Birds, from the in-prep dinosaur book (Patreon)

Hi everyone. Not getting much stuff done at the moment due to workload ... literally haven't touched any of my in-prep books and such for weeks now. But to show that things are still happening with the DinoPedia book (see previous posts), I thought you'd appreciate seeing another completed section from the work: this time, the 'Birds' Section. Any thoughts appreciated... (I've inserted some pictures - all from The Big Book, not from DinoPedia! - just to make it look more interesting).

Birds

A diverse animal group (containing over 10,000 living species) that occurs worldwide today and is famous for the feathers, toothless, beaked jaws and flight ability of the majority of its constituent species. Crown-birds (also called modern birds or neornithines) – that is, those birds which belong to the most inclusive clade delimited by lineages containing living species – are superbly adapted for flight. The skeleton is pneumatised, a keel on the sternum anchors large muscles which power the wings, the enormous feathers of the forelimbs and tail form flight surfaces which the animal can tightly control, the eyes and brain are unusually large, and the feet (typically equipped with a hallux which fully opposes the other three toes) are specialised for perching. The average size for a crown-bird is around 30g. Birds are coelurosaurian theropods, specifically, part of the maniraptoran group Pennaraptora closely related to dromaeosaurids and troodontids. This means that dinosaurs did not become extinct at the end of the Cretaceous and that dinosaurs have been substantially successful throughout the so-called Age of Mammals. 

The technical name long preferred for birds is Aves. Some palaeontologists use this term for all members of the bird lineage. An alternative take is that ‘Aves’ is best restricted to crown-birds (since these alone have the combination of features typically regarded as special to birds), and that the whole lineage is best termed Avialae. Unfortunately, experts disagree on this, meaning that Aves and Avialae are both currently in use for the clade that includes Archaeopteryx, crown-birds, and all lineages in between. Because Avialae has been used as a branch-based name applied to all taxa closer to crown-birds than to other maniraptorans, such pennaraptorans as anchiornithines and scansoriopterygids (which are further from crown-birds than Archaeopteryx) are also included in Avialae. Should we really be using the term ‘bird’ for all theropods belonging to this branch? While there’s no formal agreement on this issue, most experts do indeed use the word ‘bird’ for all members of Avialae… even though Archaeopteryx is more like a Microraptor than a pigeon.

It’s been agreed since the 1800s that birds are ‘glorified reptiles’. More specifically, it’s obvious that birds are archosaurs, since they share anatomical and behavioural characters with crocodylians. An even more specific view – that birds might be dinosaurs – was mooted during the 1860s when famed ‘Darwin’s bulldog’ Thomas Huxley noted the bird-like hips and legs of the coelurosaur Compsognathus and the ornithischian Hypsilophodon, and consequently the idea that birds are allied to dinosaurs became popular during the late 1800s and early 1900s. Experts of this time tended to have vague views on how animal groups were related, so the idea of a bird-dinosaur affinity was consistent with the possibility that birds might also be close to early archosaurs or even pterosaurs. 

By the 1920s, the consensus was that birds had evolved from a group of archosaurs (termed ‘pseudosuchian thecodonts’) that were themselves ancestral to dinosaurs and other archosaur lineages too. One work in particular – Gerhard Heilmann’s 1926 book The Origin of Birds – did much to establish this view as the one best supported by evidence. An ironic aspect of the book’s success in convincing palaeontologists is that Heilmann was an amateur scientist and artist, ridiculed and ignored by scientists in his native Denmark, but mostly unknown elsewhere and thus assumed to be authoritative. Anyway, the ‘pseudosuchian’ view became textbook wisdom for the next few decades.

This was eventually overturned in the late 1960s following John Ostrom’s work on Deinonychus. Today, it’s fantastically well supported by tens of Jurassic and Cretaceous fossils. These include bird-like non-birds in addition to a vast number of early birds, many of which possess anatomical traits which are ‘intermediate’ in shape and proportion between those of dromaeosaurs and those of crown-birds. Prior to the 1980s, the early bird record consisted of Archaeopteryx, the toothed Cretaceous seabirds Ichthyornis and Hesperornis and little else. Today we know of several bird lineages which are archaic relative to crown-birds (many have teeth and clawed fingers), but closer to them than is Archaeopteryx. The most important of these are the enantiornithines (sometimes called ‘opposite birds’), a large, diverse radiation of archaic birds that includes species superficially recalling seabirds, waders, hawks and finches. The most archaic fossil birds show that birds were initially just one of several pennaraptoran lineages, all of which looked highly similar during the earliest stages of their history, and all of which started their history as small generalist predators or omnivores. Only later did birds evolve the peculiarities associated with crown-birds, and in fact the most profound transition in bird evolution is the one between crown-birds and their relatives. This event occurred during the Cretaceous, and we know this because early members of the main crown-bird lineages – including forms related to ducks and gallinaceous birds – are known as Upper Cretaceous fossils.

Working out how the numerous crown-bird lineages are related has been a formidable task, a vast literature exists on this topic, and numerous ideas have been put forward. A consensus has emerged since 2006 thanks to several major genetic studies. These agree in showing that palaeognaths (ratites and tinamous) form the sister-group to remaining crown-birds, that anseriforms (ducks and kin) and galliforms (gallinaceous birds) are united within Galloanserae, and that remaining crown-birds diverged in the following order: swifts, nightjars and kin (Strisores), bustards, pigeons and kin (Columbaves), cranes, rails and kin (Gruiformes), waterbirds, seabirds and kin (Aequorlitornithes), raptors (Accipitriformes), owls (Strigiformes), woodpeckers, rollers and kin (Coraciimorphae) and parrots, falcons, songbirds and kin (Australaves). Songbirds – properly, Passeriformes – are the largest single bird group, containing over 60% of living bird species. A significant percentage of bird species are endangered by human activity and it looks likely that entire lineages will be lost within coming decades. This book is about extinct animals, not those alive today, but as a conscientious person interested in the natural world do remember to do what you can to ensure that birds and other animals persist into the future.

See also Archaeopteryx; Birds Are Not Dinosaurs; John Ostrom; Maniraptorans.