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HONR 259C "Fearfully Great Lizards": Topics in Dinosaur Research

Fall Semester 2017
Dinosaurs in the Tree of Life


Early sauropodomorph dinosaur Buriolestes in the Late Triassic by Maurílio Oliveira

Key Points:
•Life emerged in the water, and it took a series of adaptations (defense against gravity and desiccation; new forms of respiration, reproduction, locomotion, and senses; etc.) for taxa to colonize the land.
•Among vertebrates, it was "amphibian"-grade tetrapods which partially made the transition to life on land, but only the amniotes that lived their entire life cycle on land.
•The amniotes had evolved a shelled egg, keratinized skin, and claws (among other traits) which allowed them to be successful at living on land.
•Different groups of amniotes radiated at different times: the first group were the basal members of the Synapsida (the lineage that contains mammals) in the Carboniferous and Early Permian; followed by their descendants (the Therapsida) in the Middle and Late Permian; then the Pseudosuchia (crocodile-line archosaurs) in the Triassic.
•Within Amniota (the terrestrial vertebrates with a shelled egg), dinosaurs are nested with the increasingly exclusive groups Sauropsida, Diapsida, Sauria, Archosauromorpha, Archosauriformes, Archosauria, Avemetatarsalia, Ornithodira, Dinosauromorpha, and Dinosauriformes.
•There is a shift towards obligate bipedalism, parasagittal (upright) posture, digitigrade stance, a grasping manus, and a perforate (open) acetabulum on the lineage up to Dinosauria.
•Dinosauria is comprised of three major clades: Ornithischia, Sauropodomorpha, and Theropoda. Traditionally, sauropodomorphs and theropods were recognized to form a clade Saurischia. However, recent discoveries have reduced the support for this hypothesis, and alternative relationships are possible.
•Dinosauria first appears by early in the Late Triassic, and may have actually originated even earlier. Many (although not all) of the oldest dinosaurs are from the southern parts of Pangaea, but this may be due to a sampling bias.
•There are several Triassic (Herrerasauria; Eodromaeus; Tawa + Chindesaurus; etc.) and even Jurassic (Chilesaurus) dinosaurs whose position within the clade are presently very uncertain.


I. Dinosaurs Among the Vertebrates
Dinosaurs are, obviously, animals. But what kind of animal are they?

Dinosaurs are vertebrates: they have an internal skeleton; a dorsally-located neural chord; skulls with eyes, nasal capsules, ear chambers, and brains; and many other traits shared with lampreys & hagfish, sharks & their kin, ray-finned fish, coelacanths, lungfish, and the terrestrial vertebrates. They are furthermore gnathostomes (vertebrates with jaws and teeth), osteichthyians (gnathostomes with lungs and an internal skeleton of bone), sarcopterygians (osteichtyians with support bones running down their fins/limbs, and choanates (sarcopterygians with a choana, or internal nostril opening).

Ancestrally, all the above vertebrate groups lives strictly in the water. In other words, they were "fish" (a paraphyletic assemblage).

The transition to vertebrates that we wouldn't call "fish" occurred during the Devonian Period (419.2-358.9 Ma). The animals involved were simply among the great diversity of "fish" at the time (the Devonian has been nicknamed the "Age of Fishes" due to the number of lineages present). Like some of the other sarcopterygians (and later some of the other fish groups), the new forms could move on land for limited amounts of time: to find prey, escape predators, move from pond to pond, and so on. Unlike other fish, though, this new group called stegocephalians had evolved wrists to give them extra "push" and digits (that is, fingers and toes) to give them better purchase on land.

Many of these Late Devonian stegocephalians still lived their lives essentially only in the water (such as Acanthostega), and thus were essentially fish-with-fingers. Others (such as Ichthyostega) may have gotten most of their food from land. It was from these latter sort that the more fully terrestrial vertebrates--the Tetrapoda ("four footed ones"), would evolve.

