Palaeognathae is one of the two major divisions of living birds, encompassing ostriches, emus, cassowaries, rheas, kiwi, tinamous, and their extinct relatives such as moa and elephant birds. The name, from the Greek for “old jaw,” refers to a distinctive bony palate structure that sets these birds apart from all other living birds, which belong to the vastly more species-rich Neognathae. What makes palaeognaths so interesting to biologists is not just their ancient lineage but how thoroughly genomic evidence has overturned the old story about their evolution, particularly the assumption that their most famous trait, flightlessness, arose only once.
The Palate That Names the Group
The defining anatomical feature of palaeognaths is the structure of their bony palate. In most birds (neognaths), certain palatal bones are fused and mobile in ways that allow the upper bill to flex. In palaeognaths, the palate retains a more rigid, less fused arrangement that was long thought to be a “primitive” holdover from early bird evolution. Researchers have debated exactly how primitive these features really are. A detailed study of palatal bone variation across birds found developmental evidence against the idea that the palaeognath palate simply results from arrested development, and comparison with fossil birds like Archaeopteryx and Hesperornis suggests that some of the palaeognath palatal features are genuinely ancestral rather than secondarily simplified.1BioOne Complete (Annals of Carnegie Museum). Variation in the Os Palatinum and Its Structural Relation to the Palatum Osseum of Birds (Aves) That same study argued that the old composite palatal categories used to classify birds (palaeognathous, desmognathous, and so on) are too blunt an instrument, and that analyzing individual palatal bones character by character is more informative for understanding how birds are related.
Regardless of the ongoing technical debate, the palate remains the trait that gives the group its name, and it is consistent enough across palaeognaths to serve as a reliable anatomical marker. Beyond the palate, palaeognaths share other features, many of which are tied to their most conspicuous characteristic: most of them cannot fly.
Who Belongs to Palaeognathae
Living palaeognaths fall into five broad groups. The ostriches of Africa are the most familiar and the largest living birds. The rheas of South America are the second largest. Emus and cassowaries share the Australasian region. Kiwi, small and nocturnal, are endemic to New Zealand. And then there are the tinamous, a group of about 45 species of ground-dwelling birds scattered across Central and South America. Tinamous stand out from the rest because they can fly, at least in short bursts, making them the sole living palaeognaths that have retained powered flight.
The extinct members of the group were among the most spectacular birds ever to exist. New Zealand’s moa included species ranging from turkey-sized to the giant Dinornis, which stood around 3.6 meters tall. Madagascar’s elephant birds (Aepyornis and relatives) were enormously heavy and laid the largest eggs of any known animal. Both groups were wiped out after human arrival in their respective islands.
The flightless palaeognaths were historically grouped together as “ratites,” a name derived from the Latin word for raft, referring to their flat breastbone that lacks the prominent keel found in flying birds.2Diversity. Embryonic Development of the Avian Sternum and Its Morphological Adaptations for Optimizing Locomotion For a long time, ratites were treated as a natural group that shared a single flightless ancestor. That story has been comprehensively dismantled by molecular evidence.
Flight Was Lost Repeatedly, Not Once
The old picture was tidy: a single ancestor lost flight, then its descendants were separated by the breakup of the supercontinent Gondwana, drifting apart on fragments that became Africa, South America, Australasia, and New Zealand. Molecular phylogenetics demolished this narrative. Analyses of twenty unlinked nuclear genes showed that tinamous, the flying palaeognaths, are nested phylogenetically within the flightless ratites, making ratites not a natural group but a collection of lineages that independently lost flight.3PubMed Central. Phylogenomic evidence for multiple losses of flight in ratite birds Mitochondrial genome analysis reached a consistent conclusion, placing tinamous as sister to the extinct moa and inferring that flight was lost multiple times in close association with the mass extinction event that ended the Cretaceous period.4Systematic Biology. Tinamous and Moa Flock Together: Mitochondrial Genome Sequence Analysis Reveals Independent Losses of Flight among Ratites
If tinamous sit inside the ratite family tree rather than outside it, the simplest explanation is that the common ancestor of all palaeognaths could fly, and that flight was lost independently in the lineages leading to ostriches, rheas, kiwi, emus and cassowaries, moa, and elephant birds. That is a minimum of six separate transitions to flightlessness within a single bird group, a striking case of convergent evolution.
