WHAT IS AN ORNITHOPOD?
The Incredible Shrinking Ornithopoda
Traditionally, Ornithopoda ("bird feet") comprised all ornithischians that weren't stegosaurs, ankylosaurs, or neoceratopisans. Eventually,
pachycephalosaurs were recognized as their own distinct clade, and psittacosaurids as ceratopsians. With the development of cladistic analysis,
it was recognized that Scutellosaurus and Scelidosaurus belonged with the stegosaur-ankylosaur clade, and that Pisanosaurus
and Lesothosaurus were primitive ornithischians outside of all the other major groups.
But even at the dawn of the 21st Century, Heterodontosauridae was still generally considered as sharing a more recent common ancestor with the "hypsilophodonts" and iguanodontians than with any other group of dinosaur: thus, the heterodontosaurids were thought to be the oldest branch of Ornithopoda. More recently, however, heterodontosaurids have been recognized as splitting from other ornithischians at a very basal divergence, and thus are no closer to ornithopods than to marginocephalians or to thyreophorans. So there are at present no recognzied Late Triassic or Early Jurassic ornithopods.
Even more recently, Middle Jurassic taxa (such as Chinese Hexinlusaurus and Agilisaurus ) and Late Jurassic western North American Othnieliosaurus (all of which were considered to be primitive hypsilophodont-grade ornithopods in many cladistic analyses) now seem to be basal members of Neornithischia. This latter clade includes marginocephalians, true ornithopods, and all taxa closer to them than to Thyreophora. Among various other cranial (and pelvic) characters, neornithischians share asymmetrical teeth with enamel on only one side, allowing them to be ever-sharpening.
(The recognition of the non-ornithopod status of these Jurassic forms comes in great part with the discovery of Eocursor, Strombergia, and other Late Triassic/Early Jurassic ornithischians. The data from these fossils are helping to sort out the relationships of the bird-hipped dinosaurs. Expect additional changes and refinements in the near future.)
(A note on the name "Ornithopoda": advanced iguanodontians do indeed have three-toed feet something like birds, as seen in these tracks. But basal ornithopods have four forward-facing toes, and no ornithopod seems to have the backwards-facing digit I of birds. In fact, it is kind of a lousy name for the clade, but rather late in the game to change it...)
MAJOR GROUPS OF ORNITHOPODS
So what an ornithopod, then? Ornithopoda contains the clade Iguanodontia and a series of primitive forms traditionally
called "hypsilophodonts". (Note that Agilisaurus and Othnieliosaurus were also included among the "hypsilophodonts" until recently.)
Some paleontologists consider the hypsilophodonts to be a clade, but most recent work demonstrates that they are a paraphyletic series in
relation to Iguanodontia.
The removal of Heterodontosauridae and the basal neornithischians from Ornithopoda means that the shared derived characters that unite true Ornithopoda are a bit muddled at present. However, here is a list of some that seem to apply:
Ornithopods:
"HYPSILOPHODONTS" The primitive ornithopods do not seem to form a single clade, but instead are either more or less closely related to the Iguanodontia. The first discovered and best known of this range of ornithopods is little 2 m long Hypsilophodon of Early Cretaceous Europe (and possibly North America). Most of these early forms have the same general grade of organization: bipedal dinosaurs 1-2 m long with small triangular heads. They seem adapted for browsing on low vegetation and for running as a defense. The "zephyrosaur" Oryctodromeus of the Early Cretaceous of western North America) is known to have dug burrows, and recently described burrows from Early Cretaceous Australia might also be from "hypsilophodonts".
Relationships among the "hypsilophodonts" are not yet well resolved. Some groups seem to include:
The Late Cretaceous Argentine ornithopods including Gasparinisaura and Talenkauen might be "hypsilophodonts" or they might be closer to Iguanodon than to Thescelosaurus (and thus by definition be primitive iguanodontians). In fact, a few recent analyses suggest many of the Gondwanan (in this case, South American and Australian) ornithopods of the Cretaceous represent a single clade of primitive iguanodontians.
One puzzle: all the currently known "hypsilophodonts" are from the Cretaceous, but their phylogenetic position predicts that representatives of this clade should have been around in the Middle and Late Jurassic.
IGUANODONTIA
The members of Iguanodontia were transformed from their "hypsilophodont" cousins by a number of features:
All retained some bipedal ability, but many of the iguanodontians were facultative bipeds only, spending a sizeable fraction of time on all fours.
The oldest iguanodontian known is the Middle Jurassic dryosaurid Callovosaurus. Iguanodontians become more common in the Late Jurassic, but really come into their own in the Cretaceous. In many ecosystems the iguanodontians are the most abundant large animals, displacing sauropods and stegosaurs.
Among the diversity of primitive iguanodontians are:
The last two represent the transition from the more primitive iguanodontians to the specialized Styracosterna. Their snouts have become longer and broader-ended with a better developed pleurokinetic hinge, while their hands have become better adapted for absorbing weight. These transformations are more fully developed in the hadrosauriforms.
