The Saurischians

Before we start talking about the saurischians, a brief introduction to dinosaurs as a group is necessary. Dinosaurs are a very diverse group of animals. They are grouped into two main groups: the Saurischia ('lizard-hip') and the Ornithischia ('bird-hip'). The saurischian hip has a pubis bone that is directed forwards, and the ischium towards the tail, while ornithischian hips have the pubis directed backwards, parallel with the ischium.  

Saurischian hip (from Wikimedia Commons user
 Muriel Gottrop)

Ornithischian hip (from Wikimedia Commons user
Muriel Gottrop)

The saurischians include some of the earliest dinosaurs like Eoraptor, and Herrerasaurus, as well as the sauropodomorphs and theropods. The early forms were likely carnivorous, and are found in Ischigualasto, Argentina dating back to the Middle Triassic, about 231 million years ago. Sauropodomorphs are the large long-necked dinosaurs and their relatives. Not all sauropodomorphs were the large animals often talked about, many of them were smaller forms like Saturnalia, Thecodontosaurus, and possibly even Eoraptor. 

Pantydraco, a sauropodomorph from the British Isles. Image from Wikimedia Commons (user ArthurWeasley)

Of course, the sauropodomorphs also include the sauropods, the largest animals to have ever lived on land. These include animals like Apatosaurus (check out our video on 'Whatever happened to Brontosaurus?'), Diplodocus, and the largest known from significant remains, Argentinosaurus. There are some unusual sauropods as well, like Amargasaurus, which had several large spines on its neck and back, and Saltasaurus, an armoured sauropod with armour like the ornithischian ankylosaurs. 

Saltasaurus, an armoured sauropod from the Late Cretaceous

The second group of saurischians is the theropods. The theropods include almost all truly carnivorous dinosaurs, as well as some herbivorous and insectivorous groups. Early theropods include Coelophysis, which is known from a large bone bed at Ghost Ranch, New Mexico, and Dilophosaurus, also from the U.S. The largest land carnivores ever were also theropods, like Tyrannosaurus rex, Spinosaurus, and Giganotosaurus. A large group of theropods is the dromaeosaurids, which are characterised by their large sickle-shaped claw on their second toe like Velociraptor, and Deinonychus. Like all dinosaur groups, theropods had their share of bizarre ones. There's Therizinosaurus, a herbivore which has metre-long claws used possibly to claw at tree bark, the ornithomimids, or bird-mimic dinosaurs, and the oviraptorosaurs. They were originally thought to be egg-thiefs, because of a fossil which was thought to be an Oviraptor stealing from a Protoceratops nest. However, it was later discovered that it was actually protecting a nest of its own eggs. 

Nothronychus, a therizinosaur. Image from Wikimedia Commons user DinoGuy2.

The final group of theropods is the birds. Birds evolved from the deinonychosaurs, a group that includes the dromaeosaurids and troodontids. Derived deinonychosaurs can be very similar to early birds, and can sometimes be hard to distinguish the two. Early birds include Archaeopteryx and Jeholornis, both from the Jurassic period. 

The Berlin specimen of Archaeopteryx. Image from Wikimedia Commons user Vesta.

Saurischians at Jurassic Forest:

Albertosaurus

Apatosaurus

Coelophysis

Ornitholestes

Troodon

Tyrannosaurus

Spinosaurus

Utahraptor 

References:

http://palaeo.gly.bris.ac.uk/communication/boulton/evolution.html

What is a dinosaur?

People often mistake the word 'dinosaur' as being a general term for any big, extinct reptile. Earlier this week, we had a great talk from Victoria Arbour, a PhD student from the U of A on pterosaurs. She started out by defining exactly what a dinosaur is, and explaining why a pterosaur is not a dinosaur, and I thought that was a great idea, so here we go. 


