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Mass Extinction in the Beginning of the Age of Dinosaurs

Early Jurassic World

Map of the Early Jurassic

Pangea was still mostly intact, although the rifting process was proceeding so that by 180 million years ago, the sea was working its way into the rift valleys formed between North America and Africa.

The four assemblages we will look at in this lecture and the next lecture are: 1), the Newark Supergroup, 2) the Glen Canyon Group; 3) the Stormberg Group; and 5) the Lias of England. Also shown is 4, the Lufeng Formation of China.

Triassic Provinciality

The distribution of various tetrapod groups during the Late Triassic is not at all uniform. This is despite the continuity of land in Pangea. We know that climate zonation was very pronounced during the Late Triassic, with a narrow humid equatorial zone and broad arid zones at roughly 30 degrees, north and south latitude. Humid temperate climates extended from around 50 degrees to the poles. During the Late Triassic, prosauropods had a distribution that corresponded roughly to the temperate zones of both hemispheres (although possibly not extending to the poles). A large range of large "amphibians" shared the prosauropods' ranges. Phytosaurs were limited to the northern hemisphere and the north-coastal portions of the southern hemisphere (i.e. India). Rauisuchians, crocodylomorphs, and aetosaurs seem to have been cosmopolitan, with some genera (if not species) seemingly global. In the equatorial region, the diversity of large "amphibians" was very, very limited, but ceratosaurian, and possibly ornithischian, dinosaurs were relatively common.

In plants the Dicroidium-dominated flora remained established in the southern hemisphere, while in the northern hemisphere the Late Triassic saw a rise in a group of seed-bearing conifers called the Cheirolepidiaceae or "cheiroleps". These plants ranged from in size from shrubs to large trees. They tended to have fleshy leaves and may well have been deciduous. Their rise coincides with the rise to prominence of the sauropodomorphs: first the prosauropods of the Triassic and Early Jurassic and then the giant sauropods, later in the Jurassic and Cretaceous. As the cheiroleps became less common though the Cretaceous, so did sauropods. However, the correspondence of the geographic distributions of plant and animals is far from perfect. During the Late Triassic, in India, the Late Triassic faunas resemble those of Europe, while the contemporary flora is of strikingly Gondwanan affinities.

Jurassic assemblages

By the Earliest Jurassic, the biological world had changed significantly. Most notable was a mass extinction event that wiped out a large proportion of both continental and marine animal diversity. First we will look Newark Supergroup tetrapod assemblages of Early Jurassic age, and then concentrate on the mass extinction event itself.
 

The Newark Supergroup

The rift system that formed during the Triassic as part of the huge Atlantic rift zone continued to subside and accumulate sediment during the Early Jurassic. Although depauperate in good skeletal remains compared to the other deposits we will consider here, the rifts of eastern North America, containing the Newark Supergroup, are famous for the many tens of thousands of dinosaur tracks that have been found over the last 160 years. 

The most famous of the early collectors and researchers to work on these footprints was Edward Hitchcock, who ultimately became President of Amherst College. From 1836 to 1865 he collected over 20,000 footprints of crocodilians and ceratosaurian and ornithischian dinosaurs. The bulk of these came from the Connecticut Valley rift basin of Connecticut and Massachusetts. Since that time an amazing number of dinosaur tracks have been recovered in Jurassic strata of the Newark Supergroup, from Virginia to Nova Scotia.

The very first track he described (1836) was Eubrontes giganteus, which was also the first dinosaur track to be described from anywhere.

Edward Hitchcock

A particularly fine display of Eubrontes giganteus tracks is at Dinosaur State Park in Rocky Hill, CT. There hundreds of fine tracks are exhibited in situ.

Trackways at Dinosaur State Park

Eubrontes giganteus trackways at Dinosaur State Park.

Theropod dinosaurs walked along the shores of drying lakes in the Hartford basin rift valley. A slight increase in lake level allowed wave action to cover the tracks with sand without eroding the tracks themselves.