During the early part of the Carboniferous Period (358.9-298.9 Ma) the early tetrapods became established. Unlike earlier stegocephalians, these had necks (that is, the skull was not attached to the scapula), which allowed them to bend their heads down to capture food on land. Like fish, these early tetrapods still laid their eggs in the water and their young (tadpoles) often had gills. As adults, though, many were more terrestrial.

In that way, the early tetrapods were "amphibians" in the broad sense of the term. However, these were often scaly-skinned (like fish), unlike the naked-skinned Lissamphibia (frogs, toads, salamanders, etc.: the modern clade of amphibians). Carboniferous and Permian (299-251 Ma) Period primitive tetrapods included a great diversity: snake-like forms; primarily aquatic boomerang heads; alligator-like forms; and many others. But even for those forms which spent most of their adult lives on land had to come back to the freshwater to reproduce.

During the Carboniferous, however, one group of tetrapods evolved an adaptation that allowed them to break free of the pond: the amniotic egg. Instead of being a "naked" egg laid in a pond or stream, the amniotic egg had:

Additionally, amniotes have claws (which aid in digging nests on land, among other behaviors) and intromittent sexual organs (i.e., a phallus of some form) allowing fertilization while the eggs are still within the female.

In other words, tetrapods were now freed from the water. As such, the tetrapods with an amniotic egg did not have a larval ("tadpole") stage, and so they were terrestrial for their entire life cycle. This adaptation marks the evolution of the most successful clade of tetrapod, the Amniota:

There are two major clades within Amniota. Synapsida includes mammals and all taxa closer to us than to the sauropsids. The synapsids were the first group to radiate, becoming diverse in the Late Carboniferous and Permian. Synapsids are characterized by a skull with a complete infratemporal fenestra with a lowerbar but no supratemporal fenestra.

During the Early Permian Epoch primitive synapsids radiated into many different forms:

These early synapsids would have had the sprawling stance found in primitive tetrapods in general. They almost certainly would have been "cold-blooded" (the ancestral state for vertebrates). So traditionally these animals have been considered "reptiles". However, they lack the shared derived features of reptiles (see below), and are instead simply primitive synapsids. (In traditional taxonomy, these were put in the group "Pelycosauria", but this is a paraphyletic group: all synapsids except for therapsids). New discoveries show that even early synapsids had some parental care.

The early synapsids evolved themselves into extinction: that is, they were replaced in the Middle and Late Permian Epoch by the Therapsida: the advanced synapsids. Once called the "mammal-like reptiles", they are not true reptiles. Instead, they are the advanced branch of the synapsid phylogeny. They differed from earlier synapsids by:

The Middle and Late Permian therapsids included:

Ancestrally, all vertebrates are cold-blooded (warm their bodies primarily using sunlight). However, some evidence suggests that the advanced therapsids of the Late Permian may have had elevated metabolisms (that is, were at least partially warm-blooded):

Additionally, some therapsids seem to have had parental care of the young, keeping them in burrows.

The other great clade of amniote is the Sauropsida (sometimes referred to by the older, but more problematic name "Reptilia"). Sauropsids tended to be relatively rare in the Carboniferous and Permian Periods. Sauropsida is characterized by a number of particular skeletal features (which we aren't going to deal with here, as they are fairly technical). Modern sauropsids (and by inference, their concestor and all of its descendants) share a number of soft-tissue features:

Parareptilia is a major clade of basal sauropsids which flourished alongside the synapsids during the Permian, but became rarer in the Triassic. The most primitive of these groups (the aquatic Mesosauridae) was thought (from 2012 to 2014!) to have an infratemporal fesestra, but this appears to be a mistake. Most other basal parareptiles have the no-fenestra (anapsid) condition, although a few derived ones have a fenestra (lacking the lower bar), which is best thought of as independently derived from synapsids and diapsids. Because most parareptiles had the anapsid condition, they were traditionally called "Anapsida", but now called "Parareptilia" (or sometimes "Proganosauria"). Parareptiles underwent a radiation in the Permian, producing:

For many years it was argued that turtles were a branch of parareptile. And it is true that the oldest known animals more closely related to turtles than any other named group is a wide-bodied Permian form with broad ribs. New discoveries of this animal (Eunotosaurus) and of Triassic proto-turtles show that the turtle lineage is a type of diapsid, however, and not parareptiles at all.

