Dinosauria

Introduction: The Dinosaurs

Painting © Doug Henderson, reproduced with permission.

Originally Palaeos had no dinosaur chapter as such, so the present admittedly brief unit was added as a place to introduce everyones' favorite multi-ton reptiles. Probably no creatures that have lived upon this Earth have excited the imagination more than the Dinosauria. For some 150 million years they dominated ever medium to large to gigantic terrestrial vertebrate ecological niche, evolving into a wide range of forms and populating every continent.

Perhaps because they often had such bizarre forms, perhaps because they sometimes grew to such huge size, perhaps because after ruling this earth for so long they suddenly vanished, seemingly without a trace; all things add to the appeal of the Dinosauria in the popular -- and scientific! -- imagination.

As wonderful as the dinosaurs are, even these facts have to be qualified. True, many dinosaurs did have rather strange forms, but were they any stranger in appearance than, say, a giraffe or an elephant?   True, many of them did grow to be quite large and even enormous, but no dinosaur ever rivalled even the smaller baleen whales in size. It would in fact be physiologically impossible for a land animal of more than 100 tonnes to exist (it's legs would have to be so massive they would touch, leaving no space for the body between!). So much for Godzilla and the dinosaurs of One Million Years B.C. (a cult film better known for a much smaller, but equally impressive, 60 kg and 180 cm Rachel Welch in a fur bikini) - such creatures belong to the realm of Hollywood fantasy. The majority of dinosaurs were actually medium-sized creatures, equivalent to modern medium to large mammals in size. Finally, as for dying out without a trace, this is also incorrect. One lineage of small insectivorous or /carnivorous dinosaurs did survive the great Mesozoic terminal extinction and are still with us today. They're called birds.

Some ecological niches the dinosaurs didn't invade. They never established themselves in the small terrestrial vertebrate niche (this was already taken over by mammals and lizards), nor (contrary to popular belief) did any of them ever adopt a marine or aquatic mode of life. They did however take over the air with style; and so successfully that their descendents are still the most numerous and diverse of the tetrapod land-living vertebrates) even today. (MAK 010930)

Dinosaur Groups

Traditionally, the dinosaurs were divided into two orders, depending on the structure of the hip bones. Those that had a reptilian-like pelvic bone were put in the Order Saurischia or "lizard-hips"; while the ones with a bird-like pelvic bone made up the Order Ornithischia or "bird-hips".  (Paradoxically, it was from "lizard-hipped" and not the "bird-hipped" forms that birds evolved). This classification is still adhered to in some (especially older) popular and academic books, but has pretty much been rejected in favour of the cladistic interpretation.

The earliest proto-dinosaurs [1] (basal Ornithodira) were a group of small early dinosaur-like archosaurs, known only from a few scrappy Argentinian fossils of Mid-Triassic age. These are neither saurischian nor ornithischian. These creatures, previously considered ornithosuchid thecodonts, are not even formally considered dinosaurs (although they are dinosauromorphs, which means dinosaurs and a few related forms more closely related to dinosaurs then to pterosaurs.). They are the stem forms from among which which the dinosaurs evolved. It has also for some years been felt that they are closely related to the Pterosaurs flying reptiles). There is however a rival theory which derives the pterosaurs from prolacertiform "lizards," or perhaps even more distantly related stock.

The Saurischia or "lizard hipped" dinosaurs are those more closely related to birds than to Triceratops. Conventionally, they are divided in turn into two groups, one largely carnivorous,  the other herbivorous. The first of these are the Theropoda, the bipedal carnivorous dinosaurs, with their bird-like legs and necks. Theropoda means "beast-feet", a rather inappropriate name; "bird(-like) feet" would have been better. Included in this huge and diverse group are both small forms (including the birds themselves) and large predators such as Allosaurus and Tyrannosaurus.

The other group of Saurischia, the Sauropodomorpha, were herbivores. There are two main subgroups, the Sauropoda the inappropriately named "lizard-feet"), and their ancestors or uncles, the Prosauropoda "before the sauropods"). Although the prosauropods were relatively small, the more advanced types, and all of the sauropods, were elephantine giants with tiny heads, very long necks and tails, massive bodies, and pillar-like legs. This group includes the famous "Brontosaurus" (or Apatosaurus) and its relatives. Like modern-day elephants, they relied on their great size as a defense against carnivores.

The Ornithischia ("bird hipped" dinosaurs), or Predentata so called beacuse they posses a unique extra predentary bone in front of the jaw, which served as a sort of beak) were a more diverse group of herbivores. Being much smaller than the sauropods, they survived because they evolved various other means to avoid becoming fast food for their meat-eating theropod contemporaries. The ornithopods, for example,  depended on fleetness of foot and acute sight and hearing. The Ceratopsian dinosaurs (Triceratops, etc) were the rhinoceroses of the dinosaur world, their formidable horns at least appear to be ample protection against even the largest and fiercest carnivores. There is a substantial body of opinion that these horns were more decorative than functional -- although we have probably not heard the end of this issue. The Stegosaurs and Ankylosaurs evolved armour plates, spikes, and tail-clubs as defensive and offensive weapons.

The Ornithischia -- unlike saurischian dinosaurs, reptiles and birds -- possessed mammal-like cheek muscles and cheek pouches to aid in chewing. In this respect they paralleled the mammalian form. Certainly, many Ornithischia filled ecological roles similar to those of the mammalian ungulates.

So we see among the dinosaurs the tendency towards both an avimorphisation or bird-form-tendency in the Theropods, and a theromorphisation or mammal-form-tendency) in the Ornithischia. The dinosaurs of the Mesozoic era in a sense presaged the birds and mammals of the Cenozoic era.

Dinosauromorpha	Saurischia  (Lizard Hipped dinosaurs)		Ornithischia (Bird Hipped dinosaurs)
Lagosuchidae	Theropoda	Sauropodomorpha	Predentata

A word now about birds. Most palaeontologists and dino-enthusiasts today also consider the Birds to be a subgroup of dinosaurs. Cladistically (phylogenetically) speaking this is correct: birds evolved from dinosaurs, so if dinosaurs are to retain their monophyletic status they must include birds. MAK010930.  Revised ATW050609.

Aves

[1] The discussion here assumes a more or less conventional view of archosaur evolution. We are aware of unpublished work which might drastically alter this view. For both scientific and historical reasons, the data sets used to produce the present consensus tree of the archosaurs are weak. Some of those factors are discussed elsewhere. A complete restructuring of archosaur evolution, based on new data, is not at all out of the question.