GEOL 104 Dinosaurs: A Natural History
Fall Semester 2016
The Worlds of Dinosaurs: Dinosaur Paleoecology
We've looked at the dinosaurs from an evolutionary perspective. Now we'll take a look at them from an ecological and historical one.
Here are some important terms concerning paleoecology:
- Fauna: the assemblage of animals that occupy a particular region. (The equivalent for plants is "flora"). Faunas might be as broad as "the Australian fauna" or as specific as the fauna of a particular patch of park.
- Paleofauna: the assemblage of fossil animals that occupied a particular region during a particular interval of geologic time. (Paleofloras, for plants). Generally, the operational level for paleofaunas would be the fauna of a particular formation, or perhaps a particular member of a particular formation. However, we might refer to continental-scale faunas, too.
- Biogeography: the study of the distribution of organisms by region and the factors controlling that distribution
- Paleobiogeography: the study of the distribution of fossil organisms by region over time and the factors controlling that distribution
Example of Autecology: Migration in Camarasaurus
The isotopic composition of water varies from spot to spot due to temperature, evaporation, and other factors. In a study geochemist Henry Fricke and colleagues compared the isotopic composition of clay minerals and the teeth of the macronarian sauropod Camarasaurus from several sites in the Morrison Formation (Late Jurassic, western North America). While the clays only had the isotopic composition of the lowland environment (not surprisingly, since that's where they formed and that's where the teeth were found), but the Camarasaurus teeth show a broader spread of values, including upland signatures. Looking within individual teeth, it was found that some succession of layers show lowland values and then swtich to show upland values. This has been interpreted as indicating migration between uplands and lowlands
within the species.
Another Example of Autecology: Infection in Tyrannosaurus
Many specimens of Tyrannosaurus (and other tyrannosaurids) show lesions in the lower jaw. These were first considered to be
bitemarks from other tyrant dinosaurs, but closer examination reveals that these are not punction marks, but resorbed bone.
In fact, their closest analog in the modern world are the lesions produced by avian trichomonosis (infections in birds from the
protozoan Trichomonas gallinae). Modern birds of prey get this infection primarily from eating infected prey, and so it is quite likely that the tyrant infections were similarly picked up from infected food items.
Synecology of a Single Fauna: the Hell Creek (latest Cretaceous, western North America)
How do we determine the animals that make up a paleofauna? Rather simply: the animals whose fossils are found in a formation. But not all formations are equally well-explored, so while we have long faunal lists for some of these, and very few species known in others. Also, there are complicating factors:
In a recent study of the latest Cretaceous Hell Creek Formation of western North America, the distribution of different dinosaur species within different micro-habitats was examined. The researchers compared the relative occurrences of different species in sandstones (representing the rivers and near-river environments) and mudstones (which generally represented the floodplains further from the river). They found that the ceratopsians of the Hell Creek (the chasmosaurines Triceratops and Torosaurus and the leptoceratopsid Leptoceratops were more common in the upland environments; that the ornithopods (the hadrosaurine Anatosaurus and the parksosaurid Thescelosaurus) more common near the rivers; and that other dinosaurs did not show a statistically significant preference between the habitats (a possibility that ornithomimids were more common in the uplands, but not statistically significant). In the case of the apex predator of the environment, Tyrannosaurus, this lack of preference has strong statistical support: this makes sense, because meat is meat, so its food base would be found in both settings. In contrast, however, the particular plants favored by ceratopsids and ornithopods may have been linked to their favored micro-habitats.
- Formations are deposited over some period of time, sometimes longer than the duration of a species. So just because two species are found in the same formation doesn't mean that they were contemporaries: one could have gone extinct or evolved into something else prior to the second appearing.
- Formations only record the depositional part of the environment, not the erosional (or merely non-depositional) part. So animals that live in the lowest parts of the environment and/or closest to moderate-to-non-moving water will be preferentially preserved, while those in the higher lands will be less common parts of the fossil record, regardless of their relative abundance in the actual environment.
- Regardless of their micro-habitat preferences, not all species live at the same abundance in a community. Some are going to be more common than others. The more common animals will have a better chance of becoming fossils; the rarer the species, the less likely it will become a fossil or that its fossils will be found.
(Neither the oviraptorosaur Anzu nor the dromaeosaurids Acheroraptor and Dakotaraptor were known when this study was done.)
Few other formations have been subjected to this level of analysis, but hopefully this will be done in the near future.
Inter-faunal Comparisons: Late Jurassic Sauropod-Dominated Communities
One can compare faunas to faunas just as we can compare micro-habitats to micro-habitats. In a recent study, different sauropod-dominated communities of the Late Jurassic from around the world were compared. These sites were from different formations on different continents representing different habitats of the Late Jurassic.
The researchers found that arid environments were dominated by high-browsing macronarians; humid environments by lower browsing forms (including diplodocoids, stegosaurs, and ornithopods); and no surprisingly semi-arid environments had a mixture of these. Once again, the carnivores were largely independent of the different types of environment. (This isn't really that much a surprise: consider that leopards, tigers, mountain lions, and (in the historic past) jaguars and lions range from tropical jungles through arid plains to cold forests, while the herbivores they feed on are often restricted to just one habitat.)
