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Long unedited MM #2 excerpt on extinction, part 2 of 2



Continued from part 1 ---

The Pliensbachian-Toarcian (Pb-T) Extinction

The Pb-T extinction occurred toward the end of the Early Jurassic
epoch. It was primarily a marine extinction affecting European
invertebrates, and because of the truly meager fossil record there is
only equivocal evidence that terrestrial megafaunas in Europe or
elsewhere were significantly altered.

Another factor greatly affecting dinosaur diversity came into play
during the Middle Jurassic: the rifting of Pangaea into Gondwana and
Laurasia. As the two smaller supercontinents separated (and, later,
themselves proceeded to rift into the familiar continents of today),
their dinosaurian faunas decoupled and began to diverge. Thus
extinction events that affected one supercontinent did not necessarily
affect the other the same way, and groups that evolved in one
hemisphere remained absent from the other. Such faunistic differences
are more starkly apparent in the Cretaceous fossil record than in the
Jurassic, because the vicariance effect had had more time to
operate. The Pb-T extinction was probably the last extinction before
the K-T extinction to afflict all archosaurs more or less the same
way.

One consequence of the Pb-T extinction was the disappearance of
prosauropods from the fossil record, although Anchisaurus and
Ammosaurus may have survived into the Toarcian of North America. They
were completely replaced by sauropods primarily
cetiosaurids which had begun to replace them even before the
extinction. Vulcanodontid and barapasaurid sauropods seem to have
vanished along with the last prosauropods, but the latter groups have
a very restricted Lower Jurassic fossil record thus far, so their
extinction is probably of minor significance. Generally speaking, as
far as we know, sauropods suffered no major extinction at the
Pliensbachian-Toarcian boundary.

Other casualties of the Pb-T extinction may have been the more
primitive ceratosaurians (Podokesauridae and Halticosauridae), which
were replaced by advanced ceratosaurians (Megalosauridae and
Ceratosauridae), coelurians (Coeluridae), and the earliest carnosaurs
(Eustreptospondylidae) and tyrannosaurians
(Compsognathidae). Stegosaurians such as Huayangosaurus and early
kentrosaurids, and some rare primitive nodosaurids, seem to have
displaced scelidosaurids as large armored herbivores in the northern
hemisphere, but the presence of peculiar ankylosaurians in the Early
Cretaceous of Australia (Minmi) argues for the persistence and further
evolution of scelidosaurians in Gondwana. Recently discovered
ankylosaurians in Antarctica may be related to this
group. Heterodontosaurids, never very numerous in the fossil record,
may have disappeared entirely following the Pb-T extinction (although
Echinodon and a small, unnamed Fruita Formation form [G. Callison,
pers. comm.] were possible Late Jurassic members of this family). A
closely related lineage that eventually evolved into Stenopelicidae,
Psittacosauridae, and Homalocephalidae must have existed from as early
as the Middle Jurassic. The existence of a genuine pachycephalosaurid
from Madagascar (Majungatholus) indicates that Homalocephalidae and
Pachycephalosauridae extend as distinct families back to before the
separation of Laurasia and Gondwana, even though their Jurassic fossil
records are entirely nonexistent. Finally, ornithopods, in the form of
primitive hypsilophodontids, appear for the first time in the fossil
record following the Pb-T extinction.

Whereas the Middle Jurassic was a major period of dinosaurian
radiation, dinosaurian evolution throughout the remainder of the
Jurassic exhibits a bradytelic taxonomic turnover with no known major
radiative events. It is likely that the ornithischian core group of
small (no larger than 1 m long) lesothosaurians, heterodontosaurians,
and ornithopods diminished greatly and became extinct during the
Middle to Late Jurassic epochs, perhaps as a result of adverse
competition from primitive herbivorous mammals or possibly from
theropod predation. With the sole exception of Ceratopia, no new
ornithischian Bauplane are known to have originated later than the
Middle Jurassic (in the northern hemisphere, at any rate); all the
ornithischian lineages that survived into the Cretaceous (Ornithopoda,
Stegosauria, Ankylosauria, Pachycephalosauria) were notably
conservative. Their evolution was entirely nontachytelic. Ceratopia,
which probably branched away from Heterodontosauria in the Late
Jurassic, may be the only instance of a genuinely new archosaurian
Bauplan originating bradytelically rather than through tachytelic
core-group evolution.

