New Theory as to Why Bipedalism Evolved in the Dinosauria

Iconic dinosaurs such as Tyrannosaurus rex, Velociraptor, Coelophysis and Carnotaurus were all bipeds.  That is, they evolved the ability to walk on their hind legs.  Bipedalism is a trait that seems to have evolved early in history of the Dinosauria and it is a characteristic that is widespread amongst both lizard-hipped and bird-hipped forms.  It had been suggested that bipedalism arose in the ancestors of dinosaurs, to allow the forelimbs to be freed from a locomotive role, allowing them to have other uses, primarily to seize and grasp prey.

However, a team of scientists from the Department of Biological Sciences at the University of Alberta (Canada), have put forward a new theory to explain why some dinosaurs stood on two feet instead of four.  The ancestors of the dinosaurs had a “need for speed”!

The Basal Dinosauriform Marasuchus (M. lilloensis) is Typical of a Bipedal Ancestor of the Dinosauria

The basal dinosauriform archosaur Marasuchus of the Late Triassic. A potential ancestor of the Dinosauria.

Picture credit: Pterosauriablog (author Taylor Reints)

The picture above shows the crow-sized Marasuchus, fossils of which come from the La Rioja Province (north-eastern Argentina), specifically from the Chañares Formation.  This fast running, bipedal reptile lived some 242 – 235 million years ago and the University of Alberta researchers argue that the presence of big muscles (the caudofemoralis), associated with the back of the legs and tail were central to driving the evolution of bipedalism amongst the archosaurs that were to eventually lead to the evolution of the dinosaurs.

From All Fours to Just Two Legs

Publishing in the academic periodical “The Journal of Theoretical Biology” lead author, Scott Persons and is co-worker Professor Phil Currie, argue that basal dinosauriforms were essentially quadrupeds but they evolved to stand upright, a characteristic that was passed onto their descendants the dinosaurs.

The scientists challenge the idea that bipedalism came about in order to help with the seizing of prey.

Persons stated:

“While that works for some of the very, very early dinosaurs, which were certainly carnivorous, you see a bunch of herbivorous dinosaurs evolve later on and a good many of those groups actually keep their bipedal stance, which is a little strange.”

Big Muscles in the Tail

The researchers argue, that strong muscles at the base of the tail helped to power the hind legs of these prototype dinosaurs.  This allowed them to run faster and for longer.  Hind legs evolved to become longer, whilst the forelimbs became shorter to reduce body weight and to improve balance and agility.  Some of these proto-dinosaurs gave up quadrupedalism entirely.

However, as all young dinosaur fans will happily tell you, there were lots of four-legged dinosaurs, examples being Triceratops, Stegosaurus and Diplodocus.  These dinosaurs were herbivores, they evolved heavy defensive weapons, horns and armour which meant that a faster, cursorial lifestyle was sacrificed.  As plant-eaters, they evolved ever larger stomachs and digestive tracts to help them process the tough plant material they ate, bigger guts also led to a reversion back to being a quadruped.

Palaeontologist Scott Persons added:

“In the groups where speed was no longer a concern, they often went back to being a quadruped”

A Rearing Diplodocus – A Four-Legged Dinosaur

Model was introduced in 2013.

Picture credit: Everything Dinosaur

The picture (above) shows a rearing Diplodocus in the CollectA model range.

To view this range: CollectA Prehistoric Life Figures.

Studying Sauropods (Dinosauria)

If you take the lizard-hipped, Sauropoda for instance, these herbivores evolved into a myriad of forms, but they all had the same basic body plan and there was a trend towards gigantism within this infraorder.  However, it has been suggested that those strong, muscular tails and powerful back legs enabled these dinosaurs to rear up to feed higher up in the vegetative canopy.  It has been suggested that baby sauropods may have retained the ability to run on their hind legs, probably to help them escape predators.

