Evolution: Palaeontology

Basic Ideas: Origin of birds

Contents

In this article the "Urvogel" (original bird) Archaeopteryx and the theropods (two-legged carnivore dinosaurs) will be introduced as possible ancestors of birds and critically discussed. In addition, theories about the origin of feathers and the flight of birds will be introduced and the pros and cons explained.

Theropod dinosaurs as ancestors of birds?

Dinosaurs with feathers?

The driving force for the evolution of feathers and flight

Criticism of the theropod theory

Explosive variety of Cretaceous birds

Discontinuity at the Border Cretaceous-Tertiary

Summary

Today's birds are clearly differentiated from other recent vertebrates. For a long time the famous early bird Archaeopteryx (fig. 313) from the Late Jurassic almost exclusively gave an impression of how an evolutionary transition from reptiles to birds may have occurred, since it shows a distinct combination of reptile-like as well as bird-like features (fig. 314). It is therefore obvious that the “Urvogel” appears to be a mosaic and transition form (cf. Definition of mosaic and transitional forms, which, on the other hand, cannot be certainly assessed as an evolutionary transition form (see below). All the other fossil birds which were found up to the end of the 1980s were clearly discernible from reptiles.

Still, Archaeopteryx is the oldest indisputable fossil bird. Further fossil birds are unknown in strata lower than the Early Cretaceous (for the stratigraphy see fig. 31). Most have been found since the beginning of the 1990s, mainly in China. Meanwhile more than thirty bird species from the Cretaceous are known. Apart from this an increasing number of small dinosaurs walking on two legs (theropods) with features typical for birds have been found. Thus, the gap between reptiles and birds among fossils has become significantly smaller.

First and foremost through the works of J. Ostrom in the middle of the 1970s the theropods gained (see above) the status of the most probable group of bird ancestors. Therefore today's idea, that birds are feathered dinosaurs, is hardly any longer disputed among evolutionary theorists. However, the fossil sequence does not fit into this view (see below). The central argument for a transition dinosaurs – birds rather originates from the shape analogy of species of different age (for the evolutionary argument of the similarity see Similarities in morphology and anatomy). If the dinosaur-bird theory is indisputably valid for many biologists this must be seen in the light of the fact that an evolutionary transition from any reptiles to birds is required to be a prerequisite. Within this premise certain groups of dinosaurs among the theropods fit best.

The most important commonalities of many theropods with the earliest birds are bipedalism, the anatomy of the hind legs with a backward oriented toe and partially intergrown metatarsal bones, the anatomy of the front legs showing a hand with three fingers, a long downwards directed pubic bone, possession of a collarbone (molten clavicles) as well as a long vertebral tail. Due to antilogies of features, however, it is not determinable with certainty which species among theropod dinosaurs is the most eligible ancestor of Archaeopteryx. Each of the candidates has features which do not fit. For example the species Compsognathus from the Late Jurassic (fig. 314) has many features in common with its contemporary Archaeopteryx, but still it has a strongly specialized and reduced hand with only two fingers and therefore—apart from the contemporaneous occurrence—cannot be considered as ancestor. Similar discrepancies also occur with other species.

Theropod dinosaurs as ancestors of birds?

At the end of the 1990s the dinosaur-bird-theory gained considerable momentum when theropods were found showing feathers or feather-like skin attachments. However, all these fossils stem from the Lower Cretaceous and are, thus, considerably younger than Archaeopteryx whose feathers and entire plumage resemble those of recent birds in almost every aspect. With reference to the feature of feathers it is still true that at the beginning of fossil tradition there are perfect feathers. Still, about the nature of dinosaur feathers from the Early Cretaceous and the taxonomic classification of the species in question it is often not possible to make any statements with certainty.. A group of researchers showed with the help of stages of disintegration of tissue of the cover of the bodies of different reptiles that collagen fibers leave behind a feather-like print in the skin during the disintegration process. These stages of disintegration show striking similarities to "proto feathers" (=proto feathers) of fossil dinosaurs. The interpretation of covers of dinosaur bodies as proto feathers (prefeathers) is therefore not regarded as a fact.

