What is a Maniraptoran?
Maniraptorans are a group of theropod dinosaurs. Theropods include most known meat-eating dinosaurs, but many maniraptorans have evolved away from hunting large prey, instead being herbivores, omnivores, or insectivores. Maniraptorans also tend to have longer forelimbs (with some exceptions) and larger brains for their body size than non-maniraptoran theropods.
Among the theropods, maniraptorans are closely related to the ostrich-like ornithomimosaurs and carnivorous tyrannosauroids. In fact, the scientific definition of the group Maniraptora amounts to "all species more closely related to modern birds than to ornithomimosaurs".
Like in some other theropods, feathers are widespread in maniraptorans. In fact, all maniraptorans for which evidence of body covering has been found are known to have feathers. In the earliest maniraptorans, these feathers were probably hair-like tufts that served mainly in insulation, with little to no aerodynamic function.
The oldest known definite maniraptoran fossils come from the beginning of the Late Jurassic, about 163 million years ago. This would have been slightly less than halfway through the Mesozoic Era, popularly known as the "Age of Dinosaurs". However, given that those fossils are identifiable as members of specific major subgroups of maniraptorans, the group as a whole must have originated somewhat earlier than that.
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| A phylogenetic tree of maniraptorans. Although most recent studies favor the phylogeny I've shown here, the relationships between alvarezsaurs, therizinosaurs, and pennaraptorans remain an open question. A dagger (†) indicates groups that are extinct. |
One of the main groups of maniraptorans is the alvarezsaurs. They were small dinosaurs ranging from pigeon- to rhea-sized, and those that lived in the Late Cretaceous (between 66 and 100.5 million years ago) are known for their diminutive forelimbs with a single greatly enlarged claw on each hand. Despite their reduced length, the forelimb bones had large attachment points for muscles, suggesting that they were quite powerful for their size. It is widely thought that alvarezsaurs may have used their forelimbs break into hard substrates like rotting wood as they searched for insects to eat. They are obviously the best dinosaurs.
Late Cretaceous alvarezsaurs had various bird-like features in the skull and limbs, and they were once thought to be fairly close relatives of modern birds. However, discoveries of early alvarezsaurs convincingly show that they evolved from more "typical" theropods and were only distantly related to birds among maniraptorans.
Another group of maniraptorans is the therizinosaurs. The shape of their skulls and teeth indicate that they were primarily herbivorous. They tended to have wide bodies, likely to house larger intestines for digesting plants. They are also notable for the large claws on their hands, which may have functioned in feeding and defense. Therizinosaurs included the largest known maniraptorans, with the biggest species having weighed an estimated 5 tons.
Most other maniraptorans belong to a group called the pennaraptorans, which are characterized by the presence of large feathers on their arms, hands, and tail. Birds are pennaraptorans, and in most birds these feathers are of course important for generating lift during flight. However, there is little evidence that the earliest pennaraptorans could fly. Instead, they may have used their wings for display or to help them maneuver while running. Perhaps not coincidentally, pennaraptorans also have a half-moon-shaped wrist bone (the semilunate carpal) that increases the folding ability of the hand towards the forearm, which probably helps keep the wing feathers clear of the ground.
One of the major lineages of pennaraptorans is the oviraptorosaurs. These strange dinosaurs had short, boxy skulls with few or no teeth, suggesting that they were mainly herbivores or omnivores. Their tails were relatively short but muscular, which may have been useful for shaking their tail feathers in display. Most oviraptorosaurs were smaller than an adult human, but the largest known species weighed more than a ton.
Several oviraptorosaur specimens have been discovered sitting on nests of eggs. They were originally thought to have been preserved in the act of stealing eggs from other dinosaurs (hence "oviraptorosaur" translating to "egg thief lizard"), but findings of oviraptorosaur embryos within those eggs suggest that they were more likely to have been protecting their own eggs, similar to how most modern birds sit on their own nests today.
Near Birds
Another major group of pennaraptorans is the paravians, which survive today as modern birds. On average, paravians are smaller than other maniraptorans; even the largest known species did not exceed a ton in body mass. Early paravians were pigeon- to chicken-sized predators that hunted even smaller animals. Possibly to help dispatch their prey, they had a retractable second toe on each foot, which would have kept the corresponding claw sharp. (Modern birds have lost the ability to retract the second toe.)
