The History of Life on Earth: Mesozoic Vertebrates2018-01-23T11:55:09+00:00

The History of Life on Earth: Mesozoic Vertebrates

We will now look at the aftermath of the P-T Extinction on terrestrial vertebrate life, in other words look at what the vertebrates of the Mesozoic were like. The most famous representatives are, of course, the dinosaurs, so we will look at their origins and what vertebrates they were coexisting with.

A short look at the early mammals will follow, before examining the demise of the dinosaurs in the K-T Event.

As we saw in the last talk, the Late Permian saw the radiation of the diverse therapsids, but they were unlucky in that the P-T Event was underway and killed them off in their infancy.

They were replaced by the archosaurs. In the Carboniferous and the Permian, archosaurs were small, carnivorous diapsids and the disappearance of the therapsids allowed them to take over the same ecological roles. Triassic archosaurs quickly became large (e.g. Erythrosuchus). Just as with the therapsid radiation, archosaurs diversified into many types: most were carnivores; some reverted to a semi-aquatic lifestyle; some became herbivorous; one group came up with flying; another group became specialised bipedal, fast-moving insectivores.

The archosaurs are the group that includes the crocodiles and the dinosaurs (and thus birds).

At the end of the Triassic, another extinction event happened which killed off many of the archosaurian groups.

Three notable ones survived:

  • the Crurotarsi, of which one group survived, the crocodiles.

  • the small insectivores, of which only one group survived and gave rise to the dinosaurs.

  • the pterosaurs, who are the first flying vertebrates.

Additionally, several other groups radiated due to the end-Triassic extinction: the turtles, the earliest mammals and the modern amphibians, as well as lizards and snakes.

In the Triassic, the dominant archosaurian group was the Crurotarsi, of which there were four groups, see the purple box above. Of these, only the crocodylomorphs survived the Triassic extinction.

As the name suggests, these are the crocodile ancestors – however, these are not what we refer to when we call crocodiles “living fossils”

[sidenote: “living fossil” is a ridiculous, non-scientific term, but it’s quite popular and the concept is easy to grasp…]. Modern crocodiles originated in the Late Cretaceous. These early crocodilians were more diverse in their lifestyles, ranging from marine to fully terrestrial (most of them) and all grades in between, unlike today’s amphibious crocodiles.

The other group of Triassic archosaurians is the Avemetatarsalia, also variably called the Ornithodira or Ornithosuchia. This is the taxon that lumps together the pterosaurs and dinosaurs.

The pterosaurs were the first flying vertebrates, as well as the largest ever. Their main flight adaptation is their elongated fourth finger which supported the flight membrane.

They all had a huge elongated head, but their bones were all pneumatic, i.e. hollow, to reduce weight. They were active, flapping fliers, not gliders, which may be surprising given the colossal sizes they could reach.

As a sidenote, the ones in Jurassic Park are Pteranodon.

Diagram source: Claessens, L. P. A. M, O’Connor, P. M. & Unwin, D. M. 2009Respiratory evolution facilitated the origin of pterosaur flight and aerial gigantismPloS One 4, e4497.

As another side note and to highlight some recent research, pterosaurs layed eggs. Their level of parental care was similar to today’s crocodiles, not the birds. Birds spend an immense amount of energy to produce their eggs, going on calcium binges to manage to build a fortified shell. Their eggs can in some cases make up more than 25% of the bird’s weight.

In pterosaurs though, the eggs were membranous, as can be seen in the right picture above (“ie”). Pterosaurs layed their eggs quickly, not bothering to form a big protective shell, because lugging around a giant baby would seriously compromise their flying and render them vulnerable and unable to hunt and survive.

Pictures source: Lü, J., Unwin, D. M., Deeming, D. C., Jin, X., Liu, Y. & Ji, Q. 2011An Egg-Adult Association, Gender, and Reproduction in PterosaursScience 331, 321-324.

And now we come to the popular attraction, the dinosaurs. On a historical note, the name dinosaur can traced back to Sir Richard Owen, who coined it in 1842 to accommodate these three animals: Iguanodon (top), Ankylosaurus (bottom left) and Megalosaurus (bottom right). Besides recognising numerous similarities in their anatomical structure, Owen also referred to their tetrapedalism (Megalosaurus was reconstructed as walking on four limbs back then – false).