Dinosaurs with feathers?

This does not apply to Protarchaeopteryx and Caudipteryx (fig. 315). The attachments of their bodies show a central shaft and a densely closed fan. The feathers, however, are symmetric, a fact contradicting their capability to fly. Birds with capability to fly always have asymmetric feathers, symmetric ones are only known from birds that cannot fly. Therefore many researchers assume that Protarchaeopteryx and Caudipteryx stem from species having the capability to fly and are, thus, not eligible bird ancestors. Their feathers can also not be interpreted as early stages of feathers. This view is consistent with the fact that the front extremities definitely are too short for the capability to fly.

The driving force for the evolution of feathers and flight

An unsolved problem so far is the question which selection pressures might have been favorable for the forming of feathers. The majority of researchers tends to regard the first function of feathers not as the possibility to fly but one of the other numerous functions of feathers. Especially the thermal insulation is regarded as original function, but this would not require such complicated structures as feathers. In addition: Selection with regard to thermal insulation rather aims towards down feathers instead of wing feathers and leads away from the capability to fly. Hypotheses towards selection conditions for acquiring feathers can hardly be tested. Various hypothetical scenarios exclude each other. The complexity of recent feathers is shown in figures 316 and 317.

The question about the way to acquire the capability to fly has also not been solved so far. In this case the Ground-up theory (cursorial theory; from running to flight) and the Tree-to-Ground theory (arboreal theory; from gliding down to active flying) stand opposite to each other. The cursorial theory on one hand is more likely since most theropods, which are close to birds, were two-legged runners. However, opposed to that are severe aerodynamic problems. Nearly all theropod dinosaurs are too heavy as original species for a start from the ground. Only the Microraptor zhaoianus probably lived in trees and, in addition, it only had the size of a crow. Some of its features are more bird-like than those of Archaeopteryx. Thus, it is too specialized to serve as ancestor. The start from an elevated point (Tree-to-Ground theory) would have been of advantage from an aerodynamic point of view. In this case it would be reasonable to expect that the hind extremities would have been included into the flight apparatus, but this is not the case with birds as is well-known. Nevertheless, there are fossil birds with feathered hind legs; meanwhile it has been shown that this was also the case with Archaeopteryx. It must remain speculative how the gap between lethal falls and a sailing down movement at the least could have been bridged - a problem which does not occur when starting from the ground. A transition from gliding to actively flying is also questionable as far as construction is concerned (and it did not occur with other gliders among vertebrates). On the other hand a direct acquiring of actively flying from elevated locations is almost impossible. All in all significant discrepancies arise in any scenario of the evolutionary theory; the question about an evolutionary start of avian flight cannot be regarded as solved.

Criticism of the theropod theory

The idea that birds are descendants of theropod dinosaurs is being criticized by a minority of researchers. Some points of criticism will be drawn up as follows.

• Birds most resembling theropods stem from the Late Cretaceous and are, thus, far younger than Archaeopteryx from the Late Jurassic. Jurassic theropods are less suited to be ancestors of birds or they are too fragmentarily available to allow for phylogenetic conclusions.
• Nearly all theropods are, because they run on two legs and due to their heavy weight, not suited to be ancestors of birds due to aerodynamic reasons since the cursorial theory (start from the ground) would have to be applied to them. The only exception in this case seems to be Microraptor.
• Some researchers regard a hypothetic evolutionary rearrangement of therapods to the skeleton structure of birds to be partially implausible because of constructive reasons. However, these objections are disputed. While the bird’s hand corresponds to fingers II, III and IV the theropod has fingers I, II and III. A homeotic transformation as explanation for this dislocation is hypothetic and hardly to establish due to the missing selection pressure. Another example being discussed controversially is the structure of the lungs. Fossil findings point to the fact that theropods had crocodile lungs which cannot be altered into the completely differently structured lungs of a bird according to the opinion of some researchers.

