Chapter Fifteen

Mammals

A shrew-like animal that snagged insects from ferns lining the shores of freshwater lakes 160 million years ago,  might be one of the first "true" mammals to walk the Earth.

15.1 It was mentioned earlier how life from the oceans needed to develop new characteristics to cope with the new challenges of life on land, gravity and dessication, and some of the brilliant inventions the plants, then arthropods, then tetrapods came up with to solve these problems. One of the plants’ most brilliant inventions, in the thoughtful opinion Colin Tudge, was that of the seed, because it enclosed the spores in a protective environment complete with food and enabled some preliminary development in the parent before being sown. Tetrapods faced a similar challenge: how to give birth to their eggs outside a watery environment and prevent the young from quickly becoming dessicated? The answer was the invention of eggs with a tough shell. The clade of tetrapod vertebrates that came up with this idea are called Amniotes, from the Greek ‘membrane surrounding the fetus’, and include reptiles, birds and mammals that lay their eggs on land or retain the fertilized egg within the mother. They are distinguished from the anamniotes (fishes and amphibians), which typically lay their eggs in water.

15.2 The ancestors of true Amniotes, such as Casineria kiddi, which lived about 340 million years ago, evolved from amphibian reptiliomorphs and resembled small lizards. At the late Devonian mass extinction 360 million years ago, all known tetrapods were essentially aquatic and fish-like. Because the reptiliomorphs were already established 20 million years later when all their fishlike relatives were extinct, it appears they separated from the other tetrapods somewhere during Romer's gap, when the adult tetrapods became fully terrestrial (some forms would later become secondary aquatic). Rather unfortunate that such a crucial evolutionary change is hidden in Romer’s gap – exactly how did the reptiliomorphs arrive from their fish-like tetrapod ancestors in just a few million years? Watch this paleontological space, I guess.

15.3 These modest-sized ancestors of the Amniotes laid their eggs in moist places, such as depressions under fallen logs or other suitable places in the Carboniferous swamps and forests; and dry conditions were probably not the reason the soft shell emerged. Indeed, many modern day amniotes are dependent on moisture to keep their eggs from desiccating. Although some modern amphibians lay eggs on land, they all lack advanced traits like an amnion.

15.4 The big new invention, the amniotic egg was formed through a series of evolutionary steps. After internal fertilization and the habit of laying eggs in terrestrial environments became a reproduction strategy amongst the amniote ancestors, the next major breakthrough appears to have been a gradual replacement of the gelatinous coating covering the amphibian egg with a fibrous shell membrane. This allowed the egg to increase both its size and the rate of gas exchange, permitting a larger, metabolically more active embryo to reach full development before hatching. (Just like the tree seed – thanks Colin!) Further development of the design, like extraembryonic membranes (amnion, chorion, and allantois) and a calcified shell, were not essential and probably evolved later. It has been suggested that shelled terrestrial eggs without extraembryonic membranes could still not have been bigger than about 1 cm in diameter because of diffusion problems, like the inability to get rid of carbon dioxide if the egg was larger. The only way for the eggs to increase in size would be to develop new internal structures specialized for respiration and waste products. As this happened, it would also affect how much the juveniles could grow before they reached adulthood.

15.5 The first true amniotes, descendents of the reptiliomorphs mentioned above and referred to as "basal amniotes", resembled small lizards and evolved around 312 million years ago, in the swampy forests of the Carboniferous period. Their newly evolved eggs could survive out of the water, allowing amniotes to branch out into drier environments. Most mammals do not lay eggs, but corresponding structures are to be found inside the placenta. So the amniotic egg represents a critical divergence within the vertebrates, one enabled to reproduce on dry land—free of the need to return to water for reproduction as required of the amphibians. It was perhaps this invention, more than any other, that enabled the amniotes to spread around the globe, eventually to become the dominant land vertebrates.

