Introduction to the second half of the semester:

In the first half of the semester, we studied the earliest life in the Archean Eon through the extinction at the Permian-Triassic boundary. The extinction at the Permian-Triassic was the greatest extinction the earth has experienced, and life changed in many fundamental ways after this boundary. Thus, in the second half of the semester we started at the Permian-Triassic and studied the Mesozoic and Cenozoic Eras. Although these two Eras span only 250 million years, by contrast with nearly 4 billion years in the first half, the latter 250 million years contains much of the exciting developments in vertebrates such as dinosaurs and large mammals.

 

Mesozoic life

 

Record of imporant events that occur in Mesozoic Era

(Triassic, Jurassic, and Cretaceous Periods)

 

Information to know

The basis for the information

Explanatory comments

Triassic Period:

 

 

World recovering from giant mass extinction at Permian-Triassic boundary, possibly caused by extraterrestrial impact

there is new evidence from just the past few months that the extinction was rapid, that there are chemicals at the boundary believed to be associated with impacts, and that erosion increased due to loss of plant community

we also talked in class about the “big burp” hypothesis in which stagnant sea-water overturned, causing high levels of CO2 to be released from the ocean; this idea and the impact may be related

 Great supercontinent of Pangea exists, with relatively dry climate

the rock record, including paleomagnatism, indicate that all the world’s continents were together

 

“Mammal-like reptiles” and “thecodonts” dominate land ecosystems

vertebrate records from around the world, including esp. South Africa

thecodonts are the ancestors of dinosaurs (and eventually birds) and mammal-like reptiles are ancestors of mammals

 Mammals and dinosaurs both evolve by the mid-Triassic.

 

vertebrate fossil record

mammals remained small during the next 150 million years, and diversified into larger animals only after the dinosaurs were extinct

Jurassic Period:

 

 

By the end of the Triassic dinosaurs have begun dominating land ecosystems and Pangea has begun splitting apart

dinosaur bones become more numerous and others less numerous

 

Pangea continues splitting apart

among other evidence, there are fossil rift valleys along the east coast of the U.S. where the continent ripped during splitting

these rift valleys contain most of the dinosaur remains in the Northeastern U.S., particularly including footprints

 Large dinosaurs, marine reptiles, and pterosaurs all flourishing

the U.S., Britain, and northern Europe (and many other parts of the world) contain excellent Jurassic sequences, both terrestrial and marine

these include the well-known Morrison Formation where Apatosaurus, Stegosaurus, and other well-known dinosaurs were unearthed

theropod, sauropod, and stegosaur dinosaurs are among major dinosaurs

the relative abundance of dinosaurs is estimated from the relative abudance of their bones, though of course there are some preservational biases

 

birds and early flowering plants appear

bird skeletons are known from Germany, China, and elsewhere; some of the first very tiny flowering plants are just appearing

 

Cretaceous Period

 

 

The groups of large reptiles continue and grow in size still further

 

 

Ceratopsian and hadrosaur dinosaurs become more common than previously, perhaps filling the role that herds of hoofed mammalian plant eaters (antelopes, for example) in the Cenozoic

some well known assemblages have been found in North America, Africa, and Argentina, and elsewhere, including dinosaur nests that suggest herding and caring for young

 

Flowering plants become common and start to dominate floras

floral fossil remains begin to change

 

Time of extremely high sea level and warmth, but cooling toward end of of period

a shallow seaway spread from the south up through North America, leaving behind sediments

 

Cretaceous-Tertiary boundary

 

 

Large extraterrestrial impact, probable cause for extinction

 

 

Numerous other reasons have been suggested for the cause of the extinction

some of the most problematic hypotheses explain the disappearance of only one group

 

 

 

 

Major types of Mesozoic reptiles

 

You will be responsible for recognizing the major groups of reptiles and a few other kinds of animals for which we looked at pictures and videos in class. For example, you should be able to recognize sauropods, theropods, hadrosaurs, ceratopsians, stegosaurs, pterosaurs, ichthyosaurs, plesiosaurs, primitive birds, and mastodons. Recommendation: To see images of these animals, open a new window in your browser software and go to www.google.com, then click on the tab that says “images.”  Then type in a name you wish to see pictures of, and it will give you “thumbnail” pictures that are available on the Internet. These images may be sufficient to remind you what, say, an ichthyosaur looks like.

 

 

The major groups of Mesozoic “reptiles”: Archosaurs (a group of diapsids) and synapsids

A tiny bit of anatomy: There are a number of groups of reptiles that are distinguished in part of the basis of skull anatomy. One particular distinction involves the number of cavities in the skull. Those with one hole in particular places behind the eye socket are known as synapsids (the line including mammals), and those with two holes are known as diapsids (the line including birds and dinosaurs). These two groups seem to have diverged early in the history of reptiles (the Carboniferous), and the Mesozoic and Cenozoic are the story of dominance by these two groups.

 

Synapsids

        

- Synapsids are a group including mammals and their ancestors. You are a synapsid. Synapsids were very successful in the Permian-Triassic, and included the sail-backed pelycosaurs.

