32.1: Saurischians

Theropods

Figure \(\PageIndex{1}\): Drawing of Coelophysis compared to an adult human. (Image from Petrified Forest National Park.)

Saurischian dinosaurs have two major subdivisions: theropods and sauropodomorphs. The theropods are a diverse and complex group that are famous for meat eating. There is evidence of other dinosaurs being omnivores, and there are some theropods that strayed from a carnivorous diet, but all of the famous dinosaurs with big sharp teeth are theropods. Early (basal) theropods were swift and small like Coelophysis. Its name, which means ‘hollow form’, is a reference to an important trait that all theropods share: hollow bones. This made them lightweight, and therefore presumably more agile, which is helpful in chasing down prey. A less obvious but related trait they share is a series of air sacs used throughout the body used in respiration, at least partially filling the empty spaces in bones. This mechanism, seen in their living descendents today (birds), also helped allow a high level of activity while releasing heat more efficiently. Their other most notable shared trait is the three-toed feet with sharp claws.

Figure \(\PageIndex{2}\): Cast of Dilophosaurus skeleton, found in Arizona, from the Royal Ontario Museum. (Photo by Eduard Solà via Wikimedia.)

Evolutionarily, the theropods trended larger over time, a development first seen in Dilophosaurus. Contrary to its petite portrayal in the Jurassic Park movie franchise, it was over 20 feet long. As features such as a neck frill and the ability to spit poison are difficult to preserve in the fossil record, there is no direct scientific evidence for either. By the end of the Jurassic, these early large theropods diversified into famous forms like Ceratosaurus.

Figure \(\PageIndex{3}\): An illustration of Carnotaurus (by Andrey Atuchin via Wikimedia.)

Ceratosaurs would become truly massive in the Cretaceous, especially in South America, which was an isolated continent at the time. This is most exemplified by Carnotaurus, a name that means “the meat-eating bull,” which had incredibly small vestigial arms. A sister group to the ceratosaurs, Tetanurae, was a more advanced branch of the theropod tree. This would have several branches, including the allosaurs (Allosauroidea), which lead to animals like Allosaurus, and the megalosaurs (Megalosauroidea), which had the crocodilian-like spinosaurs and the first dinosaur ever scientifically described, Megalosaurus. Interestingly, the allosaurs also got large and fierce in South America, with animals like Carcodontosaurus and Giganotosaurus, which were both estimated to be up to 40 feet long. These are arguably the largest land-dwelling theropods ever.

Figure \(\PageIndex{4}\): Yutyrannus tail fossil with feather-like impressions. (Photo by ★Kumiko★ from Tokyo, Japan via Wikimedia.)

The third branch off Tetanurae is Coelurosauria. This group is best known for the evolution of feathers. While skin and related features are rare to preserve, there is significant evidence that scientists have been able to estimate that all members of this group had some form of feathers. Some of the larger members may have shed them with age (like the birds do with down that keeps them warm as juveniles) or evolved them away, but those details are difficult to discern from the fossil record. Most of the largest members of the group, where skin impressions have been found, show no feathers at all. This was complicated in 2012 when Yutyrannus (meaning ‘feathered tyrant’) was described. It was clearly a coelurosaurian, and an early tyrannosaur, but was almost 30 feet long and covered in early feather-like structures.

Figure \(\PageIndex{5}\): Largest Tyrannosaurs compared to a human. (Drawn by PaleoGeekSquared on Wikimedia.)

The earliest group of the coelurosaurians are the tyrannosaurs. Starting out small in the Jurassic, with many early members smaller than a full-grown human, their descendents became massive by the late Cretaceous. Famous examples include Daspletosaurus, Albertosaurus, Tarbosaurus, and Tyrannosaurus. All of the large members of the family had short arms with two digits, though evidence suggests that the arms still had some function. What that function is conjures much debate, from predation to mating. The group appears to have evolved in Asia, and although many of the larger, later members are famous from western North America, they also lived in Asia, on the east coast of the United States, and possibly even Australia.

Figure \(\PageIndex{6}\): Reconstruction of Deinocheirus (by FunkMonk via Wikimedia.)

Heading along the coelurosaur family tree, the next major group is Compsognathus and relatives, which were very small and delicate and probably ate insects and small animals like early mammals and lizards. Next are the ornithomimosaurs, whose name means “bird mimic lizards.” These dinosaurs had a body plan similar to ostriches, including a toothless beak. No teeth means a meat-eating diet is not an option, so these animals were mainly herbivores. They were only moderately sized, with the exception of Deinocherius, who was truly strange! It was 35 feet long, including a 3.5 foot long head with a duck-like bill, and the tail vertebrae which implies tailfeathers. It also had 8 foot long arms (the longest of any theropod) with giant claws and a sail on its back.

Figure \(\PageIndex{7}\): Therizinosaurus reconstruction (by Nobu Tamura via Wikimedia.)

A sister group to the ornithomimosaurs is the maniraptorans, or ‘thief hands.’ They have several adaptations that make them more bird-like, including the wishbone, which only they have. Dinosaur groups within Maniraptora include the short-armed, one-fingered alverezsaurs, the therizinosaurs who had the longest claws in the history of life (at over 3 feet), the poorly named (not egg eating or stealing but brooding) oviraptors, and the group paraves, which means ‘next to birds.’ Subdivisions of Paraves include Eumaniraptora and Deinonychosauria.

Figure \(\PageIndex{8}\): Various dromeosaurs (raptors) compared to humans. (Image by Fred Wierum via Wikimedia.)