15.6 Very early in their evolutionary history, in fact almost as soon as they appear!, towards the end of the Carboniferous or around 315 Ma, the Amniotes diverged into two main lines: the Sauropsids, and the Synapsids, both of which persist into the modern era. The Sauropsids went on to found the dynasty of dinosaurs which ruled the world during the entire next era, the Mesozoic – ‘The Era of Dinousaurs”. The term Sauropsida from the Greek ‘lizard faces’ has a long history, and hails back to Thomas Henry Huxley, and his opinion that birds had risen from the dinosaurs. He based this chiefly on the fossils of Hesperornis and Archaeopteryx, that were starting to become known at the time. The Synapsids wererso called because they have a ‘temporal fenestra’, an opening low in the skull roof behind each eye, leaving a bony arch beneath each. The Synapsids in turn were to rule the subsequent, and present, entire era, the Cenozoic – ‘The Era of the Mammals’ because mammals are the only living synapsids. So between the Sauropsids and the Synapsids they had things pretty much tied up. The Synapsids however had to bide their time for a while, as small furtive creatures (none were more than a metre long) living at night, hiding in trees and generally doing their best to keep their heads down and out of sight of their dinosaur predators, who were, like, everywhere. This tactic required some patience because the dinosaurs kept a pretty tight rein on things for around 200 million years.

15.7 But then in 66 Ma a meteorite slammed into the Gulf of Mexico at a place called Chicxulub creating a 180-kilometre-wide crater and enough fall-out of dust and debris to cause a global winter making it impossible for plants and plankton to carry out photosynthesis. It is estimated that 75% or more of all species on Earth vanished as a result including all tetrapods weighing more than 25 kg.

15.8 As it turned out this was rather a blessing for mammals because none of them then weighed more than 25 kg whereas pretty much all their predators did. In the geologic record, this event is marked by a thin layer of sediment containing high levels of the metal iridium, which is rare in the Earth's crust but abundant in asteroids, which can be found throughout the world in marine and terrestrial rocks. We now know what happen 66 million years ago as a result of work done 26 years ago by a team of scientists led by Luis Alvarez. Their impact hypothesis, was based on an interpretation of this iridium layer, and was hotly disputed by the scientific community for many years. It was more than 10 years later that the discovery of the Chicxulub crater provided the conclusive evidence. However, some scientists continue to maintain the extinction was caused or exacerbated by other factors, such as volcanic eruptions, climate change, or sea level change, separately or together.

15.9 The oldest known fossil synapsid is Protoclepsydrops, and the oldest known Sauropsid is probably Paleothyris, both from the end of the Carboniferous period around 310 million years ago. After the Cretaceous–Paleogene extinction event placental and marsupial mammals diversified into many new forms and ecological niches throughout the next and most recent 66 million years, the Paleogene and Neogene periods, by the end of which all modern orders had appeared.

15.10 The evolution of mammals has passed through many stages since the first appearance of their Synapsid ancestors in the late Carboniferous period. The most ancestral forms in the class Mammalia are the egg-laying mammals in the subclass Prototheria. By the mid-Triassic (around 230 Ma) there were many Synapsid species that looked like mammals. The lineage leading to today's mammals split off in the Jurassic (200 – 150 Ma). Later on, the eutherian and metatherian lineages separated; the metatherians are the animals more closely related to the marsupials, while the eutherians are those more closely related to the placentals.

15.11 The first true mammal may never be known, but the Genus Morganucodon and in particular Morganucodon watsoni, a 2-3 cm long weasel-like animal whose fossils were first found in caves in Wales and around Bristol (UK), but later unearthed in China, India, North America, South Africa and Western Europe is a possible contender. It is believed to have lived between 210 and 200 Ma. However Gondwanadon tapani reported from India on the basis of a single tooth in 1994 may be an earlier contender for the title, with a claimed date of 225 Ma.

15.12 These early mammals were small, insectivorous, nocturnal, hairy and warm-blooded. Warm-bloodedness is believed to have first evolved among the cynodonts, a late but successful group of mammal-like reptiles from which the mammals evolved. The cynodonts were the only mammal-like reptiles to survive to the Jurassic, and definitely coexisted with many of the major dinosaurs.

15.13 During the Jurassic the mammals remained small and mainly nocturnal, living beneath the 'metaphorical' feet of the great dinosaurs. These early mammals were more like small monotremes (monotremes are mammals that lay eggs ‘Prototheria’ instead of giving birth to live young like marsupials ‘Metatheria’ and placental mammals ‘Eutheria’) and probably still laid eggs. Marsupials and placental mammals (cats, dogs, you and me) did not evolve for another 70 million years.

15.14 Towards the end of the Jurassic a group of mammals known as 'multituberculates' appeared. These were to prove to be the most successful of the primitive mammal groups with species still alive 30 Ma. This means they had survived as a group for 130 million years. Some of the later multituberculates possessed marsupial-like bone structures which indicate that they had pouches like marsupials, suggesting a similar life cycle involving live birth of very premature young. The fossil record gets a little better from the Cretaceous onwards, with a few complete skeletons such as Kamptobrator from Mongolia, however as far as we know mammals were still playing a fairly insignificant role in the ecology of the time.