Primitive Middle Jurassic forms in all the dinosaur groups were
replaced by more derived and often larger Late Jurassic forms. The
theropodomorph core group continued to evolve ever better-adapted
volant forms (culminating in and in all likelihood surpassing in
flying ability such animals as Archaeopteryx), but their evolution
into larger cursorial, ground-dwelling forms was to some extent
stymied by the persistence of medium-size and large ceratosaurians and
carnosaurs that had preoccupied those ecological niches.  It is
extremely difficult to disentangle which, if any, of the Jurassic
large-theropod groups might have evolved "directly" from the
theropodomorph core group and which might have evolved from
preexisting large-theropod groups. For example, did allosaurids evolve
as derived large eustreptospondylids or sinraptorids, or did they
evolve independently from within a group of smaller theropods (such as
coelurids resembling Ornitholestes: Paul, 1988b) closer to or within
the theropodomorph core group? The fossil record is not yet good
enough to resolve this question satisfactorily.

  The Jurassic-Cretaceous (J-K) Extinction

The J-K extinction is by no means well understood  it was, again,
mainly marine but one of its major consequences among terrestrial
animals seems to have been the disappearance of large euhelopodids,
brachiosaurids, and diplodocids from the northern
hemisphere. Dicraeosaurids and the smaller diplodocids, euhelopodids,
and brachiosaurids weathered the extinction event with only moderate
diversity reduction. Laurasian cetiosaurids, such as
Haplocanthosaurus, passed away, but Gondwana cetiosaurids "survived"
by diversifying into various subgroups of andesaurids and
titanosaurids during the Early Cretaceous. Austrosaurus, from the
Albian of Australia, may have been a relict Cretaceous cetiosaurid
(R. E. Molnar, pers. comm.) related to the titanosaurid radiation.

Most other dinosaur groups survived across the Jurassic-Cretaceous
boundary, although with their compositions altered a la the
sauropods. Laurasian stegosaurians diminished greatly, partially
replaced in the Early Cretaceous by nodosaurids and polacanthids such
as Hylaeosaurus, Polacanthus, and Hoplitosaurus; but Gondwana
stegosaurians seem to have survived as a relict fauna of kentrosaurids
right to the end of the Cretaceous in India (Yadagiri & Ayyasami,
1979). Their almost complete disappearance following the Jurassic and
their evident lack of Bauplan diversity signal the absence of
stegosaurian core-group evolution. Similar considerations, involving
different eras, point to the absence of core-group evolution in
Ankylosauria as well.

Ornithopods split into Laurasian and Gondwanan branches, including
several lineages of hypsilophodontids on both supercontinents. In
Laurasia, the ornithopods included Tenontosaurus and large
iguanodontids such as Iguanodon and Probactrosaurus; in Gondwana, they
included Australian and Antarctic hypsilophodontids, Ouranosaurus, and
Muttaburrasaurus. As usual, a spotty fossil record makes it difficult
to unravel which larger forms were directly descended from within
hypsilophodontid groups and which were descended from groups of
intermediate-size ornithopods such as Dryosauridae or
Camptosauridae. It is most significant that small ornithopods less
than about 1 m long are entirely absent from the Cretaceous fossil
record of Laurasia, even though this is the best prospected of all
dinosaurian faunas. This supports, albeit weakly, the idea that the
ornithischian core groups of the Late Triassic and Early Jurassic
faded away during the latter epochs of the Jurassic, leaving a
"residue" of larger forms that underwent moderate further evolution
but eventually fell victim to the great K-T extinction at the end of
the Mesozoic. The Early Cretaceous of Australia has yielded small
hypsilophodontids (Leaellynasaura, Fulgurotherium, Atlascopcosaurus),
so it is possible that an ornithischian core group survived the J-K
extinction in the southern hemisphere. If so, we should be prepared
for the eventual discovery of new ornithopod Bauplane in the
Gondwana Cretaceous.

