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GEOL 104 Dinosaurs: A Natural History

Fall Semester 2017
Theropoda III, Eumaniraptora: Dinosaurs of the Air


The dromaeosaurid Dakotaraptor by Emily Willoughby (2015)

Key Points:
•Eumaniraptora is a clade of (mostly) small-bodied feathered coelurosaurs. Based on their body size and feathered limbs, at least ancestrally they could probably fly (at least to some extent).
•Relationships among the eumaniraptorans has been difficult to determine with certainty, because the primitive members (especially the Late Jurassic forms) are all so similar in form.
•Deinonychosauria (the raptor dinosaurs) is characterized by a hyperextensible sickle claw. It contains within it the diverse Dromaeosauridae and the Troodontidae.
•Avialae is the clade comprised of modern birds and those taxa closer to them than to deinonychosaurs.
•The oldest avialians--such as Jurassic Anchiornis and Archaeopteryx--lacked most of the flight adaptations of modern birds. But over the course of the Cretaceous in stepwise fashion different groups of avialians evolved more sophisticated flying features.


Simplified cladogram of Eumaniraptora


More detailed phylogeny of Eumaniraptora

MAJOR GROUPS OF EUMANIRAPTORANS

Eumaniraptora is just one clade within Pennaraptora (and thus Maniraptora). The actual shared derived characters that unite them as a whole are a bit problematic, due to the uncertainty of the position of Archaeopteryx and other Jurassic taxa and the discovery that basal dromaeosaurids and troodontids are far more Archaeopteryx-like than are the "classic" dromaeosaurids and troodontids. Some probable transformations at the base of Eumaniraptora include:

IMPORTANT NOTE: A note on the use of the word "bird": this English term unfortunately carries with it a lot of intellectual baggage. If we hear that a fossil was from a "bird" we might assume that it had modern "bird" habits and adaptations (flight, song, tree nests, seed or insect diets, etc.). However, not all these characteristics are even true of all modern birds, much less of earlier branches of the lineage. So let's avoid calling Archaeopteryx a "Jurassic bird". There is no evidence that it was a seed eater; there is no evidence for song; it almost certainly nested on the ground; and as we will see it might not have been able to fly any better than early dromaeosaurids.

Eumaniraptora is traditionally divided into two clades: Deinonychosauria and Avialae. However, this situation has become complicated. On the one hand, there are several primitive eumaniraptorans whose exact placement is uncertain. For example, it is uncertain where the late Middle Jurassic (or early Late Jurassic) Pedopenna fits. It might be a basal avialian, or it might be outside Eumaniraptora. Similarly, the scansoriopterygids (discussed last lecture) may be primitive avialians.

But more problematically, a new studies fail to recover the traditional grouping of Dromaeosauridae plus Troodontidae as a monophyletic "Deinonychosauria". And even worse, these studies disagree if Troodontidae or Dromaeosauridae are closer to Avialae. So for this course we are going to discuss three clades for Eumaniraptora: mostly carnivorous Dromaeosauridae; Troodontidae; and Avialae. (Furthermore, some studies in the 2010s pointed to an "Archaeopterygidae" comprised of Archaeopteryx and taxa here regarded as basal troodontids, and this whole group was uncertain as to where they fit. The latest rounds of studies, though, are followed here.)

In recent years a series new taxa have been found in China which seem to be closer to each other than to all other known taxa. These were once thought to form a clade with Archaeopteryx (Archaeopterygidae), but other studies place them as a clade of basal troodontids. These are the latest Middle Jurassic/earliest Late Jurassic Anchiornis, which shows the presence of long leg feathers as in Archaeopteryx, basal dromaeosaurids, and various basal birds; Xiaotingia; Aurornis; and Eosinopteryx. Anchiornis was first thought to be an avialian basal to Archaeopteryx, then to be a basal troodontid, then as an archaeopterygid, and now as the basalmost avialian.

At present, it appears that the following three alternative positions for these taxa are quite possible, and there is not strong evidence to discount any of these:

For the moment, we are regarding Anchiornis at least as a basal avialian. Additionally, it is not unlikely that at least two (if not three or four) of these genera will just turn out to be the same taxon (Anchiornis by priority).

One comforting aspect of this uncertainty: the similarity of the morphology of the basal troodontids, the microraptorine dromaeosaurids, and Archaeopteryx shows us what our best model for what the common ancestor of all Eumaniraptora looked like. (Basically, something very similar to Anchiornis and Archaeopteryx.) The basal members of all three clades are crow-sized animals.