Other primitive Carboniferous and Permian sauropsids show the anapsid condition, but were eureptiles (the clade containing diapsids [see below] and their closest relatives).

Relatively rare during this time are the oldest known members of Diapsida. This would eventually evolve into the major clade of eureptiles, and the dominant clade of amniotes during the Mesozoic. Changes from the ancestral sauropsid condition to Diapsida include:

During the late Paleozoic diapsids were only a very minor part of the ecosystem.

In general, compared to typical Mesozoic and Cenozoic ecosystems, the late Paleozoic land vertebrates were smaller (few ox- or hippo-sized, none larger), slower (no real speed specialists), and close to the ground (only a few gliders and no powered fliers; few tree-climbing specialists).

At the end of the Late Permian, the greatest extinction in the history of life clobbered ecosystems on land and sea. This event totally changed the make-up of the diversity of life, and forms the boundary between the Permian Period of the Paleozoic Era and the Triassic Period of the Mesozoic Era. Consequently, it is called the Permo-Triassic Extinction. Perhaps 95% of all species died out.

The cause seems to ultimately have been the Siberian Traps, a monumentally huge series of volcanic eruptions. These raised global temperatures through extremes of greenhouse gasses; this further triggered additional greenhouse gasses being released from the sea floor. Together, extremes of heat and of carbon dioxide and very low levels of oxygen caused mass deaths.

Regardless of precise scenario, extinction reorganizes the world. In the immediate aftermath, there was a great reduction in size and diversity of the animals present.

Lost were many of the primitive therapsids and many of the primitive reptiles.

During the Early Triassic diversity started off very low in land and sea, and recovered over the next ten million years or so. On land, more advanced carnivorous therapsids were the dominant predators, and piglet to ox-sized herbivorous therapsids were common. But the sauropsids (and especially diapsids) began to radiate into the number of different forms:

Within Archosauromorpha is the clade Archosauriformes, characterized by the antorbital and mandibular fenestra. The former is related to sinuses in the snout; the latter for jaw muscles. Early archosauriforms ranged from cat to cow size. Within the archosauriforms was Archosauria ("ruling reptiles"), the dominant group of Mesozoic sauropsids.

Archosauriforms are also characterized by the "semi-improved" or "semi-erect" posture, so that they could move sometimes without having to twist from side-to-side; by interlocking gastralia that may have worked to help pump extra oxygen through the body: belly-breathing; and possibly by early versions of the air sac system (more about that in the later part of the course)


II. Archosaur Diversity, Past and Present
Simplified cladogram of Archosauria.


More detailed phylogeny of Avemetatarsalia, with concentration on the early dinosaurs.

It is not certain if these early archosauriforms had the behavioral traits found in both the living groups of Archosauria (that is, crocodylians and birds). Because both crocodylians and birds share the following derived traits, however, it is fairly certain that at least the concestor of all Archosauria had them, and passed them on to its descendants:

During the Middle and Late Triassic, the archosaurs displaced the therapsids as the dominant group of large terrestrial amniotes.

Archosaurs are divided into two main branches:

It is the pseudosuchians which dominated the Middle and Late Triassic. This group is characterized by a complex heel-based ankle (the crurotarsal ankle). This group radiated into a number of forms:

The pseudosuchians include some of the first terrestrial animals to exceed the size of oxen and hippos. Most of them could stand with a semi-erect posture of the limbs, and a few had the fully-erect (that is, parasagittal gait).