Paravians can be divided into three main lineages:
- Dromaeosaurids, which include names familiar to just about anyone with a passing interest in dinosaur paleontology, such as Velociraptor and Deinonychus. They have become popularly known as "raptors" thanks to the Jurassic Park franchise. With stiffened tails for balance and particularly well-developed toe claws, they were likely agile predators. Ranging from pigeon- to grizzly-bear-sized, dromaeosaurids appear to have adopted a wide variety of lifestyles, including semi-aquatic fish eaters, small flying forms, and hunters of relatively large prey, which could potentially attack animals even bigger than themselves.
- Troodontids, which generally had shorter forelimbs and longer legs than dromaeosaurids did. In general, they probably kept the ancestral paravian habit of being small game hunters, with no known species having grown larger than a wolf. Some troodontids may have even been omnivores instead of strict carnivores. They had some of the largest brains relative to body size of any dinosaur (other than birds), and thus are often hailed as being some of the smartest dinosaurs in popular media.
- Avialans, by far the most diverse group, being the one that includes modern birds. Most avialans are flight capable, with very long forelimbs relative to body size. They also tend to be "baby-faced", retaining a juvenile skull shape into adulthood, giving them shorter faces than other dinosaurs.
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| A phylogenetic tree of select paravian lineages. As noted in the main text, the position of Archaeopteryx is debated, but it is most commonly found to be an early avialan. A dagger (†) indicates groups that are extinct. |
A particularly successful group of avialans is the pygostylians, which are characterized by having a much shorter tail than most other dinosaurs. In addition, the vertebrae at the tip of their tail are fused into a structure called the pygostyle. This reduction in tail length may be beneficial for decreasing the amount of drag experienced during flight. Modern birds belong to this radiation of short-tailed avialans.
It is in paravians that we see the earliest evidence of at least rudimentary flying abilities in dinosaurs. The latest studies at the time of writing favor the possibility that several lineages of small paravians independently started experimenting with aerial locomotion. However, it has also been suggested that the last common ancestor of all paravians was a flying animal, in which case flightless paravians such as Velociraptor would have had flight-capable ancestors.
A Bird in the Hand
The oldest known pygostylian fossils come from the Early Cretaceous, roughly 131 million years ago, and it appears that by this point they had already evolved into a wide variety of forms. One specific group of pygostylians that would continue to flourish throughout the Cretaceous (and onward) is the ornithothoraceans.
Ornithothoraceans exhibit increased adaptations for flight, including enlarged breastbones for the attachment of flight muscles and shoulders that allow the wings to be raised higher above the body during flapping. Furthermore, their wrist bones are more consistently fused to their palm bones compared to those of other maniraptorans, and their fingers and front claws are reduced in size. This stiffens their hands and makes them of limited use for grasping objects, but provides them with a more efficient flight stroke. Ornithothoraceans also became more specialized for spending time in trees: their innermost toe on each foot points fully backward as an adaptation for gripping small branches.
One of the two main lineages of ornithothoraceans are the enantiornitheans, which translates to "opposite birds" because some of the ways in which their bones connect to each other are opposite from those of modern birds. The most completely known enantiornithean fossils suggest that most of them were small, tree-dwelling dinosaurs that may have fed on insects, but there are fragmentary specimens that suggest that there may have also been flightless, semi-aquatic, and raptorial species. Unlike most modern birds, enantiornitheans appear to have been capable of flight shortly after hatching. Several specimens of juvenile enantiornitheans have been found entombed in amber, making them some of the best-preserved Mesozoic dinosaur fossils of all time.
Modern birds are members of the other major ornithothoracean branch, the euornitheans. The tail of euornitheans sports a fan-shaped array of feathers that can be spread to form an additional lifting surface during flight. (In contrast, enantiornitheans usually had few or no large tail feathers.) Another notable feature of euornitheans is that at least the tips of the upper and lower jaws are toothless and instead covered by a beak. A subset of euornitheans also adopted a supercharged growth rate, typically reaching adulthood in less than a year (whereas other dinosaurs took several years to mature). This type of rapid growth is still seen in most modern birds. In general, Cretaceous euornitheans appear to have been more inclined towards ground-dwelling or semi-aquatic niches compared to enantiornitheans, and are known to have produced flightless, seagoing forms.
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| A phylogenetic tree of select ornithothoracean lineages. A dagger (†) indicates groups that are extinct. |
Modern birds form a group of euornitheans called neornitheans. Numerous features distinguish neornitheans from most other euornitheans, but perhaps the most obvious one is that the beak, which had been present only on the jaw tips of ancestral euornitheans, has expanded backward to take up most of the jaw, and teeth have been lost entirely. The timing of when neornitheans originated remains a subject of great debate, but a handful of fossils indicate that they had appeared by the end of the Cretaceous, though they may have only been present in relatively small numbers.