DEINONYCHOSAURIA
For many decades the "raptor dinosaurs" broadly defined (Troodontidae and Dromaeosauridae) were grouped together as Deinonychosauria ("Deinonychus lizards"). Some studies fail to recover this clade, and instead have Troodontidae or Dromaeosauridae closer to birds. (In fact, some analysis find both groups paraphyletic with respect to birds!)

Deinonychosauria is united by a few traits, including features of the mandible. However, for this course we will focus on one in particular (and indeed one of the first features noted to be shared among them): a sickle claw on the extremely retractable pedal digit II.

At present all known deinonychosaurs are either dromaeosaurids or troodontids. However, some analyses in the past found archaeopterygids as a basal deinonychosaur clade.

There are no definite deinonychosaur body fossils from the Jurassic. Some teeth and skeletal material from the Late Jurassic of western North America and from the Middle Jurassic of England have been referred to either Troodontidae or Dromaeosauridae, but we can't definitely show these aren't from a basal avialian group instead. That said, two-toed footprints from the Middle Jurassic of Iran are pretty convincing evidence that deinonychosaurs were present.

DROMAEOSAURIDAE
Basal dromaeosaurs (Unenlagiinae and Microraptorinae) include some crow- to turkey-sized taxa: Rahonavis of the Late Cretaceous of Madagascar (and initially thought to be a bird); Microraptor of the Early Cretaceous of China (the first eumaniptoran for which the tail feathers were known); long-snouted Buitreraptor of Late Cretaceous Argentina; and others. Not all of these were small, however. One of the most unusual of these basal dromaeosaurids is giant Austroraptor of the mid Late Cretaceous of Argentina: a long-snouted, short-armed polar bear-sized unenlagiine. The very small size of unenlagiine teeth suggest that they ate only small bodied animals; we have definite evidence that at least some microraptorines ate their avialian cousins.

The more derived dromaeosaurids form a group called Eudromaeosauria. These were coyote-to-grizzly bear sized. Eudromaeosaurs include a major clade: robust Dromaeosaurinae. Dromaeosaurinae includes coyote-sized Dromaeosaurus of Late Cretaceous western North America; lion-sized Achillobator of mid-Late Cretaceous Asia; and grizzly-sized western North American Utahraptor of the Early Cretaceous and the just named (October 30, 2015) Dakotaraptor of the latest Late Cretaceous, at present the largest known eumaniraptorans. The remaining eudromaeosaurs sometimes cluster together as a clade Velociraptorinae; alternatively, some of the North American forms fall out as branches outside Dromaeosaurinae + Velociraptorinae. In general the non-dromaeosaurine eudromaeosaurs are more gracile than dromaeosaurines. Well-studied examples of include coyote-sized Velociraptor of the Late Cretaceous deserts of Mongolia; wolf-sized Deinonychus of the Early Cretaceous of western North America; and jackal-sized Acheroraptor of latest Cretaceous western North America.

Dromaeosaurid caudals were more tightly interlocked than in typical theropods, and in microraptorines and eudromaeosaurs extensions from the chevrons and neural arches grew extremely long. The tail was thus an extreme dynamic stabilizer.

Some basal dromaeosaurids had elongate metatarsi (indeed, they have a primitive form of the arctometatarsus), but the majority had relatively short stout metatarsi and tibiae. This suggests that they had sacrificed speed, perhaps for agility (better able to turn quickly while pursuing prey or escaping predators, especially with the help of the stiffened tail). Despite certain books and movies to the contrary, the dromaeosaurids show no signs of being speed specialists.

While the small primitive forms may have eaten small prey (skewering it with the sickle claw in the manner of modern secretary birds, perhaps), the larger forms were predators of dinosaurs. The "Fighting Dinosaurs" specimen of Velociraptor shows it in combat with Protoceratops, the hands used to grasp the head of the herbivore while the sickle claw was ripping into the throat: very similar to the attacks used by large cats. Some (contraversial) evidence suggest that Deinonychus may have attacked the much larger iguanodontian Tenontosaurus in groups (packs or mobs).

Note that the sickle claw may have also been used to climb: either up trees, or up the sides of victims! Additionally, they may have been used to pin smaller prey down on the ground while the jaws and hand claws were used to kill it: the raptor prey restraint (RPR) model.

As discussed next lecture, at least some of the smaller basal forms may have had limited flight capability: indeed, they show somewhat better flight adaptations than Archaeopteryx and the winged troodontids! The majority of dromaeosaurids, however, probably lived their lives on the ground (although hiding/sleeping in the trees may have been possible, especially for juveniles).