Within Paraves, the most notable dinosaur group is the dromaeosaurs, colloquially known as the raptors. The trait that most identifies them is their second toe sickle claw, which was held above the ground. Velociraptor is perhaps the most famous member of the group due to the Jurassic Park films, but the animal in the movie is actually patterned after Deinonychus, a larger raptor. In fact, the animal that most matched the movie depiction is Utahraptor, which ironically was published and named just days after the film’s premiere. Deinonychus is also important because when it was described in 1969 by Jon Ostrum and Robert Bakker, it helped spark the Dinosaur Renaissance, which led to our modern ideas of dinosaurs as dynamic animals instead of the lumbering beasts of the early 20th Century.

Video by PBS that breaks down how dinosaurs are related to birds.

Figure \(\PageIndex{9}\): The famous Berlin Specimen of Archaeopteryx. (Picture by H. Raab (User: Vesta) via Wikimedia.)

A related group to the dromaeosaurs is avialae, which can be considered (depending on definitions) to be birds. It is assumed that all members of this group were capable of powered flight. Archaeopteryx, traditionally known as the first bird, would be an early member of this group. This group originated in the Jurassic and became quite prolific in the Cretaceous, and led to the group Neornithies, which is the clade that includes all modern birds. Current analyses consider the ratites, the group which includes large flightless birds like ostriches and emus, to be the most primitive in the modern bird phylogeny.

Theropods: Did I Get It?

Exercise \(\PageIndex{1}\)

Besides feathers, what unique adaptation do theropods have that was taken advantage by birds?

a. Sharp claws

b. Long Necks

c. Teeth

d. Hollow bones

Answer

d. Hollow bones

Exercise \(\PageIndex{2}\)

Which theropod group is closest to birds?

a. Dromeosaurs (raptors)

b. Tyrannosaurs

c. Alverezsaurs

d. Carnosaurs

Answer

a. Dromeosaurs (raptors)

Exercise \(\PageIndex{3}\)

Which theropod was earliest (evolutionarily oldest)?

a. Megalosaurus

b. Allosaurus

c. Carnotaurus

d. Coelophysis

Answer

d. Coelophysis

Sauropods

Figure \(\PageIndex{10}\): Skeleton of Plateosaurus. (Photo by Ryan Somma via Wikimedia.)

The other group of saurischian dinosaurs, the sauropods, are thankfully much less complicated than the theropods. Sauropods became the largest animals to ever walk the earth, but they started out relatively small like other dinosaurs. Early sauropods were traditionally called prosauropods, but that term has started to become unpopular. The preferred term, to encapsulate the entire sauropod clade (all true sauropods and their ancestors) is sauropodomorphs. Early sauropodomorphs (now called Plateosauridae) were bipedal and probably omnivores, with sharp claws on their hands that could have helped tearing vegetation or flesh. The best example of an early sauropodomorph is Plateosaurus. It had blunt teeth and was about twice as tall as a human. It also had an elongated neck, allowing it to reach leaves that most other species at the time could not access.

Figure \(\PageIndex{11}\): Restoration of Melanorosaurus (by Mario Lanzas via Wikimedia.)

Over time, the sauropodomorphs got larger and larger. The important step from early sauropodomorphs to true sauropods is when their weight became so large that they were permanent quadrupeds. There is debate as to which genus represents this transition, but an animal like Melanorosaurus or Vulcanodon are good candidates. It has more robust limbs, with specifically longer hind limbs with less-useful claws, to support its increasing weight. Interestingly, the teeth of these animals were not what you would expect for a tree browser. They kept their omnivore-like needle teeth and used gastroliths, small rocks they would swallow and store in their gizzards to help digest the tough plant material, instead of grinding and chewing their food in their mouths.

Figure \(\PageIndex{12}\): Diplodocus life reconstruction (by Fred Weirum via Wikimedia.)

Early sauropods include the club-tailed Shunosaurus and the incredibly-long-necked Mamenchisaurus. All sauropods still have a front toe claw, a legacy of their bipedal beginnings. More derived sauropods split into two major groups: The first group are the diplodocids (Diplodocidea), which had a long body plan. This group includes Apatosaurus/Brontosaurus (which may be the same animal or may be different animals depending on who you ask), the spiny-necked dicraeosaurs, and the namesake of the group, Diplodocus.

Figure \(\PageIndex{13}\): Life sized model of Argentinosaurus at Clore Garden of Science in Weizmann Institute of Science in Israel. (Picture by MathKnight and Zachi Evenor via Wikimedia.)

The other major group of sauropods is Macronaria. These are animals that had a more upright stature, most famously demonstrated by Brachiosaurus. A subgroup of macronaria are the titanosaurs. As the name suggests, these were generally very large animals that commonly had some armored skin coverings made from bone. Included in this subgroup is Argentinosaurus, which is the best candidate for the largest known dinosaur, based on decent fossil remains of a leg and spine. Upper estimates for its size is 130 feet long and 100 tonnes. Other titanosaurs, such as Sauroposeiden and Patagotitan, may be larger but are estimated from more incomplete remains.

Sauropods: Did I Get It?

Exercise \(\PageIndex{1}\)

Which group covers the most animals?

a. Prosauropod

b. Sauropod

c. Sauropodomorph

d. Sauropodiform

Answer

c. Sauropodomorph

Exercise \(\PageIndex{2}\)

What feature do all sauropods have that is a remnant of their early history on two legs?

a. Sharp teeth

b. Hollow bones

c. Long necks

d. Toe claw

Answer

d. Toe claw

Exercise \(\PageIndex{3}\)

What separates the major groups of advanced sauropods?

a. Meat-eating verses plant-eating

b. Heavy verses light

c. Long verses upright

d. Spikes verses no spikes

Answer

c. Long verses upright