The Classification of Dinosaurs

FAS Astronomers Blog, Volume 31, Number 13.

As noted in An Introduction to Dinosaurs, Richard Owen (Owen 1842) created the designation Dinosauria meaning “Fearfully great lizards” or “terrible lizards” to describe the new found dinosaurs. Dinosaurs lived during the Mesozoic era and dominated the planet in the Jurassic and Cretaceous periods.

In 1887, Harry Seeley (Seeley 1887) divided dinosaurs (Dinosauria) into two orders based on the structure of their hip bones.

• The “lizard hipped” Saurischians were some of the more famous dinosaurs. They are further divided into Sauropods such as Brachiosaurus and Theropods such as Tyrannosaurus rex. Saurischians had pubic bones pointed down and slightly forward and their pubis and ischium bones were split apart.
• The “bird hipped” Ornithischians were smaller plant-eating dinosaurs such as Stegosaurus and Triceratops. They had pubic bones pointed backward with their pubis and ischium bones close together.

Historically, dinosaurs have been classified using Seeley’s “hipped based” division and the period in which they lived.

Dinosaurs, as well as other animals, have also been classified using the “traditional” taxonomy first developed by Carolus Linnaeus in 1758 (Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species). As an example, T. rex is assigned to:

Domain: Eukarya / Kingdom: Animalia / Phylum: Chordata / Class: Dinosauria / Order: Saurischia / Family: Tyrannosauridae / Genus: Tyrannosaurus / Species: T. rex.

Unfortunately, this misses a few important groupings such as Diapsida, Reptilia, Archosauria, and Therapoda.

During the mid to late 20^th century, traditional classifications have given way to the use of cladistics – an approach first developed by Willi Hennig in 1950 and translated into English in 1966 (Hennig 1966). It was applied to dinosaurs by Robert Bakker and Peter Galton in 1974 (Bakker and Galton 1974), and eventually became accepted beginning with the work of Michael Benton and others in the mid-1980s. Cladistics uses cladograms and groups dinosaurs into nested nodes (clades) based on shared traits and common ancestors.

Cladogram nodes are characterized as monophyletic (including all descendants from a common ancestor such as dinosaurs and birds) or paraphyletic (including some descendants from a common ancestor such as “non-avian” dinosaurs). Dinosaur groups can also be polyphyletic (an arbitrary grouping with or without common traits, ancestors, or descendants).

More recently a group of paleontologists challenged Seeley’s classification and proposed a revised structure (Baron, et al. 2017). Their analysis was based on 457 traits found in 74 species rather than just on dinosaur hip bones. They concluded that the long-necked Sauropods are their own branch and the Theropods are more closely related to “bird-like hip” Ornithischian dinosaurs. Therefore, according to Baron, the large meat-eating Theropods are grouped together with the smaller plant-eating Ornithischian dinosaurs under the order Ornithoscetida.

A Deeper Dive

Given all this, the classification of dinosaurs is, well, complicated. At a high level, a few of the more common groupings can be further defined.

Theropods (Theorpoda) were the large well-known meat-eating dinosaurs such as Allosaurus and Tyrannosaurus rex. Most, such as T. rex, Giganotosaurus, Spinosaurus, and Albertosaurus were dominant during the Cretaceous period. Allosaurus, however, lived near the end of the Jurassic period.

Sauropods (Sauropoda) were large, long-neck plant-eating dinosaurs such as Brontosaurus/Apatosaurus and Brachiosaurus. These dinosaurs lived primarily during the Jurassic period, although Titanosaurs such as Argentinosaurus, Puertasaurus, and Patagotitan were found later during the Cretaceous.

The bird-hipped Ornithischians include many smaller plant-eating dinosaurs. This included Iguanodontians (e.g., Iguanodon) and hadrosaurs (large duckbilled dinosaurs such as Hadrosaurus and Edmontosaurus). There were also “horned face” dinosaurs including Triceratops and “thick headed” dinosaurs such as Pachycephalosaurus. There were “spiked” Jurassic dinosaurs Stegosaurus and Kentrosaurus, and similar looking “armored” Cretaceous dinosaurs such as Ankylosaurus.

Although, Ornithischians had bird hips, it was found that birds are more closely related to the lizard hip Saurischians and, in particular, Theropods.

However, as I tried to dig into this some more, it became evident that there are many more groupings and hierarchies used to classify dinosaurs. I found it to be very confusing and more than somewhat difficult to follow. I’ve captured what I know in the chart below.