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re:K/T extinction, Champsosaurs
Responding (very, very briefly--I gotta make a living)
to Thom Holtz and Peter Sheehan:
Thomas: champsosaurs are currently Diapsida Insertae sedis. I wouldn't
call them archosaurs. Erickson (1985, J.V.P. 5(2):111-127) wussed out
and didn't even consider their systematic position. I believe that's the last
n the group I've seen.
-But to the point, they'reanother common Late Cretaceous group that sailed trh
he K/T boundary.
Responding to Peter Sheehan:
I hate to be construed as a spokesman for "gradualists" because I
prefer to consider myself a theoretical anarchist. Nevertheless, I will
gingerly argue a few things which affect groups I study.
Basically, I see many keystone Maastrichtian taxa which were either
little affected or unaffected by anything occurring at the K/T. I have
no primary data to support or refute dinosaur extinction, inoceramid,
rudist, ammonite, or most other apparent sudden extinctions at the
-But this begs the question and may explain why one gives the impression
of being a die-hard gradualist.
Given that all geological time boundaries were originally based on either
major sedimentary or extinction-diven events (i.e., the time-rock units
were so-defined and the time units followed the former), we expect to
see extinction, indeed "mass extinction" at every period boundary. Perhaps
all were catastrophic; but if so, there ought toi be a lot of bolide
signs in the record which are not apparent.
-another salient: as Thomas Holtz noted, champsosaurs were oblivious to
the event. So were several common crocodilians (Leidyosuchus, Crocodylus,
Allognathosauchus, perhaps Brachychampsa, Thjoracosaurus>[D>[D>[D>[D>[D>[D>[D>[D
and others which don't come to mind. Belemnoids, gryphaeids, multituber-
culates, nautiloids, and many others, already declining through the Maastr.
persisted to the Paleocene or Eocene.
Their survivirship doesn't prove or disprove catastrophic events at the
K/T boundary, but it does argue more reasonably for a degradation of the
marine or paralic environment through the latest Maastr. rather than for
a wham bam incident.
-Finally, I have never understood by what means one may claim that anything
was "instantaneous" given the error factor in radiometric dating
-combined- with the uncertainties of correlating marine sections with
terrestrial sections. If you can put a "precise" date on the K/T
event in Montana I would be amazed. If you can correlate it with, say
Gubbio or Stevns Klint or any deep marine section, -other than by
isotopes (with 5% error factors, several million year's worth) I'd be
By the way, too, one can persuade "gradualists." My doubts about the
surviving taxa still impress me,m but >[D>[A>[C>[C>[C>[C>[B>[C>[D>[D>[D>[D>[D>
strong that there was indeed an impact event around the K/T boundary.
Just a bit more hard evidence may be enough from myu view.