To read an article about proposed bipedalism in juvenile sauropods: Facultative Bipedalism in Young Sauropods.

The powerful caudofemoralis muscle provides a greater source of propulsion to the back legs and it is the presence of this tail muscle that may have given the impetus to developing a two-legged stance.

Modern Lizards Provide a Clue

Modern facultatively bipedal lizards offer an analog for the first stages in the evolution of dinosaurian bipedalism.   In biology, the term facultative refers to the ability of many organisms to do things by choice rather than by obligation.  Facultatively bipedal lizards may spend most of the time on all fours, but when the need arises, such as to escape danger, these reptiles can revert to a bipedal stance.  An example of a living facultative biped lizard is the Australian frilled lizard (Chlamydosaurus kingii).  Extant lizards are capable of remarkable feats of locomotion.

An Example of a Living Reptilian and Remarkable Locomotion

An example of lizard locomotion. Picture credit: Everything Dinosaur.

Picture credit: Everything Dinosaur

Why Don’t Fast Mammals Run on Two Legs?

Biomechanically, running on four legs is more efficient than running on two legs.  However, the University of Alberta researchers concluded that the behaviour of synapsid reptiles, distant ancestors of today’s fast running horses, cats, camels, and grey hounds, during the Permian, may explain the lack of bipeds amongst the Mammalia.

In the Permian geological period, it seems some animals started losing the reptilian trait of a strong leg-powering tail muscle.  Around that time, many creatures were becoming burrowers, (adapting to a fossorial lifestyle), so they needed strong front limbs for digging.  Big back legs and a long, powerful tail would have made it tough to manoeuvre in the confines of a burrow, as well as making it easier for a pursuing predator to snatch them by their tail.

The scientists postulate that living underground may have helped those proto-mammals survive the Permian mass extinction.  Their descendants would have evolved to run fast, but without the tail muscles that would have caused them to stand upright.

The Biological Merits of Tetrapods (Not Just the Dinosauria)

Commenting on the biological merits of tetrapods evolving to favour one set of legs for running Scott Persons stated:

“That’s a really funny and strange adaptation.  Why would you choose to use just one set of limbs to help you run away when you’re most desperate?  And the answer has to do with that great big tail muscle [caudofemoralis].  It effectively sort of overpowers the back legs relative to the front legs.  What the lizards are effectively doing is popping a wheelie as they speed off.”

That cursorial advantage explains the relative abundance of cursorial facultative bipeds and obligate bipeds among fossil diapsids and the relative scarcity of either amongst the Mammalia and their close relatives.  Having lost their caudofemoralis in the Permian, perhaps in the context of adapting to a fossorial lifestyle, the mammalian line has been disinclined towards bipedalism, but, having never lost the caudofemoralis of their ancestors, the basal dinosauriforms and their relatives were more inclined to adopting a bipedal stance.

A Tale in a Tail!  Researchers Explain the Presence of Bipedalism within the Dinosauria

Faster and slightly more nimble when compared to the biggest Albertosaurus dinosaurs.

Picture credit: Everything Dinosaur

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Plant-Eating Dinosaurs Probably Played an Important Role in Seed Dispersal

Time to catch up on our reading and first on the list of papers is this fascinating insight into the relationship between plant-eating dinosaurs and seed dispersal.  An international team of scientists from Portugal, Spain and Argentina have described a new species of primitive, bird-hipped dinosaur and an assessment of the body cavity led to the discovery of the dinosaur’s last meal.  Permineralised seeds, most of which having been identified as coming from cycads (Cycadales), suggest that herbivorous dinosaurs played an important role in seed dispersal, just as many plant-eating mammals do today.

The New Argentinian Dinosaur Isaberrysaura mollensis is Related to Kulindadromeus from Siberia

A 1:1 scale model of Kulindadromeus.

