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Mr. Bois and the Oviphagus Mammalian Radiation
Without doubt, one of the greatest challanges of paleontology
lies in the inability to test hypothoses by observation. Instead the
paleontologist must rely heavily upon modern analogues for comparison.
Blaire Van Valkenburgh (1994) makes note of this and urges caution,
lest we make false assumptions based upon "uniformitarianism". Current
studies in paleoecology, Valkenburgh adds, are designed to test
theories based upon modern systems. While the arguments of Mr. Bois
are both stimulating and inciteful, it is apparent that there exists
many misunderstandings of extant communities and the ecological
processes that shape them. Also, it would seem that the sampling bias
of the fossil record is being ignored. Along, with grade level
reasoning (vs. cladistic reasoning), these two confounding variables
amplify faulty assumptions of the impact of egg predation, and in
particular, the Eutherian mammals as egg predators, upon Dinosaurian
Community structure is derived from a myriad of biotic and
abiotic factors, of which recognized as major interactions are
predation, symbiosis, competition, and stochastic events (i.e.
cataclysmic events, enviromental perturbuations, and arrival order of
colonists). Note that predation and competition are treated as
distinct factorials of community structure. Mr. Bois would like to sit
on both sides of the proverbial fence. At one point he claims that
"mammals of the K-T" (?) were major predators of Dinosaurian eggs. And
at another time he claims that "mammals may or may not have played a
role in dino nest predation. But inasmuch as they were able to simply
run away from what I believe was an increasing egg-predation load
toward the end of the K-T, they were the victors not in a war of
direct competition, but of reproduction-- of survival of the fittest"
Nonetheless, neither of Mr. Bois claims can be substantiated
employing modern communites as models, and accounting for fossil
record bias. Predation, not competition, is the major force shaping
species composition of a community. Thus, temporarily ignoring Mr.
Bois account of "survial of the fittest", we now seek to validate Mr.
Bois' hypothetical theory of a radiation of oviphagus mammals. The
problem is we cannot. James I. Kirkland (1994), while identifying two
potential Mammalian dino-egg predators (Gobiconodon and Didelphodon),
indicated that there is NO direct evidence of such predation having
taking place. Note that Didelphodon is a Metatherian and the
Triconodonts are not mammals sensu stricto, but would appear to be
more closely allied with Mammalia than Cynodontia (Novacek, 1993).
Kirkland, like Valkenburgh, warns against making hsty interpretations
of the fossil record.
"For extinct organisms, the association of potential egg
preadtors in or near recognized dinosaur nesting sites provides one
tenative line of evidence. This association, however, must be wieghed
against the possibility that the nesting organism brought it as a food
item for the young, or that the fossil material represents temporal or
This emphasizes the importance of multipule and independent
observations, however limited these may be in the fossil record,
before making hypothetical claims (vs. tenative proposals).
Mr.Bois also claims that birds were likely predators of
Dinosaurian eggs, and Mr.Wagner that some Pterosauria were also likely
oviphagus canidates. However, it is not thought plausible that either
of these groups could successfully dig open a dinosaur nest, and even
less likely for those canidates that were morphologically capable,
could do so unnoticed by a defending parent. Instead, it is proposed
that if such activities did occur, they would likely be scavenging
abandoned nests. Tentative dino oviphagus predators proposed by
Kirkland include cursorial Mesosuchian crocodiles, Varanid lizards,
and possibly medium sized Theropoda. The evidence of these animals as
had having preying upon dino eggs is circumstantial, but using modern
analouges of egg eaters as models, opportunistic feeding on dinosaur
eggs is a possibility. Kirkland had also cast a shawdow of doubt on
Varanids as potential dino egg predators because of their relatively
small size (in relation to dinosaur eggs). However, I have personally
seen captive-raised Varanus exanthematicus tackle chicken eggs bigger
that it's gape- with more than moderate success.
Mr. Bois demonstrates "classic" grade thought when he infers that
mammals are more stealthy, intelligent, and aware (?) than their
Amniote bretheren. Varanus niloticus is known to work in pairs in
order to lure a nest-defending Crocodylus niloticus away from her
eggs. In addition, given the gregarious nature of many of the large
herbivorus dinosauria that Mr. Bois would like to limit our discussion
to, it would be "gradist" to speculate their intelligence as
homologous to the extant Squamata and Chelonia. And whether or not
Mesozoic Mammalia were more "aware" then the Dinosauria, if we are to
use modern analogues as models, we would first have to decide if the
mammals of Cenozoic are "aware" and we would have to come to an
agreement of what it means to be "aware"! (for example, I assume my
dog is smarter than a mouse, but that doesn't mean the mouse isn't
aware that it is alive).
