Chapter Fourteen

Evolution of reptiles and birds

Hylonomus (hylo- "forest" + nomos "dweller") is an extinct genus of reptile that lived 312 million years ago during the Late Carboniferous period. It is the earliest unquestionable reptile.

14.1 The evolution of tetrapods (four legged animals) began about 395 million years ago in the Devonian Period with the earliest tetrapods evolved from lobe-finned fishes. Tetrapods include all living and extinct amphibians, reptiles, birds, and mammals. While most species today are terrestrial, little evidence supports the idea that any of the earliest tetrapods could move about on land, as their limbs could not have held their midsections off the ground.

14.2 The change from a body plan for breathing and navigating in water to a body plan enabling the animal to move on land is one of the most profound evolutionary changes known. It is also one of the best understood, largely thanks to a number of significant transitional fossil finds in the late 20th century combined with improved phylogenetic analysis, as described below.

Lungs before land.

14.3 The lung/swim bladder originated as an outgrowth of the gut, forming a gas-filled bladder above the digestive system. In its primitive form, the air bladder was open to the alimentary canal, a condition called physostome which is still found in many fish. The primary function is not entirely certain, but it was certainly not to breath air on land. One possible purpose was to provide additional buoyancy in the oceans to compensate for the increasingly heavy scale armour which the early bony fishes developed to defend themselves from an ocean increasingly full of predators. Cartilaginous fishes such as open sea sharks, which lack a swim bladder, need to swim constantly to avoid sinking into the depths, the pectoral fins providing lift. Another possible purpose was oxygen consumption to support the extra energy needed for these big, heavy vertebrate fish to move under water. Ambient oxygen was relatively low in the early Devonian (400 Ma), possibly about half of modern values and in any case per unit volume, there is much more oxygen in air than in water.   Vertebrates are active animals with a high energy requirement compared to invertebrates of similar sizes. Additionally the Devonian saw increasing oxygen levels which opened up new ecological niches to any group able to exploit the additional oxygen to develop into active, large-bodied animals. Particularly in tropical swampland habitats, atmospheric oxygen is much more stable, and may have prompted a reliance of lungs rather than gills for primary oxygen uptake. In the end, both buoyancy and breathing may have been important, and some modern physostome fishes do indeed use their bladders for both.

14.4 The first tetrapods probably evolved in coastal and brackish marine environments, and in shallow and swampy freshwater habitats. Well that’s a surprise – didn’t we see the same location used when early plant life came ashore – a little like Lake Annecy where shallow land meets a stable body of fresh water. In 2010, the oldest known tetrapod tracks, preserved in marine sediments at Holy Cross Mountains in Poland, were found to come from the Middle Devonian, about 395 Ma, 18 million years earlier than the oldest known tetrapod body fossils. Additionally, the tracks show that the animal was capable of thrusting its arms and legs forward, a type of motion that would have been impossible in tetrapodomorph fish like Tiktaalik.

14.5 Tiktaalik was a lobe-finned fish from about 375 Ma, having many features akin to those of tetrapods. Its fossils were found in 2004 on Ellesmere Island in Nunavut, Canada. It is thought to represent an evolutionary transition from fish to amphibians, and so to reptiles, birds, and mammals. It is one example of several lines of ancient sarcopterygian (from the Greek for ‘flesh + fin’) fish which developed adaptations to the oxygen-poor shallow-water habitats of its time, environmental conditions thought to have led to the evolution of tetrapods.

14.6 By the late Devonian, around 360 Ma, land plants had stabilized freshwater habitats, allowing the first wetland ecosystems to develop, with increasingly complex food webs that afforded new opportunities. Freshwater habitats were not the only places to find water filled with organic matter and choked with plants with dense vegetation near the water's edge. Swampy habitats like shallow wetlands, coastal lagoons and large brackish river deltas also existed at this time, and there is much to suggest that this is the kind of environment in which the tetrapods evolved.

