What Are Dinosaurs?
Coelophysis, and early form of theropod dinosaur from the Triassic period. Image credit: Dr. Jeff Martz/NPS.
A common misconception is that any large or extinct creature is classified as a dinosaur. This notion, however, is incorrect. Dinosaurs possess a distinct set of characteristics and traits that define them. In the following sections, we outline the evolutionary lineage of dinosaur ancestors that ultimately led to the true dinosaurs (including the defining features that characterize what a dinosaur is).
Archosaurs – The Great Ruling Reptiles
Around 250 million years ago, in the aftermath of the catastrophic Permian–Triassic extinction event, a remarkable group of reptiles known as the archosaurs—meaning “ruling reptiles”—rose to ecological prominence (Benton, 2015). These animals would go on to dominate the Mesozoic Era and give rise to some of history’s most iconic creatures. Archosaurs split into two main evolutionary branches: the pseudosuchians, which include the ancestors of modern crocodiles and their extinct relatives, and the avemetatarsalians, which would eventually lead to pterosaurs and dinosaurs (Nesbitt, 2011). Distinct anatomical traits set archosaurs apart from other reptiles, including the antorbital fenestra—a hole in the skull in front of the eyes that may have lightened the skull or provided space for air sacs or muscles—socketed teeth firmly rooted in the jaw, and advanced ankle joints that allowed for powerful and efficient movement (Brusatte et al., 2010).
Avemetatarsalia – The “Bird Line” Archosaurs
Emerging in the Early to Middle Triassic, the avemetatarsalians represent the “bird line” of archosaurs, encompassing both the pterosaurs and the ancestors of dinosaurs (Nesbitt et al., 2010). This group is so named because all living birds ultimately descend from dinosaurs, which themselves arose within Avemetatarsalia. Avemetatarsalians possessed straight-hinged ankle joints that allowed a more upright and efficient gait, setting them apart from their sprawling, crocodile-like relatives. Their lighter body structures and evidence of higher activity levels suggest they were built for agility and endurance—traits that foreshadowed the energetic lifestyles of later dinosaurs and birds (Benton, 2015).
Dinosauromorphs – Almost Dinosaurs
By the Middle to Late Triassic, a more specialized group within Avemetatarsalia evolved known as the dinosauromorphs. These small, lightly built reptiles were predominantly bipedal and highly active, bridging the evolutionary gap between early avemetatarsalians and true dinosaurs (Sereno, 1999). Notable examples include lagerpetids, which were slender, long-legged insectivores, and silesaurids, which were small herbivores or omnivores closely related to the first dinosaurs. Dinosauromorphs exhibited several key adaptations leading toward the dinosaur condition, such as elongated legs for efficient running, modified hip joints for a fully upright posture, and in some forms, reduced forelimbs, emphasizing bipedal locomotion (Irmis et al., 2007).
Dinosauriformes – The Closest Precursors
Within dinosauromorphs, the dinosauriformes were the closest precursors to true dinosaurs. Appearing in the Middle Triassic, these animals, such as the Argentine species Marasuchus, already displayed many of the hallmark traits of dinosaurs (Sereno & Arcucci, 1994). Dinosauriformes were characterized by an even more upright stance, shorter forelimbs relative to the hind limbs, a stiffened tail used for balance while running, and hip bones that were beginning to resemble the configuration seen in dinosaurs. By this stage, nearly all the foundational features of Dinosauria were in place—only a few skeletal refinements remained before true dinosaurs emerged.
The First True Dinosaurs
A dinosaur is a member of the clade Dinosauria, a group of primarily terrestrial archosaurs defined by specific skeletal features.
True dinosaurs appeared around 233 million years ago during the Late Triassic, marking a turning point in vertebrate evolution (Irmis et al., 2011). They rapidly diversified into three main groups: theropods, which were primarily carnivorous and eventually gave rise to birds; sauropodomorphs, the long-necked herbivores that included the giants of the Jurassic; and ornithischians, diverse plant-eaters characterized by a “bird-hipped” pelvis, despite not being directly ancestral to birds (Benton, 2015).