Picture credit: T. Hubin/RBINS

Plant-eating Dinosaurs

The researchers which include lead author, Leonardo Salgado (CONICET – Universidad Nacional de Río Negro), conducted a phylogenetic analysis and assigned this new dinosaur species to the basal Ornithopoda, suggesting that it is closely related to Kulindadromeus (K. zabaikalicus), fossils of which are known from geologically younger strata found in Siberia.  However, this new dinosaur, named Isaberrysaura was much larger, with an estimated body length of approximately six metres.

The Feeding Habits of Herbivorous Dinosaurs

Despite some two-thirds of all the dinosaurs described to date being plant-eaters, there is little direct evidence of the feeding habits of herbivorous dinosaurs that matches the stomach contents preserved within a carcase.  Most unaltered gut content that has been found is associated with much later dinosaurs -hadrosaurids and ornithopods.  This is the first time that gut contents have been identified in association with the remains of a basal neornithischian.

For models and replicas of ornithischians and other dinosaurs: Prehistoric Animal Models and Dinosaur Figures.

The specimen, representing a single individual, consists of an almost complete skull, vertebrae, part of the left shoulder blade (scapula), along with ribs and elements from the pelvic girdle.  It was excavated from the Los Molles Formation (Neuquén Basin, Argentina), from sediments that suggest a coastal-delta environment, the presence of the zone ammonite Sonninia altecostata in the same fossil bed, indicate that Isaberrysaura lived some 170 million years ago (Early Bajocian faunal stage of the Middle Jurassic).

These are the first dinosaur remains found in this geological unit and the one of the oldest dinosaurs known from the Neuquén Basin.  In addition, this is the first neornithischian dinosaur known from the Jurassic of South America.  The skull and the teeth show some affinity with basal stegosaurids which suggests that both the Thyreophora and neornithischian dinosaurs shared a lot of similar features (potential evidence of convergent evolution amongst plant-eating dinosaurs).

Isaberrysaura mollensis – Views of the Skull and Teeth

The skull in (a) dorsal and left lateral view (c) with corresponding line drawings (b and d). Premaxillary tooth (e) and maxillary teeth (e and f).

Picture credit: Scientific Reports

Why Isaberrysaura?

This month, we have once again been celebrating International Women’s Day (March 8th), it is pleasing to note that the female form of “saurus” has been chosen when it came to naming this new dinosaur, as the genus has been erected to honour Isabel Valdivia Berry, who reported the finding of the holotype material.  In the body cavity, the researchers were able to identify a mass of permineralised seeds.  These were identified as belonging mostly to the Cycadales group of plants.  These tough seeds would have passed through the dinosaur’s gut and would have been deposited in the dung.  This fossil discovery suggests a possible and unexpected role of bird-hipped dinosaurs, that of Jurassic seed-dispersal agents.

An Analysis of the Gut Contents of Isaberrysaura

Permineralised seeds identified in the gut cavity of Isaberrysaura mollensis.

The photograph above shows images of the body cavity showing evidence of the seed fossils.

Some of the fossils show that their fleshy seed-coats are still intact (sc = sarcotesta), this indicates that these seeds were swallowed whole with little or no chewing action in the mouth.

The scientific paper: “A New Primitive Neornithischian Dinosaur from the Jurassic of Patagonia with Gut Contents”, published in the journal “Scientific Reports”.

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Knoetschkesuchus – Living on an Island

Last month, scientists published in the on-line academic journal “PLOS ONE” a paper that provided details of the discovery of a new species of Late Jurassic terrestrial crocodile, but this animal was no ground-shaking giant, like most of its kind (atoposaurids), it probably had a maximum length of under a metre and it would have weighed a couple of kilogrammes or so.

Lead author of the research paper, Daniela Schwarz (Leibniz Institute for Evolutionary and Biodiversity Research, Germany), in collaboration with colleagues, re-examined fossils including skull material (an adult and a juvenile specimen), that had previously been assigned to the atoposaurid Theriosuchus – a genus of crocodile-like reptile that is known from a large number of fossils dating from the Late Jurassic and into the Early Cretaceous from places as far apart as Thailand and southern England.  However, when the beautifully preserved fossils were studied using CT scans and three-dimensional images of the fossil material created, a number of autapomorphies were identified to allow the erection of a new genus.