14.7 Until the 1990s, there was a 15 million year gap in the fossil record between the late Devonian tetrapods (360 M) and the reappearance of tetrapod fossils in recognizable mid-Carboniferous amphibian lineages (345 Ma). It was referred to as "Romer's Gap", after the palaeontologist who recognized it. It was an unfortunate gap for palaeonotologists because a lot of interesting tetrapod evolution took place during that time. During the "gap", tetrapod backbones developed, as did limbs with digits and other adaptations for terrestrial life. Ears, skulls and vertebral columns all underwent changes too. The number of digits on hands and feet became standardized at five, as lineages with more digits died out. Thus, those very few tetrapod fossils found in this "gap" are all the more prized by palaeontologists because they document these significant changes and clarify their history.

14.8 The transition from an aquatic, lobe-finned fish to an air-breathing amphibian was about as significant and fundamental as you can get in the evolutionary history of the vertebrates. For an organism to live in a gravity-neutral aqueous environment, then colonize one that requires it to support its entire weight and possess a mechanism to mitigate dehydration required significant adaptations (getting better at what they were designed to do) or exaptations (getting good at something they were not designed to do) within the overall body plan. Remember these were exactly the same challenges as for the plants, mentioned earlier, and we saw what clever inventions they came up with to do the job.

14.9 Eryops, is an example of an animal that made such changes, refined many of the traits found in its fish ancestors. Sturdy limbs supported and transported its body while out of water. A thicker, stronger backbone prevented its body from sagging under its own weight. Also, through the reshaping of vestigial fish jaw bones, a rudimentary middle ear began developing to connect to the piscine inner ear, allowing Eryops to amplify, and so better sense, airborne sound.

14.10 Fossils of Eryops, named from the Greek ‘drawn-out + face’ because most of its skull was in front of its eyes, were found in early Permian (295 Ma) rocks of Archer County, Texas, but have also been found in late Carboniferous period rocks (310 Ma) from New Mexico. Several complete skeletons of Eryops have been found in lower (260 Ma) Permian rocks, but skull bones and teeth are its most common fossils.

14.11 Eryops could grow up to 3 metres making them among the largest land animals of their time. We saw with the arthropods earlier, who were just about as successful as you could get as a species by their vast numbers and adaptability, that being able to grow to a large size was beyond them, and would bring tremendous evolutionary advantages to a competitor in the battle to survive. Adult Eryops weighed about 90 kg and the skull was proportionately large, being broad and flat and reaching lengths of 60 centimetres. It had an enormous mouth with many curved, enamelled teeth whose shape and cross section made them exceptionally strong and resistant to stresses.

14.12 The skeleton of Eryops was much more strongly built and sturdy than its rivals. The limbs were especially large and strong. The pectoral girdle was highly developed, with a larger size for both increased muscle attachments. Most notably, the shoulder girdle was disconnected from the skull, resulting in improved terrestrial locomotion. The texture of Eryops skin was revealed by a fossilized "mummy" described in 1941. This mummy specimen showed that the body in life was covered in a pattern of oval bumps.

14.13 The Permian–Triassic extinction event, colloquially known as the Great Dying, occurred around 252 Ma. It is the Earth's most severe known extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct. It is the only known mass extinction of insects, where some 57% of all families and 83% of all genera became extinct. Amphibians and reptiles were strongly affected by the Carboniferous rainforest collapse (CRC) which brought the Carboniferous period to a close. The Carboniferous period has long been associated with thick, steamy swamps and humid rainforests. The sudden collapse of the vital rainforest ecosystem profoundly affected the diversity and abundance of the major tetrapod groups that relied on it. The CRC was a self-reinforcing and very rapid change of environment where the worldwide climate became much drier and cooler overall

14.14 The outcome of this animal reduction was a crash in global carbon dioxide levels, which impacted the plants even more. The aridity and temperature drop which resulted from this runaway plant reduction and decrease in a primary greenhouse gas caused the Earth to rapidly enter a series of intense Ice Ages.

