Classification of Dinosaurs

This article focuses on the categorization of creatures from antiquity, including dinosaurs, marine reptiles, and Pterosaurs (flying reptiles), which constituted the largest organisms of their time. The task of assigning a novel species of dinosaur to its appropriate group is less straightforward than the mere act of naming it. Similar challenges are encountered in the classification of flying reptiles and marine reptiles. Within this article, we delve into the methodologies employed by paleontologists to differentiate a new prehistoric animal, considering its order, suborder, genus, species, and temporal context. Furthermore, we shall elucidate this process through intriguing examples from the field of paleontology.

Taxonomy in the Classification of Dinosaurs

Tyrannosaurus rex
Sue, the Tyrannosaurus skeleton at the Field Museum in Chicago.

Taxonomy serves as the principal framework for the classification of organisms, wherein different classes of life are scrutinized based on their resemblances and compatibility with preceding living entities. For instance, within the primate class, which encompasses both monkeys and humans, taxonomy delineates two distinct subclasses, namely prosimians and anthropoids. These subclasses, in turn, exhibit various subgroups, like Platyrrhini, which comprises New World monkeys. In cases where the class does not adequately encompass the organism, a superorder is employed.

The subsequent stages of the taxonomic hierarchy involve the classification of prehistoric animals, specifically the assignment of genus and species. A “genus” is a classification of animals that only have one known specimen, such as the Diplodocus. For example, the dinosaur Diplodocus carnegii, abbreviated as D. carnegii, belongs to a distinct species.

In regard to dinosaur classification, Saurischians, or “lizard-hipped dinosaurs,” encompass both theropods, such as the two-legged predator Tyrannosaurus Rex, and sauropods, which include four-legged herbivores like Brachiosaurus. On the other hand, Ornithischians, or “bird-hipped dinosaurs,” comprise all herbivorous dinosaurs, including Triceratops and Hadrosaurs, with the Shantungosaurus from the Shandong peninsula being classified as a Hadrosaur.

Marine reptiles also undergo classification into superorders, orders, and suborders. Notable families within this group include Pliosaurs, Plesiosaurs, Ichthyosaurs, and Mosasaurs.

Pterosaurs, commonly known as flying reptiles, are broadly categorized into two primary superorders: Rhamphorhynchus, representing early long-tailed forms, and Pterodactyloids, corresponding to short-tailed ones. The latter superorder comprises substantially larger members compared to the former.

Classification of Saurischian Dinosaurs

The taxonomic classification of the dinosaur group known as Saurschia indicates its division into two distinct subgroups: Firstly, the Theropods, characterized by their bipedal stance and primarily carnivorous diet, and secondly, the Sauropods, encompassing the Prosauropods and Titanosaurs.

The Saurschia group, which derives its name from “lizard-hipped,” is comprised of dinosaurs exhibiting a pelvic bone structure reminiscent of that found in lizards. Notably, elongated necks and asymmetrical digits are additional characteristics of Saurschia dinosaurs.

Among the subgroups within Saurischia, the Theropods, or “beast-footed” dinosaurs, were particularly dominant during the Jurassic and Cretaceous epochs. This group boasted some of the most renowned predators in history, such as the famous T-Rex. It is worth noting that the Theropod lineage has endured to the present day, having evolved into the modern class of vertebrates known as “aves,” or birds.

