Thursday, 9 August 2012

Dinosaurs and other ancient creatures

I realise that this is covered on many other sites, but I thought I would do a post on some Dinosaur basics, seeing as many of my posts are Dino-centric. For the purposes of this article the word Dinosaur refers to Non-avian Dinosaurs, as opposed to Avian Dinosaurs which are also known by the much less kludgy term, birds.

The word Dinosaur was first coined by the famous British Anatomist Richard Owen, and is derived from the greek Deinos meaning 'terrible', or 'fearfully great' and sauros meaning lizard. You may remember that there is one particular Dinosaur that features 'deinos' in its name, Deinonychus, or 'terrible claw'. The word deinos also implies to be 'inconcievable' or 'unknowable'. When Owen coined the term Dinosaur he proposed its use in a report:

'The combination of such characters, some, as the sacral ones, altogether peculiar among Reptiles, others borrowed, as it were, from groups now distinct from each other, and all manifested by creatures far surpassing in size the largest of existing reptiles, will it is presumed, be deemed sufficient ground for establishing a distinct tribe or suborder of Saurian Reptiles for which I would propose the name of 'Dinosauria'.'

'The sacral ones', Owen refers to in this report are the fused sacral vertebrae that had been noted, by him, in specimens of Iguanodon and Megalosaurus. Marine lizards like the three I have described below, Ichthyosaurs, Mosasaurs and Plesiosaurs, flying lizards (the Pterosaurs) and crocodiles do not have a fused sacrum. Interestingly Mammals, such as ourselves, have a fused sacrum, but it is fused in a different way.

The Iguanodon sacrum referred to by Richard Owen (imaged sourced from wikipedia here

The other details of determining what is and isn't a Dinosaur can become complicated, but there are a few differentiations that are fairly straight forward. Dinosaurs are mostly land based, and all descend from a common Archosaur ancestor. They also have two distinctive pelvic types, Saurischian (lizard hipped) and Ornithischian (bird hipped). On a side note, oddly enough, it is the Saurischians that gave rise to modern birds!  They were either bipedal (like the therapods) or quadrapedal (like sauropods) and laid eggs, and lived during the Mesozoic era (that is between about 251-64 Million years ago (Ma)).

Ok so now we have a loose definition of what a true Dinosaur is lets take a look at some creatures that are often confused with, or lumped in with, Dinosaurs.

Ichthyosaurs: Ichthyosaurs (meaning fish lizard) are marine reptiles, and so technically, not a Dinosaur. These amazing creatures were first discovered by Mary Anning, a working class, self taught geologist and anatomist who lived in Lyme Regis in the 1800's. She had many things working against her, she was female in a male oriented society, working class, and without her father for much of her life. For me this only makes her discoveries more impressive. I will write more on Mary Anning at a later stage! Nevertheless, she found several different Ichthyosaur remains. Ichthyosaurs look a bit like a modern dolphin, although they don't share a common ancestor, and provide a good example of convergent evolution. They actually evolved from land-based Archosaurs, and so even though they look very much like fish, or dolphins, they evolved their anatomy completely independently.

One of Mary Anning's Ichthyosaur specimens, published in an 1824 paper by London Geological Society member Conybeare

Plesiosaurs: Plesiosaurs (meaning near-to lizard) were also marine reptiles, and therefore not Dinosaurs. They were also first discovered by Mary Anning. The initial incredibly long necked specimen Mary found was thought to be a forgery because the eminent scientist Georges Cuvier did not believe that a creature containing an astounding 38 vertebrae in its neck alone could have existed! On further study Geologists proclaimed that it was most definitely real, and since this time both long and short necked Plesiosaurs have been found. They do share one very interesting aspect with Dinosaurs, and that being the caches of Gastroliths found with Plesiosaur remains. Gastroliths are gizzard stones, and have been found with  Sauropod remains. They sat in the gizzard of the animal, just like in modern birds, and help to break up tough foods for digestion.

Mary Anning's Plesiosaur, published in Conybeare's 1824 paper

Mosasaurs: Mosasaurs were another marine lizard. They were so well adapted to life in the water that they gave birth to live young. Therefore, they are excluded from being a Dinosaur on at least two counts. Mosasaur means 'Meuse river lizard', paying tribute to the area where it was first found in a quarry in Maastricht on the Meuse river in 1764. It took quite a bit of time before it was properly understood, and confirmed as being closely related to lizards and snakes, by Georges Cuvier in 1808. Its internal organs appear to have been arranged a little similarly to that of cetaceans, probably due to its fully marine lifestyle. Its head and jaw however most closely resemble that of a snake, sporting a double hinged jaw and flexible skull. This adaptation would have allowed a Mosasaur to ingest large prey items in much the same way as a modern python.

Sketches of several Mosasaur skeletons (image sourced from  here)

Dimetrodon: This amazing animal often gets mixed up with little plastic Dinosaur models, or appears as a lolly in jelly Dinosaurs. Dimetrodon is a type of synapsid (meaning it had one lower temporal opening in its skull), Dinosaurs on the other hand are diapsids, having two temporal openings in their skulls. Dimetrodon falls in the order of Pelycosauria which means 'two measures of teeth', and is in fact more closely related to mammals than it is to Dinosaurs. Dimetrodon had been extinct for around 30 Ma before Dinosaurs evolved. It had amazingly long spinous processes which supported a large sail, the function of which is thought to be temperature control related.

Dimetrodon (image sourced from wikipedia here)

Pterosaurs: Pterosaur means 'winged lizard' and although they are often referred to as 'flying dinosaurs' in the media, and lived at the same time, they do not share the common ancestor of saurischian or ornithischian Dinosaurs. This excludes pterosaurs from the true Dinosaur club. Pterosaurs are fascinating creatures in their own right and were discovered by an Italian naturalist in 1784. He thought that they were a sea creature, using their wings as paddles, it wasn't till 1801 that Cuvier suggested they were in fact flying creatures. Cuvier also came up with the name 'ptero-dactyle', which is now only used to describe one group of pterosaurs. Pterosaurs are thought to be the first vertebrates to develop powered flight and came in all sizes from that of a squirrel to the enormous Quetzalcoatlus who had a wingspan of somewhere between 10 and 21 metres. Quetzalcoatlus is regarded as the largest known flying creature of all time.

Etching of the original Pterosaur found in 1784 (image sourced from wikipedia here)
This post is a culmination of a lot of recent reading, and inspired by the book 'The Dinosaur Hunters' by Deborah Cadbury. This wonderful book details the rivalry between Gideon Mantell (who discovered and named Iguanodon) and Richard Owen, and describes in detail much of the paleontological world of 19th century Britain. Other sources used include 'Dinosaur Plots' by Leonard Kristhtalka and an 'Introduction to Paleobiology and the Fossil Record; by Michael Benton and David Harper. I am eagerly awaiting my latest purchase from Book Depository 'Dinosaur Paleobiology' by Steve Brusatte. I might do a few non-dino posts over the next week or so. I have lots of interesting swamp pics, bone finds and mammal anatomy information just bursting to be turned into articles!

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