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extinction in China



     In 1998 Zhao produced a paper about "excitations" and iridium
anomalies in dinosaur eggshells of putative K-T age.  His purpose was not to
corroborate the impact scenario, as he had already attributed extinction to
dessication, trace mineral concentration and poisoning.  Zhao's principal
motivation was to strengthen the case for continuous deposition during the
K-T interval in the Nanxiong basin.  Most researchers, e.g. Russell, maintain
that the Nanxiong (or Pingling) - Shanghu contact is disconformable, an
opinion apparently based at least partly on the lack of an iridium anomaly
there.  By 1993 Zhao was virtually alone in arguing for continuous
deposition.  All of his geochemical results are questionable.  So much for
poisoning, "excitations" and a conformable contact.  With regard to the
latter, however, the hiatus may be brief, inasmuch as chron 29R seems extant,
above and below the boundary.
      The Nanxiong may shed light on dinosaur extinction.  The evidence is
not consistent with sudden death by impact.  The egg type Nanshiungoolithus
disappears at a lower stratigraphic level than Stromatoolithus,
Shixingoolithus
and Ovaloolithus, which vanish at a lower level than
Macroolithus.  
The latter alone persists to the top of the Nanxiong, which is
apparently not quite the end of the Cretaceous.  The egg record in China, as
in Europe, does not support sudden, catastrophic extinction of all or most
taxa at the very end.
      Does it support a specific alternative?  It is interesting that
Mikhailov suggested the Macroolithus egg type was laid by T. bataar, inasmuch
as it is the largest of the elongatoolithid eggs of presumed theropod
affinities, and abundant, like T. bataar, in the Nemegt svita e.g. at Tsagan
Khushu.  So perhaps Tyrannosaurus gradually annihilated and outlasted the
Nanxiong herbivores, just as it may have done to earlier Nemegtian taxa and,
to a lesser extent, to the mid-Maastrichtian fauna of America.  Etc.
      Tyrannosaurus seems to have been an intruder in south China, where the
local endemics were not co-adapted and soon succumbed.  Although teeth from
the upper Nanxiong indicate the presence of a tyrannosaur as big as T. rex,
the contemporary Nanshiungosaurus was only 6m long.  That was far smaller
than Therizinosaurus, which faced T. bataar, a smaller hunter than T. rex.  
Nanshiungosaurus
was even smaller than Segnosaurus, which confronted the
diminutive Alectrosaurus.  In no other Asian environment was there such a
mismatch.  The little Nanshiungosaurus could have prolonged its existence by
fleeing rather than fighting T. rex, but its prospects did not seem bright.  
Microhadrosaurus
may have been equally challenged.  Sometimes compared to
Edmontosaurus,
it might have been the archaic Tanius, inasmuch as the
Nanxiong egg type Ovaloolithus laminadermus also occurs in the Wangshi
series, and south China may have been isolated for most of the intervening 12
m.y. (the presence of the primitive Alioramus in the Nemegt svita at Nogoon
Tsav suggests gradually increasing contact between formerly isolated regions
of Asia.)
      How could Tyrannosaurus have outlasted its dinosaur quarry long enough
to account for the higher occurrences of Macroolithus?  Perhaps one or more
other taxa nested elsewhere, and the predator subsisted to an extent on
pterosaur nests, turtles, etc., for a time, accounting for various other
extinctions, although such a doomed, marginal existence might also explain
Macroolithus
eggshell pathologies.
      The Nanxiong record may be relevant to global extinction in that it
demonstrates Tyrannosaurus radiated into paleoenvironments where it did not
belong c. 65 Ma, with devastating results.
      --Tim Donovan