  • Herrerasauridae family: The Herrerasauridae family constitutes a group of carnivorous Saurischian dinosaurs, comprising five distinct species, with Staurikosaurus and Herrerasaurus being the most renowned among them. As one of the earliest dinosaur lineages, Herrerasaurs exhibited distinctive anatomical traits, including the presence of two sacral vertebrae attached to their spine and a relatively primitive hand structure compared to later Theropods. This unique anatomy has sparked debate among paleontologists regarding the proper classification of this creature within the dinosaur group. Herrerasaurs became extinct at the close of the Triassic Period, prior to the advent of the Jurassic and Cretaceous periods.
  • Ceratosauridae family: In contrast to the more primitive Herrerasaurs, the Ceratosauridae family is recognized as authentic dinosaurs. They are characterized by distinctive features such as a pneumatic bone structure, an “S”-shaped curved neck, and a distinctive jaw anatomy. Appearing millions of years before the emergence of birds, they display certain resemblances to their avian descendants. Notable species within this family include Ceratosaurus, Dilophosaurus, and Coelophysis.
  • Coelurosauria clade: Coelurosaurs, constituting a clade representing a group of organisms descending from a common ancestor, are set apart from other Theropods due to their closer affinity with birds than their sister group, Carnosauria. Encompassing a diverse array of dinosaur species, ranging from Velociraptor to Ornithomimus and Tyrannosaurus Rex, Coelurosaurs exhibit distinctive skeletal characteristics, particularly notable in the tail vertebrae, tibia, and ulna.
  • Carnosauria clade: Although it may be tempting to assume that all members of the Carnosauria dinosaur group were fearsome carnivores akin to Tyrannosaurus Rex, this is not the case. Carnosaurs can be distinguished based on their dietary preferences, whether carnivorous or herbivorous, and other anatomical features. Their distinctive characteristics include proportional legs, tibia and hip bone structures, eye socket sizes, and skull shapes. Moreover, their relatively large forelimbs set them apart from the lineage to which T. Rex belongs. Prominent examples of carnivorous Carnosaurs encompass Allosaurus and Spinosaurus.
  • Therizinosauridae family: Formerly known as Segnosaurs and placed with uncertainty on the evolutionary tree, the Therizinosauridae family has been recently recognized as closely related to birds and subsequently included in the Theropod lineage. These omnivorous dinosaurs stand out for having noticeably long claws, pubic bones that are angled backward (like those of birds), four toed feet, and generally large sizes. Among the most notable representatives of this group are Therizinosaurus and Segnosaurus.
  • Sauropodomorpha Suborder: Among the herbivorous dinosaurs, the most recognized ones are the Sauropods and Prosauropods, known for their astonishing heights and weights. It is believed that they emerged in an area that corresponds to present-day South America.
    1. Sauropods, like the famous Brachiosaurus and Diplodocus, were truly colossal, often reaching lengths of over 100 feet and weighing several tons. Their long necks and tails, coupled with their massive bodies, made them one of the most iconic groups in dinosaur history.
    2. On the other hand, Prosauropods, while still impressive, were not as gigantic as their Sauropod cousins. These dinosaurs were generally smaller and had a more bipedal posture, although some could also walk on all fours. Nevertheless, their significance lies in being one of the early herbivorous dinosaur groups that paved the way for the later, more massive Sauropods.

Classification of Bird-Hipped Dinosaurs (Ornithischian)

Parasaurolophus
Image: Wikimedia.

The Ornithischian dinosaur clade comprises the majority of herbivorous dinosaurs from the Mesozoic Era, encompassing ceratopsians, ornithopods, and hadrosaurs, each of which will be further expounded upon in the clade sections.