The dinosaurs sensu stricto (not counting “dinosauromorphs”) are split into two groups, the Saurischia and the Ornithischia. They share a last common ancestor, and the Dinosauria are a monophyletic grouping (in the past, it was thought that they originated independently).

To tell them apart, one has to look at the pubis and the hip structure. Saurischians are “lizard-hipped” while ornithischians are “bird-hipped”.

The saurischians contain the gigantic herbivorous sauropods and the carnivorous theropods, while the ornithischians contain the armoured herbivores.

As a sidenote, birds originated from the theropods – the “bird-like hip” evolved independently in them.

We have come a long way since Owen’s seminal work on the dinosaurs and can now certainly place certain synapomorphies as diagnostic for the dinosaurs. These are all changes in the skeleton involved with locomotion and musculature.

The first is the elongate deltopectoral crest, allowing larger muscles to attach at the “shoulder”.

The acetabulum is where the hip articulates with the pubic bone. In mammals, they are practically integrated, but in the dinosaurs, there is a large space between them. This gave dinosaurs more articulation, allowing faster running.

fossa is a crater in the skull, indicative of muscle attachment. All dinosaurs have one in front of the supratemporal fenestra, which basically means that they had muscles for their jaw going from the top of the skull. This is the reason why dinosaurs had such an enormous bite force.

Another locomotion-related character is the asymmetric fourth trochanter.

By the late Triassic, the earliest true dinosaurs were here, and were already diversified, as seen in the depiction above.

We will now look at the more famous dinosaurian groups, starting with the ornithischians.

As I said, the ornithischians are the grouping with the armoured herbivores. We will look at the stegosaursankylosaurshadrosaurs and ceratopsians.

Diagram source: Sereno, P. C. 1997The Origin and Evolution of DinosaursAnnual Review of Earth and Planetary Sciences 25, 435-489.

Stegosaurs are immediately recognisable from the plates on their backs. These are not extensions of the vertebrae; they’re not even made of bone. They are derived from the scales that make up the skin. Therefore, we can hypothesise that they were also filled with blood vessels and innervated, and a role in thermoregulation becomes likely.

Another proposed role is for mate attraction.

One other thing to notice is the tiny head. There isn’t much room for brain there, and it is in fact proposed that most dinosaurs (many of which had similarly small heads) were “stupid”. This is not a certain fact, for the most part because there is no proper non-anthropocentric definition of intelligence, and also because we don’t have any dinosaur brain casts. Available evidence of dinosaurian behaviour contradicts the “stupidity” scenario, as many exhibit parental care and collective behaviour, especially on the saurischian side of the spectrum.

The ankylosaurs are another group of ornithischians, famous for their heavy armour plating. These were obviously defensive.

The tail also had an enormous club on it which it used to whack predators in the face. This is not imagination, as we have Tyrannosaurus skulls with ankylosaur-tail-club-shaped damage on them.

The ceratopsians are well-known for their head shield and horns (which could be of variable number). They were herbivorous and characterised by their beak.

Another famous group are the hadrosaurs, recognisable by the crest on their heads. This crest was probably used for communication.

The other major group of dinosaurs are the saurischians, split into the sauropods and the theropods.

The sauropods are the largest land animals to have roamed the Earth, with their characteristic extra-long neck.

Gigantism is a hallmark of the sauropods. In the picture above, Argentinosaurus is the sauropod and weighed up to 90 tonnes – large enough for an elephant to fit under their tail. Looking at the lower diagram, we see that the sauropods occupy the largest size class on their own; the closest other animals are the whales (mammals; aquatic, so not as impressive) and the largest carnivorous theropods (T-Rex, titanosaurs).

Of course, having such large sizes opens up many very interesting questions. How did they pump blood around the whole body and especially up to the head? How did they move? Were they cold- or warm-blooded? What was their rate of growth?

Diagrams source: Sander, P. M., Christian, A., Class, M., Fechner, R., Gee, C. T., Griebeler, E.-M., Gunga, H.-M., Hummel, J., Mallison, H., Perry, S. F., Preuschoft, H., Rauhut, O. W. M., Remes, K., Tütken, T., Wings, O. & Witzel, U. 2010Biology of the sauropod dinosaurs: the evolution of gigantismBiological Reviews 86, 117-155.

At the heart of many of these questions lies the question of what sauropods ate to get their energy. They most likely fed on Equisetum, which can form very large grassland-like fields. The sauropod can stand on the edge and just graze on them by moving its neck. Alternatively, Araucaria, a modern one pictured, formed forests in the Mesozoic and is digestible enough to be considered good food. Not much is known about the Cheirolepidiaceae except their spores, but they are abundant at all sauropod localities, so they could well have served as food plants.

