Omnivore

Omnivory refers to the dietary strategy in which an individual or species consumes both plant and animal matter. However, the category of 'omnivore' encompasses far more complexity than a simple binary distinction suggests. A 2023 study published in Proceedings of the Royal Society B (DOI: 10.1098/rspb.2022.1062) demonstrated that mammalian omnivory can be subdivided into invertivorous omnivory and vertebrate-predation-based omnivory, each exhibiting distinct evolutionary pathways and transition patterns. This finding underscores that omnivory is best understood as a dietary spectrum rather than a monolithic feeding category.

In modern ecosystems, prominent omnivores include bears (Ursidae), pigs (Suidae), corvids (Corvidae), raccoons (Procyonidae), and humans (Homo sapiens). These species flexibly adjust the ratio of plant to animal food depending on seasonal availability, geographic location, and energetic demands. National Geographic Education defines an omnivore as 'an organism that eats a variety of other organisms, including plants, animals, and fungi,' emphasizing the breadth of dietary intake rather than fixed proportions.

Omnivorous animals exhibit a suite of morphological adaptations that enable the processing of diverse food types. In dentition, heterodont tooth rows—combining sharp incisors or canines for cutting with broad, flat molars for grinding—are characteristic. The digestive system typically features one or more stomach chambers and an intermediate gut length, longer than that of strict carnivores but shorter than that of dedicated herbivores.

In dinosaurs, the morphological indicators of omnivory (and the related shift toward herbivory) were systematically quantified by Zanno & Makovicky (2011) in a landmark study published in PNAS. By applying character correlation tests within a phylogenetic framework to 90 coelurosaurian theropod taxa, the authors identified 21 skeletal features exhibiting statistically significant correlations with extrinsic fossil evidence of herbivory (EEH), such as stomach contents and gastric mill stones. These correlated herbivorous traits (CHTs) include tooth symmetry, conical-subconical tooth morphology, loss of ziphodonty (serrated tooth edges), ventral deflection of the dentary symphysis, progressive tooth loss, evolution of a rhamphotheca (keratinous beak), and cervical elongation through increased vertebral count.

Critically, the study found that the initial evolution of a beak in coelurosaurian theropods correlates with the adoption of plant material into the diet, and that region-specific tooth alteration universally precedes tooth loss—suggesting a consistent developmental sequence in the transition from carnivory through omnivory to herbivory.

Determining the diet of extinct animals without direct observation presents a formidable challenge. Multiple categories of evidence are employed to infer omnivorous habits in dinosaurs.

Coprolites (Fossil Feces): Coprolites containing both plant tissue and animal remains (bone fragments, invertebrate cuticle) provide direct evidence of mixed diets. However, attributing coprolites to specific taxa remains difficult without associated body fossils.

Gut Contents: Fossilized stomach contents represent the most compelling direct evidence of diet. The giant ornithomimosaur Deinocheirus mirificus offers perhaps the most spectacular example: Lee et al. (2014, Nature) described specimens preserving over a thousand gastroliths alongside fish remains (vertebrae and scales) in the abdominal region, confirming that this 11-meter theropod was a 'megaomnivore' inhabiting mesic environments of Late Cretaceous Mongolia.

Gastroliths and Gastric Mills: The presence of deliberately ingested stones (gastroliths) arranged as a functional gastric mill provides indirect evidence of significant plant consumption, as these structures mechanically break down fibrous plant material in the absence of complex chewing dentition. Zanno & Makovicky (2011) established strict criteria for recognizing a true gastric mill: stones must occur as a definitive mass, lack high polish, conform to predicted body-mass-to-gastrolith-mass ratios, and be exotic relative to the entombing sediment.

Tooth Morphology and Microwear: Tooth shape, size, replacement patterns, and surface wear textures serve as indirect proxies for diet. Heterodont dentitions mixing conical and lanceolate teeth, or teeth with reduced serrations, may suggest omnivorous habits. Holtz et al. (1998) analyzed Troodon tooth morphometrics and proposed possible omnivory based on the small, coarsely serrated teeth that differed from typical hypercarnivorous theropod dentition.

Comparative Skeletal Anatomy: Comparisons between fossil specimens and the skeletal features of living omnivores provide analogical evidence, though this method is subject to ambiguity, particularly when extinct taxa lack close modern analogs.

Oviraptorosauria: These theropods possessed powerful, parrot-like beaks with few or no teeth. Zanno & Makovicky (2011) detected multiple CHTs across the clade, supporting herbivorous to omnivorous diets. However, the original Oviraptor specimen was found atop a nest initially thought to belong to Protoceratops, and lizard bones have been found associated with oviraptorosaur specimens, suggesting animal protein also featured in their diet. Representatives include Oviraptor, Citipati, Caganathus, Conchoraptor, and Heyuannia.

Ornithomimosauria ('Ostrich Dinosaurs'): Characterized by their ostrich-like body plans and edentulous beaks, ornithomimosaurs have long been subjects of dietary debate. Barrett (2005, Palaeontology) conducted a comprehensive review and concluded that multiple dietary hypotheses remain plausible, though evidence points toward significant plant consumption. Sinornithomimus specimens preserve evidence of a gastric mill, confirming substantial herbivory. Deinocheirus mirificus, the largest known ornithomimosaur at approximately 11 meters and 6.4 tonnes, was directly confirmed as omnivorous through preserved stomach contents combining gastroliths and fish remains (Lee et al. 2014).