- Mammal-like reptiles are now known as “protomammals” are not considered true reptiles, but an independent offshoot of early land-dwelling vertebrates.

- Many possess anatomy that contains characteristics of both early vertebrates and mammals. This group was relatively abundant and dominant in many ecosystems into the mid-Triassic, then declined substantially.

- Mammals themselves arise in the mid-Triassic and stay small until the early Cenozoic Era. We know about these animals primarily through teeth, which preserve relatively well compared to weak bones.

 

Dinosaurs

 

Dinosaurs: Dinosaurs are a particular group of reptiles with a common ancestor.

This is evident from unique aspects of the anatomy of all species of dinosaurs. There are many such traits, but they include (1), upright stance, with legs held directly beneath the body (unlike, for example, sprawling lizards) and (2), a particular kind of ankle (“metatarsal”) bones. They existed from the Midde Triassic to the end of the Cretaceous, and occupied many ecological niches on land.

 

Dinosaurs are NOT simply any large prehistoric vertebrates.

Animals that are not dinosaurs include pterosaurs, any marine reptiles, mastodons and mammoths, and pelycosaurs such as Dimetrodon. Why is this distinction important? Imagine trying to make sense of human history if we mistakenly ascribed people to incorrect families or cultures, maybe even in the wrong intervals of time.

 

Dinosaurs are divided into two large groups based on the shape of the hip (and other anatomical traits), known as the saurischians and ornithischians; the significance of noting the shape of the hip is simply that it indicates common ancestry.

 

         First dinosaurs: The very first dinosaurs, ancestors of dinosaurs to follow, do not fit cleanly into these two dinosaur groups, as they have anatomical characteristics intermediate between the two groups. The first dinosaurs were small bipedal (walking on two legs) meat-eaters, similar to the theropods.

 

         Saurischians: “Lizard-hipped” dinosaurs. These include the seemingly unlikely pairing of meat-eating theropods and long-necked behemoths. Early sauropods (prosauropods) do not look much like their descendants, as they are bipedal and relatively small.

 

Theropods: Meat eating bipedal dinosaurs. They include everything from Coelophysis, the little late Triassic dinosaur, to Tyrannosaurus, from the latest Cretaceous. There were many different groups of theropod dinosaurs, including for example the “raptors” of Jurassic Park fame.

 

         Birds are, believe it or not, now classified by many as a kind of theropod dinosaur. This is because the building consensus suggests that birds are descendents of these dinosaurs. (A more traditional view is that, even if this evolutionary relationship is correct, it makes more sense to elevate birds to a new group on par with reptiles, rather than classify them a subset of dinosaurs.) The evidence for this relationship involves dozens of aspects of anatomy – the skeletons of primitive birds are almost exactly like theropod dinosaurs – together with new discoveries of feather-like down on small theropod dinosaurs. Feathers thus apparently developed for thermoregulation (to help keep warm-blooded dinosaurs warm) and were only later developed for other purposes, such as flight.

 

         Sauropods: The sauropods are the “long-necked” giants, including familiar dinosaurs such as Apatosaurus (formerly Brontosaurus), Brachiosaurus, Diplodocus, and many others. These dinosaurs had small heads and teeth adapted for stripping branches of leaves rather than chewing leaves; digestion apparently occurred in a “gizzard” filled with stones (called gastroliths).

 

         Ornithischians: The “bird-hipped” dinosaurs are entirely plant-eaters, including all the armored dinosaurs and the bipedal plants eaters, the “ornithopods.” Don’t be confused – although described as “bird-hipped” in the late 1800’s. their hips are only superficially bird-like; it is the lizard-hipped dinosaurs that gave rise to birds.

                  Stegosaurs: plates running down the back.

                  Ankylosaurs: armor over the back.

                  Ceratopsians: frills over the neck and horns

                  Ornithopods: bipedal plant eaters

                           Hadrosaurs: bipedal plant eaters with duck-shaped faces

 

 

Other things to know about dinosaurs:

 

Dinosaurs were diverse. We tend to see a limited number of types of dinosaurs that are especially well-known and familiar to the public showing up in popular books, but we have discovered over 600 species and we continue to find new ones every year. The most important limiting factor to continuing discovery is simply the human and financial resources available for finding and studying their remains.

 

Dinosaurs were very successful. There is a media tradition of using dinosaurs as a symbol of something outdated and perhaps somehow slow, stupid, and poorly adapted. In fact, dinosaurs dominated land ecosystems around the world for 150 million years – they are by far the most dominating group of land animals of all time.

 

Dinosaur species did not all live simultaneously. Firstly, it is evident that there were trends in abundance of different groups of dinosaurs. Dinosaurs of late Triassic, Jurassic, and Cretaceous were very different. Furthermore, species evolve and go extinct on a regular basis, geologically speaking. There is a misconception that perhaps all dinosaur species (or even all dinosaur individuals!) that had ever existed until the Cretaceous-Tertiary extinction; this is not at all true.