An example of a natural cast in sandstone of one of these tracks is shown on the right. Note quarter for scale.

Dilophosaurus

Eubrontes

As part of the exhibit, they have a fine reconstruction of Dilophosaurus (on left). This model presently stands in front of an excellent mural depicting the Early Jurassic scene in which the tracks were formed.

Tiny Grallator track
In addition to large theropod tracks, the Newark Supergroup has produced many smaller theropod tracks belonging to various track species of Grallator

On the left is one of the very smallest ever found. It is a natural cast from Nova Scotia. (Photograph courtesy of Donald Baird.)

There are two other major kinds of dinosaur tracks found in the Newark Supergroup. The most abundant of these is a small track that was certainly made by ornithischian dinosaurs similar to Lesothosaurus or Scutellosaurus.

Anomoepus is the name given to the type of tracks seen at the right. When first found these were the source of much confusion for Hitchcock. On the right is one of the first dinosaur tracks to be recognized as not a bird. Hitchcock thought it was a kangaroo-like marsupial, but now we know it belonged to a small primitive ornithischian dinosaur. Note the hands and the heel marks made by the sitting animal. The specimen shown here is from the Deerfield basin in Gill, Massachusetts.
Anomoepus

Otozoum

A much larger track is Otozoum moodii (above), first described by Edward Hitchcock in 1847. It was probably made by a large prosauropod dinosaur, most likely one of the types found in the Newark Supergroup. In the above trackway, which is a natural cast, "b" is the best track and indicates a dinosaur walking plantigrade.

Anchisaurus
Skeletal remains have also been found in the Newark Supergroup Jurassic as well. The first examples found were described before the concept of the Dinosauria had crystallized, and in fact constituted the first dinosaur remains described from the New World.

These early skeletal remains were described by Edward Hitchcock and later by O. C. Marsh and given the names Anchisaurus (see left) and Ammosaurus.

These were prosauropod dinosaurs, similar to Plateosaurus. Partial skeletons have been found in Connecticut, Massachusetts, and Nova Scotia.

Skeletal reconstruction on left by Marsh.

Much more recently abundant skeletal remains have been found in Nova Scotia. In addition to Ammosaurus and perhaps Anchisaurus are beautifully preserved material of sphenodontids, along with trithelodont synapsids, crocodylomorphs, and scrappy remains of ornithischians and theropods.

On the right is a nearly complete skull of a sphenodontian. Sphenodonts are lizard relatives. Very common globally in the Mesozoic, only one genus survives today, the Tuatara of New Zealand.

Surprizingly, the sphenodontid from Nova Scotia is indistinguishable from a species found in the Stormberg of Southern Africa. In fact, faunas of the Early Jurassic show remarkable homogeneity, and this contrasts strongly with the Triassic situation.

Sphenodontian

Photo courtesy of Hans Sues. 

Mass Extinction at the end of the Triassic

There was an overall proliferation of forms through the Late Triassic with an overall increase in diversity, both in the oceans and on land. Many odd amniote groups, including giant rauisuchians, phytosaurs, and herbivorous aetosaurs, vied for dominance with the dinosaurs. However, at about 201 million years ago about 50% of all terrestrial vertebrate families were wiped out at the end of the Triassic, including the afore-mentioned non-ornithodiran groups. A similar mass extinction is seen in the marine record.

graph of extinctions from Sepkoski

Graph of extinction rate against time for shelly marine invertebrates.

The sudden drop in animal diversity corresponds to a dramatic change in the plants, with a diverse plant community giving way to a very low diversity community almost entirely dominated by the cheiroleps (at least in the tropics) in the earliest Jurassic.

An especially detailed record of the continental aspects of this mass extinction is preserved in the Newark Supergroup. In large part, this is because of the time control afforded by ubiquitous sedimentary cycles. These cycles record the rise and fall of lakes that periodically filled the rift valleys. The rise and fall of the lakes was, in turn, under the control of climate cycles governed by celestial mechanics. The most obvious of these cycles is one that has a period of about 20,000 years.