Troodontidae: Small, bird-like theropods with relatively large brains and acute senses. Their small, coarsely serrated teeth have been interpreted as consistent with omnivory (Holtz et al. 1998), though the clade's precise dietary habits remain contested. Jinfengopteryx, a basal troodontid, preserves seeds in its abdominal region, providing direct evidence of plant consumption. A comprehensive review of troodontid diet published in Biological Reviews (2026) confirmed that the dietary habits of this clade remain a subject of active debate.

Therizinosauria: These bizarre theropods possessed enormous claws (up to approximately 70 cm in Therizinosaurus), small heads, long necks, and broad, pot-bellied bodies. While primarily interpreted as herbivorous in their derived forms, their ancestry among carnivorous theropods strongly implies an omnivorous transitional stage. Zanno et al. (2009, Proceedings of the Royal Society B) described Nothronychus graffami, the first North American therizinosaurid, and discussed the role of herbivory in 'predatory' dinosaur evolution. Therizinosaurs uniquely retained lanceolate cheek teeth alongside a rostral beak throughout their evolutionary history—a pattern that Zanno & Makovicky (2011) associated with retention of high-fiber folivory.

The landmark Zanno & Makovicky (2011) PNAS study fundamentally revised understanding of theropod ecology by demonstrating that herbivory was widespread among Coelurosauria, with hypercarnivory being the exception rather than the rule. Quantitative evidence for herbivory was identified in 44 coelurosaurian species spanning six major subclades. The study further revealed that ornithomimosaurs and oviraptorosaurs followed a statistically significant common succession in the acquisition of herbivorous traits, suggesting intrinsic developmental or functional constraints guiding the morphological adaptation to plant eating.

However, the specific sequence of trait acquisition differed between lineages. In Ornithomimosauria and Oviraptorosauria, tooth loss and rostral projection of the dentary symphysis preceded ventral symphyseal deflection, whereas in Therizinosauria, a downturned and convex dentary was a primary adaptation, with tooth loss occurring subsequently. This plasticity in order of appearance, combined with statistical concordance, indicates that while correlated, these characters maintain a degree of underlying independence and represent discrete adaptations.

Barrett, Butler & Nesbitt (2011) examined the roles of herbivory and omnivory in early dinosaur evolution, finding that herbivorous and omnivorous dinosaurs were rare during the Carnian stage of the Late Triassic, but diversified rapidly during the succeeding Norian stage. This pattern suggests that the ability to exploit plant resources was a key factor in early dinosaurian ecological success and diversification.

Omnivory occupies a complex position in macroevolutionary theory. Research on mammalian dietary evolution (DOI: 10.1098/rspb.2022.1062) has shown that omnivory often functions as an 'evolutionary sink'—a state into which lineages transition readily but from which they rarely escape to respecialize. Most omnivore diversity derives from transitions into omnivory from specialist diets (whether carnivory or herbivory), with comparatively low rates of transition back out. Studies in avian evolution have produced similar findings, with omnivory in birds identified as a macroevolutionary sink associated with reduced speciation rates compared to dietary specialists.

However, a 2025 study in Evolution (DOI: 10.1093/evolut/qpaf174) on phyllostomid bats challenged this narrative, highlighting omnivory as a key transitional stage in the evolution of dietary specialization rather than a terminal state. This suggests the macroevolutionary role of omnivory may vary across clades and ecological contexts.

In the context of dinosaur evolution, omnivory appears to have served primarily as a transitional stage facilitating the shift from ancestral faunivory to derived herbivory in multiple coelurosaurian lineages. The evolution of amniote herbivory is broadly considered to originate via omnivory (Reisz & Sues 2000; Barrett 2000), and the repeated, independent evolution of herbivorous traits across disparate theropod lineages underscores the macroevolutionary importance of omnivory as a gateway dietary strategy.

One of the most significant implications of the Zanno & Makovicky (2011) study is that herbivory preceded the origin of Avialae (birds), with multiple theropod lineages ancestral to or closely related to birds having already established plant-based or mixed diets. The repeated, independent evolution of edentulous beaks—initially correlated with herbivory—subsequently enabled dietary diversification as beaks were coopted for a stunning array of functions. Modern birds exhibit extraordinary dietary diversity, from the granivorous sparrow to the piscivorous pelican to the hypercarnivorous raptor, all facilitated by beak morphology that originated in the context of herbivorous or omnivorous theropod ancestors.

Among modern birds, corvids (crows, ravens, jays) exemplify the combination of high intelligence and omnivorous feeding, while the hoatzin (Opisthocomus hoazin)—the only bird with active foregut fermentation—shares cranial morphological traits with herbivorous non-avian theropods, as noted by Zanno & Makovicky (2011).

Advantages: Omnivory provides ecological resilience through dietary flexibility. When one food resource declines due to seasonal change, drought, or habitat disturbance, omnivores can shift to alternative resources. This buffering capacity reduces extinction risk during periods of environmental stress. Omnivores can also avoid direct competition with dietary specialists for specific food resources and can supplement nutritional gaps—for example, obtaining essential amino acids from animal protein that are difficult to derive from plant sources alone.

Constraints: The digestive system of omnivores is typically less efficient at processing any single food type compared to dietary specialists. Cellulose digestion, for instance, is far less effective in omnivores than in dedicated herbivores with specialized fermentation chambers. Additionally, the cognitive and energetic costs of locating, identifying, and processing diverse food types may be higher for omnivores, potentially selecting for larger brains and more complex behavioral repertoires—a pattern consistent with the relatively large brain sizes observed in some putatively omnivorous theropods such as troodontids.