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Re: "Abominable Mystery": Rise of Angiosperms in the Cretaceous

Thomas R. Holtz, Jr. <tholtz@umd.edu> wrote:

> Something worth keeping in mind, however, is that the great diversity (and in 
> the first two phases, essentially the ENTIRE
> diversity) of angiosperms were as herbs and very small shrubs. Angiosperm 
> trees only show up in the later part of the Cretaceous.

Yes, this is true.  According to Coiffard &c, terrestrial angiosperms
really only began to become "real" trees in the Cenomanian, and then
went on to dominate the floodplains to become forests in their own
right.  Prior to this, floodplains angiosperms were part of the
understorey - literally "in the shade" of the conifers.  Although the
ancestors of modern oaks and magnolias were around in the early part
of the Cretaceous, it's difficult to gauge just how "tree-like" they
actually were.  Nevertheless, conifer trees were abundant, on the
floodplains and marshes, and became even more so in the Albian, with
those in the floodplains forming canopies.

So there were "trees" (ginkgoes, conifers, and increasingly
angiosperms) around, which potentially afforded branches for birds to
perch on.  However, for most of the Cretaceous terrestrial floras were
dominated by "cycadophytes", which offered little in the way of refuge
(although they likely offered other enticements, such as nutrient-rich
fructifications).  These particular gymnosperms were pushed aside by
the rise of angiosperms.  Coincidentally or not, it was in the later
Cretaceous that we see avialans that are the most specialized for
perching, such as the avisaurids.

> Furthermore, tree-nesting is not as widely distributed in even crown-group 
> Aves as people think. Not all birds are tree-nesters, and
> assuming Mesozoic avialians were mostly tree nesters because many neoavians 
> are is similar to assuming that Mesozoic eutherians
> lived in herds because many modern ungulate placentals do.

Again, I agree.  By "refuge" I was referring to merely perching on
tree-branches: for roosting, or avoiding terrestrial predators, or
keeping an eye on the ground below (such as for targeting small prey).
 So far, there is no direct evidence that any Mesozoic theropod
(avialan or non-avialan) nested in trees; although if crown neognaths
appeared at the end of the Cretaceous, it is likely that some
Cretaceous birds did nest in trees.