The first important thing to understand is the structure of the skull. Amniotes, that is animals that have an amniotic egg (reptiles, mammals, and birds) are divided into three main groups with respect to the 'temporal fenestra', or simply 'holes', in the back of their skull. If you feel above your cheekbone, in your temple, you'll feel that it's not hard and bony, but soft and full of muscle. This is what remains of one of those holes. The first group is the anapsids. These animals have no holes in the back of their skull, and consist mainly of turtles. The second group is the synapsids, which have a single hole in the back of the skull, just like us. This includes mammals, and the extinct reptiles closely related to mammals, like Dimetrodon. That's right, Dimetrodon is not a dinosaur, but more closely related to us and other mammals than a dinosaur! The third group is the diapsids, and like it's name suggests, they have two holes in the back, one on top and one below. All remaining reptiles are diapsids, including dinosaurs, pterosaurs, and birds (which are the direct descendants of dinosaurs, and therefore technically dinosaurs). There is a fourth group you will sometimes hear called the euryapsids, which have only one hole up top (in a different position than the one synapsid hole). These were most likely diapsids that lost the bottom hole over time, and include animals like ichthyosaurs. 

The front hole (on the left) is the nostril, while the large middle hole is for the eye. The holes at the back are the temporal fenestrae. A. Anapsid skull with no holes, B. Synapsid skull with 1 hole, C. Diapsid skull with 2 holes. From Fossil Wiki

So, dinosaurs are diapsids: they have two holes in the back of their head. But so do lots of other reptiles, including pterosaurs. So what makes a dinosaur a dinosaur? Unfortunately, that's a bit of a hard question to answer. First of all, it's important to know that dinosaurs are part of a group called the archosaurs, which includes crocodiles, pterosaurs, and birds. This group is united by a character called the ant-orbital fenestra, which is a small hole in front of the eye (not the nostril), not found in any other group. They are also part of a group called the Ornithodira, which includes just pterosaurs and dinosaurs. Ornithodirans adopted an upright stance, with their legs directly under their body rather than splayed out to the side as seen in modern reptiles. Although pterosaurs do this as well, they have several other characters that separate them from dinos. Now: dinosaurs. Dinosaurs share a number of features that are unique to them. For example, they have a set of jaw muscles that attaches to the top of their skull, not seen in any other group. They also have well developed processes or crests on their humerus (upper arm bone), hip, knee, and ankle. 

Reptilian posture: Top shows a typical reptile with the legs splayed to the side,  while the bottom shows the legs directly under the body, as seen in dinosaurs. Image from Freethought Forum

There are many other characters that dinosaurs share, which are all highly technical and primarily are features of the hind limbs. These features are not found in pterosaurs, which is one of the reasons why pterosaurs, the flying reptiles contemporaneous to the dinosaurs, are not dinosaurs. Pterosaurs also have several features that are not found in the dinosaurs, including an elongated fourth finger that forms their wing, and a new bone called the pteroid. Dinosaurs were (and are still today when including birds) an amazingly diverse group of reptiles. There were huge sauropods that include the largest animals to ever walk on land, and small theropods like Compsognathus, that were close to chicken-size.  Dinosaurs are divided into two main groups: the saurischians (or 'lizard-hipped'), and ornithischians ('bird-hipped'). Saurischians include all of the sauropodomorphs (all large long-necked dinosaurs and their relatives), and the theropods (the carnivores, and other strange creatures), while ornithischians are an almost purely herbivorous group including the hadrosaurs ('duck-billed' dinosaurs), ceratopsians ('horned'), the armoured ankylosaurs and stegosaurs, and many more. Next week, I'll talk a bit about the diversity of dinosaurs, and introduce all the different groups. For now, here's an example of a cladogram, which is kind of like a family tree, for the dinosaurs. 

Cladogram showing the relationships and diversity of dinosaurs. From UCMP Berkeley

References:
National Museum of Natural History

The Mesozoic

When people talk about the Mesozoic, what are they talking about anyways?
The Mesozoic literally means 'middle animals', because it is the middle Era in which animals thrived on Earth. It is from 251-65.5 million years (Ma) ago and includes the Triassic (251-199.6 Ma), Jurassic (199.6-145.5 Ma) and Cretaceous (145.5-65.5 Ma) Periods. During the Mesozoic, both the landscape and life on Earth changed drastically. It saw the breakup of Pangaea, the large supercontinent that had existed previously, and the introduction of many new plants and animals. By the end of the Cretaceous, the continents had started to move to the relative positions seen today, and the major land masses were present.  

