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Guest Lancelot Arnold

The Origin and Diversification of Birds 1

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Guest Lancelot Arnold

Birds are one of the most recognizable and diverse groups of modern vertebrates. Over the past two decades, a wealth of new fossil discoveries and phylogenetic and macroevolutionary studies has transformed our understanding of how birds originated and became so successful. Birds evolved from theropod dinosaurs during the Jurassic (around 165–150 million years ago) and their classic small, lightweight, feathered, and winged body plan was pieced together gradually over tens of millions of years of evolution rather than in one burst of innovation. Early birds diversified throughout the Jurassic and Cretaceous, becoming capable fliers with supercharged growth rates, but were decimated at the end-Cretaceous extinction alongside their close dinosaurian relatives. After the mass extinction, modern birds (members of the avian crown group) explosively diversified, culminating in more than 10,000 species distributed worldwide today.
Main Text
Introduction

Birds are one of the most conspicuous groups of animals in the modern world. They are hugely diverse, with more than 10,000 extant species distributed across the globe, filling a range of ecological niches and ranging in size from the tiny bee hummingbird (∼2 grams) to the ostrich (∼140,000 grams). Their feathered bodies are optimized for flight, their supercharged growth rates and metabolism stand out among living animals, and their large brains, keen senses, and the abilities of many species to imitate vocalizations and use tools make them some of the most intelligent organisms on the planet [1].

This begs a fascinating question: how did birds achieve such great diversity and evolutionary success? For much of the last two centuries this was a mystery, but over the past two decades a wealth of new fossil discoveries, molecular phylogenetic analyses of living birds, and quantitative macroevolutionary analyses have revolutionized our understanding of bird origins and evolution. This new information reveals a surprising story: birds evolved from dinosaurs and have a deep evolutionary history, during which their signature body plan evolved piecemeal over ∼100 million years of steady evolution alongside their dinosaurian forebears before many of the modern groups of birds explosively diversified after the non-avian dinosaurs went extinct 66 million years ago (Figure 1) (e.g. 2, 3, 4).
Figure 1. Summary phylogeny (genealogical tree) of birds.

The phylogeny shows where birds fit into the larger vertebrate family tree and the relationships of the earliest birds and their closest dinosaurian relatives (based on [2] and other studies cited therein). Timescale values are in millions of years; thick red line denotes the mass extinction at the Cretaceous–Paleogene boundary caused by asteroid impact (denoted by fireball on the right); arrows denote lineages that survived the extinction; circles represent species known from a particular point in time; thick line sections of branches indicate direct fossil evidence and thin lines are temporal distributions implied by phylogenetic ghost lineages; Cz, Cenozoic interval after the end-Cretaceous extinction. Silhouette anatomical features in the lower part of the figure are plotted approximately where they evolve on the phylogeny. Species silhouettes at the top of the image are from phylopic.org and designed by (from left to right): Nobu Tamura, Anne Claire Fabre, T. Michael Keesey, Steven Traver, Andrew A. Farke, Mathew Wedel, Stephen O’Connor/T. Michael Keesey, Brad McFeeters/T. Michael Keesey, Scott Hartman, T. Michael Keesey, Scott Hartman, Scott Hartman, Matt Martyniuk, Matt Martyniuk, Matt Martyniuk, Matt Martyniuk, Nobu Tamura/T. Michael Keesey, Matt Martyniuk, J.J. Harrison/T. Michael Keesey. ‘Bipedal posture’ silhouette by Scott Hartman.

The origin of birds is now one of the best understood major transitions in the history of life. It has emerged as a model case for using a combination of data from fossils, living species, genealogies, and numerical analyses to study how entirely new body plans and behaviors originate, and how prominent living groups achieved their diversity over hundreds of millions of years of evolution 2, 3. Here, we review what is currently known about the origin, early diversification, and rise to dominance of birds, and the various lines of evidence that piece together this story.