Early pachycephalosaurians (Yaverlandia) and ceratopians
(Psittacosaurus) first become evident in the Early Cretaceous fossil
record, but as noted previously, the pachycephalosaurian lineage
almost certainly extends back to the Middle Jurassic, when Laurasia
and Gondwana were still connected. The psittacosaurids probably
diverged from that lineage in Laurasia sometime during the Late
Jurassic, but little is known of this branch of ornithischian
evolution until its Early Cretaceous debut and its Late Cretaceous
diversification into Protoceratopidae and Ceratopidae. The problematic
Gondwana genus Notoceratops is most likely not a ceratopian, but it
may belong to a southern-hemisphere heterdontosaurian lineage. As with
Stegosauria and Ankylosauria, there is no evidence for core-group
evolution in Ceratopia, but there were probably many more genera of
smaller-size ceratopians like Protoceratops, Leptoceratops, and
Montanoceratops than are presently known in the fossil record.

Among pterosaurs, the rhamphorhynchoids, which had been a declining
pterosaur core group during the Late Jurassic, vanished completely
across the J-K boundary. They were gradually being replaced by the
pterodactyloids in a process that evidently finished at or just after
the onset of the Early Cretaceous. As noted earlier, pterodactyloids
were tailless, unstable fliers that were more efficient and
maneuverable than their predecessors, which probably accounts entirely
for their success throughout the Cretaceous. That the extinction of
the rhamphorhynchoids coincides with the close of the Jurassic
presently seems to be fortuitous.

The theropods underwent intriguing changes across the J-K boundary,
but (as usual) the fossil record of the smaller members of the lineage
is simply terrible. The continual emergence of new theropod families
during the Late Jurassic and Early Cretaceous indicates one or more
thriving and evolutionarily active theropodomorph core groups from
which the small and large theropods and various clades of birds could
diverge. Ceratosaurians (Ceratosauridae, Megalosauridae) became
extinct in Laurasia across the J-K boundary, but some survived in
Gondwana, where throughout the Cretaceous they radiated into
neoceratosaurians (abelisaurids, noasaurids, alvarezsaurids, and
velocisaurids), spinosaurians, and likely many other as-yet-unknown
distinctive cursorial and graviportal theropods. The hypothesis of a
direct ceratosaurian ancestry for most of the large Gondwana theropods
is supported by the discovery of a tetradactyl manus in the
abelisaurid Carnotaurus (Bonaparte, Novas & Coria, 1990), and it would
not surprise me if the other Gondwana groups enumerated above all turn
out to have had tetradactyl manus as well.

Whereas ceratosaurians prevailed in Gondwana, they were entirely
replaced by more "advanced" theropods, including coelurosaurs and
carnosaurs, in Laurasia a process that had begun in the Middle
Jurassic. It is odd that in Gondwana, ceratosaurians prevailed over
sinraptorid theropods such as Piatnitzkysaurus, and that the
"advanced" theropods left no known Cretaceous
descendants. Representatives of some of the Gondwana ceratosaurian
groups found their way into Europe during the Cretaceous via a
sweepstakes connection between southern Europe and northern Africa
across the Mediterranean branch of the Tethys Sea. These include
abelisaurids (Tarascosaurus: Late Cretaceous of France) and
spinosaurians (Baryonyx: Early Cretaceous of England). Some
iguanodontids (Gravisaurus, as yet undescribed) closely related to the
Wealden Iguanodon may have entered Africa from Europe along this same
route. Spinosaurians (Spinosaurus: late Early Cretaceous of northern
Africa) were probably ceratosaurian descendants via forms like
Afrovenator; they remained restricted to Gondwana throughout their
known evolutionary history. The fact that several kinds of unrelated
Gondwana dinosaurs evolved tall neural spines, which may have
supported webs of skin for thermoregulation, during the Early
Cretaceous (Rebbachisaurus and Amargasaurus sauropods;
Ouranosaurus ornithopod; Spinosaurus theropod) may indicate that
Gondwana was then almost inhospitably hot.