A New Beginning
About 66 million years ago, a giant asteroid the size of Manhattan impacted the Earth. The impact itself would have been devastating, but more concerning to organisms that lived far from its immediate vicinity would have been the copious amounts of debris that it threw into the atmosphere. The ejecta would have blocked incoming sunlight, wreaking havoc on food webs and climate. These dramatic changes resulted in a mass extinction, marking the end of the Mesozoic Era and the beginning of our current geologic era, the Cenozoic. Most of the dinosaurs, including most of the maniraptorans, were wiped out in this cataclysmic event. However, a handful of neornitheans survived.
How neornitheans managed to live through the end-Cretaceous mass extinction when all other dinosaurs perished is not clear. Certainly being small enough to take cover from the immediate aftermath of the asteroid impact and require less food than the larger dinosaurs was a likely factor, but this would also apply to many of the maniraptorans that died out, such as the enantiornitheans. Perhaps the toothless beaks of neornitheans allowed them to crack open seeds as a food source in the absence of green vegetation. It also appears that, unlike the largely arboreal enantiornitheans, early neornitheans were likely ground-dwelling birds, which would have also helped them cope with the loss of forests.
Regardless of how they did it, at least three neornithean lineages made it past the end of the Cretaceous and would eventually give rise to the more than 10,000 species of birds alive today. Those three lineages are the following:
- Paleognaths, which include large flightless birds like ostriches and emus, as well as the flight-capable tinamous. Traditionally, it was thought that the flightless paleognaths shared a flightless ancestor, but it is now considered more likely that they lost flight several times.
- Galloanserans, which include the anseriforms (ducks and their kin) and galliforms (chickens and their kin). Most are ground-feeding or semi-aquatic omnivores. They have produced several giant flightless lineages that are now extinct.
- Neoavians, which include all other modern birds. They account for over 95% of living bird species.
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| A phylogenetic tree of neornitheans. |
The interrelationships among neoavians remain one of the biggest outstanding questions regarding dinosaur evolution, but recent genetic studies generally agree in identifying the following major groups:
- Mirandornitheans, the flamingos and grebes. The close relationship between the long-legged flamingos and diving grebes was surprising when first discovered, but it has since been noted that the two groups share several anatomical similarities, such as flattened, nail-like claws on their feet.
- Columbimorphs, the mesites, sandgrouse, and pigeons. Unfortunately for paleontologists, they have an incredibly poor fossil record.
- Otidimorphs, the turacos, bustards, and cuckoos. Whereas bustards and some cuckoos are mostly ground-dwelling birds, the fruit-eating turacos are highly arboreal. However, fossils of early turacos suggest that they descended from terrestrial ancestors.
- Strisoreans, the nightjars, swifts, hummingbirds, and their kin. They are some of the most remarkable dinosaurs, and I would stand by that even if I weren't biased. They include superb fliers, as well as large-mouthed, nocturnal insect eaters that barely look like real animals.
- Opisthocomiforms (not pictured below), the hoatzins. The single living species is a highly unusual, leaf eating bird that lives in South America. More than any other living bird, it represents the "problem child" of bird phylogenetics, having been linked to almost every other bird group under the sun. Even sophisticated genetic studies have done little to quell the debate so far.
- Gruiforms, the cranes, rails, and their kin. Most members of this group are omnivorous, ground-dwelling birds that feed near water.
- Charadriiforms, the shorebirds. These include plovers, sandpipers, gulls, auks, and more. Although many species are specialized for picking food off mudflats and beaches, they are highly diverse, having also evolved into inland and open ocean foragers.
- Natatoreans, one of the greatest radiations of aquatic dinosaurs. They include a wide range of predatory birds that tend to feed in water, including loons, penguins, albatrosses, cormorants, herons, and more.
- Telluravians, a massive assemblage of mostly tree-dwelling birds. They likely diversified quickly into arboreal niches shortly following the end-Cretaceous mass extinction. Most studies find that they split into two main lineages: afroavians (hawks, owls, hornbills, kingfishers, woodpeckers, and more) and australavians (falcons, parrots, songbirds, the extinct terror birds, and more)
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| A phylogenetic tree of neoavians. Given how contentious neoavian phylogeny is, I have chosen to depict most of their interrelationships as ambiguous. |