Key traits defining dinosaurs include a fully perforated acetabulum (open hip socket), which allowed the femur to articulate directly beneath the body and supported a fully upright posture. They also possessed three or more sacral vertebrae fused to the pelvis, creating a reinforced and weight-bearing hip structure. Additional diagnostic features include elongated deltopectoral crests on the humerus for powerful forelimb musculature, modified ankle joints (particularly the mesotarsal ankle in many groups) that permitted efficient parasagittal locomotion, and distinctive limb proportions optimized for sustained terrestrial movement.
Together, these anatomical innovations enabled greater locomotor efficiency, endurance, and biomechanical stability compared to many contemporaneous archosaurs. As a result, dinosaurs diversified extensively following the Late Triassic extinction and went on to dominate terrestrial ecosystems for over 160 million years, occupying herbivorous and carnivorous niches across nearly every continental landmass.
Around 250 million years ago, in the aftermath of the catastrophic Permian–Triassic extinction event, a remarkable group of reptiles known as the archosaurs—meaning “ruling reptiles”—rose to ecological prominence (Benton, 2015). These animals would go on to dominate the Mesozoic Era and give rise to some of history’s most iconic creatures. Archosaurs split into two main evolutionary branches: the pseudosuchians, which include the ancestors of modern crocodiles and their extinct relatives, and the avemetatarsalians, which would eventually lead to pterosaurs and dinosaurs (Nesbitt, 2011). Distinct anatomical traits set archosaurs apart from other reptiles, including the antorbital fenestra—a hole in the skull in front of the eyes that may have lightened the skull or provided space for air sacs or muscles—socketed teeth firmly rooted in the jaw, and advanced ankle joints that allowed for powerful and efficient movement (Brusatte et al., 2010).
Avemetatarsalia – The “Bird Line” Archosaurs
Emerging in the Early to Middle Triassic, the avemetatarsalians represent the “bird line” of archosaurs, encompassing both the pterosaurs and the ancestors of dinosaurs (Nesbitt et al., 2010). This group is so named because all living birds ultimately descend from dinosaurs, which themselves arose within Avemetatarsalia. Avemetatarsalians possessed straight-hinged ankle joints that allowed a more upright and efficient gait, setting them apart from their sprawling, crocodile-like relatives. Their lighter body structures and evidence of higher activity levels suggest they were built for agility and endurance—traits that foreshadowed the energetic lifestyles of later dinosaurs and birds (Benton, 2015).
Dinosauromorphs – Almost Dinosaurs
By the Middle to Late Triassic, a more specialized group within Avemetatarsalia evolved known as the dinosauromorphs. These small, lightly built reptiles were predominantly bipedal and highly active, bridging the evolutionary gap between early avemetatarsalians and true dinosaurs (Sereno, 1999). Notable examples include lagerpetids, which were slender, long-legged insectivores, and silesaurids, which were small herbivores or omnivores closely related to the first dinosaurs. Dinosauromorphs exhibited several key adaptations leading toward the dinosaur condition, such as elongated legs for efficient running, modified hip joints for a fully upright posture, and in some forms, reduced forelimbs, emphasizing bipedal locomotion (Irmis et al., 2007).
Dinosauriformes – The Closest Precursors
Within dinosauromorphs, the dinosauriformes were the closest precursors to true dinosaurs. Appearing in the Middle Triassic, these animals, such as the Argentine species Marasuchus, already displayed many of the hallmark traits of dinosaurs (Sereno & Arcucci, 1994). Dinosauriformes were characterized by an even more upright stance, shorter forelimbs relative to the hind limbs, a stiffened tail used for balance while running, and hip bones that were beginning to resemble the configuration seen in dinosaurs. By this stage, nearly all the foundational features of Dinosauria were in place—only a few skeletal refinements remained before true dinosaurs emerged.