STYRACOSTERNA
This represents the clade comprised of Hadrosauridae and all taxa closer to hadrosaurids than to Camptosaurus. The primitive styracosternans were
once all grouped together as "Iguandontidae", but that family turned out to be paraphyletic with respect to hadrosaurids. In fact, the genus Iguanodon as
it was used up to the 1990s has turned out to be paraphyletic, as mentioned below. Styracosterna is by far the most successful radiation among the ornithischians.
Styracosternans show the following transformations from the ancestral state:
The combination of their great size, ability to walk on their hindlegs or all fours, and powerful beaks with grinding teeth allowed styracosternans to be excellent browsers of both low and high vegetation. At least some seem to have lived in herds.
Styracosternans are known from most of the Cretaceous world, but are most particularly common or diverse in Europe, North America, Asia, and (in the Early Cretaceous) northern Africa. Among the diversity of Early Cretaceous styracosternans are:
One subset of styracosternans (Hadrosauria) in particular shows a seires of transformations including an increase in the number of tooth positions in the jaws and expansion of the snout. These dinosaurs are on the lineage which leads to the duckbilled dinosaurs (Hadrosauridae). Among the precursors and cousins of the hadrosaurids are Early Cretaceous Jinzhousaurus, Equijubus, and Probactrosaurus of China; Early-to-Late Cretaceous Protohadros of western North America; Late Cretaceous Telmatosaurus of Europe and Bactrosaurus of Asia. (These last two, in particular, are sometimes included in an expanded vesion of Hadrosauridae).
HADROSAURIDAE
True Hadrosauridae is the most speciose and specialized branch of the ornithopods. All known members of Hadrosauridae proper are from the
Late Cretaceous. Although known from Europe, South America, and Antarctica, the main diversity of hadrosaurids is in Asia and North America.
The transformations of hadrosaurids relative to their ancestors include:
Hadrosaurids include some definite herd dwellers. The entire life cycle of hadrosaurids is preserved: nests, eggs, embryos, hatchlings, juveniles, subadults, and adults. Hadrosaurid footprints and isolated hadrosaurid teeth are among the most common Late Cretaceous fossils of North America. Skin impressions and even mineralized soft tissue are known for duckbills.
The latest on-going phylogenetic analyses show two major subclades of Hadrosauridae: crested Lambeosaurinae and broad-snouted Saurolophinae. The latter group is more traditionally called "Hadrosaurinae", but Hadrosaurus proper seems to lie outside the Lambeosaurinae-Saurolophinae clade and so this other, less commonly seen name is used in this course. (However, a note of caution: some preliminary studies suggest that "saurolophines" may be paraphyletic with respect to Lambeosaurinae). Both clades are known from a great number of excellent skeletons.
Lambeosaurines are characterized by a hollow crest covering the nasal passage. These crests, which vary between species, may have had both a visual and sound display function. Baby lambeosaurines lacked this structure.
Differences in crest size and shapes within some populations may reflect sexual and/or ontogenetic varations.
Among the better known lambeosaurines are Nipponosaurus, Olorotitan, and Charonosaurus of Asia and Parasaurolophus, Corythosaurus, Hypacrosaurus, Velafrons, and Lambeosaurus of North America.
Saurolophines differ from their relatives by greatly flared snouts and greatly expanded nares. Some hadrosaurines had (relatively) short snouts: North American Gryposaurus and Brachylophosaurus, for instance. Others had longer snouts: North American Maiasaura and Prosaurolophus and transcontinental (Asia and North American) Saurolophus. The extreme development of the duckbill can be found in the the edmontosaurs, a group containing Edmontosaurus proper and species sometimes considered separate genera (Anatosaurus and Anatotitan), but sometimes all considered Edmontosaurus. However, the babies of even the long snouted saurolophines had relatively short faces.
Both saurolophines and lambeosaurine produced giants of greater than 13 m in length. These represent the largest animals other than sauropods that have ever lived on land, and the heaviest bipeds in Earth's history.
EVOLUTIONARY PATTERNS IN ORNITHOPODA
Feeding adaptation transformations:
Locomotory changes:
Social behavior in Ornithopoda:
Ornithopods (in particular iguanodontians (in particular hadrosaurids (in particular lambeosaurines))) have abundant evidence for
socially-related adaptations, including: herding; visual (and possibly aural) displays; species recognition structures; possible sexual dimorphism. We will discuss these
more fully in the third section of the course.
Heterochrony, size, and ornithopod history:
In general, peramorphosis seems to play an important role in ornithopod evolution. Hatchling iguanodontians tend to resemble adult "hypsilophodonts",
while hatchling hadrosaurids tend resemble young primitive iguanodontians, and young hadrosaurids tend to resemble the immediate out
groups of Hadrosauridae.
Basal ornithopods were small (comparable to basal members of other ornithischian groups). But at the base of Iguanodontia and the base of Styracosterna there are major size increases. Additionally, various different styracosternan lineages independantly achieved very large (>12 m) size.
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