Using the extant Crocodylia as modern analogues (despite their
largely non-social behavior) the only animals capable of forcing a
female croc off her nest are armed humans or a very determined large
bull male croc (although I suppose an elephant or hippo could too, but
I wouldn't count on them to be in the mood for a crocodile egg omlet).
Thus, as indicated by Mr. Bois, a stealthy approach would be required
to approach and raid a dinosaur nest; a nest defended by a very
dangerous and very large mother (or father or both)- but Mr. Bois has
failed to demonstrate that mammals are indeed superior egg predators!
Of even greater significAnce is that Mr. Bois has failed to elucidate
why the Squamata and Chelonia, groups that were just as prevelant
during the late Cretaceous, and often do NOT defend their nests, were
not the overwhellming victims of this proposed oviphagus onslaught.
Nor does he discuss this impact upon the Aves or the Pterosauria.
Mr. Bois points out that egg-layers do have a somewhat precarious
moder of reproduction. However, it is a fact that non-lactating
egg-layers are by far more speciose than Mammalia, even more so than
Eutherian mammals. But this may still have little to do with
reproductive potentials and competition- but more of a reflection of
inter-specific variation in ecological attributes usually associated
with resource utilization. Mr. Bois fails to recogonize that there are
both disadvantages and advantages to all reproductive modes (even
sexual vs. asexual), and the adaptive mode of reproduction is often a
reflection of the community (I feel this is best represented by the
Lissamphibia). Given the aforementioned success of the non-Mammalian
Amniota, it would seem incorrect to conclude an adaptive superiority
of the Eutherian reproductive mode. This is especially relevant when
you consider that the "generalized" reproductive mode of the
Dinosauria is that of an R-strategist (as in Aves and Squamata which
did not go extinct). In fact, large herbivorous Sauropods, had higher
reproductive potentials than rodents and other small mammals. In
addition, mammalian reproductive rates scale differently than the
Dinosauria, with allometric increases in body size. Larger mammals
tend to have decreasing fecundity, while there is no change or an
actual increase in fecundidty in the Dinosauria.
Mr. Bois had mentioned that there is no apparent disadvantages to
the pregnant Eutherian. It has been demonstrated that gravidity
decreases locomotor performance in lizards. While perusing through the
EVOLUTION OF PARENTAL CARE, I could find no such examples of a
corresponding effect in mammals. However, I do not accept this
negative evidence as proof of Mr. Bois statement, and I ask that a
Mammalogist out there post such info on the Locomotor performance of
pregnant mammals. As an obvious example though, examine the marked
decrease in locomotor performance of preg. human females. In addition,
many Mammalian young are quite helpless post-partum and require
intensive parental investment- which means that they cannot invest
that energy into producing more offspring. Likewise, it is believed
that the parental investment of the Dinosauria was highly variable-
ranging from precocial nestlings to alticial nestlings with a likely
equivocal range of nesting stratagies.
Despite it's loss of universal acceptance, as "the explanation
for community structure" (Zug, 1993), competition, or more
specifically, competitive exclusion, may further decrease the
possibility of, or the limiting of an oviphagus late Cretaceous
Mammalian radiation. As millions of years of Dinosaurian evolution had
already passed, we would expect that one or more of the aforementioned
potential egg-predators, or an ecomorphologically similar taxon, to
have filled this dino-oviphagus niche, excluding and or limiting a
Mammalian radiation by limiting their access to the resource.
In conclusion, I see no reason to positively conclude the
presence of oviphagus mammals during the late Cretaceous, for a
multitude of reasons: (1) There is nothing in the fossil record to
idicate such events; (2) There is no sound reason to expect that small
mammals would be any more successful at dino-egg predation than any
other forms that may have existed; (3) a mammalian radiation into such
an ecological niche would have been inhibited by competitive
exclusion; and (4) There is no explanation of why the Squamata,
Chelonia, and Aves would not suffer similar fates. Unfortunately this
brings us full circle- why did the Dinosauria, the Pterosauria, the
Mososaurs, etc... go extinct and not the Squamata, Chelonia, Aves, and
the Mammalia? Why did they dissapear in such a short time span.
Clearly, the breadth of the extinctions over such a large geographic
scope, indicates a cataclysmic crisis or a series of smaller ones.
However that still does not answer why some groups go extinct and not
others. Perhaps we will never know, but let the debates continue.
B.S. Animal Sciences, University of Illinois
--------I will have to post my references in full later as I must
leave from Chicago to Louisville, KY about an hour ago.
The Biology of the Amphibia- Duellman and Trueb
The Evolution of Parental Care- Clutton Brock
The Skull, VOl.2- Hanken and Hall
Mass Extinctions- S. K. Donovan
Dinosaur Eggs and Babies - Carpenter, Hirsch, and Horner
Ecological Morphology- Wainwright and Reilly