Sometime toward the end of the Late Jurassic, the theropodomorph core
group generated the first true birds, which I earlier defined as
theropodomorphs in which the forelimb elements were fused into a wing,
the sternum was enlarged and keeled, and a true furcula with
hypocleidium could have been present. One must imagine these
characters as existing in a few related lineages among the many other
contemporary lineages of volant core-group theropodomorphs in which
the forelimbs and pectoral girdle evolved various different anatomies.
There were volant theropodomorphs with three functional, independent
digits in the forelimb, such as Archaeopteryx, and there were probably
volant theropodomorphs with two or even just one functional forelimb
digit. Members of any of these "pre-avian" groups could have evolved
into larger, cursorial forms, and such forms would presently be
classified as theropods. Among these, I contend, are Oviraptorosauria,
Ornithomimosauria, Tyrannosauria, and the dromaeosaurid branch of
Archaeopterygiformes, all of which seem to have originated during or
before the Late Jurassic but do not become evident in the fossil
record until the Cretaceous. The differences in forelimb anatomy among
these well-known groups would thus derive from the differences in wing
architecture of their ancestral volant forms. Dinosaurs of these
groups seem to have been competitively superior to the small theropods
of the Jurassic and Early Cretaceous (here called Coeluria, for want
of a better name and phylogeny), for they became widespread in
Laurasia during the Cretaceous.  One branch of Theropodomorpha,
perhaps Archaeopterygiformes itself, evolved into enantiornithid
"birds," known from the Late Cretaceous of both Laurasia (Gobipteryx
and Avisaurus) and Gondwana (Enantiornis, Nanantius, Yungavolucris,
Soroavisaurus, and others). They were fully volant, powered, unstable
fliers quite capable of spanning the strips of ocean opening up
between the continents. Martin (1991) called this lineage
Sauriurae. Another branch, Ornithurae, including such animals as
Ichthyornis, Hesperornis, and Sinornis, ultimately developed into
modern birds, with a wing built around three manual digits fused
together.

  The Albian-Cenomanian (A-C) Extinction

This was another primarily marine extinction, in the sense that it is
made known to us mainly from the fossil record of marine
invertebrates. The principal cause is held to be widespread oceanic
anoxia, but there is also an iridium anomaly of this age, suggesting
asteroid impact (Raup, 1991). The contemporaneous fossil record of
terrestrial archosaurs is rather poor, but in view of the changes in
dinosaur faunas that can be documented across the boundary between
Early and Late Cretaceous, there is room to speculate that a mass
extinction of some kind occurred. One of the characteristics of the
A-C extinction was a greatly elevated sea level, which flooded North
America and Europe with epicontinental seas that persisted until late
in the Maastrichtian, the final stage of the Cretaceous. It is quite
possible that the mass extinctions of terrestrial vertebrates were
caused entirely by this massive transgression.

North America seems to have been hardest hit by the A-C extinction.
Practically none of the dinosaur groups that populated North America
during the Early Cretaceous survived into the Late Cretaceous. North
American dinosaurs of the Late Cretaceous (tyrannosaurids,
hadrosaurids, lambeosaurids, pachycephalosaurians, ceratopians,
ankylosaurids) seem all to have come from somewhere else, primarily
eastern Asia, across the Bering land bridge. Indigenous dromaeosaurids
such as Deinonychus were replaced by forms such as Saurornitholestes,
much more closely related to the eastern Asian
Velociraptor. Ornithopods like Tenontosaurus were decimated; sauropods
vanished entirely.

The Late Cretaceous epoch was thus one of decreased dinosaur
diversity, although paradoxically the existence of numerous
well-prospected Upper Cretaceous dinosaur-bearing formations
(Weishampel, 1990a) makes it seem as if dinosaur diversity peaked
then. Among the archosaurs, not one core group remained  the last
of them, the theropodomorph core group, had transmuted entirely into
birds and it is within the core groups that most diversity
resides. All the Late Cretaceous dinosaurian lineages became
bradytelic, evolving new species at a stately pace but never arriving
at any truly new Bauplane. The fossil record shows a plethora of
medium-size to large forms distinguishable from one another in
relatively trivial ways: different horns, frills, crests, and
dermal-armor patterns; occasionally, taller neural spines; and so
forth.

