Episode 21 – Dinosaurs, an Introduction

Listen to Episode 21 on PodBean, iTunes, or Stitcher!

There may be no group of prehistoric organisms more iconic, more widely-studied, or more often-requested by our listeners, than the subject of this episode … Dinosaurs.

In the news
More Dinosaur Colors!
An in-depth analysis of the coloration of Sinosauropteryx reveals hints of how it hid itself within its environment. [Link]
How Herbivores Change the World
This study compares plant remains from before and after the rise of herbivorous dinosaurs, revealing how big herbivores affect the spread of plant nutrients. [Link]
An Indian Ichthyosaur
The first Jurassic ichthyosaur from India is also one of the most complete! [Link]
Saber-tooth DNA

Genetic analysis of two famous saber-toothed cat genera reveals interesting new details about the cats’ evolutionary histories. [Link]

Meet the Dinosaurs

 

DinosaurTree_Holtz
A simple dinosaur family tree. This is a very diverse group! Image from Thomas Holtz; skeletal artwork by Scott Hartman.

Dinosaurs, like any group of animals, are defined by their shared features and ancestry. The shape of their hips and limbs, the position of their legs, the curve of their necks, and more – these are the traits that unite Dinosauria. They sit on the archosaur family tree, opposite crocodylomorphs and very close to pterosaurs.

Commercial enterprises – Hollywood, toy stores, etc. – have a hard time keeping Dinosauria straight. Animals that don’t share those specific features, and who don’t share dinosaurian ancestry, are NOT dinosaurs, including: crocodiles, pterosaurs, mosasaurs, plesiosaurs, ichthyosaurs, Dimetrodon, mammoths, and everything else that doesn’t fit on that tree in the image above.

These are the major groups of dinosaurs:

Thyreophora – The armored dinosaurs: the stegosaurs and ankylosaurs.

Stegosauria
Stegosaur armor comes in the form of plates and spikes on the back and tails, and sometimes shoulders. Images: Stegosaurus (left, Nobu Tamura); Kentrosaurus (right, LoKiLeCh) from Wikimedia.
Ankylosauria
Ankylosaurs are tanks, with armor knobs and spikes all over the back, neck, tail, and more. Images: Euoplocephalus (left, Wilfried Berns); Sauropelta (right, John Conway); from Wikimedia.

Marginocephalia – the ceratopsians and pachycephalosaurs.

Ceratopsia
Ceratopsians share skull ornamentation in the form of frills, spikes, horns, and beaks. Images: Psittacosaurus (top left, Vinther et al 2016); Triceratops (bottom left, Nobu Tamura;) A variety of ceratopsid skulls at NHMU (right, skinnylawyer); from Wikimedia.
Pachycephalosauria
The pachycephalosaurs have thick-boned skull domes ridged with spikes and knobs. Images: Pachycephalosaurus (left, Kabacchi); Stegoceras (top right, cherrysweetdeal; bottom right, FunkMonk); from Wikimedia.

Ornithopoda – diverse, specialized herbivores.

Ornithopoda
Ornithopods come in a huge variety: generally specialized herbivores capable of walking on two or four feet. Images: Tenontosaurus (top, Nobu Tamura); Parasaurolophus (left, Lisa Andres); Hypsilophodon (right, FunkMonk), from Wikimedia.

Sauropoda – long-necked, often enormous herbivores.

Sauropoda
With small heads atop long necks, sauropods are the largest land animals in history, and among the most successful dinosaurs. Images: Plateosaurus (top left, FunkMonk); Amargasaurus (top right, Nobu Tamura); Argentinosaurus (bottom, Sellers et al); from Wikimedia.

Theropoda – bipedal carnivores (mostly), including the only living dinosaur group: the birds.

Theropoda
Theropods are mostly carnivorous, bipedal dinosaurs, and the most diverse and successful group of dinosaurs. Images: Spinosaurus (top left, Durbed); Allosaurus (top right, The_Wookies); Deinonychus eating a small ornithopod (bottom left, Emily Willoughby); from Wikimedia. And an owl from Pixabay.

Early Dinosaurs

Dinosaurs appeared in the Triassic Period, with the oldest definite members of the group dating to around 230 million years ago. Even as they diverged into the lineages that would ultimately give rise to all the diversity you see above, early dinosaurs looked very similar. In fact, paleontologists have a very difficult time figuring out exactly how early dinosaurs are related to each other, and to later groups, which is why there’s been a lot of talk about Ornithoscelida recently!

EarlyDinosaurs
Early dinosaurs were bipedal omnivores, herbivores, and carnivores with the ancestral dinosaurian grapsing hands. Images: Eoraptor (top left, Nobu Tamura); Camposaurus (bottom left, Tomozosaurus); Herrerasaurus (right, Dallas Krentzel); from Wikimedia.

Dinosaurs belong to Archosauria, the group of reptiles that includes crocodylomorphs and relatives on one side, and dinosaurs and pterosaurs (and relatives) on the other. Dinosaurs themselves evolved among a diverse group of closely-related dinosauromorphs, as well as lots of early crocodylomorphs and therapsids (early relatives of mammals).

Dinosauromorphs
These are NOT dinosaurs, but the very closely-related dinosauromorphs that lived alongside them in the Triassic. Images: Silesaurus (top left, Dmitry Bodganov); Lagosuchus (bottom left, ArthurWeasley); Lagerpeton (right, Nobu Tamura); from Wikimedia.

After the end-Triassic extinction, most of the dinosaurs’ terrestrial competition had been wiped out, clearing the way for this one group of ruling reptiles to take over the dominant niches on land, and hold them for nearly the next 150 million years.

If you want a fantastic source of general dinosaur info, check our Dr. Tom Holtz’s very thorough and up-to-date course, Dinosaurs: A Natural History.

Also, for a great depiction of the diversity and scale of Mesozoic dinosaurs, check out this great video!

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