III. Origin of the Dinosaurs
Modern birds and all archosaurs closer to them than to crocodilians are the Avemetatarsalia (bird metatarsals). Just recently the most basal branch of avemetatarsalians was discovered: the Aphanosauria ("hidden reptiles"). Only recognized in 2017, these were medium-sized (2-3 m long or so) quadrupedal, long-necked Middle Triassic archosaurs. Best known of these is Teleocrater of Tanzania.

The remaining avemetatarsalians belong to the Ornithoidira ("bird necks"). These were generally small-bodied animals that evolved under the shadow of their pseudosuchian cousins, differing from typical diapsids by having elongate tibiae and metatarsi (suggesting that they were even faster than typical sauropsids.) Their name is derived from the fact that their cervical vertebrae were greatly modified so that they look very different from dorsal vertebrae, but instead could allow the neck to form an S-shaped curve.

One branch of the ornithodirans that appeared in the Late Triassic are the Pterosauria: the first group of powered fliers among the vertebrates. We'll look more at the pterosaurs in the third section of the course.

The other major branch of ornithodirans are the dinosauromorphs. The oldest dinosauromorphs are known from the Middle Triassic in terms of body fossils, but footprints show that some very small dinosauromorphs were present as early as the Early Triassic. Dinosauromorphs modified the hindlimbs further than the ancestral ornithodirans to evolve:

The combination of these traits allowed the little dinosauromorphs to run and accelerate in a bipedal mode all the time, not just at top speeds like typical diapsids. The parasagittal gait and hinge-like ankle also allowed dinosauromorphs the ability to move more actively and constantly rather than only in short bursts of speeds; thus, they were striders. Additionally, although early dinosauromorphs were small (~30 cm long for Marasuchus, 1-3 m long for "silesaurs" and basal dinosaurs, etc.), the presence of limbs directly underneath the body meant that this lineage to grow to much larger size than any previous clade while still remaining terrestrial and mobile (sprawlers relegated to a semi-aquatic life if they became too big to support their weight).

Early dinosauromorph lineages were typically a meter or less long. These include the lagerpetids, such as Ixalerpeton, Dromomeron, and Lagerpeton; and the lagosuchids, such as Marasuchus and (possibly) Saltopus.

The largest dinosauromorphs other than dinosaurs are the Silesauridae. These were quadrupedal animals typically about 1-2 m long (although fragmentary remains of one at least 3 m long has been found in Tanzania). They and dinosaurs share the specialization of elongate pubes and ischia (which may have aided in increased respiratory ability: more air per breath). Primitive forms look to have been carnivorous, but most silesaurids were herbivores. Silesaurids had been known from teeth and bones of the Middle and Late Triassic from around the world, but had been thought to be from primitive dinosauromorphs, ornithischians, sauropodomorphs, and/or theropods until the more complete specimens of Asilisaurus and Silesaurus itself let people know what this group looked like. At least some have a toothless portion of the front end of the dentary, likely covered by a rhamphotheca (beak).

Also occurring in the Middle Triassic (of Tanzania, in this case), is Nyasasaurus. Incompletely known, what data is available shows that it is either a dinosauromorph closer to Dinosauria than are silesaurids, or (possibly) the oldest known dinosaur. (At least one study provisionally put it quite far up the dinosaur tree, as a massospondylid "core prosauropod". If true, this would mean a tremendous part of the diversification of Dinosauria had occurred by the Middle Triassic that is not yet documented in the fossil record.)


IV. Dinosaur Groups and Relationships
Simplified cladogram of Dinosauria

Silesaurids and Nyasasaurus are the closest relatives to Dinosauria, but they do not seem to be true dinosaurs: that is, they lack the shared derived features expected in the concestor of Iguanodon, Diplodocus, and Megalosaurus and its descendants. (But see below.) Those features (the shared derived features of Dinosauria) are:

A possible shared derived feature of Dinosauria is the presence of at least some fuzz rather than scales. This rather startling concept is because basal members of one lineage and more derived members of the other show the presence of such structures. While many dinosaurs are preserved with impressions of scales, there are several with other forms of integument. These include:

At present we have no definite evidence of fuzz or other non-scales in sauropodomorphs. But it is possible that a dinosaurian evolutionary novelty is the ability to generate at least some quill, tuft, or branching integument rather than just scales. However, different clades of dinosaurs express this ability in different fashion.