History, Extinction, and Paleobiogeography: the Many Worlds of the Late Cretaceous
For much of dinosaur history, the same dinosaur groups were found in nearly every continent: that is, the faunas were cosmopolitan. This makes sense for the early part of dinosaur history, when the landmasses were still joined as Pangaea. But even with the birth of the Atlantic at the Triassic-Jurassic boundary, the spreading of the Atlantic during the Middle and Late Jurassic, and the break up of the southern continents (Gondwana: today's South America, Africa, Antarctica, Australia, India, and Madagascar) and the rise of higher sea levels during the early Early Cretaceous and late Early and early Late Cretaceous, there is still a generally shared pattern of the types of dinosaurs found worldwide. Different groups come and go, but they do so around the world:
- Late Triassic: Pseudosuchians and therapsids still very abundant (and include dominant predators and herbivores); among dinosaurs, basal theropods, basal neotheropods present among carnivores; basal ornithischians (incl. heterodontosaurids) present at least in Gondwana, but very very rare); only common dinosaurs are basal sauropodomorphs, core prosauropods, and near-sauropods (and at least in Gondwana, a few basal sauropods): nothing else was high-browsing, so this niche was unexploited by anything else.
- Early Jurassic: After the Tr/J extinction, dinosaurian competitors lost. Basal neotheropods ("dilophosaur-grade") apex predators; core prosauropods, near-sauropods, and rare basal sauropods dominate as herbivores. Ornithischians begin to diversify, especially basal thyreophorans.
- Middle and Late Jurassic: the Golden Age of dinosaurs, huge diversifications. Basal neotheropods disappear, replaced by apex predators in the form of megalosauroids (esp. megalosaurids) and carnosaurs. Early ceratosaurs and coelurosaurs (including basal avialians) present as minor parts of the predatory fauna. Non-eusauropod sauropodomorphs disappear. Tremendous diversification of eusauropods (esp. neosauropods). Stegosaurs fairly common, ankylosaurs present but rare. Ornithopods and basal ceratopsians present as small-to-medium sized herbivores.
- Early part of Early Cretaceous: Spinosaurids and carcharodontosaurs become the apex predaors, displacing more primitive megalosauroids and carnosaurs (respectively). Ceratosaurs and (esp.) coelurosaurs continue their diversification: great increase in the number of non-carnivorous coelurosaurs. Iguanodontids and ankylosaurs become very common, rivaling or (in some cases) surpassing sauropods as dominant herbivores. Stegosaurs vanish over time. Ceratopsians diversifying in Asia.
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 (Europe, Asia, North America) and Gondwanan dinosaurs of this interval became even more distinct. While some groups were shared (ankylosaurs; iguanodontians; titanosaurs; spinosaurids; carcharodontosaurids; megaraptorans), others were more provincial. The odd regions out seems to be parts of Laurasia. In North America, ceratosaurs and spinosaurids have yet to be confirmed. In Asia (in particular) the ankylosaurines, 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 Hadrosauria.
Also at the end of this time western North America's fauna receives a number of immigrants from Asia: ankylosaurines, advanced neoceratopsians, advanced tyrannosauroids, and ornithomimids.
The situation changes, however, with the later part of the Late Cretaceous. Several changes occur during this time:
- Large-bodied carcharodontosaurids and spinosaurids go extinct (a smallish spinosaurid is known in Asia after this, and the megaraptoran carnosaurs survive for some time, but big Carcharodontosaurus and Giganotosaurus disappear)
- Large-bodied abelisaurids and tyrannosaurids evolve from their smaller-bodied ancestors: the former are present in Gondwana and Europe; the latter in Asia and North America
- Large-bodied ceratopsians and big ankylosaurines evolve from smaller-bodied ancestors; dome-headed pachycephalosaurs appear
- In many parts of the world, the very largest titanosaurs disappear.
- Non-titanosaur sauropods (except maybe a stray rebbachisaurid lineage) died out.
Why these faunal changes occurred remains to be explained. It may be related to a general cooling of the world, and perhaps a loss of food at the base of the food chain (which supported the largest sauropods, which supported the giant carnivores). For whatever reason, the world left in this faunal change is rather distinct: it is provincial (that is, there are distinct faunas in different region).
There are several distinct biogeographic regions during the late Late Cretaceous (85-66 Ma):
Thus, the Late Cretaceous world was more like the modern one, with different groups of animals in different regions.
- Asiamerica: central and eastern Asia and Laramidia (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 ankylosaurines, 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 ankylosaurines
- Wetter environments (most of western North America, but also some parts of China, Mongolia, and Siberia) had larger dinosaurs. True ceratopsids are limited to these regions of Asia.
- Eastern North America (Appalachia): 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. What little is known of African dinosaurs from the later part of the Late Cretaceous is similar (abelisaurids and titanosaurs).
- 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 (of an endemic European struthiosaur radiation) 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: 15 August 2016