Explosive variety of Cretaceous birds

Some descriptions of the evolution of birds show variation sequences of some Cretaceous species displaying the regression of reptile features and the increase of bird-like features, as there is the transition from Archaeopteryx via Sinornis (fig. 318) to Iberomesornis (Las Hoyas bird). Those descriptions, however, represent only a small excerpt from the variety of Cretaceous birds. Even in the Early Cretaceous there is a contemporaneous coexistence of quite different types of birds so that there is reason to assume the existence of two or three separate lines of birds from the beginning of the tradition of fossil birds. Among the many spectacular findings in the Early Cretaceous of Yixian (China) there is Confuciusornis a species with a "modern" horned beak and a modern linkage of elbow and wrist (important for flying). On the other hand it had an "ancient" pelvis - an unexpected combination. The temporal region of the skull is constructed diapsid (cf. fig. 319) and, thus, more ancient than that of Archaeopteryx according to evolutionary understanding. Liaoningornis retrieved from the same strata shows a contrary mosaic of features. It had a toothed beak ("primitive feature"), the prerequisites for flying were even more "modern" than those of Confuciusornis. Many other examples of such contradicting combinations of features might be listed.

From the Lower Cretaceous we already know Hesperornithiformes, aquatic birds which on one hand had recessed wings and were in this regard derived and were also rather "modern" with regard to other features, but on the other hand they had teeth. Although having become unable to fly they were spread all over the globe. Likewise with teeth were the Ichthyornithiformes. These groups were neither related to each other nor were they related closer to recent birds or theropods.

Even this small preview gives an impression of the variety of Cretaceous birds which occur out of a sudden on the stage of the birds existing as fossils - many of them nearly at the same time or even earlier than theropod dinosaurs which are supposed to be the closest relatives of bird ancestors. The spectrum of features of this variety of forms rather appears to be a confusing network than a family tree. This is expressed in the occurrence of numerous convergences (=development of similar features emanating from dissimilar pre-stages) (cf. Similarities in morphology and anatomy)

Discontinuity at the Border Cretaceous-Tertiary

The bird species from the Cretaceous disappear at the end of the Cretaceous to be replaced explosively by bird species well known to us at the beginning of Tertiary. The relations between them and the Cretaceous birds are not clear. As a rule they are clearly distinct from each other from the beginning of their being available as fossils - a result that is regularly demonstrated in other animal groups (cf. Cambrian explosion).

Summary

1. Among the two-legged theropod dinosaurs of the Cretaceous there are numerous mosaic forms with different features of birds and reptiles. Therefore they are considered to be the best candidates for bird ancestors.

2. The increasing similarity to birds of some species supports the evolutionary theoretical interpretation. All in all the features of the respective species are distributed in a mosaic way to such a high degree that in many cases convergences and reversions (=retrogression) have to be stipulated, even with regard to some key features.

3. The oldest fossil feathers occur in their finished form with Archaeopteryx. Other fossil findings of feather or hair-like structures are clearly younger and their interpretation is being controversially disputed. It is regarded as possible that some of the Cretaceous forms have lost their capability to fly.

4. It is controversial which selection pressures may have fostered the acquirement of feathers and the capability to fly.

5. During the Cretaceous as well as at the beginning of the Tertiary numerous groups of birds emerge out of a sudden and with remarkable discontinuities

The previous text is an abbreviated version of the chapter "The Origin of Birds" from "Evolution—a critical textbook" (http://www.wort-und-wissen.de/lehrbuch.html). Literature references can be found under the following link: http://evolutionslehrbuch.wort-und-wissen.de/anhang/literatur.html, provided in section VI.14.5.

Translator: Isa Sefzick, 09.06.2013

Author: Reinhard Junker

 
© 2013