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 recocgnzied Late Triassic or Early Jurassic ornithopods.
Even more recently, Middle Jurassic taxa (such as Chinese Agilisaurus and Hexinlusaurus, Middle/Late Jurassic Siberian Kulindadromeus, and Late Jurassic western North American Othnielosaurus (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 and a diastema (gap) between the premaxillary teeth/beak and the maxillary ones.
(The recognition of the non-ornithopod status of these Jurassic forms comes in great part with the discovery of Eocursor, Stormbergia, 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...)
Kulindadromeus is a noteworthy animal: not so much in terms of its skeleton (which is boringly standard for a neornithischian), but for its behavioral evidence and integument. Firstly it was found in bonebeds of many dozens of individuals, so it is quite possible it lived in groups. More intersting than that, though, is its body covering. It is found in an environment which fine details can be preserved. It has some parts of its body (bottoms of the feet) there are simple scales; on the front of the legs and on top of the tail are more plate-like scales; on other parts of the body are simple filaments; and then there are odd plates with fuzz coming off them (unlike any structure known in other dinosaurs). So small ornithischians show complext types of integument beyond scales (and beyond fuzz).
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:
As might be apparent from all the above, the phylogenetic relationships among the ornithopods is currently in a state of flux. There are numerous analyses out there, and they generally agree that there is a grade of small-bodied forms outside the clade of large-bodied forms. But the exact relationships among the "hypsilophodonts" and among the iguanodonts is very much unsettled.
"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 orodromine 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, but getting better. Some recent studies put Hypsilophodon proper as closer to a diverse clade, the Thescelosauridae. Thescelosaurids includes the aforementioned burrowing Orodrominae (including Early Cretaceous Oryctodromeus and Zephyrosaurus and Late Cretaceous Orodromeus) and a clade (Thescelosaruinae) that contains primitive forms once called "changchunsaurs" or "jeholosaurs" from the late Early to early Late Cretaceous of eastern Asia (including Changchunsaurus, Haya, and Jeholosaurus: all once included in a clade "Jeholosauridae") and larger derived taxa of the Late Cretaceous of North America, including Parksosaurus and long-snouted Thescelosaurus.
However, other studies find the "orodromines" to be the basalmost ornithopods, with Jeholosauridae, Hypsilophodon and a reduced Thescelosauridae as successively closer to Iguanodontia; and Parksosaurus and the South American small ornithopods as basal iguanodonts (with Iguanodontia being defined as "Iguanodon and all taxa sharing a more recent common ancestor with it than with Thescolosaurus or Hypsilophodon").
The Early Cretaceous small ornithopods of Australasia, including ultra-long tailed Leaellynasaura, are of uncertain position. Some studies suggest one or more these might actually be basal ornithischians rather than ornithopods; others ally these taxa with the Late Cretaceous Argentine ornithopods including Gasparinisaura and Talenkauen in a clade closer to Iguanodon than to Thescelosaurus (and thus by definition be primitive iguanodontians). Analyses to sort out these relationships are ongoing.
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.
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 grinding jaws, while their hands have become better adapted for absorbing weight. These transformations are more fully developed in the hadrosauriforms.
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:
The relationships among the styracosternans shown here is very tentative. Different analyses disagree wildly with respect to the interrelationships of the taxa in the list above. Some recent ones find a good old-fashioned Iguanodontidae containing the Early Cretaceous European Iguanodon, Barilium, Mantellisaurus (including Dollodon), and Proa with the Chinese Bolong and Jinzhousaurus: other analyses find these all as a paraphyletic grade.
One subset of styracosternans (Hadrosauria) in particular shows a series 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, Gongpoquansaurus and Probactrosaurus of China; Early-to-Late Cretaceous Protohadros of western North America; and Late Cretaceous Bactrosaurus and Plesiohadros of Asia. (These latter two fall out within Hadrosauridae proper in some analyses). There are many others, and more are being named every year.
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 see the fullest expression known of the ornithopod grinding mechanism. As mentioned above, it was once thought that the motion was relative simple: the side unit would move outwards when the lower jaw was brought up, giving a side-to-side grinding of the teeth during chewing. This model (proposed during the 1980s) was called "pleurokinesis" (or "side-motion"). Here is a video of a computer animation of this interpretation:
However, more detailed study using CT scans and more complete computer models show the motion is really a LOT more complex. Pleurokinesis plays a part in it, but there are other directions as well. No name is given at present for this form of jaw mechanics, but below is a preliminary model of how it works:
Their exceedingly-complex tooth form--which were made of six different tissue types, rather than the standard two of most tetrapods--maintained a good girding surface as they wore down.
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 Hadrosaurinae. The latter group has sometimes been called "Saurolophinae", because in some analyses Hadrosaurus proper (and the very similar Italian Tethyshadros and Telmatosaurus of Romania) seems to lie outside the Lambeosaurinae-Hadrosaurinae clade (Euhadrosauria) (However, a note of caution: some preliminary studies suggest that "hadrosaurines" may be paraphyletic with respect to Lambeosaurinae). Both the major 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.
CT scans allow for the pathways of these passages to be studied in greater detail:
Differences in crest size and shapes within some populations may reflect sexual and/or ontogenetic varations.
Among the better known lambeosaurines are Nipponosaurus, Olorotitan, Tsintaosaurus, and Charonosaurus of Asia and Parasaurolophus, Corythosaurus, Hypacrosaurus, Velafrons, Lambeosaurus, and GIGANTIC Magnapaulia of North America.
Hadrosaurines (aka "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 sauropod-sized Shantungosaurus (largest of all ornithischian dinosaurs) of China and North American Edmontosaurus proper and species sometimes considered separate genera (Anatosaurus and Anatotitan), but sometimes all considered Edmontosaurus. (There is recent work to show that there are only two species, each with different growth stages: personally, I am fine using the old name Anatosaurus for the geologically-younger annectens.) Babies of even the long-snouted hadrosaurines had relatively short faces.
Both hadrosaurines 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.
Microwear analysis of the teeth of hadrosaurids is consistent with their complex chewing patterns. In at least the broad-billed edmontosaurs there is a great degree of scratching on the teeth, suggesting that they were primarily low browsers of tough vegetation ("grazers"). (Given the wide snouts of Edmontosaurus, Anatosaurus, and so forth, this makes a lot of sense.) Studies have not yet been published to see if most hadrosaurids were primarily low browsers, or if some of them might have been mostly high browsers. Given the diversity of bill shapes and snout lengths (and the diversity of species overall), there was probably a number of different diets among the hadrosaurids.
EVOLUTIONARY PATTERNS IN ORNITHOPODA
Feeding adaptation transformations:
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 outgroups 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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