Sorry about the quality: the captions from top left are Permian, 225 million years ago, Triassic, 200 Ma ago, Jurassic, 150 Ma ago, Cretaceous, 65 Ma again, and Present Day
Image from http://pubs.usgs.gov/gip/dynamic/historical.html

At the end of the Permian/beginning of the Mesozoic, there was a large mass extinction, the biggest mass extinction ever. This is known as the Permo-Triassic extinction, and estimates suggest that at least 90% of marine species and 70% of terrestrial species went extinct. This allowed for the large diversification of new forms in the Mesozoic. 

The Mesozoic is commonly known as "the age of the dinosaurs", or in terms of plants, "the age of the cycads". Dinosaurs first appeared in the Triassic, and survived until the end of the Cretaceous, living for over 150 million years. Pterosaurs, which are often mistakenly called dinosaurs, lived during the same time as the dinosaurs, also going extinct 65.5 Ma ago. Another major group that evolved during the Mesozoic was birds. Birds evolved from small theropod dinosaurs, and are therefore dinosaurs (biologically speaking). This is why you will sometimes see the distinction between avian (birds) and non-avian dinosaurs. The first birds include animals like Archaeopteryx, a beautiful fossil that shows features of both dinosaurs and birds, and may have been able to fly. They appear during the Jurassic Period. Mammals also evolved, with the first ones appearing at the end of the Triassic, 205 Ma. Mammals were relatively minor aspects of the Mesozoic fauna, and only became very diverse and common after the non-avian dinosaurs went extinct at the end of the Cretaceous. 

A fossil of Archaeopteryx, one of the first birds from the Solnhofen limestone in Germany.
Picture from Wikimedia Commons

Plants changed substantially during the Mesozoic as well. For most of the Mesozoic, only gymnosperms existed. Gymnosperms are the 'naked seed' plants, because of the unenclosed nature of their unfertilised seeds, vs. angiosperms (aka flowering plants) that have their ovaries encased within an ovule. Gymnosperms in the Mesozoic include ferns, cycads, ginkgophytes and other strange plants no longer present today, and were common during the Triassic. Conifers, a very common group of modern gymnosperms, started to appear in the Triassic, with modern groups in the Cretaceous. Angiosperms didn't evolve until the Early Cretaceous, with much diversification during the Late Cretaceous. This means that most herbivorous dinosaurs weren't eating leaves like what we think of today from big deciduous trees, but rather things like ferns (including some giant ferns), ginkgos, cycads, and conifers. Check out our short educational video on 'Plants of the Mesozoic Era' for more information.

Typical Mesozoic landscape from Germany from Wikimedia Commons

The end of the Mesozoic is marked by another large mass extinction, in which all non-avian (non bird) dinosaurs, pterosaurs, and many other animals went extinct. Two main debates about the cause of this extinction exist and consist of a large meteor impact that hit on the Yucutan Peninsula in Mexico (the Chicxulub crater), and hugh volcanic eruptions that occurred in Indian (the Deccan Traps). Although most scientists believe it was caused by the meteor impact, there are a number of people that believe the volcanic eruptions were the main cause, or at least somewhat to blame. Take a look at our 'K-T Extinction Event' video to learn more.

References:

http://www.ucmp.berkeley.edu/mesozoic/mesozoic.php

http://en.wikipedia.org/wiki/Mesozoic -> Of course use caution when using Wikipedia as a source, as anyone can change it. That being said, this page seems to be quite accurate, and provides lots of information I didn't include! 

Mesozoic Mondays Introduction

Welcome to Mesozoic Mondays!

Just a quick introduction about what we're planning on doing here. Mesozoic Mondays is a blog set up for Jurassic Forest, an educational animatronic dinosaur park near Edmonton, Canada. That being said, it is of course open to everyone! The more the merrier!

We're hoping to use Mesozoic Mondays as another way to teach people about creatures from the past and present, but of course, mainly from the Mesozoic. Each Monday, we will post about a creature, complete with pictures and sometimes links to some scientific websites where you can learn more. Throughout the week, we will answer any questions that are posted, and we encourage people to ask any questions they have. We also encourage suggestions or requests for what creatures to talk about.

Mesozoic Mondays is run primarily by a graduate student in palaeontology. She studies pterosaurs, the flying reptiles that lived along side dinosaurs, but has background in many aspects of vertebrate palaeontology, including dinosaurs.

We hope you enjoy!