In North America, sauropods disappeared completely at the A-C boundary
(Lucas & Hunt, 1989). Only one Late Cretaceous sauropod genus,
Alamosaurus, is known from North America, and it was a Maastrichtian
titanosaurid thought to have immigrated from South America across an
Antillean land bridge from "Venezuela" to "Florida" with the ebbing of
the abnormally high Maastrichtian sea level. (One other such genus,
the recently described Dyslocosaurus, may be from the Upper Cretaceous
Lance Formation but is more likely from the Upper Jurassic Morrison.)
Some hadrosaurids (Kritosaurus australis) probably entered South
America from North America via the same route, although others
(Secernosaurus) were distinctly more primitive and may represent an
indigenous Gondwana hadrosaurid fauna. Dicraeosaurids persisted in
eastern Asia (Nemegtosaurus and Quaesitosaurus), as well as the
peculiar camarasaurid(?) Opisthocoelicaudia, but most sauropod
evolution took place in Gondwana. The titanosaurids of South America
were notably diverse, and better prospecting in the future will
certainly reveal a comparable diversity among the titanosaurids of
Late Cretaceous India and Africa. Titanosaurids also found their way
into Europe during the Cretaceous via the trans-Tethys route from
Africa, and it is eminently possible that the Asian sauropods just
noted entered Eurasia from Gondwana, too, because they show more
affinity with Gondwana forms than with the Late Jurassic and Early
Cretaceous northern-hemisphere forms.

In Mongolia, segnosaurians appear in the fossil record as a new
Bauplan, but as I noted earlier, their lineage probably extends back
to the Late Triassic.  Scanty material referable to Segnosauria has
also been found in Canada (Currie, 1992), indicating that the group
may have been widespread.  The A-C extinction affected ornithischians
least, although aside from fragmentary material from India
(Dravidosaurus and an undescribed Maastrichtian specimen) there is no
evidence that stegosaurians survived across the A-C
boundary. Ankylosaurids diversified somewhat after the A-C event, and
nodosaurids persisted as well. Polacanthids, however, seem to have
vanished. Pachycephalosaurians underwent a minor diversification in
the northern hemisphere (eastern Asia and western North America), but
as heterodontosaurians they already had a nearly worldwide
distribution, and there is no reason to believe they did not also
diversify elsewhere. With respect to western North America, the most
spectacular Late Cretaceous diversification occurred within
Ceratopidae, which evolved into dozens of genera there and nowhere
else. Among the ornithopods, hypsilophodontids evidently persisted
without significant change in both hemispheres; the existence of
relatively primitive ornithopods such as Thescelosaurus in the late
stages of the Late Cretaceous suggests that many hyspilophodont-like
lineages remain hidden from the fossil record. Hadrosaurids and
lambeosaurids, both derived from within Iguanodontidae, appear for the
first time in the fossil record following the A-C event. There is no
evidence suggesting that their lineages extend back beyond the Early
Cretaceous; like Ceratopidae, the two groups of duckbill dinosaurs
seem to have arisen and evolved bradytelically exclusively in the Late
Cretaceous. They were not, however, anatomically different enough from
their iguanodontid ancestors to constitute a new Bauplan.

The ecological roles of small to large terrestrial predators continued
to be filled by various kinds of theropods. Dromaeosaurids,
ornithomimosaurs, and oviraptorosaurs were not adversely affected by
the A-C extinction and underwent minor diversifications in the
northern hemisphere, as did coelurosaurs in the form of their most
derived family, Troodontidae. The only known avimimiform, Avimimus, is
so birdlike that it could well be considered a flightless true avian
rather than a theropod (Chatterjee, 1991). Our understanding of these
groups is, of course, plagued by their dismal fossil record outside
Mongolia and western North America. Among the larger theropods, one
suborder in particular, Tyrannosauria, stands out. Its lineage
evidently extends back at least to the Early Cretaceous and perhaps
even to the Late Jurassic (Compsognathus), but only in the Late
Cretaceous did the group become a substantial part of the fossil
record. In eastern Asia and western North America, all the large
theropods belonged to the single family Tyrannosauridae. They are
generally held to be the most derived carnosaurs known, but no known
earlier carnosaur (allosaurid or sinraptorid) makes a convincing
ancestral tyrannosaurid, and evidence that tyrannosaurians may have
been more closely related to ornithomimosaurs and troodontids is
accumulating (Currie, 1992; Holtz, 1994a).

Outside eastern Asia and western North America, the fossil record of
Late Cretaceous dinosaurs is terribly incomplete. We know that
Gondwana theropods diversified as abelisaurids, noasaurids,
velocisaurids, alvarezsaurids, and probably other as-yet-undiscovered
forms, almost uncannily convergent in gross morphology and presumed
ecological roles with the Laurasian lineages (noasaurids even had
"killer claws" on their feet like those of dromaeosaurids and
troodontids). Dryptosauridae, a poorly documented family of
slender-limbed large theropods with relatively big, well-clawed
forelimbs, is known from the eastern United States
(Dryptosaurus). These may have been massive, tyrannosaurid-like
coelurosaurs.

