Aves Translation and Pronunciation Guide

Introduction

Ben Creisler

Dividing Birds from Dinosaurs

Should birds be called dinosaurs? This question raises issues involving both taxonomy and nomenclature. Breakthrough research by John Ostrom in the late 1960s carefully and convincingly documented the striking similarities between the anatomy of the Late Jurassic Archaeopteryx and Early Cretaceous Deinonychus, and led to the logical conclusion that Archaeopteryx evolved from a dinosaur closely related to Deinonychus. According to the more traditional evolutionary classification advocated by Ernst Mayr, Gaylord Simpson and others, birds may have evolved from theropod dinosaurs, but nonetheless deserve to be treated as a separate Linnaean class, distinct from reptiles, based on the unique anatomy and physiology they have developed. Cladists, however, advocate a more narrow approach to classification that insists on establishing strict phylogenetic lineages only. The specialized anatomy and physiology that distinguish modern birds are an issue separate from their classification--if birds evolved from theropod dinosaurs, they ARE theropod dinosaurs, and Linnaean hierarchies are an artificial hold-over from a creationist view of Nature. The most devoted among the cladists make a distinction between avian and non-avian dinosaurs, and thus do not recognize birds as a separate class of vertebrates. A number of researchers have taken a middle ground, accepting the methods of cladism, but rejecting wholesale revision of the nomenclature--birds remain a branch of the theropod dinosaurs, but are nontheless referred to as birds in the traditional sense, and are granted class status.

A smaller group of researchers remains unconvinced that birds evolved directly from dinosaurs, and have argued for a kind of parallel evolution--birds and dinosaurs share a common ancestor among the archosaurs, but have distinct evolutionary histories probably leading back to the Late Triassic, or even earlier. Despite many similarities between birds and theropod dinosaurs in the construction of their skulls and limbs, these researchers propose that birds may have evolved instead from thecodonts such as Euparkeria, from early crocodilians, or from a group of arboreal reptiles similar to Megalancosaurus. To date, evidence for a pre-Late Jurassic origin for birds is not considered convincing by most researchers, the Late Triassic Protoavis being a controversial, if intriguing, exception. The discovery of a wide variety of bird fossils from the Early Cretaceous that combine features of Archaeopteryx such as gastralia, teeth, clawed wings, etc., with more advanced modifications of the limbs for flight and perching strongly suggest that major bird evolution began during, or shortly, before the Late Jurassic, though Archaeopteryx, with its long bony tail, may represent a distinct lineage not directly ancestral to modern birds. To date, only fragmentary limb bones, and isolated teeth have been recognized as possible evidence for Jurassic dromaeosaurs and troodontids, though it seems certain that relatives of such advanced maniraptoran theropods must have existed before the Cretaceous if the theropod origin of birds during the Jurassic is correct.

The Dinosaur-Bird Problem

Although most researchers now classify birds as a highly specialized branch of theropod dinosaurs, or as a clade embedded within the Theropoda, the identification of a number of genera as either birds or dinosaurs is not settled. Archaeopteryx is sometimes treated as a feathered dinosaur, or as the earliest well documented bird. Debate rages as to whether Mononykus should be classified as a birdlike dinosaur, or as a highly specialized dinosaur-like flightless bird--Novas (1994) has proposed that the Argentine genus Alvarezsaurus (originally identified as theropod dinosaur) and the Mongolian Mononykus are closely related, and belong together in the Alvarezsauridae, considered a family within the birds. Other forms known from incomplete or difficult of interpret material have been shifted between the birds and the dinosaurs. Avisaurus, proposed as theropod dinosaur, has been reidentified as an enantiornithine bird, a now widely accepted classification. The poorly known Wyleyia, Bradycneme, Heptasteornis and Elopteryx, based mainly in fragmentary limb material, have been classified either as birds or as maniraptoran dinosaurs. Chatterjee has placed Avimimus among the birds, while his proposed avian genus Protoavis remains controversial, with some researchers speculating that most of the defined material includes one or more types of small dinosaurs. New discoveries and further research may clarify the status of each of these taxa. Commonly disputed forms are indicated by [dino-bird] at the end of the list entry.