Note that throughout this review, we use the vernacular term ‘birds’ to refer to a specific group, which is defined in a phylogenetic sense as the most inclusive clade containing Passer domesticus (the house sparrow) but not the extinct bird-like dinosaurs Dromaeosaurus albertensis or Troodon formosus. This clade includes all living birds and extinct taxa, such as Archaeopteryx and Enantiornithes. Some researchers refer to this group as Avialae (e.g. 2, 5), but others use the name Aves (e.g. [6]). In this review, we avoid these debates by referring to this group as ‘Avialae/Aves’ and its members as ‘avians’. We use Neornithes to refer to the avian crown group, which comprises all living birds and the descendants from their most recent common ancestor.

The Dinosaur–Bird Link: Once Controversial, Now Mainstream

What did birds evolve from and where do they fit into the family tree of life? For much of the 19th and 20th centuries these questions were hotly debated. The first hint that birds evolved from reptiles appeared in 1861, only a few years after Darwin published On the Origin of Species, with the discovery of an exquisite skeleton of a Late Jurassic (ca. 150 million year old) bird from Germany. Named Archaeopteryx by British anatomist Richard Owen, this fossil possessed a curious mixture of classic bird features, such as feathers and wings, but also retained sharp claws on the hands, a long bony tail, and other reptilian characteristics [7]. Over the next two decades, Thomas Henry Huxley — Owen’s great rival and Darwin’s most vociferous early supporter — argued that Archaeopteryx bore remarkable similarities to small dinosaurs like Compsognathus, supporting an evolutionary link between the groups 8, 9. This idea gained some acceptance, but fell out of favor during the early 20th century, largely as a result of an influential book by Danish anatomist Gerhard Heilmann [10]. Up until the 1960s most scientists held that birds originated from a nebulous ancestral stock of reptiles called ‘thecodonts’.

The debate over bird origins was reinvigorated in the 1960s–1980s, as a new generation of paleontologists spearheaded the ‘Dinosaur Renaissance’ [11]. John Ostrom discovered fossils of the astonishingly bird-like dinosaur Deinonychus in western North America [12], Robert Bakker and colleagues argued that dinosaurs grew fast and had active metabolisms like living birds [13], and Jacques Gauthier and colleagues used the revolutionary new technique of cladistics to place birds within the family tree of dinosaurs [14]. By the 1990s the vast majority of paleontologists accepted the dinosaur–bird link, but many ornithologists remained skeptical. The discovery in the late 1990s in China of fossils from thousands of bona fide dinosaurs covered in feathers provided the most definitive visual evidence for the dinosaur–bird link 15, 16, 17, convincing most of the remaining skeptics (Figure 2A–C). It is now widely accepted, even by ornithologists, that birds evolved from dinosaurs [18], with the two groups linked by hundreds of shared features of the skeleton, soft tissues, growth, reproduction, and behavior 2, 3, 19, 20, 21, 22. Most amazingly, it is now known that many non-bird dinosaurs were feathered and would have looked much more like birds than lizards or crocodiles (Figure 3).
Figure 2. Montage of feathered, bird-like non-avian theropod dinosaurs.

(A) The four-winged dromaeosaurid Microraptor gui (photo by Mick Ellison). (B) The small long-armed dromaeosaurid cf. Sinornithosaurus (photo by Mick Ellison). (C) The large short-armed dromaeosaurid Zhenyuanlong suni (photo by Junchang Lü). All specimens from the Early Cretaceous (130.7–120 million years ago) Jehol Biota of Liaoning Province, China.
Figure 3. A troodontid dinosaur, one of the closest relatives to birds.

Reconstructed, artistic and scientifically informed appearance of a small troodontid dinosaur and its surrounding environment, illustrating the incredibly bird-like appearance of derived non-avian dinosaurs close to the common ancestor of birds. The male (left) is shown displaying to the female. The environment (Tiaojishan Formation, Middle-Late Jurassic, Liaoning, China) is a seasonally dry woodland dominated by bennettites and cycads. Illustration by Jason Brougham (http://jasonbrougham.com/). Other artistic illustrations and interpretations for these advanced paravian dinosaurs exist in the literature, with various degrees of reptilian and avian features reconstructed, but all depictions are remarkably bird-like.