The First True Dinosaurs
A dinosaur is a member of the clade Dinosauria, a group of primarily terrestrial archosaurs defined by specific skeletal features.
True dinosaurs appeared around 233 million years ago during the Late Triassic, marking a turning point in vertebrate evolution (Irmis et al., 2011). They rapidly diversified into three main groups: theropods, which were primarily carnivorous and eventually gave rise to birds; sauropodomorphs, the long-necked herbivores that included the giants of the Jurassic; and ornithischians, diverse plant-eaters characterized by a “bird-hipped” pelvis, despite not being directly ancestral to birds (Benton, 2015).
Key traits defining dinosaurs include a fully perforated acetabulum (open hip socket), which allowed the femur to articulate directly beneath the body and supported a fully upright posture. They also possessed three or more sacral vertebrae fused to the pelvis, creating a reinforced and weight-bearing hip structure. Additional diagnostic features include elongated deltopectoral crests on the humerus for powerful forelimb musculature, modified ankle joints (particularly the mesotarsal ankle in many groups) that permitted efficient parasagittal locomotion, and distinctive limb proportions optimized for sustained terrestrial movement.
Together, these anatomical innovations enabled greater locomotor efficiency, endurance, and biomechanical stability compared to many contemporaneous archosaurs. As a result, dinosaurs diversified extensively following the Late Triassic extinction and went on to dominate terrestrial ecosystems for over 160 million years, occupying herbivorous and carnivorous niches across nearly every continental landmass.
Plateosaurus, a basal (early) sauropodomorph from Triassic period. Such forms were once known as "prosauropods'' but this term is considered outdated. Image credit: Total Dino.
These Are Not Dinosaurs
A Pterosaur (top left), Dimetrodon (top right), and a plesiosaur (bottom). These animals all superficially resemble dinosaurs. However, they are not actually dinosaurs at all.
Pterosaurs such as Pterodactyl, marine reptiles like the long-necked plesiosaurs, and the sail-backed synapsid Dimetrodon are frequently grouped with dinosaurs in popular media. Due to this many people assume these animals are dinosaurs. However, this is not true in a cladistic sense.
True dinosaurs are defined by a specific suite of skeletal features (which were outlined above).
Pterosaurs, while closely related to dinosaurs within Ornithodira, represent a separate archosaur lineage that evolved powered flight independently (Benton, 2015). Plesiosaurs belong to Sauropterygia, a distinct group of marine reptiles outside Archosauria altogether (Benton, 2015). Dimetrodon, often mistaken for a dinosaur because of its reptilian appearance, is even more distantly related. It is a Permian synapsid more closely allied with the lineage that ultimately gave rise to mammals than with reptiles or dinosaurs (Reisz, 1986; Brusatte, 2018). Thus, while these animals shared prehistoric ecosystems at different points in deep time, they fall outside Dinosauria and reflect the broader diversity of amniote evolution rather than dinosaurian ancestry. In short, none of them are dinosaurs.
True dinosaurs are defined by a specific suite of skeletal features (which were outlined above).
Pterosaurs, while closely related to dinosaurs within Ornithodira, represent a separate archosaur lineage that evolved powered flight independently (Benton, 2015). Plesiosaurs belong to Sauropterygia, a distinct group of marine reptiles outside Archosauria altogether (Benton, 2015). Dimetrodon, often mistaken for a dinosaur because of its reptilian appearance, is even more distantly related. It is a Permian synapsid more closely allied with the lineage that ultimately gave rise to mammals than with reptiles or dinosaurs (Reisz, 1986; Brusatte, 2018). Thus, while these animals shared prehistoric ecosystems at different points in deep time, they fall outside Dinosauria and reflect the broader diversity of amniote evolution rather than dinosaurian ancestry. In short, none of them are dinosaurs.
