GEOL 104 Dinosaurs: A Natural History
Fall Semester 2015
In the Shadow of the Dinosaurs: Mesozoic Mammals and Mesozoic Plants
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
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:
- Covered with fur
- Sweat glands
- Mammary glands
- Parental care of young
On the other hand, some mammalian features are preservable:
- Only two sets of teeth: deciduous ("baby") and permanent
- Highly specialized teeth divided into incisors, canines, premolars, and molars
- Teeth highly distinctive, can recognize a species from only one or two cheek teeth (premolars and molars)
- Lower jaw comprised only of dentary; jaw joint is dentary-squamosal, not
- Respiration using diaphragm (so that dorsal vertebrae are divided into thoracic and lumbar sections)
- And more
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:
- Evidence for parental care in basal synapsids, as well as more derived therapsids
- Division of teeth into incisiors, canines, and cheek teeth in early therapsids
- Diaphragm breating in more advanced therapsids
- Predator-prey ratios of therapsid communities suggest elevated (or at least non-cold blooded) metabolisms
- No direct evidence of fur outside Mammaliformes, but a likely osteological correlate to whiskers and a mobile wet nose are present outside Mammaliformes. (Developmentally these are also linked to mammary glands, too!) More primitive versions of fur may go down much further in synapsid history, but we don't yet have fossils to constrain this.
Living mammals are divided into three clades:
- Monotremes: the last surviving egg-laying mammals. Rare and surviving only in Australasia
- Marsupials: reproduce by young born live but very poorly developed, which then suckle and grow within a pouch. Dominate Australasia, common in South America, present in early Cenozoic Europe and North America
- Placentals: stay in the womb until more developed, fed by a placenta. The dominant group in the Americas, Africa, and Eurasia.
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):
- Generally relatively stumpy legs and tails, even in modern forms
- Oldest from Middle Jurassic onward
- Oldest monotremes proper from Early Cretaceous (perhaps actually a platypus! it has the same electrosensory apparatus); survive today in Australasia as platypus and echidna
- Prototheres were never a dominant group, but were once moderately common in Gondwana (present in Asia, too)
- Retain ancestral sprawlling posture and egg-laying reproduction
- Range from tiny to
- Relatively common mammals from the Jurassic and Cretaceous in Laurasia and northern Africa
- Omnivores and carnivores: at least one has gut contents containing the remains of
- At least one lineage produced gliders
- In their time, were major parts of the larger mammal fauna: ecologically comparable to opposums and raccoons in the modern U.S.
- Wiped out at end of Cretaceous
- Mode of reproduction unknown
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
- Until recently known only from isolated jaws and teeth, which indicated a gnawing (rodent-like) diet
- As the name implies, known only from the southern continents
- New discovery of nearly complete skull of Vintana of the Late Cretaceous of Madagascar shows at least some gondwanatheres had relatively big brains, very large eyes, and well-developed olfaction. Also, Vintana is quite large (about beaver-sized), making it among the largest of Mesozoic mammals
- Some gondwanatheres survived in South America until the Miocene Epoch (a mere 18 million years ago!)
- At least some studies place Gondwanatheria WITHIN Multituberculata
- Oldest multituberculate fossils Middle Jurassic; survived into early Cenozoic (about 35 Ma) when they became extinct
- Mode of reproduction unknown
- Many were good climbers
- Extremely common in Laurasia
- Specialized molars and gnawing teeth: convergent with rodents, and a major radiation in this way of life before the true rodents evolved
- Recently discovered trace fossils shows that some of these gnawed on dinosaur bones!
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):
- Oldest fossils Early Cretaceous
- Survive today in opossums of the Americas and great diversity in Australasia (and in Cenozoic were even more diverse in South America)
- In marsupials, birth barely-formed young that develop in pouch: not certain yet when non-marsupial metatheres evolved that form of
- During Mesozoic, metatheres were very common mammals in both Gondwana and Laurasia
- Some very small but some were among the largest mammals of the Mesozoic (badger-sized)
- Recent phylogenetic analyses show that there are no definite members of Marsupalia during the Mesozoic, although they are present so early
in the Cenozoic that they probably had evolved before the end of the Cretaceous
Eutheria (placentals and our extinct relatives):
- The earliest Late Jurassic Juramaia and the Early Cretaceous Eomaia were once thought to be eutheres. However, new analyses place these two outside Theria. As a result, the oldest known eutherians presently are from the Late Cretaceous, but the clade must extend down into the Early Cretaceous.
- Survive today as most diverse group of mammals (including us!)
- Placental mammals reproduce by keeping young in womb until birth, fed by placenta: not certain how non-placental eutheres reproduced
- Mesozoic eutheres were small; many were herbivorous, omnivorous, and insectivorous
- True placentals are not yet known older than the end of the Cretaceous (in fact, the most recent comprehensive study shows that none of the Cretaceous eutheres definitely belong to Placentalia),
but it is quite possible that the major divergences had already happened before the end of the Age of Dinosaurs
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
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
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:
- Cycads, once widespread, but now more common in the tropical regions. They
look something like palms and something like pineapples, but are neither. The include small tree- and shrub-sized species. They were VERY common in the Mesozoic.
- Ginkgoes, trees that were widespread in the Mesozoic
and Cenozoic but now limited to a single species
- Conifers, highly diverse tree-and-shrub sized species including:
- Bennettitaleans, including
tree and shrub forms and the only fully extinct group listed in this page!
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.
- 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
- 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.
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.
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Last modified: 15 August 2016