15.15 Juramaia, the earliest known eutherian, lived 160 million years ago in the Jurassic, this divergence must have occurred in the same period.  Juramaia is an extinct genus of one of the first eutherian mammals found in deposits of western Liaoning, China. It is a small shrew-like mammal of body length approximately 10 cm, like a small rat. Juramaia is known from the holotype BMNH PM1343, an articulated and nearly complete skeleton including incomplete skull preserved with full dentition. It was collected in the Daxigou site, Jianchang. It was first named by Zhe-Xi Luo, Chong-Xi Yuan, Qing-Jin Meng and Qiang Ji in 2011. The discovery of Juramaia provides new insight into the evolution of placental mammals by showing that their lineage diverged from that of the marsupials 35 million years earlier than previously thought. Furthermore, its discovery fills gaps in the fossil record and helps to calibrate modern, DNA-based methods of dating the evolution. Based on climbing adaptations found in the forelimb bones, it has been suggested that the first Eutheria were arboreal, in a manner resembling that of modern rats.

15.16 So there we have it – we are not descended from Apes, but from Rats.

15.17 Most of the evidence for the above consists of fossils. For many years, fossils of Mesozoic mammals and their immediate ancestors were very rare and fragmentary; but, since the mid-1990s, there have been many important new finds, especially in China. The relatively new techniques of molecular phylogenetics have also shed light on some aspects of mammalian evolution by estimating the timing of important divergence points for modern species. When used carefully, these techniques often, but not always, agree with the fossil record.

15.18 Although mammary glands are a signature feature of modern mammals, little is known about the evolution of lactation as these soft tissues are not often preserved in the fossil record. Most research concerning the evolution of mammals centers on the shapes of the teeth, the hardest parts of the tetrapod body. Other important research characteristics include the evolution of the middle ear bones, erect limb posture, a bony secondary palate, fur, hair, and warm-bloodedness.

15.19 Molecular phylogenetics uses features of organisms' genes to work out family trees in much the same way as paleontologists do with features of fossils — if two organisms' genes are more similar to each other than to those of a third organism, the two organisms are more closely related to each other than to the third. Molecular phylogeneticists have proposed a family tree that is very different from the one with which paleontologists are familiar. Like paleontologists, molecular phylogeneticists have different ideas amongst themselves about various details.

15.20By the end of the Cretaceous (66 Ma) 15 mammal families were in existence that we know about. The end of the Cretaceous however saw another mass extinction. So many species dying out left huge niche vacancies in the habitat. Following this disaster it was the mammals alone of the remaining groups of animals who diversified to take advantage of this new situation. Over the next 15 million years the remaining 10 mammal families (five became extinct with the dinosaurs during the K-T event) expanded to become 78 families by the early Eocene. The number of genera increased from about 40 to over 200 during the same time. This sudden massive increase in species from a single stem group is an example of what is called 'adaptive radiation'.

15.21 By the middle of the Eocene (45 Ma) all the major groups of mammals alive today had come into existence, though not necessarily as we know them now.

15.22 Primates for instance have been around since the beginning of the Paleocene, 65 Ma, but the distant bipedal ancestors of man only occurred for the first time 5 Ma. The same applies to all the other major groups. Although this is the ‘Era of Mammals’ it is worth noting that over the last 10 million years, 6 of the 24 major mammal groups to come out of the Eocene have died out. This is 25% of the existing mammals then and is important because for the previous 20 million years before that no major groups died out at all. We do not know why these groups died out but there is a remarkable coincidence with the rise of mankind.

15.23 Huge numbers of fossils have been found from the Oligocene and Miocene, including the giant terrestrial mammals like Indricotherium which was 5.4 m tall at the shoulder. Brontotherium an elephant-sized oddity with two blunt horns, and the giant sabre-toothed cat, Smilodon.

15.24 Finally, it is worth knowing that besides fossils found in rocks in bits and pieces, as is often the case, a lot of what is known from the fossil record come from Logerstatten. Logerstatten are fossil deposits of exceptional richness which include numerous species preserved in their entirety, sometimes even including hair and gut contents. One example of this is the shale deposits at Messel in Germany. Another source of remarkably well-preserved specimens are the tar pits of Texas and southern USA.

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