On the right is a photo of one of these cycles in the Hartford basin. The black layer is the deepest water unit and the gray layers on either side are shallow-water units. The red beds are very shallow sediments formed when the lake was mostly dry.

Lake level cycle
Fern spike layer

Corollina

A very thin layer (upper left) has been found in the Newark basin in Pennsylvania that has almost nothing but fern spores in it (right). Ferns are often the first colonizers after a catastrophe and the appearance of this so called"fern spike" in the fossil pollen and spore record appears to be an indication of a major ecological disaster.

Below the layer with the fern spike, pollen and spore assemblages are diverse. Above it, one pollen taxon dominates up to 99 percent of the microflora. That form is Corollina (or Classopollis) (lower left), and it was produced by the Cheirolepidiaceous conifers. 

Fern spike graph
 

This began a dominance that would last until about 120 million years ago, a span of over 80 million years.


Abundant reptile footprints in the same area show a massive change in taxonomic composition right at the same layer. At the same locality where the fern spike is observed, there are typical Triassic footprint forms within a few meters of the boundary.

On the left is a spectacular slab (natural cast) of dozens of trackways. Most obvious are tracks of medium to large sized tracks of Brachychirotherium, most likely made by a rauisuchian. Also are many three-toed, small- to medium-sized theropod tracks (Grallator and Anchisauripus). The larger of the dinosaurian tracks are the largest we see in the Triassic. Scale is 25 cm. Found by Mike Szajna and Brian Hartline. Photograph courtesy of Brian Hartline.

Footprint assemblages above the fern spike and correlative layers elsewhere completely lack the Triassic forms. It is not until these layers that we get the large theropod tracks of the Eubrontes type.

 

In Austria and Italy, what is probably the same layer as the fern spike layer in the Newark basin, has grains that have been identified as "shocked quartz". Shocked quartz is known only from known asteroid impact sites and nuclear bomb test sites. It is produced by very high, but short-lived, pressures. This suggests that the Triassic-Jurassic boundary was caused by the impact of a giant asteroid similar to the one that terminated the Cretaceous Period. But in this case, rather than wipe out the dinosaurs, the Triassic-Jurassic mass extinction may have opened the doors for the dinosaurs by wiping out their competition. The dinosaurs would have taken over by default!

Manicouagan Impact Structure from space (NASA)
A candidate for an impact event that could be the cause of the Triassic-Jurassic mass extinctions is the great Manicouagan impact structure of northern Quebec, Canada. This gigantic structure formed about 200 million years ago by the impact of a ~10 km diameter asteroid. The Manicouagan structure produced by this impact is easily visible from space and consists of a ring of shattered rock 70 km in diameter surrounding a disk of rock that was completely melted by the impact event. About 1 km of rock has been eroded from the site since the impact and its original size may have been about 100 km in diameter. The amount of energy released by this impact was probably about 10,000 times the combined energy that would be released by the simultaneous explosion of the entire nuclear arsenals of the United States and former Soviet Union!

Although a possible candidate for the cause of the end-Triassic mass extinction, the newest dates suggest that the Manicouagan impact structure could be about 12-15 million years older than the mass extinction itself, thus ruling it out as a causal agent. A different impact is still possible.

A huge eruption of lava flows occurred over about a 600,000 year time span near the Triassic-Jurassic boundary. It probably marks the very beginning of sea-floor spreading and formation of oceanic crust near Florida. This is one of the largest igneous events known. It is quite possible that CO2 emissions or sulfur aerosols associated with these lava flows may also have been associated with the Triassic-Jurassic mass extinctions. It is even possible that both an impact and volcanism could have been responsible. A similar pattern occurs at the Cretaceous-Tertiary boundary, which we shall discuss in later lectures.

REFERENCES

Fowell, S. J. 1993, Palynology of Triassic/Jurassic boundary sections from the Newark Supergroup of Eastern North America: Implications for catastrophic extinction scenarios [Ph. D. Thesis]: New York, New York, Columbia University, 133 p..


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