14.14 The entire geological era called Paleozoic, the 300 million years of evolution, from the Cambrian explosion to the Permian implosion, saw brilliantly inventive plant life and superbly adaptable animal life emerge from the oceans to conquer vast tracts of land and grow to great sizes. And then they were mostly wiped out and had to do it all over again.

14.15 The next geological era, the Mesozoic, encompasses the entire life cycle of probably the most successful and adaptable species ever known, the dinosaurs. The first few lines of primitive dinosaurs diversified rapidly through the Triassic period, quickly evolving the specialised features and range of sizes needed to exploit nearly every terrestrial ecological niche. During the period of dinosaur predominance, which encompassed the ensuing Jurassic and Cretaceous periods (around 180 million years), nearly every known land animal larger than 1 meter in length was a dinosaur. sphere and freezing of relations between global superpowers.

14.16 It’s a funny thing, then, that the term ‘dinosaur’ is used colloquially to denote a person or an institution that is incapable of adapting to changed circumstances. Yet dinosaurs emerged from the catastrophic destruction of the Great Dying, adapted to a world environment that had and has never seen such dramatic changes, and flourished for the next 200 million years responding to, and overcoming, every kind of environmental change that could be thrown at it throughout that vast era of time.   And it took the biggest meteorite impact in the past 4 billion years finally to put an end to their amazing evolutionary success. Perhaps it is about time that the word dinosaur ceased to be used perjoratively – at least by a species that has been around for less than 1% of the dinosaurs’ time and which looks every day increasingly unable to find a sustainable way to manage its environment or a peaceful way to coexist with itself. One of the more common birds around Lake Annecy today is the black-headed gull, or Chroicocephalus ridibundus, an interesting hybrid Greek and Latin name. The first name is from the Greek ‘colour + head’ and the second means laughing, from the Latin ‘ridere’, in honour of its being a noisy species, especially in colonies, with its familiar cackling "kree-ar" call producing at times a veritable chorus of laughter for all to hear. Perhaps this bird also sees the joke, since, of course all birds are survivors of the dinosaur lineage, (which is why dinosaurs are properly called ‘non-avian dinosaurs’) and have adapted superbly to every environmental shock and catastrophe thrown at them ever since. The surviving birds diversified so extensively that, today, nearly one out of every three species of land vertebrate is a bird species, displaying their beautiful adaptations in every imaginable shape, colour and size each day for all to admire and, lest we forget, calling our attention to that fact with their song each morning.

14.17 The fossil record indicates that birds are the last surviving group of dinosaurs, having evolved from feathered ancestors within the theropod group of saurischian dinosaurs. True birds first appeared during the Cretaceous period, around 100 million years ago. DNA-based evidence finds that birds diversified dramatically around the time of the Cretaceous–Palaeogene extinction event (65 Ma) that killed off all other dinosaurs. Birds, especially those in the southern continents, survived this event and then migrated to other parts of the world while diversifying during periods of global cooling. Primitive bird-like dinosaurs that lie outside class Aves proper, in the broader group Avialae, have been found dating back to the mid-Jurassic period. Many of these early "stem-birds", such as Archaeopteryx, were not yet capable of fully powered flight, and many retained primitive characteristics like toothy jaws in place of beaks, and long bony tails.

14.18 The evolution of birds began, then, in the Jurassic Period (around 180 Ma), with the earliest birds derived from a group of dinosaurs named Paraves. Birds are categorized as a biological class, Aves. The earliest known is Archaeopteryx lithographica, from the Late Jurassic period, though Archaeopteryx is not commonly considered to have been a true bird. Modern phylogenies place birds in the dinosaur clade Theropoda. According to the current consensus, Aves and a sister group, the order Crocodilia, together are the sole living members of an unranked "reptile" clade, the Archosauria.

