I. 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 toward the end of the course.

(Pterosaur origins are somewhat obscure. Although most researchers do consider them a sort of ornithodiran archosaur, a few hypothesize that they split from other diapsids before the pseudosuchian-ornithodiran split. If so, then they did evolve several similarities with true ornithodirans. Unfortunately, at present we have no pterosaur-lineage animals which are not already highly derived for flight, so we can't yet trace the transformations from walking to flying in this group.)

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 parasagittal stance: the erect posture with the hindlimbs positioned directly underneath the body rather than sprawling to the sides
  In this posture, the muscles running from the femur to the tail are still the main source of propulsion, but that motion is limited to front-to-back motion
• Simple hinge-like ankle joints (the "advanced mesotarsal" condition)
• Digitigrade posture: standing on the "balls of the feet" with the metatarsal held up, allowing for a longer stride

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). (As will be discussed later on, an alternative hypothesis is that silesaurids ARE dinosaurs!)

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.)

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 highly modified large manus with
  • Semi-opposable thumb
  • Grasping ability of digits II and III
  • Reduced digits IV and V
• Because of the transformation of the hand, dinosaurs were obligate bipeds (at least at first)
• An enlarged muscle attachment surface (the deltopectoral crest, "dp" in this figure) on the humerus, again suggesting specialization of the forelimb from the ancestral walking condition. (This trait is also present in Nyasasaurus).
• A perforate acetabulum: the medial wall of the hip socket formed by the pubis and ischium in other amniotes (including silesaurids) is missing, and was simply cartilage. (In all but the most primitive dinosaurs, the wall formed by the ilium is also removed.) So, as preserved, there is just a hole between the ilium, pubis, and ischium in dinosaur pelves
• Expansion of the space for the supratemporal fenestra forward onto the skull roof
  • Traditionally considered for increasing the power of the temporal muscles, a recent hypothesis is that the dorsal portion of this is to expand the surface for heat-dumping vascularized tissue. (As we will see later in the course, dinosaurs show a trend for developing adaptations to remove excess heat from the body.)

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:

• The fuzz of the heterodontosaurid Tianyulong
• The many types of fuzz, bristlescales, and other projections on the neornithischian Kulindadromeus
• The quills of the primitive ceratopsian Psittacosaurus
• The fuzz on the megalosauroid (or is it a coelurosaur?) Sciurumimus
• The fuzz on all primitive coelurosaurs for which the integument is known
• Pennaceous feathers in ornithomimosaurs and maniraptorans

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:

• Ornithischia ("bird hips"): Iguanodon and all taxa sharing a more recent common ancestor with Iguanodon than with Megalosaurus or Diplodocus
• Theropoda ("beast-footed ones"): Megalosaurus and all taxa sharing a more recent common ancestor with Megalosaurus than with Iguanodon or Diplodocus
• Sauropodomorpha ("sauropod forms"): Diplodocus and all taxa sharing a more recent common ancestor with Diplodocus than with Iguanodon or Megalosaurus.

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 some new analyses in 2016 and 2017 have shown this is probably a silesaurid, not a dinosaur (although others show it is is indeed one.) (As we will see, there is an hypothesis where it is BOTH a silesaurid and an ornithischian!)

Ornithischia is characterized by the following traits:

• The predentary bone: a single midline bone joining the left and right dentaries:
  • Covered in life with a horny beak
  • Allowed slight rotation of dentaries to help them chop up food
  • Similar structures known in a few other animals, including paired edentulous (toothless) anterior dentary ends in silesaurids
• Broad phyllodont dentition (that is, broad leaf-shaped teeth with large denticles)
• Five or more sacral vertebrae
• Inset tooth row and ridges along the maxilla and dentary suggesting that ornithischians had a muscular or skin cheek.
  • If so, this would help keep food in the mouth while munching. However, this hypothesis has its detractors, and some suggest the alternative of a large horny surface along the sides of the mouth instead.)
  • The latest and most comprehensive work shows that a large muscle from the temporal region to the dentary occupied this space, associated with chewing ability. Thus, this wasn't a cheek, but also there was some "meat" in this area.
• A Opisthopuby (backwards-pointing pubis) (the "bird-like" aspect of their hips that gives this group its name)
  • NOT present in Pisanosaurus and silesaurids (if one or the other are ornithischians), but present in all the others.
  • Allows for greater gut capacity for digesting plants without making the body too wide
  • Backwards pubis evolves independently among different advanced saurischian groups, as we will see later
• Epaxial (above the vertebrae) ossified (turned to bone) tendons, stiffening the back and (perhaps) acting as a "spring" to absorb and release energy while running

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.)

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 silesaurids, 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:

• Specialized projections in the vertebrae
• Hollow chambers in the cervical vertebrae for a series of air sacs: in derived groups of both sauropodomorphs and theropods, posterior vertebrae are also invaded by the air sacs
• An enlarged hand in which metacarpal I is reduced and the thumb claw is enlarged
• Elongated neck in many forms

(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.)