A View of the Larger Specimen of Knoetschkesuchus langenbergensis

Knoetschkesuchus langenbergensis fossil material.

Picture credit: PLOS ONE

Knoetschkesuchus langenbergensis

The little crocodile has been named Knoetschkesuchus langenbergensis, the fossils come from the famous Langenberg Quarry, located near the town of Goslar, Lower Saxony, northern Germany.  The limestones and marls that form the quarry site, were laid down in shallow inlets associated with an island archipelago.  The Knoetschkesuchus material comes from Bed 83, the same location as the fossils of the dwarf sauropod Europasaurus (E. holgeri).

For replicas and models of ancient crocodilians and other prehistoric animals: Mojo Fun Prehistoric and Extinct Models.

Lots of Terrestrial Crocodiles in the Mesozoic

Knoetschkesuchus langenbergensis fossil material has been dated to the Upper Kimmeridgian stage of the Jurassic, approximately 154 million years ago, like most members of the Atoposauridae it had large eyes and it may have been a fast runner.  The researchers conclude that the Langenberg Quarry fossils represent a new species because of unique features of the skull, such as openings in the jaw bone and in front of the eye, as well as the shape and placement of the animal’s tiny teeth.

The teeth are heterodont in nature (different shapes), at the tip of the snout they are pointy and needle-like, very typical of a small crocodilian.  Towards the back of the jaws they are broader and more rounded, particularly in the lower jaw.  It has been suggested that these teeth were adapted for crushing hard-shelled prey, such as snails, which are known from abundant gastropod fossils associated with Bed 83.

Line Drawings Showing Various Views of Both the Adult and Juvenile Skull Specimens

Knoetschkesuchus skull drawings.

Picture credit: PLOS ONE with additional annotation by Everything Dinosaur

Note

Elements of the adult skull fossil have been drawn based on scaled-up material from the juvenile specimen.

Dr Schwarz commented:

“The study describes a new diminutive crocodile Knoetschkesuchus langenbergensis that lived around 154 Million years ago in north-western Germany.  Knoetschkesuchus belongs to the evolutionary lineage that leads to modern crocodiles and preserves for the first time in this group two skulls in 3-D, allowing us detailed anatomical studies via micro-CT images.  Our research is part of the Europasaurus-Project which studies the remains of a unique Jurassic island ecosystem in northern Germany.”

A Unique Island Ecosystem

The Langenberg Quarry preserves the remains of a unique Late Jurassic European ecosystem.  The islands were relatively small and this led to a divergence between residents of these islands and their ancestors which lived on the larger landmasses to the east.  For example, in response to limited food resources and space, the sauropod Europasaurus became a dwarf form.

The atoposaurid crocodiles probably filled a number of ecological niches within the food chain, including that of arboreal predators.  These distant ancestors of today’s crocodiles were in turn preyed upon by a variety of theropod dinosaurs, the majority of which are only known from fragmentary teeth.  What is quite remarkable, is the diversity of the theropod teeth found in Upper Jurassic deposits of northern Germany.  Studies have suggested that representatives of the Tyrannosauroidea, as well as Allosauroidea, Megalosauroidea and probably Ceratosauria roamed this part of the world some 155 to 150 million years ago.

The genus name (Knoetschkesuchus) is a combination of the family name of Nils Knötschke, and suchus (from the Greek meaning crocodile).  The genus name honours of Nils Knötschke (Dinosaurier-Freilichtmuseum Münchehagen), who collected, prepared, and curated several important fossil specimens from the Langenberg Quarry.

Visit the Everything Dinosaur website: Everything Dinosaur.