Early dinosaurs were all around 1-2 m long bipeds with grasping hands. Footprints that might come from dinosaurs are found in the Middle Triassic of Argentina, but these may be from early members of the dinosaur lineage that evolved before the origin of Dinosauria proper.

The oldest known definite dinosaurs are from about 230 Ma, at the very beginning of the Late Triassic Epoch. The very oldest dinosaurs are from Argentina (the Ischigualasto Formation) and Brazil (the Santa Maria Formation), and from India; fossils from the same age from Europe, North America, and elsewhere have turned out to be non-dinosaurian.

Dinosauria contains three major clades:

Until recently essentially all dinosaur researchers considered the most basal split in Dinosauria to be between Ornithischia and a clade named Saurischia ("lizard hips"), the latter defined as Megalosaurus, Diplodocus, and all taxa closer to these taxa than to Iguanodon. However, recent studies call into question that relationship: we'll go into that in a little more detail shortly.

Well look at the shared derived characters of Sauropodomorpha and Theropoda in later lectures. But let's look a bit at the base of Ornithischia:

Simplified cladogram of Ornithischia:

Ornithischians are, at present, not definitely present in the Triassic. Since the 1970s the oldest and most primitive ornithischian was thought to be Pisanosaurus of the early Late Triassic Argentine Ischigualasto Formation. The fossil is incomplete, so many aspects of its anatomy are uncertain. But new analyses in 2016 and 2017 have shown this is probably a silesaurid, not a dinosaur.

Ornithischia is characterized by the following traits:

Based on their tooth form and the retroverted pubis, ornithischians were herbivorous. (That doesn't mean that they were exclusively plant eaters, of course! In the modern world, many "herbivorous" sauropsids and mammals eat some meat.)

Major Alternative Dinosauria Arrangement I: Silesaurids are the Triassic Ornithischians: There are a number of derived features of the teeth and jaws shared by Ornithischia and Silesauridae. The traditional interpretation is that these are convergences, since silesaurids like the shared derived characters of Dinosauria. That said, a few analyses actually place Silesauridae as a basal branch in Ornithischia. An advantage of this hypothesis is that it explains where the Triassic ornithischians are. However, if true than the features regarded above as shared derived dinosaurian traits (such as the modified hand and the perforated acetabulum) either evolved independently in derived ornithischians and saurischians, or were lost in Silesauridae. We'll see how this proceeds.

Simplified cladogram of Saurischia:

Saurischians are named after a primitive trait ("lizard hips" = "forwards-pointing pubis"; the ancestral condition), but they are in fact united by some shared derived traits. These include modifications of the snout and of the vertebrae: however, these are rather technical features and we're not go into them in detail in this course. Additionally, some of the traits long thought to be basal saurischian traits are now popping up in early ornithischians and in silesaurs, and so are not actually saurischian shared derived characters.

The saurischians were more common in the Late Triassic than their ornithischian sisters. The theropods include the long-necked herbivorous Sauropodomorpha and the (ancestrally carnivorous) Theropoda. Among other traits shared among the saurischians are:

However, additional possible shared derived features uniting all saurischians are less certain due to contradictory information between the basalmost sauropodomorphs and the basalmost theropods. We will explore sauropodomorph and theropod diversity in weeks to come.

(A special note: Owen's Dinosauria traits re-examined: If we look at some of the traits that Owen used to recognize Dinosauria, we now find that some were actually inherited from pre-dinosaurian ancestors (e.g., parasagittal stance, from the early dinosauromorphs) and others evolved convergently between Iguanodon, Hylaeosaurus, and Megalosaurus (giant size; more than two sacrals). Still, even though the traits to recognize them have changed, we still use the name "Dinosauria" to unite these reptiles.)

