GEOL 104 Dinosaurs: A Natural History

Fall Semester 2019
In the Shadow of the Dinosaurs: Mesozoic Mammals and Mesozoic Plants

Detail from "In the Shadow of Giants" by Vlad Konstantinov (2016)

Key Points:
•Mammals are a branch of therapsid synapsids.
•Most of mammalian history occurred within the Mesozoic. Many features passed on to modern mammals (loss of good color vision; whiskers; etc.) represented adaptations as mostly nocturnal animals during the Age of Dinosaurs.
•One of the most important groups of organisms to evolve during the Mesozoic are the angiosperms (flowering plants). Unlike other seed plants, angiosperms use flowers to "bribe" insects and others to spread their pollen, and fruit to lure vertebrates (including herbivorous dinosaurs) to spread their seeds.

Origin of the Fuzzballs: The First 7/10ths of Mammalian History
On land during the Mesozoic, there were plenty of organisms other than dinosaurs. Among the most important (especially for us!) were the mammals.

Mammals and their closest relatives (more properly Mammaliformes, or sometimes "Mammaliaformes") appear in fossil record the same time as dinosaurs, in Late Triassic.

Mammals are very advanced therapsids synapsids.

True mammals (Mammalia) found from Late Triassic onward.

Like birds, many of the features that characterize modern mammals don't fossilize:

On the other hand, some mammalian features are preservable:

Many features limited to Mammalia among living amniotes were probably found in their closest non-mammalian therapsids relatives. For example, we can't say for certan when warm-blood, fur, sweat & mammary glands show up. We can determine a few of these, though:

Living mammals are divided into three clades:

However, mammal diversity in the Mesozoic was MUCH different. Many groups of Mesozoic mammals have long since died out. And some Mesozoic mammal groups survived the end of the Cretaceous, but have since died out.

Most Mesozoic mammals very small (shrew-to-house cat sized, with a few badger-sized forms in the Cretaceous); mammals only become large AFTER extinction of non-avian dinosaurs.

Oldest mammaliforms of the Late Triassic and Early Jurassic were fairly small. But by Middle and Late Jurassic, there were already some specialized mammals:

Some major groups of Jurassic and Cretaceous mammals:

Prototheria (sometimes called "Australosphenida"; monotremes and their extinct relatives):

Eutriconodonta (eutriconodonts):

Allotheria (allotheres):

  • Some studies suggest that the basal, poorly known Late Triassic "haramiyids" (a paraphyletic group) and the Jurassic tree-climbing euharamiyids are part of this clade; others put these branches outside the Prototherian + Allotheria + Theria group
  • More definite members are the Cretaceous Gondwanatheria, and the diverse Multituberculata.

    There are other branches of early mammals (docodonts, symmetrodonts, dryolestoids, etc.), but the most important remaining two are joined together as the clade Theria. Therians are united by various skeletal (parasaggital stance, some dental, etc.) and soft-tissue (nipples, external ears, etc.) features. Therians include the metatheres and eutheres, which diverged by the Early Cretaceous. (Supposed Jurassic eutherian Juramaia now appears to be outside of Theria proper).

    Metatheria (marsupials and their extinct relatives):

    Eutheria (placentals and our extinct relatives):

    Prototheres, allotheres (as multitubercultates), metatheres (including the first marsupials), and eutheres (including the first placentals) all survived the great extinction event at the end of the Cretaceous.

    The Flowering of the Age of Dinosaurs: Plants of the Mesozoic
    The base of the food chain on land is plants. They are responsible for taking sunlight, carbon dioxide, and water and combining them to produce glucose and oxygen (in other words, the reverse of the aerobic respiration equation).

    Plants at the dawn of the age of dinosaurs were very different from those of the modern world: not so much of what was there as what wasn't. There was no grass, no grain, no fruit, no flowers. But by the end of the Mesozoic, these were present.

    Primitive plants are spore plants. Spore plants are a paraphyletic grade: some are more closely related to seed plants than are others. Spore plants first colonized land in the early Paleozoic Era.

    They reproduce by releasing spores, which settle onto moist surfaces, grow into plants that produce sex cells which meet in the thin film of water, and join to produce a new spore-producing plant. (Note that this is somewhat analogous to amphibian-grade tetrapods: plants that live their life on land, but need to put their sex cells in water to reproduce.)

    Various sorts of spore plants exist:

    All of these were present in the Mesozoic. For most of the Mesozoic the dominant ground cover was ferns, and tree ferns were fairly important trees in the Triassic and Jurassic.

    Spore plants lack true wood (tree ferns cheat by having many stalks growing right next to each other for support), nor do they have complex root systems.

    Those traits, however, ARE present in the seed plants. Seed plants first appear in the mid-Paleozoic Era, and become the dominant land plants in the Permian Period of the Paleozoic Era.

    They reproduce by releasing male sex cells (carried in pollen) which land on female sex organs, join with female sex cells, produce a fertilized seed, which can then be released from the plant to land in the soil and germinate on its own. (This is analogous to the amniotic egg in tetrapods, allowing plants to colonize further inland into drier regions.)

    All non-flowering seed plants are traditionally grouped into a paraphyletic grade, the "gymnosperms": some extinct gymnosperms are more closely related to flowering plants than to other gymnosperms. The living gymnosperms do form a clade (Acrogymnospermae). Various sorts of gymnosperms exist:

    As today, gymnosperms were an important group of land plants in the Mesozoic. In fact, they were even more common then! Ginkgoes, dawn redwoods, cycads, and bennettitaleans (all rare or extinct today) were major parts of the flora, and major sorts of dinosaur food.

    Some gymnosperms (conifers, cycads) wrap their seeds in a fleshy coating, and some (bennitataleans, cycads) had specialized structures around their female sex organs to attract insects. But only in the next group do we get true fruit and true flowers.

    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:

    Possible angiosperm body fossils are known from the Jurassic, and close relatives of the angiosperms go back to the Permian, but the oldest definite angiosperms are from the Early Cretaceous. Early Cretaceous angiosperm pollen and leaves are known from far off Prince George's County, Maryland and nearby in Virginia, and similar fossils are known from earlier in the Cretaceous in China. Pollen of angiosperms (or near-angiosperms) date back to the Middle Triassic, but we can't tell yet if these are from true flowering plants or pre-flowering relatives of the angiosperms.

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

    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.

    By the Late Cretaceous many modern clades of angiosperms were present (mangolias, rose-relatives, maples, etc.). Also during this time the first grass appears. Grasses include not only the stuff that grows in lawns and meadows, but all the grain-producing plants (wheat, barley, etc.), as well as bamboo. Their flowers are extremely small, and they are often wind-pollinated rather than by the help of insects.

    Grasses grow from the base of the leaf rather than the tip. They often have little bits of silica in them to persuade herbivores not to eat them. Recent discoveries in both Laurasia and Gondwana: the latter were found in titanosaur sauropod coprolites! So at leastsome Cretaceous dinosaurs were grass eaters. However, grasses seem to have been relatively rare in the Mesozoic, and did not form grasslands until much later. Ground cover in the later Mesozoic was a mixture of ferns and herbaceous angiosperms. So as far as we know, no dinosaur other than birds ever wandered in prairies or savannahs: these appear much later, long after the end of the Mesozoic.

    A relevant video:

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    Last modified: 11 June 2019

    Reconstruction of Jurassic euharamiyidan mammals (and a gliding eutriconodont) by Zhao Chuang