In the Early Cretaceous dromaeosaurids were major mid-sized predators, and in the deserts of Late Cretaceous Asia they were among the largest carnivorous dinosaurs present.

[Hey, want to make eumaniraptoran systematics MORE confusing! Some studies suggest that microraptorines, unenlagiines, and/or troodontids might be more closely related to Aves than to the eudromaeosaurs; this would make microraptorines, unenlagiines, and/or troodontids members of Avialae!]

TROODONTIDAE
Troodontids were crow-to-deer sized theropods. The oldest troodontid may be Jinfengopteryx of China: described originally as a bird, but now recognized as a troodontid. It is from the early Early Cretaceous (although for some time it was thought that the formation it is from was as old as the Jurassic). Its gut contents include small ovoid objects, that might be seeds. Like Archaeopteryx, Jinfengopteryx had long tail feathers all along its tail. (Oddly, there are a pair of as-yet unnamed Late Cretaceous troodontids that form a clade, Jinfengopteryginae, with Jinfengopteryx.) Additionally, a Late Jurassic troodontid is known from western North America, but has not yet been named or fully described. The last troodontids are from the very end of the Cretaceous. As discussed above, some studies once places Anchiornis and its kin (Aurornis, Eosinopteryx,Xiaotingia) as the basalmost troodontids. Since Jinfengopteryx is from the Early Cretaceous, the Anchiornis-clade would have been the oldest group of troodontids.

Troodontids differ from other eumaniraptorans by an increase in the number of teeth, reduced arm length, and increased distal hindlimb length. Tiny Sinovenator of the Early Cretaceous of China has a backwards-pointing pubis and lacks an arctometatarsus; most later troodontids show a reversal to a forward-pointing pubis and have an arctometatarsus. (But the newly recognized Latenivenatrix has a backwards-pointing pubis, just to be confusing...)

Troodontids have relatively lightly built snouts, suggesting that they did not tackle very large prey (particularly as their forelimbs were fairly short and lightly-built as well). The largest and most derived troodontids are not particularly large (turkey-to-deer sized), and had forward-facing eyes and leaf-shaped teeth with large denticles on the back. They may have had some plants in their diet, but tooth-marked bones show that they definitely ate meat. (NOTE: if troodontids were omnivores, this would make Dromaeosauridae the only primarily carnivorous group of maniraptoriform. If true, this would almost certain be an evolutionary reversal, and indicate that Velociraptor and Deinonychus were descendants of omnivores or even herbivores!)

The more completely known troodontids include Mei and Sinornithoides of the Early Cretaceous of Asia (and both known from specimens buried in sleeping position!); Saurornithoides, Xixiasaurus, and Zanabazar of the Late Cretaceous of Asia; and Talos and "Troodon" of the Late Cretaceous of western North America. (Okay, it is not getting complicated. The original specimen of Troodon were isolated teeth, and we can't be certain which skeleton-based taxon it belongs to. So we know recognize Stenonychosaurus and slightly older Latenivenatrix, largest known of the Troodontidae. A latest Cretaceous form known as Pectinodon exists, but we don't presently know what its skeleton looked like.

The large eyes of troodontids suggests to some that they may have been nocturnal. Possibly related to that, troodontids are by far the most common dinosaur in polar Late Cretaceous North American fossil sites (where light levels would have been very low during the winter), while they were no more common than dromaeosaurids in the rest of their range. "Troodon" (in the broad sense: the collection of Late Cretaceous western North American troodontids: Talos, Latenivenatrix, Stenonychosaurus, and forthcoming new taxa) is particularly notable by having the largest brain/body size of the non-pygostylian dinosaurs, and thus likely the "smartest" of the traditional dinosaurs.

As mentioned above, at least some phylogenetic analyses place Troodontidae as closer to extant birds than to Dromaeosauridae; in these studies, Troodontidae would be part of Avialae rather than Deinonychosauria.

AVIALAE
The remaining eumaniraptorans are the Avialae ("bird wings"), or "birds, broadly defined". (See note above, though, about the cautions of using the word "bird" for these early members of the lineage). If Anchiornis remains in Avialae (which remains a distinct possibility), than this clade is known from the earliest Late Jurassic (or maybe really the latest Middle Jurassic).

The best-studied, and longest-known, basal avialian (indeed, the longest-known Mesozoic eumaniraptoran) is Late Jurassic is Archaeopteryx. Some paleontologists regard this as a single genus and species (A. lithographica); others divide it up into several genera (Archaeornis, Jurapteryx, Wellnhoferia, as well as Archaeopteryx). (For this course, we'll provisionally regard them as a single genus.)