The 3-star-rated ones may have been fed on opportunistically or by select groups, as they are not found in all sauropod localities.

The cycads and their extinct relatives, the bennettitaleans, are poor to digest and grow very slowly. It is unlikely that they constituted a major source of food for the sauropods.

If sauropod palaeobiology is of interest to you, I highly recommend these two brand new books.

As for what ate the sauropods, those were their phylogenetic relatives, the theropods. They came in all sizes, but all were carnivorous, had small forelimbs and were bipedal.

These pictures are from a very recent (2010) exhibition at the Zoological Museum in Denmark, and you will notice that the dinosaurs have feathers.

While this may have been controversial at some point in the past, it is now accepted that the theropods developed feathers. These may have served several functions, including thermoregulation and communication/mate signalling. Some recently-developed techniques have even been able to reconstruct the colours of the feathers, but this awaits application to all theropods.

Of course, by far the most famous theropod is T-Rex.

And theropods are still alive today. Above is a picture of the Berlin Archaeopteryx, the famous “missing link” between dinosaurs and birds, of which there are nine specimens, all from the same locality in Solnhofen, Germany, a place that was once a lagoon. Other cool fossils from there include insects.

We will examine in detail how exactly we know that birds descended from dinosaurs in the next talk. Suffice it to say for now that the dinosaurs did not go extinct at the end of the Cretaceous and live on as birds.

The other great group of amniotes is the mammals. They first evolved from the cynodonts in the Late Triassic. The transition from basal synapsid to mammal involves a shift of the jaw joint into the middle ear.

In reptiles, the articular bone, only one of 6 bones of the lower jaw, is connected to the quadrate in the skull. In mammals, the lower jaw consists of one bone, the dentary, which is connected to the squamosal bone.

The transition involved the reduction of the articular-quadrate joint (seen in the cynodonts) ad a move up into the middle ear – the remnants of this reduction are the three ear bones, the hammer, anvil and stirrup, now coopted for use in hearing. Reptiles only have the stirrup.

If there is one way to annoy a vertebrate palaeontologist, it is to tell him that early mammals were all small, rat-like organisms. This was the prevalent view for a long time, but recent advances and discoveries have shown that while most early mammals were small, they were incredibly diverse and filled many ecological guilds, even the specialised ones.

Diagram source: Luo, Z.-X. 2007Transformation and diversification in early mammal evolutionNature 450, 1011-1019.

As the picture above shows, Mesozoic mammals were more diverse than they are today: only three branches remain (the placentalsmarsupials and monotremes), with the others all having died out at the border between the Cretaceous and Tertiary.

This is, of course, the same event in which most of the dinosaurs also died out: the K-T event.

This is Stevns Klint (Steven’s Cliff) in Denmark.

It’s made up of deep-water chalks, with a couple of flint-rich horizons.

And if you explore further along the cliff (actually, go up a set of nicely-marked stairs), you will come across this tiny black layer. This is the Fish Clay, named after the abundance of fish teeth and scales found in it.

Its age is quite precise at the boundary between the Cretaceous and Tertiary, 65.5 Ma. It is well-known that after that age, the dinosaurs went extinct (except for birds).

If you were to take a sample of this clay and analyse its chemical composition, you would notice that it contains an abnormally large amount of iridium, a chemical element that can only come from outer space, for example an asteroid impact.

When this iridium anomaly was first discovered by the Alvarez father-son team in the late 1980s, they very controversially suggested that the extinction of the dinosaurs was caused by a giant asteroid impact that caused a typical “nuclear winter” scenario.

The controversy this first raised is now gone, and the reality of an impact Is undoubted: we know there is an impact crater off the Yucatán Peninsula, Mexico – the Chicxulub Crater. And it is also dated at 65.5 Ma.

However, to chalk up this extinction event as due to a single asteroid is not entirely correct. While the impact may have started off the whole thing, there was a P-T-like event also happening at the time.

If you drive along Central India along the Deccan Plateau, you will notice landforms like this, and that the ground is basically all basalt, i.e. solidified lava. This lava is also of end-Cretaceous age, and it is thought that this extensive volcanism exacerbated the conditions of the K-T, if they were not the root cause of the extinction.

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