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Long unedited MM #2 excerpt on extinction, part 1 of 2



Regarding the recent thread on dinosaur extinction: My 2 cents. Please
excuse lack of italics and boldface (lost in e-mail) and actual
citations; I can furnish the citations on request via private
e-mail. I'm still rewriting this part of Mesozoic Meanderings #2, but
nothing major will change.

Archosaur Evolutionary Patterns

All cladograms and other schemes for depicting evolutionary patterns
within the animal kingdom share a fundamental topology that arises
from the speciation process itself. Because in any single lineage
there can be only a finite number of speciation events, while the time
available for these events forms a continuum, the probability of two
speciation events occurring at exactly the same time within a
population is vanishingly small. This allows a speciation event in a
phylogeny to be modeled as a single point to which a single branch,
representing the parent species, is directed, and from which exactly
two branches, representing the parent (or plesiomorphic) species and
daughter (or apomorphic) species, emerge. A node in a cladogram from
which more than two branches emerge is considered unresolved, in the
sense that a sufficiently detailed fossil record throughout a finely
enough subdivided interval of time around the node would convert such
a polychotomous node into a series of closely spaced binary
nodes. Thus, given a perfect fossil record, and assuming that
speciation is an instantaneous event, an ideal phylogeny may always be
modeled with a graph known as a directed binary tree.  This
topological constraint on phylogeny mandates just three different
evolutionary basic patterns for the archosaurs (or any group, for that
matter), because a speciation node may be the root of just three
tree-structure shapes. First (and trivially), both parent and daughter
branches may become extinct. Second, one branch may continue to
speciate while the other becomes extinct. And third, both branches may
continue to speciate. It is my guess that extinction, or at best a
narrow and minor diversification, rather than a broad diversification,
is the usual fate of a new branch, except in extraordinary times when
the customary agents of extinction are subdued.

The extinction probability of a branch during an interval of time
determines the "bushiness" of the phylogeny. If the extinction
probability is high, then the phylogeny will be dominated by
evolutionarily static stem-group lineages leading to infrequent
dichotomous radiations; if the probability is low, then extensive
dichotomous radiations will predominate. Most interestingly, the
extinction probability is never constant; when it is high (such as
during and immediately after an asteroid impact), we observe among
the survivors a period of environmental stress with highly
constrained or nonexistent speciation, and when it is low (such as
during the species-poor, several-million-year-long aftermath of a
major extinction), we observe a period of evolutionary
radiation. Articles relating extinction probabilities to phylogeny
occur with some frequency in the literature on evolutionary theory, so
there is little reason to do more than simply mention the connection
here.

As might be expected from this topological discussion, the observed
pattern of archosaur evolution in the Mesozoic is one of rapid
radiation followed by apparent evolutionary stasis, eventually
followed by mass extinction, followed by rapid reradiation and
diversification of the survivor groups.  There were seven pre-Cenozoic
extinction events that affected the archosaurs, beginning with the one
that opened the Mesozoic Era and ending with the one that closed it;
those extinctions affected some archosaurs more than others.  I do not
intend to address the questions of what caused the extinctions or of
how long the extinction episodes may have lasted. While those are
interesting questions, I am more concerned here with the effects of
the extinctions on Mesozoic archosaur phylogeny and diversification.

The longest archosaur lineages are those extending from the common
ancestor of all the archosaurs to any of the 22 species of extant
crocodylians or 9672 species of extant birds. Although we are free to
select any of them to serve as the spine of a "Hennigian comb"
archosaur cladogram (Panchen, 1991), for reasons that must be obvious
I shall choose one of the avian lineages, say the one culminating in
the American robin, Turdus migratorius. Far more than just a line on
paper from which all the other archosaur clades branch off, this line
represents a single population of archosaurs (which was once the only
population of archosaurs) that has been in continuous existence since
at least the Late Permian, changing anagenetically in response to
environmental vagaries and competition from other populations. It
survived all seven Mesozoic extinction events as well as the Cenozoic
ones: a population of tetrapods that originated as small, quadrupedal,
sprawling, lizardlike diapsids and eventually became small, migratory,
red-breasted birds. Of course, the "mesenosaur-to-robin" lineage
ultimately stretches back to primordial eucaryotes, but that earlier
segment of the lineage is not as relevant to this discussion.

Incidentally, I cannot help noting that I see no place along the
"mesenosaur-to-robin" lineage where it might, temporarily, have
evolved into large, obligatory bipeds with reduced, grasping forelimbs
before becoming small, volant bipeds with large, winglike
forelimbs. It is much more straightforward to suppose that the lineage
began as small, sprawling quadrupeds that evolved into semi-erect and
then fully erect quadrupeds, gradually adapting to a lifestyle of
climbing and leaping among trees. They started small and remained
small throughout their lineage, progressively improving their leaping
abilities and in time becoming feathered gliders, then fliers. One
hundred fifty million more years turned those primitive fliers into
today's superbly adapted Turdus migratorius. Although the lineage must
have endured its share of bottleneck events and reversals, and it
undoubtedly comprised both plesiomorphic and apomorphic segments on
its path toward robinhood, I cannot see where in the lineage the
animals might have become as large as even a small theropod such as
Rioarribasaurus, only to become bird-size again, or where they might
have acquired reduced forelimbs that later re-enlarged. Evolution
almost never takes a straight path from species A to species B, but I
believe the evolutionary random walk from small quadrupeds to small,
volant bipeds did not pass through the space of large bipeds with
grasping forelimbs. What we presently call dinosaurs instead seem very
clearly to have comprised a series of side branches of
ground-dwelling, cursorial animals off the "mesenosaur-to-robin"
lineage. Once a population left the trees and grew large, it certainly
could not have re-evolved into an arboreal population competitively on
a par with those left behind.

Let me digress further. Suppose for a moment that there were no side
branches from this "mesenosaur-to-robin" lineage. How would a strictly
cladistic taxonomy divide this lineage of morphological types into
species-level taxa?  Common sense, perhaps more than anything else,
dictates that a species changes into another species with the
acquisition or loss of suitably defined key characters. There is no
guarantee that such events will obligingly occur exactly at the branch
nodes of a lineage. Yet it is only at the branch points that cladists
"allow" the definition of new species. Without branch points, cladists
would have no choice but to declare the mesenosaur and the robin to be
members (though "early" and "late," respectively) of the same species.
This situation strikes me as rather embarrassing to strictly cladistic
taxonomy.

The "mesenosaur-to-robin" lineage is at least 250 million years
old. If, as noted earlier, there were a complete archosaurian species
turnover every 4.5 million years or less, we might expect that the
"mesenosaur-to-robin" lineage speciated anagenetically more than 55
times. Cladogenesis probably created a comparable number of side
branches, though there is of course no reason why this number should
agree exactly with the number of anagenetic speciation events. About
40% of those side branches appeared after the close of the Mesozoic
Era and account mainly for the diversity of extant passerine birds and
their near relatives; the remaining side branches account for the more
distantly related birds, the theropod dinosaurs, the other dinosaurs,
the pterosaurs, and the thecodontians. This is quite a number of
clades, but it is still not the whole story, because as a rule
populations of small animals speciate more rapidly than populations of
large animals. The estimate of 4.5 million years is based on the
species turnover among large dinosaurs in four successive faunas of
western North America. Smaller archosaurs could well have speciated
every million years or less, so the number of clades generated by the
"mesenosaur-to-robin" lineage could number in the hundreds. (Compare,
for example, the speciation rate for the genus Homo, a relatively
large vertebrate, over just the preceding one or two million years.)
While some of those clades would have suffered rapid extinction, just
as many or more could have flourished and diversified. Owing to the
poor archosaurian fossil record, we will probably never know how many
clades there were and what their histories were. We can guess,
however, that this number was large. (I cannot help noting
parenthetically that 55 speciation events would inject 54 hierarchic
levels in between the root node Archosauria and the leaf node Turdus
migratorius. Which of these would be significant enough to require
names? What criteria would enable us to decide? Can standard
typography accommodate 54 1-pica indents on a printed page?)

Viewing the evolutionary patterns of archosaurian subclades as smaller
"Hennigian combs" suggests that a single model might account for them
all.  Central to this model is the existence of core groups of small,
rapidly evolving (tachytelic), highly diverse archosaurs that comprise
evolutionary engines from which the larger, more slowly evolving
(bradytelic) forms arose.  Being small, core forms such as
Compsognathus, or animals directly on the "mesenosaur-to-robin"
lineage are seldom found as fossils, and when they are, they tend
to be perplexing animals that seldom fit into the better-known
groups. The reason I use the term core group rather than core lineage
is that a core group comprises a bundle of lineages of morphologically
similar taxa that would be extremely difficult to disentangle even if
we had a nearly perfect fossil record. For example, the extant
passerine birds form a large avian core group. If each of its several
thousand species were available to us only as a fossil, we would find
it practically impossible to reconstruct their phylogenies correctly;
it is difficult enough even when we have access to living populations
and can chemically analyze their genomes (Sibley & Ahlquist, 1990).

Because they are small animals, core forms are subject to considerable
predation pressure, often from their own larger descendants, which
tends to keep their evolutionary fecundity in check. But when an
extinction event removes this predation pressure, the lineages within
the core groups rapidly radiate to fill the void. As the new forms
become large and widespread, the likelihood of their preservation as
fossils rises, and the fossil record displays their "abrupt"
appearance. The fossil archosaurs we have so far discovered are thus
just the tip of an evolutionary iceberg.  Core groups are practically
immune from mass extinctions, though not from evolutionary
change. Large terrestrial vertebrates are unable to find shelter from
mass-extinction agents such as abrupt climatic change, asteroid
impact, and so forth. Burdened with a slow evolutionary tempo, large
animals generally respond to such agents by becoming extinct, and
since they are by virtue of their size prominent in the fossil record,
it is their disappearance that often signals the "mass extinction."
New Bauplane seldom if ever evolve among large terrestrial
vertebrates although they would, if it weren't for the
meddlesomeness and frequency of mass-extinction agents. As things
stand, the evolution of larger animals consists almost exclusively of
"variations on a theme."

Evolution in a core group occurs at a faster tempo because core-group
animals are smaller, have shorter gestation periods, take less time to
attain sexual maturity, and sometimes (particularly among mammals)
have larger litters than large animals. In the absence of predation,
this is the source of radiative evolution. But it is a wellspring of
rapid evolution under most other circumstances as well. When niches
for large forms become occupied, core-group evolution simply produces
new core-group forms. Core groups can themselves split into other core
groups, though not quite as rapidly as species can speciate. New
Bauplane are thus a comparatively frequent product of core-group
evolution. The primary agent of core-group extinction if this
process really does occur and is not simply an artifact of core-group
anagenesis seems to be continual competition for resources with
other core groups, rather than the global mass-extinction events that
wipe out the larger forms every few score million years. One peculiar
way for a core group to become "extinct" is for all its member
lineages to evolve into animals of larger size. This seems to have
happened to the ornithischians during the Jurassic Period. Core-group
extinctions (or turnovers) appear to be less episodic and occur at a
more uniform pace than the extinctions of the larger forms.

Sister groups in a phylogeny are usually of unequal diversity and
importance, but it occasionally happens that a cladogenetic event
yields two clades of roughly equal size. The evolutionary interplay
between their two competing core groups, as first one group and then
the other generates large, dominant forms is, if nothing else,
fascinating to chart. The archetypal instance of this is the eons-old
competition between the sauropsid and theropsid branches of
Amniota. During the Late Paleozoic, Theropsida was ascendant,
providing most of the world's terrestrial megafauna in the form of
pelycosaurs and therapsids. This changed drastically at the beginning
of the Mesozoic, when the P-T extinction removed most of the
therapsids and permitted the radiation of the sauropsids: the diapsids
and archosaurs. In the form of archosaurs and dinosaurs, the sauropsid
branch dominated Mesozoic megafaunas, whereas the theropsids "went
underground" and gradually transformed into mammals. When the K-T
extinction removed the large archosaurs, mammals regained the dominant
role for Theropsida, while birds and squamates remained as the two
sauropsid core groups. Extinction events do not guarantee that the
"overlord" core group will always be eclipsed and that the "underdog"
core group will always take over. There were at least five other
Mesozoic extinction events following which different kinds of
archosaurs attained center stage while therapsids and mammals remained
in the background.

The Permian-Triassic (P-T) Extinction

The Paleozoic Era ended with the tremendous P-T extinction, which may
have eliminated as many as 96% of the world's species (Raup,
1991) or, more accurately, 96% of those that lived in depositional
environments. At the time, there were, as far as we know, just two
extant archosaur orders: Proterosuchia and (probably) its immediate
descendant, Basitheropoda.  Together these made up a core group from
which all other known Early Triassic archosaurs descended; they
themselves arose from within a core group of Permian archosauriform
diapsids that they eventually partially replaced. (The diapsids that
the archosaurs did not replace went on to become today's lizards and
snakes.) The primary terrestrial tetrapod victims of the P-T
extinction were the large therapsids, including most if not all of the
world's top predators as well as many groups of rather ungainly
herbivores.  The removal of those forms fostered the first archosaur
radiation, part of a wider sauropsid radiation that included
rhynchosaurs, prolacertiforms, trilophosaurs, and other diapsids
(Benton, 1990b). Once lost, dominance seems to be not easily regained:
Although the therapsid core group remained an evolutionary engine that
continued to generate new forms, for the rest of the Mesozoic it did
not generate any animals significantly larger than Placerias, and
after the start of the Jurassic, it did not generate any animals
larger than a marten. Only during the Late Cretaceous, with the
appearance and initial diversification of placental mammals, did
Theropsida begin to regain ecological lost ground.

Once the Triassic Period was underway, the fossil record seems to show
a gradual archosaur diversification at the expense of the remaining
large therapsids and other nonarchosaurian diapsid reptiles (Benton,
1986b, 1990b; Hunt, 1991; Weems, 1992). In particular, the
proterosuchian part of the core group generated a number of
ground-dwelling predators, including early ornithosuchians,
pseudosuchians, and parasuchians. These groups, particularly the
pseudosuchians, gradually replaced the larger therapsids and other
reptilian groups as the Triassic unfolded. By the beginning of the
Late Triassic (Carnian), about halfway through the Triassic Period,
several more thecodontian orders appear in the fossil record,
including Aetosauria, Hupehsuchia, and Crocodylia.

Probably the earliest offshoot of the basitheropod core group
comprised the as-yet-unknown ancestors of Sharovipterygia and
Rhamphorhynchoidia. They became, in time, the third archosaurian core
group and, eventually, the first vertebrate group to achieve
active, powered flight. Out of ignorance of their true nature, I
informally call them "protopterosaurs" there is no point in
formally naming the group until at least one member can be
identified. The protopterosaur fossil record is still totally
nonexistent; by the time they enter the fossil record, protopterosaurs
are already well differentiated into sharovipterygians (the
plesiomorphic subclade, so far represented only by the highly derived
genera Sharovipteryx and Scleromochlus) and rhamphorhynchoids (the
apomorphic subclade).

Meanwhile, a bit later in the Middle Triassic, the basitheropod
evolutionary engine generated ancestral sauropodomorphs and
ornithischians, and also Lagosuchia. Again out of ignorance, I have
informally called the former two groups "protosauropodomorphs" and
"protornithischians." Their fossil records are (as usual) entirely
lacking, but we know that such groups must have existed in the Middle
Triassic because comparatively advanced sauropodomorphs
(Azendohsaurus, Euskelosaurus) and ornithischians (Pisanosaurus,
Technosaurus) appear in the Carnian. Both groups apparently remained a
minor part of the world's fauna until the C-N extinction, although the
protornithischians, in the form of early Lesothosauria, became the
fourth archosaurian core group. The primary fossil record of
Lesothosauria ornithischians that had not yet evolved emarginated
tooth rows to accommodate cheeks is post-Triassic and includes a
number of problematic forms described solely from teeth and jaw
elements. But their primitive aspect makes it virtually certain that
lesothosaurians had evolved before the appearance of Pisanosaurus, a
Carnian ornithischian likely a primitive heterodontosaurian from
Argentina whose relatively robust dentary had closely packed teeth and
a prominent lateral shelf (Weishampel & Witmer, 1990a).  Lagosuchians,
the earliest known cursorial ground-dwelling theropodomorphs, were
small bipedal predators with well-developed grasping forelimbs and
relatively powerful hind limbs, so they likely descended from
tail-gliders or canard-wing forms lacking more advanced volant
specializtions.

Having generated a radiation of thecodontians, the proterosuchian
evolutionary engine seems to have run out of steam without the action
of an external extinction agent toward the end of the Middle
Triassic. Perhaps this was the result of competition from
Basitheropoda and Lagosuchia, Pseudosuchia, and small therapsids and
other non-archosaurian reptiles. It is also eminently possible that
the proterosuchian core group became "extinct" in the peculiar way
alluded to earlier, by having all its member lineages evolve into
bradytelic animals of large size. Its role as primary archosaurian
core group seems to have been assumed by the more diverse
basitheropods. There is no evidence for any Late Triassic core-group
evolution no new Bauplane within Thecodontia. Instead, the
thecodontian orders continued to evolve bradytelically and
conservatively until essentially all but one vanished in the T-J
extinction. That sole surviving order was Crocodylia, which has
persisted to the present in the ecological niche of large, tropical,
riparian predators without any significant Bauplan changes. Demise of
a core group practically guarantees a decline in diversity and the
eventual extinction in subsequent mass-extinction events of its
bradytelic descendant forms.

The lack of a fossil record need not prevent us from deducing (not
just speculating) that theropodomorphs filled many niches as arboreal
and small cursorial animals during the Middle Triassic. They led
ultimately from Lagosuchia (Middle Triassic: Lagosuchus, Lagerpeton,
Marasuchus) to Herrerasauria (Carnian: Staurikosaurus, Herrerasaurus,
Eoraptor), Ceratosauria (late Carnian: Coelophysis, Rioarribasaurus),
Protoaviformes (Late Triassic: Protoavis), and eventually
Aves. Competition from and predation by thecodontian predators and
early pterosaurs may have restricted theropodomorph diversification
prior to the C-N extinction.

Most workers (Gauthier, 1986a,b; Benton, 1990a; Sues, 1990; Novas,
1992) consider Lagosuchia and Herrerasauria to be basal dinosaurs that
diverged from the "mesenosaur-to-robin" lineage prior to the
divergence of Ornithischia and Sauropodomorpha from Theropoda. To me,
lagosuchians and herrerasaurians were anatomically, ecologically, and
temporally much closer to true theropods than to sauropodomorphs or
ornithischians. It would be far more difficult to contrive a dinosaur
morphologically transitional between Herrerasauria and Ornithischia
than a basitheropod from which both groups might have descended in
separate (and longer) lineages. I cannot understand why the notion
that lagosuchians and herrerasaurians were basal dinosaurs rather than
just basal theropodomorphs has persisted as long as it has.

The Carnian-Norian (C-N) Extinction

A major extinction occurred at the boundary between the Carnian and
Norian epochs, about a third of the way through the Late
Triassic. This event has only recently been identified (cf. Benton,
1986b; Olsen & Galton, 1986; and other articles in Padian, ed.,
1986b), following improvements in the fossil record at and before the
Triassic-Jurassic transition. It is not noted as a separate extinction
by Stanley (1987) or Raup (1991), and it may eventually prove to be
merely the first of a series of closely spaced extinction events that
occurred during the Norian and Rhaetian stages that allowed early
dinosaurs to radiate group by group into freshly opened niches.

The result of the C-N extinction was the removal of most of the
remaining large therapsids and many large archosauriform
diapsids. Among the archosaurs, thecodontians declined in diversity,
although faunas with large pseudosuchians, parasuchians, aetosaurians,
and of course crocodylians did persist to the end of the Triassic. The
disappearance of many of the larger archosaurian predators probably
fostered the evolution of herrerasaurians (particularly large ones,
such as the poorly known South African Aliwalia), early ceratosaurians
(Podokesauridae, Halticosauridae), and perhaps protoaviforms. We now
know that carnosaurlike teeth frequently found in Upper Triassic
deposits belonged to pseudosuchians (such as rauisuchians and
poposaurians), herrerasaurians, and perhaps early ceratosaurians,
rather than to carnosaurs, which had not yet evolved, or to
prosauropods.

Despite their minuscule fossil record, small protoaviforms were
probably widespread and diverse following the C-N extinction, in the
form of feathered climbing and gliding forms. Cursorial
theropodomorphs  herrerasaurians and early theropods were not as
diverse, but the Bauplan of a small, bipedal predator, with grasping
forelimbs where it once had elongate, clawed canard or gliding wings,
and tridactyl feet that once had an opposable hallux with a grasping
function, became well established. This body plan persisted with great
success in the paraorder Theropoda through the end of the Mesozoic.
Among the Late Triassic prey animals seized by the theropodomorphs
were small core-group therapsids, including the earliest true mammals,
and early lepidosaurs, whose own core group eventually evolved into
present-day lizards and snakes.

Hard evidence of small (pigeon-size) Late Triassic and earliest
Jurassic dinosaurs consists mainly of tiny tridactyl footprints and
miscellaneous skeletal remains from Nova Scotia and localities in the
eastern coastal United States. These are often reported in news
articles about and television interviews with Paul E. Olsen (e.g.,
Gaudet, 1986), but to my knowledge they have not yet been described in
detail (cf. Lessem, 1992: 106 121, however, for a preliminary
account).

Competition from and predation by pterosaurs may have limited the
evolution of climbing and gliding theropodomorphs during the Norian,
but the core group nevertheless flourished. Besides the combined
Basitheropoda, Protoaviformes, Lagosuchia, small Herrerasauria, and
small Theropoda, other Norian archosaurian core groups must have
included Pterosauria (Sharovipterygia and Rhamphorhynchoidia) and
Ornithischia (Lesothosauria, Heterodontosauria, and
Ankylosauria). Although comprising primarily small cursorial forms,
the ornithischian clades may have included arboreal animals as
well. The Norian fossil record of Ornithischia runs from bad to
nonexistent, but we may infer the existence of such forms from their
somewhat better pre- and post-Norian records.

Most emphatically not core-group archosaurs, early sauropodomorphs
(basal sauropods and their descendant prosauropods) took over and
expanded the number of large-herbivore niches from the large therapsid
and other nonarchosaurian herbivores wiped out in the C-N extinction,
and as sauropods and segnosaurians they enjoyed considerable success
during the remainder of the Mesozoic. Their large body sizes and
especially their elongate necks allowed them to forage higher than
other herbivores, a competitive advantage they did not relinquish
during the Mesozoic.

I imagine that if one were to somehow trace the history of the
ornithischian core group back into the Middle Triassic to a time
before the ornithischians acquired their characteristic opisthopubic
pelvis and predentary bone one would find the sauropodomorphs
diverging from it. The most primitive known sauropodomorphs are
medium-size forms (Thecodontosauridae) whose dentition is sometimes
difficult to distinguish from that of ornithischians (as, e.g., in
Azendohsaurus; Galton, 1990a; Gauffre, 1993a). Furthermore, as pointed
out by Paul (1984b), segnosaurians exhibited a mosaic of ornithischian
and sauropodomorph characters that were probably the result of common
ancestry with both groups. Possessing an unsplit metatarsal I, like
that of herrerasaurians, segnosaurians (or their ancestral forms) must
have diverged from the "mesenosaur-to-robin" lineage earlier than any
of the theropod subclades (contrary to D. A. Russell, 1994), so
perhaps Segnosauria arose from (and thus provides indirect evidence
for the existence of) an undifferentiated core group of small Middle
or early Late Triassic dinosaurian herbivores.

The Triassic-Jurassic (T-J) Extinction

The T-J extinction, which was the third most extensive extinction to
have afflicted Mesozoic archosaurs (after the P-T and K-T
extinctions), eradicated all the thecodontian orders except Crocodylia
and possibly Parasuchia, teeth of which have seemingly turned up in
Lower Jurassic deposits of France (Buffetaut, Cuny & Le Loeuff,
1991). The complete replacement of Parasuchia by Crocodylia as large
riparian predators was, however, thoroughly finished by the end of the
Early Jurassic. The heavily armored, herbivorous aetosaurians were
replaced after the T-J extinction by early ankylosaurians such as
Scutellosaurus and Scelidosaurus, and primitive huayangosaurid
stegosaurs such as Emausaurus. Not everyone agrees that the T-J
extinction is real (Weems, 1992); it may be an artifact of our sparse
fossil record, or the terminal event of a series of small Late
Triassic extinctions. But as far as archosaurs were concerned, the
world changed radically with the onset of the Jurassic Period.

The T-J extinction finished off the last remaining big rauisuchian and
poposaurian predators (such as Fasolasuchus and Teratosaurus). Large
ceratosaurians (such as Dilophosaurus) replaced them in the Early
Jurassic.  Ceratosaurian theropods of various sizes had already begun
to replace pseudosuchians (such as ornithosuchians, rauisuchians, and
poposaurians) and cursorial theropodomorphs (lagosuchians and
herrerasaurians) during the Late Triassic. Also, the transformation of
Basitheropoda into small, arboreal theropods (such as Protoaviformes
and still-unknown small ceratosaurians) as the theropodomorph core
group was probably complete by the beginning of the Jurassic.

The larger prosauropods (Plateosauridae, Blikanasauridae) seem to have
been decimated after the T-J extinction, but the smaller, more
primitive forms (including Anchisauridae and Massospondylidae)
persisted. As-yet-unknown basal sauropodomorphs with a pentadactyl pes
(perhaps including Melanorosauridae) evolved into the earliest known
sauropods (Vulcanodontidae, Barapasauridae). Yunnanosaurids were an
advanced but short-lived sauropodomorph family endemic to China (as
far as we know) that seems to have arisen from more primitive
sauropodomorphs following the T-J extinction.  Otherwise, as is to be
expected of core groups, the smaller ornithischians and the pterosaurs
(except the sharovipterygians) survived and diversified much as they
did during the Triassic, largely unaffected by the megafaunal changes
around them. The T-J extinction established the archosaurs, and
particularly the dinosaurs, as the dominant terrestrial megafauna. The
Age of Dinosaurs, which would endure some 150 million years until the
K-T extinction, began here.

--- continued in part 2