14.19 The Jurassic (201 to 145 Ma) was important in the development of birds, one of the insects' major predators. During the early Jurassic period, the supercontinent Pangaea broke up into the northern supercontinent Laurasia and the southern supercontinent Gondwana; the Gulf of Mexico opened in the new rift between North America and what is now Mexico's Yucatan Peninsula. The laurasian and the gondwanian fauna differed considerably in the Early Jurassic but later it became more intercontinental and many species started to spread globally.

14.20 Phylogenetically, Aves is usually defined as all descendants of the most recent common ancestor of a specific modern bird species (such as the house sparrow, Passer domesticus), and either Archaeopteryx, or some prehistoric species closer to Neornithes. (Currently, the relationship between dinosaurs, Archaeopteryx, and modern birds is still under debate).

14.21 Archaeopteryx, is a genus of bird-like dinosaurs that is transitional between non-avian feathered dinosaurs and modern birds. The name derives from the ancient Greek ‘ancient + wing’. For many years Archaeopteryx has been generally accepted by palaeontologists as the oldest known bird but older potential avialans have since been identified, including Anchiornis, Xiaotingia, and Aurornis. Archaeopteryx lived in the late Jurassic around 150 million years ago, in what is now southern Germany, during a time when Europe was an archipelago of islands in a shallow warm tropical sea, much closer to the equator than it is now. It was similar in size to a Eurasian magpie, with the largest individuals possibly attaining the size of a raven. Despite their small size, broad wings, and inferred ability to fly or glide, Archaeopteryx had more in common with other small Mesozoic dinosaurs than with modern birds. In particular: jaws with sharp teeth, three fingers with claws, a long bony tail, hyperextensible second toes ("killing claw"), feathers (which also suggest warm-bloodedness), and various features of the skeleton. These features make Archaeopteryx a clear candidate for a transitional fossil between non-avian dinosaurs and birds.

14.22 Thus, Archaeopteryx plays an important role, not only in the study of the origin of birds, but in the study of dinosaurs. It was named from a single feather in 1861. That same year, the first complete specimen of Archaeopteryx was announced. Over the years, ten more fossils of Archaeopteryx have surfaced. Despite variation among these fossils, most experts regard all the remains that have been discovered as belonging to a single species, although this is still debated.

14.23 Modern birds are classified Neornithes, which are now known to have evolved into some basic lineages by the end of the Cretaceous period (65 Ma). The Neornithes are in turn classified into two further groups or ‘superorders’, paleognaths and neognaths (‘old jaw’ and ‘new jaw’.  The paleognaths include the ratites, which nowadays are found almost exclusively on the Southern Hemisphere, are large flightless birds including ostriches, rheas, cassowaries, kiwis and emus. Between them these two superorders comprise all known living species of bird, the number of which, depending on the taxonomic viewpoint, varies between 9,800 and 10,050.

14.24 The first evolutionary divergence from the remaining Neognathes class was that of the Galloanserae, the superorder containing the Anseriformes (ducks, geese and swans), and the Galliformes (chickens, turkeys, pheasants, and their allies). The majority of birds now living around lake Annecy are Anseriformes.

14.25 However, the classification of birds is a contentious issue. Sibley and Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds, although it is frequently debated and constantly revised. Most evidence seems to suggest the assignment of orders is accurate, but scientists disagree about the relationships between the orders themselves; evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem, but no strong consensus has emerged. More recently, new fossil and molecular evidence is providing an increasingly clear picture of the evolution of modern bird orders.

14.26 A 2008 study published in the journal Science examined DNA sequences from 169 species of birds that represented all of the major extant groups. The findings may necessitate a wholesale restructuring of the avian phylogenetic tree. The findings also supported unestablished relationships between orders and confirmed disputes over particular groupings.

14.27 Which just goes to show how recent is our knowledge of ancient history.

Continue Reading   Chapter Fifteen