A 2019 study combining phylogenomic, developmental, and epigenomic data from eleven palaeognath genomes, including an extinct moa, investigated how these independent losses happened at the genetic level. The study found that convergent changes in gene regulatory regions, rather than in protein-coding genes themselves, were the dominant mechanism. Over two thousand independent accelerations in conserved noncoding elements were identified along flightless lineages, with functional consequences for gene expression in the developing forelimb.5PubMed. Convergent regulatory evolution and loss of flight in paleognathous birds In plain terms, the genetic “switches” that control wing and flight-muscle development were repeatedly tweaked in similar ways across different flightless lineages, even though the coding sequences of the underlying genes stayed largely the same.
The Relationships Are Still Being Sorted Out
While the broad conclusion of multiple flight losses is firmly established, the exact branching order within Palaeognathae remains contentious. Different types of genetic data can produce different trees. A study using 27 nuclear genes and retroposon insertions placed rheas as sister to the kiwi, emu, and cassowary clade, and confirmed that tinamous are sister to moa.6PubMed Central. Multiple nuclear genes and retroposons support vicariance and dispersal of the palaeognaths, and an Early Cretaceous origin of modern birds A more recent analysis examining how different sequence types affect the tree found that the grouping of kiwi with cassowaries and emu received the strongest support at roughly 40%, but alternatives linking kiwi with rheas or with tinamous each received about 30% support.7PubMed. Effect of Different Types of Sequence Data on Palaeognath Phylogeny This kind of near-even split among competing arrangements is a signature of a rapid ancient radiation, where lineages diverged so quickly that relatively little genetic signal accumulated to distinguish one branching order from another.
The position of ostriches is less controversial. They consistently emerge as the earliest-diverging living palaeognath lineage, making them the most distant living relatives of all other members of the group.
Gondwana Alone Does Not Explain the Map
The geographic distribution of palaeognaths reads like a map of former Gondwanan landmasses: Africa (ostriches), South America (rheas, tinamous), Australasia (emus, cassowaries), New Zealand (kiwi, moa), and Madagascar (elephant birds). The classic explanation was vicariance: as Gondwana fragmented, flightless birds were stranded on the resulting continents. But the phylogenetic evidence pointed to a problem. If elephant birds in Madagascar and kiwi in New Zealand are sister taxa, as ancient DNA from elephant bird remains confirmed, then vicariance cannot be the whole story, because Madagascar and New Zealand were never connected in a way that would isolate those two lineages together.8PubMed. Ancient DNA reveals elephant birds and kiwi are sister taxa and clarifies ratite bird evolution The elephant bird–kiwi relationship strongly supports the idea that flying ancestors dispersed across oceans and then independently lost flight after arriving on different landmasses.
Ancient DNA from elephant bird eggshell pushed the timeline further, suggesting that palaeognaths underwent an explosive radiation between roughly 69 and 52 million years ago, well after the major Gondwanan landmasses had already separated.9PubMed. Eggshell palaeogenomics: Palaeognath evolutionary history revealed through ancient nuclear and mitochondrial DNA from Madagascan elephant bird (Aepyornis sp.) eggshell This timing places much of the diversification in the aftermath of the mass extinction that killed non-avian dinosaurs, when ecological niches for large ground-dwelling herbivores and omnivores suddenly opened up worldwide. Flying palaeognath ancestors appear to have colonized these newly available niches across different continents and then repeatedly abandoned flight.
One oddity of the elephant bird–kiwi pairing is the enormous difference in body size: elephant birds were among the heaviest birds ever, while kiwi are small enough to hold in two hands. Yet both groups share a disproportionately large egg relative to body size, a trait that researchers have suggested may be a shared ancestral character linking the two.10Current Biology. Phylogenomics and Morphology of Extinct Paleognaths Reveal the Origin and Evolution of the Ratites
Bodies Built for the Ground
Flightless palaeognaths share a suite of anatomical changes tied to life without flight, though these evolved independently. The most obvious is the flat sternum that lacks a keel, the bony ridge that anchors the powerful pectoral muscles in flying birds.2Diversity. Embryonic Development of the Avian Sternum and Its Morphological Adaptations for Optimizing Locomotion Wing size and pectoral muscle mass are also reduced, though the degree varies. Ostriches retain vestigial wings used in display and balance, while kiwi have wings so small they are invisible under plumage.