 

 

Flying reptiles

         The primary group of flying reptiles was the “pterosaurs” (pronounced “TER-o-saurs). These reptiles share some anatomical traits with dinosaurs that put them both in the group archosaurs. Pterosaurs are not birds, however, just as bats are not birds. Some pterosaurs had wing spans far exceeding any bird every known, and most seem to have been lived on fish. Recent discoveries have revealed that at least some pterosaurs had a thin veneer hair- or down-like material over parts of their bodies. You needn’t memorize the names, but pterosaurs that we saw in the videos included Rhamphorhynchus, the crested pterosaur Tapejara, and the gigantic pterosaur Ornithocheirus.

 

 

Marine reptiles

         Marine reptiles belonged to several groups of unrelated animals. All the groups evolved from reptiles that had dwelled on land, and in that sense “returned” to the ocean.

 

Ichthyosaurs seem to have been the most fully developed to marine life, and evidence suggests that they even gave birth to live young in theier aquatic environment. Their bodies have qualities that give them the appearance of dolphins (paddles and snouts) and sharks (vertical tail). They presumably moved their tail like a shark to swim. They seem to have been especially successful in the Triassic, but to have declined in importance through the rest of the Age of Dinosaurs. The particular genus we saw in the video was Opthalmosaurus.

 

Plesiosaurs were perhaps the dominant marine reptiles. Some were the largest predators in Earth history, in sea or on land. Others were (somewhat) smaller, with remarkable snake-like necks, giving many the idea that “Nessie” the “Lochness Monster” of Scotland must be a surviving plesiosaur. Their unique bodies were stout with short tails, but large flippers from which they got their propulsion, unlike the vast majority of marine vertebrates that are powered by their tails. It is thought pleiosaurs laid eggs on land like a returning marine turtle, but moving awkwardly like a seal. The gigantic predatory plesiosaur in the video was Liopleurodon and the smaller long-necked plesiosaur Cryptoclidus.

 

Mosasaurs are a group of marine lizards that were more common toward the end of the Mesozoic. [We didn’t see any of these in the videos.] Mosasaurs had a tail that tapered near the end, and was presumably the main form of locomotion.

 

Mammals

         Mammals first arise about the same time as dinosaurs, about 220 million years ago in the middle Triassic. Mammals remain small and rodent-like through the Mesozoic; soon after dinosaurs disappear, mammals diversify to fill the niches of most land and marine reptiles. It seems likely that mammals were not able to diversify during the Mesozoic when dinosaurs already filled the niches of large land vertebrates. Note that the greatest diversity of mammals is still today in rodent-sized animals, a niche that dinosaurs never filled.

 

Mesozoic-Cenozoic invertebrate marine life

 

        

The history of the diversity of marine animals has been broadly divided into three categories, based in part on statistically analysis of the plot of marine animal families through time.

 

1. organisms that dominate the Cambrian Period, including trilobites, rise in diversity through the Cambrian, then gradually decline through the rest of the Paleozoic.

 

2. animals that dominate the rest of the Paleozoic Era, including many of the fossils common in New York State, for example:

         * brachiopods

         * corals

         * sea lilies (crinoids)

         * [trilobites were still common in certain times and places, but not a dominant part of the fauna like in Cambrian]

 

3. animals that dominate the Mesozoic and Cenozoic Era. Some of these are animals you are more familiar with, such as:

         * clams and snails (“rudist” clams were an important reef-builder in the Mesozoic)

         * ammonites (from the Mesozoic Era)

         * crabs and lobsters

         * (“bony”) fish and sharks

 

This is interesting, in part because the kind of marine faunas that look not unlike today’s began well back into the Mesozoic, whereas the terrestrial fauna looked very little like today’s. It is also interesting that diversity seems to be higher today that it ever has been; paleontologists wonder if this has something to do with the kind of dominant marine organisms, such as the increased predation that occurs in the Mesozoic.

 

The Cretaceous-Tertiary extinction

         This extinction is best known to most people as the time the dinosaurs died, but in fact it is also a time when many other organisms went extinct. They include: most or all vertebrates >100 pounds, including all nonavian dinosaurs, pterosaurs, all marine reptiles all rudist clams, all ammonites, most planktonic foraminifera and the kind of unicellular algae that make chalk, many corals (etc.)

 

There are many explanations for the extinction, most of which suffer from one of two problems:

1) the explanation considers only one of the group of organisms that went extinct, without taking into account all of the data

2) the proposal lacks available evidence to show that it’s true or false, meaning that no one can do research on it (it could be right, but we may never know)

 

Some of the ideas have included:

a) mammals ate all the dinosaur eggs

b) diseases spread by migrating dinosaurs killed off the dinosaurs

c) cooling was too hard on the cold-blooded dinosaurs

d) volcanoes erupted, causing the spread of clouds of ash that had an affect not unlike an extraterrestrial impact

e) an extraterrestrial impact

 

The extraterrestrial impact, as strange as it sounds, is favored. This is because many people did research on evidence left behind by impacts, and found that there is evidence of the impact all over the world. Also, a crator has been found near Mexico that is the right age. The impact would have thrown dust into the air, which would have traveled via the jet stream around the world, blocking out light and killing the base of the food chain.