Finally, Late Cretaceous pterosaurs seem to be almost exclusively
large to very large forms, such as pteranodontids and
azhdarchids. Sparrow-size pterodactyloids, such as those found in the
Upper Jurassic of Germany, are not known in the Late Cretaceous fossil
record. This suggests that the pterosaur core group faded away
sometime during the Early Cretaceous, perhaps as a result of direct
competition from the burgeoning avian and enantiornithiform core
groups. The bradytelic large pterosaurs maintained their dominance in
the face of the avian challenge until they were wiped out by the K-T
extinction event that closed the Mesozoic Era.

The Cretaceous-Tertiary (K-T) Extinction

This extinction event was by far the most significant for the
archosaurs.  Quite simply, all the extant pterosaur and dinosaur
groups were eradicated, to the last species, never to return. Of the
archosaurian lineages only Crocodylia and modern Aves survived. Since
the archosaurs encompassed practically all the terrestrial megafaunal
niches, the K-T event left the earth essentially devoid of medium-size
to large predators and medium-size to large herbivores for the first
time since the P-T extinction at the beginning of the Mesozoic
Era. The abruptness and sweeping universality of the extinction have
generated intense popular interest and scientific debate concerning
possible causes. The proximate cause seems to have been the impact of
a large (6-10 km) asteroid at the edge of the Yucatan peninsula in
the southern Gulf of Mexico, but I do not believe this tells the whole
story.

The most interesting question about this extinction event is not,
"What caused it?" but, "Why didn't the archosaurs return, suitably
altered, as they had done six times before?" In terms of the model
outlined here, the answer is that the Late Cretaceous vertebrate core
groups no longer included a dinosaurian Bauplan. The small arboreal
and cursorial dinosaurs of the Triassic and Jurassic Periods were
decimated during the Early Cretaceous through adverse competition from
metatherian and placental mammals the surviving theropsid core
groups leaving only a single, increasingly well-adapted, aerial
core group, Aves, as survivors. Large, flightless avian predators and
herbivores (Diatryma, Phorusrhacos, Onactornis, and Dinornis) did
evolve at various times from several avian groups during the Tertiary,
and they bore exactly the same relationship to their volant core-group
ancestors that the large theropods bore to volant core-group
theropodomorphs.  But without the functional, grasping forelimbs, such
flightless avians were no longer theropod dinosaurs. So, after serving
as dinosaur food for two-thirds of the Mesozoic Era, mammals managed
to gain some measure of revenge.

Aftermath

It is interesting to speculate briefly about what might happen to
today's terrestrial megafauna, should the earth be struck by a 6-10
km asteroid or undergo another similarly devastating catastrophe. Past
history suggests that most if not all of the large mammals, from
antelopes through elephants to zebras, would instantly become
extinct. The human population would be decimated, but it is now so
widely dispersed that small, scattered groups would certainly
survive. Presupposing, however, that not enough humans would survive
to render the reevolution of large animals of any sort impossible, we
might observe scenarios such as these: Frogs and toads form a fairly
diverse amphibian core group that would probably survive unchanged,
but since the number of niches for amphibians is limited, I doubt
whether this group would re-create large forms resembling those of the
Paleozoic Era. Among reptiles, turtles would clearly survive with no
significant loss of diversity, just as they did across the K-T
boundary 65 million years ago. The main reptilian core group, snakes
and lizards, would also remain unchanged, but the initial lack of
large mammalian predators could well permit the evolution of predatory
lizards the size of Megalania. Most if not all large birds, such as
eagles, vultures, and ratites, would undoubtedly become extinct, but
the present-day avian core group is remarkably diverse and
vigorous. New raptorial birds, and perhaps even large flightless
forms analogues of theropod dinosaurs and the flightless predatory
birds of the earlier Cenozoic would surely evolve from the
passerines (or from pigeons) within a few million years after the
impact. Finally, the two great mammalian core groups, the bats and
especially the rodents, would generate several new orders of large
mammals, perhaps including carnivores with large, incisiform front
teeth.

--- That's all, folks!