Major Alternative Dinosauria Arrangement II: Ornithoscelida: The dinosaur research community was rocked in March 2017 by a paper by graduate student Matthew Baron and his advisors. They presented evidence that Sauropodomorpha was not the sister group to Theropoda, and instead that theropods and ornithischians were clsoer to each other than either was to sauropodomorphs. Their analysis had a greater sample of early dinosaurs and dinosauromorphs than previous ones. The resurrected an old name, Ornithoscelida ("bird legs"), for the clade comprised of Megalosaurus, Iguanodon, and all taxa closer to them than to Sauropodomorpha. Ornithoscelida is supported by numerous (if sometimes subtle) traits of the skull, vertebrae, humerus, and hindlimb. (Among these are a metatarsal I that does not reach the ankle joint.) In this scenario Herrerasauria seems to be on the sauropodomorph line, or possibly more basal.

Major Alternative Dinosauria Arrangement III: Phytodinosauria: As they used to say in the old commercials, "but wait, there's more!" Back in the 1980s a possible phylogenetic hypothesis put forth by several researchers was the union of Sauropodomorpha and Ornithischia to the exclusion of Theropoda. This was given the alternative names "Ornithischiformes" or (more commonly) "Phytodinosauria" ("plant [eating] dinosaurs"). This hypothesis is based on the shared presence of phyllodont dentition and several other skull features (including a possible cheeks and beaks), associated with herbivory. A working definition for the clade would be "Diplodocus, Iguanodon, and all taxa sharing a more recent common ancestor with them than with Megalosaurus." No recent computer-generated phylogenetic analysis has supported, but possible support for this arrangement may exist in the enigmatic Chilesaurus (more about it below). In one possible variation of this hypothesis, animals currently considered basal sauropodomorphs may actually be ancestral to ornithischians, explaining the apparent absence of bird-hipped dinosaurs in the Triassic.

Chilesaurus's Relationship Status: It's Complicated: In 2015 a new dinosaur from the Late Jurassic of Chile was described. Several individuals had been found, both juveniles and adults up to 3.2 m long. Given the name Chilesaurus, this dinosaur has a suite of traits associated with different dinosaur clades. Its short skull and long neck resemble those of basal sauropodomorphs; its backwards-pointing pubis and possible beak at the end of the dentary are similar to those of ornithischians; and various vertebral, scapular, pelvic, and hindlimb traits are comparable to theropods (and especially primitive tetanurines.) Its teeth are blunt and have fine denticles, strongly suggesting a herbivorous diet: however, they do not resemble the teeth of either basal sauropodomorphs or ornithischians. The initial analysis found little Chilesaurus to be a bizarre herbivorous tetanurine theropod; alternatively, a paper in August 2017 found it to be the most primitive ornithischian (branching off before the origin of the predentary). Just throwing this out there: it would also make a great amount of sense as a basal ornithischian derived from sauropodomorph-like ancestors under the "Phytodinosauria" hypothesis. Given its position in time, either of these latter ideas require it to be a late survivor retaining the attributes of much older Triassic ancestors. In any case, an earlier representative of the Chilesaurus-lineage would be very helpful in pinning down its place in the (already destabilized) dinosaur family tree.

V. Late Triassic Dinosaurs Of Uncertain Position
As you might notice from the further information boxes above, our knowledge of the basal relationships among dinosaurs is somewhat destabilized at present. One consequence of this is that a number of Triassic taxa cannot at present be securely placed in any of the three major clades. These include:

When our knowledge of the basal relationships within Dinosauria are better established, the positions of these Triassic basal saurischians (probably) should settle down.

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Last modified: 27 September 2017

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The Middle Triassic near-dinosaur (or primitive dinosaur) Nyasasaurus and the rhynchosaur Scaphonyx by Mark Witton