Feathers are another area of divergence. Ratite primary feathers are markedly different from those of flightless birds that belong to Neognathae, a difference that may reflect the much longer time since ratites lost flight.11Journal of Zoology. Feather structure in flightless birds and its bearing on the question of the origin of feathers Ratite feathers typically lack the interlocking barb structure that gives flight feathers their aerodynamic stiffness. Instead, they tend to be loose and hair-like, as anyone who has seen an ostrich or emu up close will recognize.
Despite their massive size, ostriches and emus grow fast. Both species reach adult size in under a year, fast enough that their bones typically do not show the annually deposited growth marks seen in slower-growing animals. Bone tissue studies show that even so, both species retain the ability to temporarily slow their growth under stress, a form of developmental flexibility considered ancestral among birds.12PubMed Central. Osteohistological description of ostrich and emu long bones, with comments on markers of growth This makes them popular comparison models for understanding how non-avian dinosaurs grew.
Tinamous and the Limits of a Small Heart
Tinamous are the only living palaeognaths that can fly, but they are famously bad at it. They flush from the ground in a panicked burst and rarely stay airborne for more than a few seconds. Part of the reason is physiological. Compared to similarly sized galliform birds like junglefowl, tinamous accumulate far more oxygen debt after exertion. After an aerobic challenge, tinamous showed roughly six times more excess post-exercise oxygen consumption than junglefowl, and their blood lactate levels were about double.13Scientific Reports. Aerobic performance in tinamous is limited by their small heart. A novel hypothesis in the evolution of avian flight Their body temperature also dropped after the challenge and stayed depressed for hours. These results point to a cardiovascular system that simply cannot sustain the metabolic demands of flight beyond a brief escape burst.
Ratites more broadly tend to have lower body temperatures and basal metabolic rates than other birds, though they share all the same basic thermoregulatory mechanisms, such as panting, vascular adjustments, and feather manipulation to control heat loss.14Australian Journal of Experimental Agriculture. Thermoregulation in ratites: a review The lower metabolic rate may be another consequence of not needing to sustain flight, one of the most energetically expensive things any animal does.
Male-Only Parental Care
Palaeognaths have reproductive habits that are unusual among birds. In most ratite species, males do virtually all the parenting: they build the nest, incubate the eggs, and rear the chicks. Mating systems tend to be promiscuous, with females depositing eggs in communal nests that a single male then tends. Male testosterone concentrations stay elevated throughout the parental care period, which is unusual because in most birds, testosterone drops during incubation.15PubMed. An updated look at the mating system, parental care and androgen seasonal variations in ratites This pattern of male-dominated care appears across ostriches, emus, rheas, cassowaries, and kiwi, suggesting it may be ancestral to the group as a whole rather than something that evolved independently in each lineage.
Kiwi and Their Extraordinary Senses
Among palaeognaths, kiwi are the most radically specialized. They are nocturnal, flightless, and probe the soil for invertebrates in a manner more reminiscent of a mammal than a bird. Their visual sense is reduced, but they compensate with a highly developed sense of smell and a specialized bill tip organ similar to that found in shorebirds, which allows them to detect vibrotactile cues produced by burrowing prey.16Animal Behaviour. The relative importance of olfaction and remote touch in prey detection by North Island brown kiwis This ability, known as “remote touch,” means kiwi can sense prey moving through damp soil without seeing or directly contacting it.
Anatomical examination of kiwi bill tips revealed sensory pits containing clusters of Herbst corpuscles, the same type of mechanoreceptor found in shorebird bills, though kiwi have fewer per pit on average than shorebirds like godwits.17PLOS ONE. The Anatomy of the bill Tip of Kiwi and Associated Somatosensory Regions of the Brain: Comparisons with Shorebirds The kiwi’s nostrils sit at the very tip of the bill, unlike virtually every other bird, an arrangement that links smell and touch at the same point of contact with the environment.
