of the Cretaceous and perished in the K-Pg mass
> extinction. Here, we describe a diverse avifauna from the latest
> Maastrichtian of western North America, which provides definitive evidence
> for the persistence of a range of archaic birds to within 300,000 y of the
> K-Pg boundary. A total of 17 species are identified, including 7 species of
> archaic bird, representing Enantiornithes, Ichthyornithes, Hesperornithes,
> and an Apsaravis-like bird. None of these groups are known to survive into
> the Paleogene, and their persistence into the latest Maastrichtian
> therefore provides strong evidence for a mass extinction of archaic birds
> coinciding with the Chicxulub asteroid impact. Most of the birds described
> here represent advanced ornithurines, showing that a major radiation of
> Ornithurae preceded the end of the Cretaceous, but none can be definitively
> referred to the Neornithes. This avifauna is the most diverse known from
> the Late Cretaceous, and although size disparity is lower than in modern
> birds, the assemblage includes both smaller forms and some of the largest
> volant birds known from the Mesozoic, emphasizing the degree to which avian
> diversification had proceeded by the end of the age of dinosaurs.
>
>
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