  • Ornithischia clade: The nomenclature of this group, signifying “bird-hipped,” pertains to the pelvic morphology of its constituent members. Intriguingly, contemporary avian species do not trace their ancestry to Saurischian (“lizard-hipped”) dinosaurs but instead share a common lineage with Ornithischian dinosaurs.
  • Ornithopoda suborder: This suborder, denoting “bird-footed,” is characterized by its members’ bird-like three-toed feet and distinctive avian-like hips, distinguishing features specific to Ornithischians. Emerged during the Cretaceous epoch, Ornithopods were agile, bipedal herbivores equipped with robust tails and typically possessed rudimentary beaks. Noteworthy representatives of this diverse suborder include Iguanodon, Edmontosaurus, and Heterodontosaurus. Hadrosaurs, commonly known as duck-billed dinosaurs, constituted a widespread family of Ornithopods that flourished during the later Cretaceous period, with renowned species such as Parasaurolophus, Maisaura, and the sizable Shantungosaurus.
  • Marginocephalia clade: The Marginocephalia clade includes dinosaur species such as Pachycephalosaurus and Triceratops, and their most distinctive feature is their adorned, large-sized skulls.
    1. Pachycephalosauria Suborder: The term Pachycephalosaurs, meaning “thick-headed reptiles,” aptly describes these creatures, who employed this characteristic for both mating displays and head-butting. Flourishing primarily during the Cretaceous period, these dinosaurs were predominantly herbivorous, although a few species displayed omnivorous tendencies. Notable examples of Pachycephalosaurs include Pachycephalosaurus, Stygimoloch, and Stegoceras.
    2. Ceratopsia Suborder: Ceratopsia Suborder is distinguished from Pachycephalosaurs by its distinctive cranial features. Ceratopsians boasted robust horns and frill-like projections on their skull tops. Some, such as Triceratops and Styracosaurus, attained colossal sizes. Functioning as formidable defenses, these horns and frills were used to safeguard against Tyrannosaurs and Raptors during the late Cretaceous era. In addition to their significant stature, these large herbivorous dinosaurs displayed behaviors reminiscent of modern-day elephants and rhinoceroses.
  • Thyreophora Clade: Thyreophorans, a minor division of ornithischian dinosaurs, comprise notable representatives such as Stegosaurus and Ankylosaurus. The etymological connotation of this dinosaur assemblage is “shield bearers.” These organisms exhibited elongated, sharp plates along their dorsal region, with certain advanced species featuring elevated, mace-like protrusions at the terminations of their tails. Notwithstanding the presence of these formidable armaments, seemingly adapted for defense, thyreophorans adhered to a herbivorous dietary regimen.

Classification of Marine Reptiles

Paleontologists have faced significant challenges in classifying marine reptiles that thrived in the Mesozoic Era. This can be attributed to the limited diversity observed in the body structures of marine organisms throughout their evolutionary development. As a consequence, different species or families may display striking resemblances in their appearance. An illustrative example of this is evident in the likeness shared between an average Ichthyosaur and a large blue-finned tuna.

This results in fewer species within each genus, making it difficult to distinguish orders and suborders.