Kiwi are also a conservation concern. Genomic analyses of New Zealand’s kiwi species show that most lineages have experienced recent demographic decline and genetic bottlenecks, yet each lineage carries unique adaptive features. Researchers have argued that the different kiwi lineages should be treated as separate adaptive units in conservation programs, meaning that managing them as interchangeable populations would miss lineage-specific adaptations that could matter for long-term survival.18PubMed Central. Demographic decline and lineage-specific adaptations characterize New Zealand kiwi
What Moa Ate and Why It Matters
New Zealand’s moa were the dominant large herbivores on the islands for millions of years. Because they left behind no close living relatives and occupied a landscape with no native land mammals, understanding their ecology depends heavily on indirect evidence. Fossilized coprolites, essentially preserved dung, have been a goldmine. Analysis of coprolites from four sympatric moa species in southern New Zealand showed that different species were partitioned by habitat and diet. Some specialized in forest plants while others grazed open herbfields. The South Island giant moa and upland moa had broad ecological niches that may reflect sexual segregation or seasonal movement between habitats.19PubMed Central. Resolving lost herbivore community structure using coprolites of four sympatric moa species (Aves: Dinornithiformes) Researchers concluded that moa have no living ecological analogs, and their extinction represents an irreplaceable loss of ecological function.
Further coprolite work revealed that moa and prehistoric kakapo consumed ectomycorrhizal fungi, suggesting these birds played a role in dispersing fungal spores critical to New Zealand’s forest ecosystems. Moa also regularly ate ferns and mosses, supplementing their broader plant diets.20PubMed Central. Coprolites reveal ecological interactions lost with the extinction of New Zealand birds The loss of moa thus did not just remove a large herbivore from the landscape; it disrupted mutualistic relationships between birds, plants, and fungi that had been in place for millions of years.
What drove them extinct was straightforward. Moa, at least the giant Dinornis species, grew slowly in reproductive terms: low fecundity, probably high longevity. That combination meant their populations were especially vulnerable to the loss of adult individuals through hunting, which is exactly what happened after Polynesian settlers arrived in New Zealand.21PubMed. Postnatal ontogeny, population structure, and extinction of the giant moa Dinornis
Eggshell as an Evolutionary Archive
Palaeognath eggshells turn up frequently in Cenozoic fossil deposits across Africa, Asia, Europe, and Oceania, making them useful for dating and reconstructing ancient environments. Traditionally, fossil eggshells were sorted into “struthionid” (ostrich-like, with circular pores) and “aepyornithid” (elephant bird-like, with slit-shaped pores) types. But this classification has problems. Some fossil and even some moa eggshells have both pore types on different parts of the same egg.22PubMed Central. Microstructural and crystallographic evolution of palaeognath (Aves) eggshells
A detailed study using electron backscatter diffraction to examine microstructure and crystal orientation in eggshells from all major palaeognath clades found that what had been called “ostrich-style” and “tinamou-style” eggshell structures evolved independently in different lineages, while the “rhea-style” eggshell appears to be genuinely ancestral, representing the condition inherited from the common ancestor of all palaeognaths.23eLife. Microstructural and crystallographic evolution of palaeognath (Aves) eggshells This means that even eggshell structure mirrors the broader palaeognath theme of convergent evolution: lineages that look similar on the surface acquired those similarities independently.
Lice Tell Their Own Story
One of the more unexpected lines of evidence about palaeognath evolution comes from their parasites. Feather lice are ectoparasites that typically co-speciate with their bird hosts, meaning lice lineages diverge whenever host lineages diverge. If palaeognaths were truly an ancient, isolated group, you would expect their lice to mirror that deep divergence. They do not. Cophylogenomic analyses showed that feather lice arrived on palaeognaths through host-switching from other bird groups at least three times. One of these switches was from an ancestor within the chicken-and-duck lineage (Galloanserae) to the ancestor of emus, and two others originated from ancestors of frogmouths and potoos.24PubMed Central. Extensive host-switching of avian feather lice following the Cretaceous-Paleogene mass extinction event This makes sense in a post-mass-extinction world where ecological communities were being reassembled from surviving lineages, and parasites were finding new hosts as old ones disappeared.
Tinamous in particular host the highest diversity of feather lice genera of any bird group and carry representatives of all four feather louse body forms. Genomic phylogenetics of tinamou lice found that while body lice on tinamous do form a single evolutionary group, the broader set of tinamou lice includes lineages that colonized from other South American bird groups, followed by extensive diversification in place.25PubMed Central. Extensive in situ radiation of feather lice on tinamous Tinamous, in other words, have served as a kind of evolutionary platform for parasite diversification across South America, accumulating lice from multiple sources and then letting them radiate into new species.