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I would like to add to the points forcefully made by Norm regarding the
outrageous statements made by T. Lipka regarding dinosaur mass extinction
(and the K-T mass extinction in general).
The K-T extinction debate has largely been driven by the fallacy that
there was a major, and sudden extinction of "dinosaurs" (i.e., large bodied,
non-avian variety). Those who profess that an extraterrestrial object
colliding with the earth caused the extinction of the "dinosaurs"
(paraphyletic sense) have assumed this to be an established fact.
Indeed Alvarez et al. (1980) assumed that dinosaur extinction was "a
given" (an established fact) when they offered up the asteroid impact
theory as a way to explain their sudden extinction. From my perspective
those who have jumped on the impact bandwagon have done so without
critically analyzing the biostratigraphic and taxonomic data.
Furthermore, many have embraced the physical (possibly circumstantial)
evidence as proof, which it is not, that an impact caused the (apparent)
mass extinction. What has been lacking in most discussions concerning
the extinction of dinosaurs, is whether or not there was a decline in the
number of species before the K-T boundary. Determination of this can only
be achieved by a critical look at the biostratigraphic distribution of
dinosaur species (not families) as been done by Dale Russell (1975, 1982).
In a publication that has been largely overlooked by the paleontological
community, particularly here in the United States, I (Sullivan, 1987)
reviewed the biostratigraphic distribution of valid "reptilian" taxa
across the K-T boundary. I concluded that based on the fossil record,
there was a decline in the number of valid dinosaur taxa by late
Maastrichtian time. Moreover, the impression that dinosaurs were
perceived as thriving organisms in the last days of the Mesozoic Era is
largely an artifact of gross biostratigraphic resolution. The principal
issue is now phylogenetic one, one that I will return to in a moment.
Peter Dodson (1990) wrote a important paper addressing the numbers of
dinosaurs. He tabulated the number of named taxa for the entire Mesozoic
Era and contrasted them to the number he felt were valid. Out of the 540
named dinosaur genera Dodson concluded that 285 were valid. Of the 800
(or so) dinosaur species named he considered 336 as being valid (I remine
you that these numbers are for the entire Mesozoic Era. He estimated that
owing to the incomplete nature of the fossil record there may have been
between 900 and 1200 genera of dinosaurs (an estimate that will never be
proven). He proposed 3 models to explain dinosaur diversity and believe
that his bottleneck model best fit the data. However, Dodson, like
Russell before him, tabulated stratigraphic occurrences in a gross way
(i.e. if a dinosaur was known solely from a single stratum, say of early
Maastrichtian age, the age [range] of this taxon became early-late
Maastrichtian). Consequently, Dodson (1990) interpreted the fossil record
as exhibiting no decline in dinosaur species, but rather a increase
during the late Maastrichian!!! In reality the opposite is true.
I have tabulated the number of valid species for the intervals late
Campanian, early-middle Maastrichtian (I've put three putative middle
Maastrichtian occurrences in with the early Maastrichtian because of
the uncertainty in the age of taxa), and late Maastrichtian. There are
presently 121 valid dinosaur species that are known from the late
Campanian to late Maastrichtian. The average number of valid "dinosaur"
species existing during the late Campanian is 69 (based on a maximum
number of 81 and a minimum number of 57- the apparent "slop" is owing to the
uncertainty of the stage/age of the stratum in which a particular taxon is
from). The average number of "dinosaur" species that existed during the
early-middle Maastrichtian is 58.5 (maximum number of 71 and a minimum of
46). The average number of "dinosaur" species that existed during late
Maastrichtian is 20 (based on a maximum number of 22 and a minimum number
of 18). This later figure may inflated because some of the taxa may be
synonymous such as Stygimoloch with Pachycephalosaurus. I believe that
there are perhaps only 17 valid dinosaur from late Maastrichtian strata!
This constitutes 6.7% of the non-avian dinosaurs that lived. The question
is this: Does the elimination of 17 species of dinosaurs (or even 22) at
the end of the Cretaceous constitute a mass extinction. I would argue
that the numbers are consistent with dinosaur turnover within the
Mesozoic. For example, if one looks at the numbers of dinosaur families (a
position I normally do not advocate) that existed in the early-middle
Maastrichtian and compare them to the number of families in the late
Maastrichtian there were 17 present in the former; and 9 in the latter
for a reduction of 8 families!! The loss of the remaining 9 families in
the late Maastrichtian has nearly the same magnitude! Keep in mind that
many of these families were composed of few species. Only the
Hadrosauridae which includes some 22 (putative) valid species (most of which
are known from late Campanian strata) can be considered an exception (note
that only four hadrosaur species (Edmontosaurus annectens, E. regalis, E.
saskatchewanensis and Anatotitan copei) are known from late
Maastrichtian strata and the validity of all three Edmontosaurus species
may also be questioned.
Then there is the classification of the Dinosauria. I think that we are
beyond treating dinosaurs as a paraphyletic group. The traditional
classification has profound implications for "dinosaur" extinction. First,
it implies that the biology of non-avian dinosaurs is somehow unique,
tying together vast groups such as the diverse ornithischians,
sauropodomorphs and theropods. Second, because of their supposed unique
nature, they were selected for extinction over the avian dinosaurs
(birds) and other vertebrates. It is generally accepted by most
phylogenetic systematists that the Aves is nested within the Dinosauria
and is not distinct from it. Common sense alone suggests that there may
be something fundamentally wrong in the way that we have traditionally
treated (classified) the dinosaurs. No other vertebrate group suffered
a mass extinction at the K-T boundary. Could this be the result of bad
taxonomy? I think so.
Their large size has often been cited as a reason for their extinction.
There are a couple of problems with this hypothesis. First, not all
"dinosaurs" were large, even those that were relatively small like
Thescelosaurus, became extinct. Numerous large-bodied dinosaurs became
extinct prior to the K-T boundary, leaving no successors, yet no one
cites a bolide impact event as the cause for their extinction! Second,
non-avian dinosaurs most certainly occupied diverse ecological niches. It
is difficult to realized that such a group of diverse animals shared some
common ecological denominator.
Finally, extinction, like evolution is a very complex phenomenon. It
cannot be explained by one pat (simplistic) answer (i.e. impact event). I
have often remarked to my students that if the "impactors" are correct,
we would not be here to debate it!
- Robert M. Sullivan
Selected references:
Dodson, P. 1990. Counting dinosaurs: How many kinds were there?
Proceedings of the National Academy of Science,
87:7608-7612.
Sullivan, R. M. 1987. A reassessment of reptilian diversity across the
Cretaceous-Tertiary boundary. Contributions in Science,
Natural History Museum of Los Angeles County, Number 391,
1-26.
Partial index: