The Age of Dinosaurs saw profound changes in the surface of the Earth due to plate tectonics, and those changes are reflected in their history.

Phase I: When Dinosaurs Shared the Earth: Late Triassic
As noted before, the earliest dinosaurs (Pisanosaurus, heterodontosaurids, and Eocursor for Ornithischia; herrerasaurs, Eoraptor, Tawa, coelophysids and other basal neotheropods, and the diverse early sauropodomorphs for Saurischia) lived in a world in which many other large terrestrial amniotes were present: in particular, the diverse forms of crurotarsan archosaurs and the less common therapsids.

During this time, the only dinosaurs to be come common and successful were sauropodomorphs. This is likely due to the fact they they had no real competition in their mode of life (as tall large tree-feeders).

During this time, all the major landmasses were still connected as the supercontinent Pangaea. Consequently, the world was characterized by relatively cosmopolitan assemblages of animals and plants: pretty similar faunas and floras in different parts of the world. (To be fair, though, the record of ornithischians and of Sauropoda proper are limited to the southern parts of Pangaea during the Late Triassic.)

The end of the Triassic Period saw a major mass extinction on land and sea. Among the therapsids, only the early mammals (mammaliforms) and their closest relatives survived: the larger types of cynodonts and the larger still herbivorous dicynodonts all died out (or may have already become extinct.) Among the Crurotarsi the large predators, the armored herbivores, the aquatic fish-and-reptile eaters, and indeed all the linneages of the group died out except for the small, fast-running ancestors of crocodilians. While some groups of dinosaurs died out (basal members of both Ornithsichia and Saurischia such as Herrerasauria and Guaibasauridae), the advanced forms persisted and inherited the world.

What caused the extinction? Some researchers have implicated an asteroid impact, citing evidence of an iridium spike similar to that at the end of the Cretaceous, or the Manicouagan astrobleme (a crater in Quebec, Canada from the Late Triassic). However, the former has not been discovered at many sites, and the latter (with a date of about 214 Ma) is far too old for the 199.6 Ma extinction event.

More convincing the possibility the extinction is linked to the Central Atlantic Magmatic Province, a tremendous series of volcanic eruptions and lava flows that happened right at the Triassic/Jurassic boundary. These eruptions are associated with the break up of Pangaea. The Permo-Triassic Extinction is also linked to a huge series of volcanic eruptions.

If the volcanic eruptions (and the massive climate changes they produced) were the cause of the Triassic/Jurassic extinction, than dinosaurs owe their later success to plate tectonics.

Phase II: Dinosaurs Inherit the Earth: Early Jurassic
Dinosaurs in the aftermath of the Triassic/Jurassic extinction experienced an adaptive radiation, as their rivals had disappeared. The Early Jurassic Epoch saw the flourishing of the last of the core prosauropods and the rise of the first large theropods (dilophosaurids). Sauropods began to become more common, and among the ornithischians are the first of the armored thyreophorans and the first neornithischians.

Among other amniotes, new early mammals appear, as well as somewhat more crocodilian-like crocodile ancestors.

The rift formed by the eruptions of the Central Atlantic Magmatic Province was actually the beginnings of the Atlantic Ocean! This split was between North America on the north and Africa plus South America in the south; however, there was apparently still land contact between North and South America, and still a largely cosmopolitan world.

As the Early Jurassic progressed, evolution within each of the various dinosaur lineages produced new groups that dominated the next phase of dinosaur history.

Phase III: Jurassic Park: Middle and Late Jurassic
During the Middle and Late Jurassic Epochs the Atlantic continued to widen; as it spread, the far margins of the continents buckled due to subduction zones: in North America, the Rockies underwent a new phase of activity; in South America, the Andes began to rise. Also, rising temperatures and the increase of activity of mid-ocean ridges raised sea level, so that low lying regions were flooded. In North America, much of the West and Midwest were drowned under the Western Interior Seaway, while Europe became an archipelago.

In this world, the dominant groups of the Early Jurassic were outcompeted by their more advanced descendants. Sauropods displaced the core prosauropods and near-sauropods; averostran theropods replaced the dilophosaurids; stegosaurs and ankylosaurs replaced the basal thyreophorans.

During this phase, the dominant group of apex carnivores were carnosaurs and primitive megalosauroids (such as megalosaurids); beneath them were ceratosaurs, and then the new coelurosaur groups as the small predators. Among herbivores, the sauropods (and especially after the Middle Jurassic, the neosauropods) were the most common, and the stegosaurs were the secondmost. Among the sauropods there was substantial niche partitioning among ways to feed.

Other groups of ornithischians were present. The ankylosaurs were around, but generally rarer than stegosaurs. The neornithischians were also present, and they diverged into basal forms, ornithopods, and marginocephalians at this time. But none of these groups were very common (the ornithopods the most common of the three).

Although the continued spread of the continents and the rise of sea level reduced some connections between landmasses, the faunas of the world were still fairly similar in the Middle and Late Jurassic. For example, Middle Jurassic English and Chinese assemblages seem fairly similar, and in the Late Jurassic the dinosaurs of the Morrison Formation of the American West, the Tendaguru beds of eastern Africa, and the formations of Europe (especially Portugal) all have very similar species of dinosaurs.

Phase IV: Triumph of the Low Brow(sers): early Early Cretaceous
The change between the Jurassic to the Cretaceous is not as dramatic as the Triassic to the Jurassic, at least for terrestrial vertebrates. Many of the same groups are still present. The main switch is in relative abudance of the herbivores: sauropods and (especially) stegosaurs become rarer, while iguanodontian ornithopods and ankylosaurs become a lot more common. Increase in the abudundance of these groups shows that many more large dinosaurs are now supported from low vegetation (although iguanodontians could reach up relatively high) than when sauropods were the most common plant-eaters. Are these changes linked to the rise of a new group of plants?

Flowering plants, called the angiosperms or anthophytes are the major clade of modern plants. With rare exceptions, if you have eaten a plant, it was an angiosperm. Angiosperms are a clade within the seed plants. Their mode of reproduction is to develop a specialized set of both male and female sex organs within a flower; pollinators are lured to the flower, pick up pollen, have pollen rub off on the flower of another plant, where they fertilize the female sex cells, and a seed is made. That seed is covered by a coating of fleshy or nutty tissue: the fruit.

The basic angiosperm life cycle hinges on co-evolution with animals:

• Bright colors, attractive smells, and interesting patterns on the flowers attract pollinators (typically flying insects). These move pollen (containing the male sex cells) to flowers of other plants (where the female sex cells are)
• Fruit remains bitter, hard, and dull colored until the seeds are ready to grow. At that point, the fruit becomes brightly colored, fleshy or nutty, and sweet and juicy. The fruit is then eaten by a vertebrate, which leaves the area and deposits the seeds in its dung.

If angiosperms evolved flowers and fruit in the Cretaceous, who were their target audiences?

• Flower targets?
  • Earliest Cretaceous "birds" (flying theropods) were still relatively rare, and none show nectar-eating adaptations.
  • Mammals were small, and may have been pollinators, but no bats (the main modern mammalian pollinators) yet.
  • Insects (including beetles, flies, lepiodopterans, and hymenopterans) were present, and were almost certainly the main pollinators in the Mesozoic just as they are today.
• Fruit targets?
  • Herbivorous mammals of the Early Cretaceous were small, and may have eaten berry-sized fruit, but no more than that
  • The same holds for herbivorous lizards
  • Hebivorous crocodyliforms were rare
  • But herbivorous dinosaurs were VERY common, and could easily eat and transport a LOT of fruit!

So thank the insects for flowers, and thank the dinosaurs for fruit.

The rise of the angiosperms occurs about the same time that low-browsing herbivorous dinosaurs (ankylosaurs, iguanodontians, rebbachisaurids) become dominant over medium (stegosaur) and high (typical sauropod) browsers. Are these changes linked? Although angiosperms were present in the Early Cretaceous, they seem to have been relatively rare then, and unlikely to have been a major food source for these groups at first. But it may be that increase in low-browsing forms favored the spread of herb-sized angiosperms. This time also sees the diversification of the non-predatory coelurosaurs (although their origins go back into the Middle Jurassic).

Carnosaurs (specifically carcharodontosaurids) and megalosauroids (specifically spinosaurids) were still the apex predators; smaller megaraptorans, ceratosaurs, and coelurosaurs continue as the medium-sized and smaller predators.

As the Atlantic widens, Pangaea has become two separate supercontinents. Laurasia in the north comprises modern North America, Greenland, most of Europe, and most of Asia other than India. Gondwana in the south comprises South America, Africa, Madagascar, India, Antarctica, and Australia (as well as bits and pieces that would later become southern Europe and parts of southeast Asia).

Gondwana does break apart into several components during the Cretaceous, but the exact sequence remains uncertain, as the evidence conflicts. Different models call for a split between the western (South America/Africa) vs. eastern (everything else) as the first division; others have Africa pull out and becomes isolated first but the rest of Gondwana remains in contact until quite late.

With the split between Laurasia and Gondwana, and the breakup of Gondwana itself, there is a trend towards more provincialized faunas: that is, assemblages of animals in one part of the world tend to be different from those in other parts. Still, there is still enough connections between the landmasses (perhaps via island systems) to find similar dinosaurs in different regions.

At present, the Gondwana communities of the early part of the Early Cretaceous are less well known than the Laurasian ones. The best studied assemblages of this time are the Wealden deposits of Europe, from which Iguanodon is known.

At the end of this interval, Stegosauria is extinct. In Asia, we have a good record of the small-bodied dinosaurs because of lake deposits that preserve their skeletons and integument in fine detail. This little-bodied dinosaur community is dominated by ceratopsians and diverse coelurosaurs, and represents the ancestors of the groups which will dominate Asia and western North America in the later Late Cretaceous.

Phase V: Hot Times in the Cretaceous: late Early and early Late Cretaceous
The later part of the Early Cretaceous and the beginning of the Late Cretaceous (basically, the block of time between about 120 and 85 million years ago) is a time of relatively high oxygen levels, fairly high carbon dioxide values, high sea levels, and high temperatures (even for the Mesozoic). This produced a climate that was nearly tropical from pole-to-pole. Some of the largest dinosaurs of all are from this time (sauropods like Sauroposeidon, Argentinosaurus and Paralititan; theropods like Giganotosaurus, Mapusaurus, Carcharodontosaurus, and Spinosaurus): it may be that the conditions were favorable to high plant productivity (more food made per unit time), which could support more herbivores, which could then support more carnivores.

During this time several equatorial dinosaurs evolved tall fins from their neural spines. This may have served as a means to dump excess heat and cool off.

Laurasian and Gondwanan dinosaurs of this interval became even more distinct. While some groups were shared (ankylosaurs; iguanodontians; titanosaurs; spinosaurids; carcharodontosaurids), others were more provincial. In Gondwana the first abelisaurid and noasaurid ceratosaurs are present, and rebbachisaurid diplodocoids are common. In Asia (in particular) the ankylosaurids, the neoceratopsians, and many diverse coelurosaurs (tyrannosauroids, ornithomimosaurs, oviraptorosaurs, therizinosaurs, and deinonychosaurs) flourish and diversify. In Laurasia more generally, iguanodontians undergo "hadrosaurification": that is, we see the correlated progression of hadrosaurid traits within Hadrosauriformes.

At the end of this interval, there are some major changes to the dinosaur communities. For reasons not yet clear, the carcharodontosaurids and nearly all the spinosaurids (last of the megalosauroids) died out in both Laurasia and Gondwana (although megaraptoran carnosaurs survive and some rare spinosaurid remains have been found in mid-Late Cretaceous China.) Non-titanosaur sauropods (execpt perhaps for a stray rebbachisaurid lineage) also die out.

Also at the end of this time western North America's fauna receives a number of immigrants from Asia: ankylosaurids, advanced neoceratopsians, advanced tyrannosauroids, and ornithomimosaurs.

Phase VI: The Many Worlds of the Late Cretaceous: late Late Cretaceous
Continued motion of the continents further separates the faunas of the world, as does rising sea level. For example, North America was divided up into a western portion (which was periodically connected to Asia) and one to two eastern portions by the Western Interior Seaway: a shallow sea that ran from the Gulf of Mexico up to the Arctic Ocean over the low-lying parts of America and Canada. Other continents were similarly divided up.

At the beginning of this interval several new types of dinosaur appeared. The immediate ancestors of the hadrosaurids may have been Asia, or they might have been North America: either way, the immigrate one direction or the other at this interval. Tyrannosauridae proper may have arisen in Asia and migrated to North America, or it may evolved in western North America and some of them migrated back to Asia. Ceratopsidae is far more diverse (as currently known) in western North America, but there are primitive ceratopsoids and at least one genus of centrosaurine in Asia, and nearly all the more basal ceratopsians are Asian. Pachycephalosaurids (domeheaded pachycephalosaurs) also appear at this time: they are more diverse in western North America, while all currently known Asian pachycephalosaurids are found to be a single clade nested among North American taxa, and thus this group as a whole seems to be initially North American.

There are several distinct biogeographic regions during the later Late Cretaceous (85-65.5 Ma):

• Asiamerica: central and eastern Asia and western North America. This region is dominated by a fauna with Tyrannosauridae as the only large predators in these regions: the only other large theropods are herbivores or omnivores (ornithomimosaurs, therizinosaurs, oviraptorosaurs). Deinonychosaurs make up the small carnivores. The large herbivores (other than big herbivorous theropods) were primarily hadrosaurids, neoceratopsians, and ankylosaurids, although titanosaurs are also present. Among the smaller herbivores are pachycephalosaurs. Within this framework are some variations:
  • The deserts of Asia lacked big predators and hadrosaurids, but had a diverse fauna of maniraptorans, neoceratopsians, and ankylosaurids
  • Wetter enivronments (most of western North America, but also some parts of China, Mongolia, and Siberia) had larger dinosaurs. True ceratopsids are limited to these regions.
  • For reasons not entirely certain, small ornithopods are present in North America but are not yet known from Late Cretaceous Asia.
• Eastern North America: primitive tyrannosauroids are the top predators, and giant primitive hadrosaurids (and possibly some titanosaurs) the large herbivores.
• South America, India, Madagascar: These landmasses share a similar fauna with each other, with abelisaurids and some late surviving megaraptorans as the top predators and titanosaurs as the most abundant herbivores. Noasaurids and small deinonychosaurs are present as the smaller carnivores. In South America at least ankylosaurs, hadrosaurids, alvarezsaurids, and large deinonychosaurs are also present. African dinosaurs from the later part of the Late Cretaceous are currently unknown.
• Australia, Antarctica: Although only poorly known, these regions have hadrosaurids, small ornithopods, ankylosaurs, and deinonychosaurs. It is not certain if the large theropods in these regions were abelisaurids or something else (late surviving carnosaurs or spinosauroids, for instance).
• Europe: As in most of the later Mesozoic, Europe is a archipelago in the Late Cretaceous. On at least some islands abelisaurids and titanosaurs were present. Nodosaurid ankylosaurs were also common, as were primitive hadrosaurids. Among the smaller theropods were deinonychosaurs and alvarezsaurs. A uniquely Late Cretaceous European radiation were the rhabdodontid ornithopods.

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Last modified: 13 July 2010