The first and second specimens of this theropod were the first fossil skeletons with feathers ever discovered (in the 1860s and 1870s). Indeed, these were the first good maniraptoran fossils of the Mesozoic ever found. Consequently, Archaeopteryx has spent most of the last century and a half being considered a "bird". But as pennaceous feathers have been found in other theropods, and the skeletons of basal deinonychosaurs have been discovered, the "birdiness" of Archaeopteryx has decreased.

Archaeopteryx came from the Late Jurassic of Europe. Isolated bones from western North America of the same age might be from a same or similar taxon.

There remain a few features that Archaeopteryx shares with Avialae (modern birds and the theropods closer to birds than to deinonychosaurs), but a new specimen also shows some features more similar to deinonychosaurs than to avialians. So at present, we can't be certain of the position of this little crow-sized dinosaur: indeed, addition of characters from what seem to be its closest relatives have really confused the issue!

With newer specimens and newer analyses, however, Anchiornis has come out as the basalmost avialian. If so, than the traits that unite Avialae are:

While those that unite the clade of Archaeopteryx and more derived avialians are:

Anchiornis is known from literally hundreds, if not thousands, of specimens (rivaling Confuciusornis as the Mesozoic theropod known from the most specimens). Research on this dinosaur is ongoing: it has already revealed phenomenal levels of soft-tissue preservation. As mentioned previously, some of the other small eumaniraptorans from the same quarries (Aurornis, Eosinopteryx, Xiaotingia, and others) may just be specimens of Anchiornis.

Despite some artistic and scientific reconstructions, there is at present no strong evidence that basal avialians were any better at flight than were dromaeosaurids like Microraptor and Rahonavis. Indeed, based on the shape and orientation of the shoulder joints, there is positive evidence that the basal avialians were in fact not very good at powered flight.

Much of our knowledge of the basalmost avialians comes from China. Jeholornis (also called "Shenzhouraptor": both names were proposed within weeks of each other; and another avialian Jixiangornis may also be the same) may be the most primitive avialian other than Archaeopteryx. It was chicken-to-turkey sized, and gut contents show that it ate both seeds and fish. Like the dromaeosaurid Microraptor, Jeholornis had long tail feathers at the end of its tail; however, it also had a cluster of long feathers at the base of the tail.

Bizarre double-sickle-clawed Balaur of Late Cretaceous Transylvania was first thought to be a velociraptorine dromaeosaurid, but new studies place it as a late-surviving long-tailed avialian.

More derived are the Omnivoropterygidae (sometimes called "Sapeornithidae"), which are the basalmost avialians to show a reduction and fusion of the distalmost caudals into a pygostyle (convergently evovled in the oviraptorosaur Nomingia, therizinosauroid Beipiaosaurus, and the ornithomimosaur Deinocheirus). They have numerous feathers coming off of the pygostyle, but not the sophisticated tail fan of euornithine birds. At present, all known omnivoropterygids are from a single formation (Jiufotang Fm. of China). At least some have stubby blunt teeth at the front of their snouts, suggesting a diet of seeds, fleshy parts of plants, and/or small invertebrates. In some recent studies omnivopterygids were found to be even more basal: more distantly related to modern birds than is Jeholornis. A systematic survey of hundreds of specimens failed to find an ossified sternum on these dinosaurs (the same is true for the troodontid Anchiornis and for Archaeopteryx).

The clade Pygostylia ("pygostyle ones") is comprised of all descendants of the concestor of Confuciusornithidae and modern birds. This group is further transformed from basal avialians by:

The Confuciusornithidae of Early Cretaceous eastern Asia are best known from Chinese Confuciusornis. Literally hundreds (if not thousands) of specimens have been discovered, making this crow-sized pygostylian the most common Mesozoic dinosaur fossil. Like modern birds, confuciusornithids had a toothless beak: however, this was convergent with Aves, since the branches between Confuciusornithidae and Aves are all toothed. These may have been at least partially plant- or fruit-eating (they co-occur with the oldest fruit!), but at least some confusiusornithids have fish in their gut contents.

Unlike Aves and their closest kin (together, the clade Euornithes), there was no tail fan in the Confuciusornithidae. Instead, most specimens show no major tail feathers, while others show two very long ones. Perhaps these were sexual signals, or growth indicators, or simply lost in the tail-featherless ones.

The hand of confuciusornithids was still a perfectly good grasping organ.