III. 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:

• Eodromaeus from the Late Triassic Ischigualasto Formation of Argentina. Generally considered among the most primitive theropods known (in fact, in some ways it appears to be the closest model to an ancestral theropod.) It is quite small (only a meter or so long) agile minor predator. However, some studies put it as either a herrerasaur and/or as a basal saurischian outside of the Sauropodomorpha-Theropoda group (properly called "Eusaurischia")
• Herrerasauria, an exclusively Late Triassic group known from South and North America. These are larger: 3 m or so long, with deeper skulls than Eodromaeus. However, herrerasaurs were not the apex predators of their environments, as larger pseudosuchian predators dwarfed (and presumably hunted) them. Some studies put them as basal theropods; other as basal saurischians; still others as basal sauropodomorphs (including some analyses that support the "Ornithoscelida" hypothesis); and yet more as outside Dinosauria proper.
• Little buck-toothed Daemonosaurus of the later Late Triassic of New Mexico. Traditionally considered a basal theropod, some studies place it as a basal (non-eusaurischian) saurischian. It should be noted that the shape of its skull is more reminiscent of early ornithischians and some basal sauropodomorphs than to typical early theropods... (A postcranium for Daemonosaurus would be helpful, to say the least.)
• Late Triassic New Mexican Tawa and its likely sister-taxon Chindesaurus shares several features with basal theropods such as a kink between the maxilla and the premaxilla (often with a corresponding large dentary fang below): this was likely a spot to hold onto narrow prey (modern crocodilians with a similar snout pattern do the same). Additionally, various hindlimb traits are shared between Tawa, Chindesaurus, and theropods. These are long-necked, long-limbed, slender animals. Most analyses have placed Tawa and/or Chindesaurus in Theropoda (at least some phylogenetic analyses place Tawa as a coelophysoid), but some put it outside Eusaurischia.
• Late Triassic Brazilian Guaibasaurus has been interpreted as a possible basal theropod in some analyses, as a basal sauropodomorph in others, and as a basal saurischian in yet more.

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

Although dinosaurs appeared in the early part of the Late Triassic ecosystems, they were just a minor part. For example, other animals were far more common in the Ischigualasto Formation than Eoraptor, Herrerasaurus, and Panphagia. Theropods were not the top predators in any known Late Triassic assemblage: that distinction goes to big pseudosuchians. By the end of the Late Triassic, sauropodomorphs HAD become the largest and most common herbivores in some ecosystems (away from the equator).

Recent work shows the earliest dinosaurs and the earliest members of most dinosaur lineages showing up in the southern continents (especially South America and southern Africa) first. New discoveries from the lower part of the Chinle Formation of the American Southwest shows that diverse non-dinosaurian dinosauromorphs were still a major part of the assemblage well into the later part of the Late Triassic.

So most of the the Late Triassic is the time WHEN DINOSAURS SHARED THE EARTH!. Pseudosuchians remained ecologically diverse and important, and (especially in the early Late Triassic) non-archosaurian archosauromorphs and therapsids were also significant. In the later Late Triassic (in Gondwana and Europe, at least) things shift: pseudosuchians remain important (especially in the form of paracrocodylomorphs, aetosaurs, and phytosaurs) as did dicynodont therapsids, but sauropodomorphs expand in diversity (and size!). In recent years it has been suggested that a major environmental shift was the cause of this. Sedimentological analysis shows that the world got considerably wetter from 234-232 Ma. Its particular trigger isn't well understood, but it affected life on both land and sea. This event occurred in the Carnian Stage, and so is called the Carnian Pluvial Event ("pluvial" = "of or concerning rain"). One of the changes on land is the diversification and spread of groups of tree-sized plants (especially conifers and the now-extinct cycadeoids). It has been argued that the accumulation of this new leafy biomass may have promoted the expansion of the sauropodomorphs, and thus of Dinosauria.

(That said, the phenomenon is NOT apparent in North America! Both the excellent body fossil record of the Southwest and the trackway record of the East show no spread of sauropodomorphs, or dinosaurs in general, in North America in the later Carnian and subsequent Norian and Rhaetian stages.)

At the end of the Late Triassic, though, there was a mass extinction that affected the entire planet: the Triassic-Jurassic Extinction. The primary trigger seems to have been a period of tremendous volcanism in the middle of Pangaea. 201 Ma saw the beginnings of the rupture of the supercontinent of Pangaea, between the landmasses that are today North America, Europe, and Asia (on the north) and South America, Africa, and the other continents (to the south). This produced the Atlantic Ocean in the long term, but for the inhabitants of the Earth at the time suffered.

A vast amount of greenhouse gasses (both carbon dioxide and methane) were released, producing catastrophic global warming. But this was preceded by a burst of sulfur dioxide, which produced a rapid phase of cooling. There is additional evidence that poisons like mercury were released into the environment.

These massive pulses of environmental change were too much for many groups of organisms to survive. This is one of the five greatest mass extinctions known, with many terrestrial and marine taxa wiped out. Among terrestrial vertebrates, some of the main victims were:

• Non-dinosaurian dinosauromorphs
• Pseudosuchians other than the direct ancestors of crocodilians and their closest relatives
• Therapsids other than the tiny direct ancestors of mammals and their closest relatives

With no serious competition remaining, dinosaurs radiated into numerous forms. From the beginning of the Early Jurassic onward, DINOSAURS RULED THE EARTH.

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Last modified: 9 October 2020