Dinosaur Tribes
Kentrosaurus, an ornithischian thyreophoran dinosaur. Photo credit: Nobu Tamura.
Dinosaurs are traditionally divided into two major groups, or tribes, based on the structure of their pelvic bones. These are the Saurischia (“lizard-hipped” dinosaurs) and the Ornithischia (“bird-hipped” dinosaurs). In saurischians, the pubis bone points downward and forward, resembling that of modern reptiles, while in ornithischians, the pubis is rotated backward, parallel to the ischium. This creates a superficial bird-like configuration (though birds actually evolved from saurischian theropods). The Saurischia include two major clades. There are the Theropoda, composed mostly of bipedal carnivores such as Tyrannosaurus and Allosaurus and Sauropodomorpha, the long-necked herbivores like Brachiosaurus and Diplodocus.
The Ornithischia, on the other hand, encompass a wide variety of herbivorous dinosaurs, including the Thyreophora (armored dinosaurs such as Stegosaurus and Ankylosaurus), Ornithopoda (bipedal grazers like Iguanodon and Parasaurolophus), and Marginocephalia (horned and dome-headed forms like Triceratops and Pachycephalosaurus). Despite debates over alternative classifications (most notably the Ornithoscelida hypothesis, which proposes that theropods are more closely related to ornithischians than to sauropodomorphs), most paleontologists continue to support the traditional Saurischia–Ornithischia division due to the strength of anatomical and historical evidence (Sereno, 1999; Baron et al., 2017; Brusatte, 2018).
Dinosaurs are considered a monophyletic group, meaning they all descend from a single common ancestor and include all of that ancestor’s descendants. This classification is based on shared derived traits (synapomorphies) that unite all true dinosaurs within the clade Dinosauria, such as an upright, fully erect stance with limbs positioned directly beneath the body, an open hip socket (the perforated acetabulum), and distinct features of the ankle and skull. These anatomical innovations distinguish dinosaurs from other archosaurs, such as crocodilians and pterosaurs, which branched off earlier in evolution. Because all recognized dinosaur lineages, including both Saurischia (including theropods and sauropodomorphs) and Ornithischia (the “bird-hipped” dinosaurs) share this common ancestry, dinosaurs form a natural, monophyletic clade rather than a collection of unrelated species grouped by superficial similarities (Benton, 2015; Brusatte, 2018).
The Ornithischia, on the other hand, encompass a wide variety of herbivorous dinosaurs, including the Thyreophora (armored dinosaurs such as Stegosaurus and Ankylosaurus), Ornithopoda (bipedal grazers like Iguanodon and Parasaurolophus), and Marginocephalia (horned and dome-headed forms like Triceratops and Pachycephalosaurus). Despite debates over alternative classifications (most notably the Ornithoscelida hypothesis, which proposes that theropods are more closely related to ornithischians than to sauropodomorphs), most paleontologists continue to support the traditional Saurischia–Ornithischia division due to the strength of anatomical and historical evidence (Sereno, 1999; Baron et al., 2017; Brusatte, 2018).
Dinosaurs are considered a monophyletic group, meaning they all descend from a single common ancestor and include all of that ancestor’s descendants. This classification is based on shared derived traits (synapomorphies) that unite all true dinosaurs within the clade Dinosauria, such as an upright, fully erect stance with limbs positioned directly beneath the body, an open hip socket (the perforated acetabulum), and distinct features of the ankle and skull. These anatomical innovations distinguish dinosaurs from other archosaurs, such as crocodilians and pterosaurs, which branched off earlier in evolution. Because all recognized dinosaur lineages, including both Saurischia (including theropods and sauropodomorphs) and Ornithischia (the “bird-hipped” dinosaurs) share this common ancestry, dinosaurs form a natural, monophyletic clade rather than a collection of unrelated species grouped by superficial similarities (Benton, 2015; Brusatte, 2018).