ORNITHOTHORACES
Before we move on, let's take a look at the postcranium of a modern bird (courtesy of Emporia State University's Vertebrate Structure and Development course:

(You'll note a lot of special terms, some of which we've already encountered.)

The remaining pygostylians form the clade Ornithothoraces ("bird chests"). These theropods are further derived in possessing:

The hands of ornithothoracines lacked grasping ability, having become true wings. I think from this point on up the cladogram, it is perverse not to call these animals "birds".

There are two main branches of Ornithothoraces: Enantiornithes and Euornithes.

The "opposite birds", Enantiornithes have a range from the early Early Cretaceous through the end of the Cretaceous. They seem to have been the most diverse and abundant group of birds during their history. Most enantiornithines were toothed. Some may have been insect eaters, some fish eaters, some fruit or seed eaters, and possibly even some meat eaters. The smallest were sparrow-sized; the largest, eagle-sized (and thus the largest flying birds of the Mesozoic). They have been recovered from all over the world, and environments from deserts to shores.

Enantiornithes retained small claws on the manus, but these were greatly reduced compared to earlier avialians. Despite many depictions to the contrary, enantiornithines do not seem to have the tail fan of feathers which characterize modern birds, and at most of a pair of long tail feathers (though rarely as long as those of confuciusornithids).

EUORNITHES
Modern birds and everything closer to them than to Enantiornithes form the clade Euornithes ("true birds": also known as "Ornithuromorpha" or "Ornithuraemorpha"). Transformations relative to basal bird groups include:

Among the early branches of euornithes are the seagull-sized fish-eating Songlingornithidae and Honshanornithidae (collectively the Yanornithiformes) of the Early Cretaceous of China and the flightless Patagopteryx of Late Cretaceous Argentina. Limited remains of a possible very large (140 kg) flightless euornithine, Gargantuavis are known from the Late Cretaceous of Europe.

Even more derived are the Carinatae ("keeled ones"), characterized by a keeled sternum to support the powerful wing muscles, as well as by 10 or more sacrals, loss of the manual unguals, a synsacrum (fusion of the sacrals together into a sacrum, fusion of the sacrum to the ilium, and fusion of all pelvic elements together), and complete fusion of the premaxillae. A notable Late Cretaceous toothed carinate is Ichthyornis of the seas of the American midwest. (It is worth noting that some studies reverse the order of this: Ichthyornis is closer to Aves than are hesperornithines, and Carinatae is thus within Ornithurae!)

Derived within the carinates are the Ornithurae ("bird tails"), in which the premaxilla makes up the majority of the upper jaw. One (extinct) branch of the ornithurines were the aquatic Hesperornithes (sometimes called "Hesperornithiformes"). Early hesperornithines (such as Early Cretaceous Enaliornis) may have still have had the power of flight, but later ones greatly reduced their wings. In fact, the most specialized forms lost their forearms altogether, with only stumps of humeri for wings. These latter include Hesperornis of the Late Cretaceous of the inland seas of midwestern North America. Even these advanced birds still retained teeth.

The most speciose branch of ornithurines is Aves.

AVES
"Birds" in the strictest sense: the concestor of modern birds and all of its descendants. Aves are distinguished from other carinates by toothless beaks (convergent with "elaphrosaurs", ornithomimids, oviraptorids, confuciusornithids, and an enantiornithine); the premaxilla makes up almost all of the beak (and thus the maxilla is extremely reduced); dentaries fused to each other (also, the tiny predentary bone is gone), and all bones in the mandible fused to each other; a pneumatic coracoid; the tibiotarsus(see above: fusion of the proximal tarsals to the tibia); 15 or more sacrals; and a number of other skeletal features.

The oldest members of Aves in this restricted sense are from the latest Cretaceous. The major groups of Aves (a clade that today contains 10,000 species, and prior to the Polynesian expansion over the last 2000 years were probably at 12,000 species!) include:

with several additional branches of uncertain relation with the others.

And with Aves, we have finished our review of dinosaurian diversity!

Aves is the only clade of Dinosauria to survive the great extinction.

EVOLUTIONARY PATTERNS IN EUMANIRAPTORANS
Feeding adaptation transformations:

Terrestrial locomotory adaptations:

Miniaturization (and a little Gigantism):

Correlated Progression:
This issue is best dealt with next lecture, as one of the main selective forces seems to be that marvelous dinosaurian success story: the conquest of the air!

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Last modified: 28 August 2017

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Detail of "The earliest birds" (featuring Archaeopteryx (left) and Aurornis (right)) by Emily Willoughby, from Callaway (2014) doi: 10.1038/516018a