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Re: [dinosaur] Review of Bois & Mullin (2017)

Mickey Mortimer <mickey_mortimer111@msn.com> wrote:

> Isn't this likely a depositional issue, much like the Jehol fauna?  Messel
> was continental, right?  It's not like continental faunas exist without
> large mammalian taxa today- something's going to move in.

For the "continental" fauna of Europe during the Paleocene, the
largest terrestrial vertebrate was a bird (_Gastornis_).  There were
no large(r) mammals.  Then again, it has been said that Paleocene
Europe was somewhat "insular" (e.g., Buffetaut & Angst, 2014
Earth-Science Reviews 138: 394-408).  Either way, _Gastornis_ survived
the major faunal turnover at the Paleocene–Eocene boundary (~PETM),
and even spread to Asia and North America (home to large mammals,
including many that invaded its home turf in Europe).  So large
mammals did "move in" to Europe in the Eocene.

So the absence of large mammals from Messel might be depositional.
_Gastornis_, the largest bird (and animal) from Messel, is known only
from a mould of a single femur.  Mayr (2016) suggests that its
scarcity in Messel is likely to be a taphonomic artefact; _Gastornis_
remains are more common in other, coeval European fossil sites.  More
generally for the large terrestrial birds of Messel, their comparative
rarity "is probably due to the fact that birds foraging on the forest
floor were less likely to end up in the lake sediments after their
death".  So the bones of any big terrestrial mammal probably had even
less chance of ending up in the lake.

However, I wouldn't dismiss the possibility that the Messel assemblage
might have lacked large mammalian herbivores.  The large browsing
pantodont _Coryphodon_ is known from elsewhere in Europe, and its
arrival in Europe is inferred to have had a major impact on the local
vegetation (Hooker & Collinson, 2012 Austrian J. Earth Sci. 105:
17-28). Maybe _Coryphodon_ (and other large mammals) never made it to
Messel.  Hooker & Collinson regard _Coryphodon_ as wholly terrestrial.
But _Coryphodon_ has been regarded as semi-aquatic by other sources
(including reports on _Coryphodon_ from Ellesmere); if true, then one
might expect it to turn up in lake sediments.