  • Ichthyopterygia Superorder: The designation Ichthyosaurs derives from the etymological origins of the Greek words “ichthys,” denoting fish, and “sauros,” signifying lizard. During the Triassic and Jurassic periods, these cetacean-like marine reptiles lived in the water. They included well-known genera like Ichthyosaurus and Ophthalmosaurus. Towards the end of the Jurassic period, their numbers dropped significantly, making way for Pliosaurs, Plesiosaurs, and Mosasaurs.
  • Sauropterygia Superorder: Termed as the “lizards with fins,” this superorder consisted of diverse marine reptiles from distinct families that commenced their aquatic existence approximately 250 million years ago, at the advent of the Mesozoic Era. Nevertheless, their lineage ultimately faced extinction roughly 66 million years ago, mirroring the fate of their terrestrial relatives, the dinosaurs.
  • Placodontia Order: Emerging during the Triassic period, approximately 250 to 210 million years ago, Placodonts represented the earliest marine reptiles. Possessing stout, short-legged bodies reminiscent of turtles, these creatures, some of which attained lengths of up to 3 meters, were adapted to inhabit shallow coastal regions rather than venturing into the depths of the oceans. Prominent members of this order included Placodus and Psephoderma.
  • Nothosauroidea Order: Believed to resemble small seals, the Triassic reptiles classified under Nothosauroidea were adept at navigating shallow waters for hunting purposes while routinely venturing onto rocky shores for various activities. With a typical length of approximately 2 meters, these reptiles displayed streamlined bodies, elongated necks, and paddle-like limbs, indicating a primary diet of fish. Nothosaurus, as the quintessential nothosaur, remains emblematic of this group.
  • Pachypleurosauria suborder: Pachypleurosaurs, an extinct assemblage of reptiles, possessed a slender and diminutive form, measuring approximately 40 to 60 cm in length, and predominantly subsisted on a diet of fish. The taxonomic placement of these marine reptiles, particularly concerning their best-preserved representative, Keichousaurus, remains a subject of ongoing contention among scholars.
  • Mosasauroidea superfamily: In the Late Cretaceous epoch, awe-inspiring and frequently colossal marine reptiles, referred to as Mosasaurs, reigned supreme. These formidable creatures epitomized the pinnacle of marine reptilian evolution, and remarkably, some analyses suggest that their only extant descendants are snakes. Among the most impressive members of the Mosasauroidea were Tylosaurus, Prognathodon, and the illustrious Mosasaurus.
  • Plesiosauria order: This group encompasses some of the most recognizable marine reptiles from the Jurassic and Cretaceous periods. Typically characterized by dinosaurian proportions, members of this order can be subdivided into two principal suborders, as delineated by paleontologists:
    1. Plesiosauroidea Suborder: Displaying the typical appearance of Plesiosaurs, this suborder comprised predatory marine creatures characterized by elongated necks, substantial flippers, and sharp dentition, all contributing to their streamlined physique. Although they were not as proficient in swimming as their close relatives, the Pliosaurs (explained later), the Plesiosaurs gracefully navigated the surfaces of rivers, lakes, and oceans. Capitalizing on their elongated necks, they effectively captured unsuspecting prey. Notable representatives within this suborder included Elasmosaurus and Cryptoclidus, both widely recognized in the scientific community.
    2. Pliosauroidea Suborder: In stark contrast to the Plesiosaurs, the Pliosaurs exhibited formidable features, including elongated, toothy heads, shortened necks, and robust barrel-shaped bodies, instilling a greater sense of fear. Many genera bear resemblances to present-day sharks or crocodiles. Possessing superior agility in the water when compared to Plesiosaurs, Pliosaurs were believed to thrive in deeper marine environments, preying on other marine reptiles and fish. Among the most awe-inspiring members of this suborder were the colossal Kronosaurus and Liopleurodon, creatures that commanded respect and terrorized the prehistoric seas.

Classification of Winged Reptiles (Pterosaur)

Reconstructed skeleton of Quetzalcoatlus in the Arizona Museum of Natural History in Mesa Arizona.
Image: Tony the Marine.

When examining prehistoric fauna, the consideration of marine reptiles becomes indispensable. The process of classifying Pterosaurs, commonly referred to as “winged reptiles,” is comparably less intricate when compared to Saurischian and Ornithischian dinosaurs. Notably, all these Mesozoic reptiles can be grouped together; however, they further segregate into two suborders, with only one being recognized as a “true” suborder from an evolutionary perspective.

Pterosaur order: The Pterosaur order was distinguished by being the first large creatures to achieve true flight. Characterized by their hollow bones, relatively enlarged brains, prominent eyes, and membranous wings extending along their forelimbs.

  1. Rhamphorhynchidae suborder: Within the taxonomic hierarchy, the Rhamphorhynchidae suborder holds a somewhat status with respect to evolutionary principles. It is posited that Pterodactyloidea (explained later) is believed to have evolved from this suborder, implying that not all suborders can be traced back to a single common ancestor. Nonetheless, paleontologists encompass within this family the smaller and more primitive Pterosaurs like Rhamphorhynchus and Anurognathus, both belonging to the Triassic period. Rhamphorhynchoids were characterized by their dentition, elongated tails, and skulls devoid of a beak.
  2. Pterodactyloidea suborder: In contrast, the Pterodactyloidea suborder comprises the well-known, large flying reptiles that thrived during the Jurassic and Cretaceous periods, among which are Pteranodon, Pterodactylus, and the colossal Quetzalcoatlus. Members of the Pterodactyloids were generally of significant size, featuring abbreviated tails, elongated wing bones, and, on occasion, elaborate bony head crests and tooth deficiencies. Regrettably, these Pterosaurs, along with their counterparts, the dinosaurs and marine reptiles, met their demise approximately 66 million years ago during the extensive extinction event.