Where Birds Nest in the Dinosaur Family Tree

Birds evolved from dinosaurs, and therefore are dinosaurs, in the same way that humans are a type of mammal (Figure 1). Birds are nested within the theropod dinosaurs, the major subgroup of mostly carnivorous species that includes the behemoths Tyrannosaurus and Allosaurus, but also smaller and obviously much more bird-like species such as Velociraptor, Deinonychus, and Troodon21, 22. Birds are members of a nested set of ever-more exclusive theropod subgroups: Coelurosauria, Maniraptora, and Paraves (Figure 1). Their very closest relatives are the mostly small-bodied, feathered, large-brained dromaeosaurids and troodontids, exemplified by the well-known Velociraptor [23].

However, the exact relationships among paravians (birds, dromaeosaurids, and troodontids) are uncertain and often vary between competing phylogenetic analyses based on morphological characters, because as more fossils are found it is becoming clear that the earliest birds were very similar anatomically to primitive dromaeosaurids and troodontids, so it is difficult to tell them apart. Thus, there is current debate about whether dromaeosaurids and troodontids form their own clade of close bird relatives, or whether one of them is more closely related to birds than the other 2, 5, 24. This means that there is also ongoing debate about which fossils are the earliest birds. The iconic Archaeopteryx is still widely considered to be among the first birds 2, 5, 24, 25, 26, but some studies have suggested that it may instead be a primitive dromaeosaurid or troodontid 27, 28. Additional studies have also found other small feathered theropods, such as Anchiornis and Xiaotingia, to be the earliest birds 24, 26, more primitive than Archaeopteryx. There is also debate about whether the bizarre, sparrow-to-pigeon-sized, long-fingered scansoriopterygids are basal-most birds or non-bird maniraptorans 2, 5, 24, 25, 26, 29.

These debates will likely continue, but the alternative answers do not change two important points: firstly, that birds first appear in the fossil record during the Middle–Late Jurassic, around 165–150 million years ago (the age of Archaeopteryx, Xiaotingia, Anchiornis, and close dromaeosaurid and troodontid relatives); and secondly, that the oldest birds and their closest relatives were small (roughly chicken-sized), lightweight, long-armed, winged, and feathered animals (Figure 4A,B). The fact that scientists are having a difficult time distinguishing the earliest birds from their closest dinosaur relatives illustrates just how bird-like some non-bird dinosaurs were (Figure 3), and how the transition between non-bird dinosaurs and birds was gradual.
Figure 4. Montage of bird-like features in non-avian theropod dinosaurs.

(A) Simple filament-like ‘protofeathers’ on the head of the compsognathid Sinosauropteryx. (B) Large, branching, vaned feathers forming a wing on the arms of the dromaeosaurid Zhenyuanlong suni. (C) Parent oviraptorosaur brooding its nest of large eggs. (D) Furcula (wishbone) of the dromaeosaurid Bambiraptor feinbergorum. (E) Hollow internal cavity in the tibia of the tyrannosaurid Alioramus altai. (F,G) Pneumatic foramina (denoted by arrows), where air sacs penetrated the bones, in a cervical vertebra (F) and rib (G) of the tyrannosaurid Alioramus altai. (H) The reconstructed brain of the troodontid Zanabazar junior (orange, olfactory bulb; green, telencephalon; blue, cerebellum; red, midbrain; yellow, hindbrain). (I) The brain of the modern woodpecker Melanerpes. Photo in (B) by Junchang Lü; images in (H,I) by Amy Balanoff; all other photos by Mick Ellison.

Mesozoic Birds: The First ∼100 Million Years of Avian History

Birds had diversified by the Early Cretaceous, evolving into a number of groups of varying anatomy and ecology [30] (Figure 1; example fossils in Figure 5). This diversification is recorded by the fossils of the Jehol Biota of northeastern China, dated between approximately 130.7 and 120 million years ago, which have yielded thousands of almost complete and fully articulated skeletons 31, 32. These are the oldest unequivocally avian fossils after Archaeopteryx and account for approximately half of the total recorded global diversity of Mesozoic bird species, with representatives of every major early avian group present [33]. Although highly diverse for its time, not surprisingly this primitive avifauna exhibited less ecological diversity than modern assemblages. Small arborealists, semiaquatic taxa, and larger generalists are present, but certain extant ecomorphs were absent, such as large aerial foragers and aquatic specialists [34].

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