THE REPTILES AND THE SYNAPSIDS EVOLVE
ABOUT 22 DECEMBER; ABOUT 977,000 METERS UP THE LINE
The year that started on 1 January with the Big Bang is now less than ten days from its end. In our usual time it is now 315 million ybp. There are now life forms that are able to move themselves about with limbs, and which are able to process oxygen gas through structures within their bodies (while retaining, in many species, the ability to breathe in oxygen through the skin). The amphibia are now firmly established on the tiny planet that had been brought forth 4,200,000,000 years earlier, and have come to occupy a great many ecological niches. Some amphibian lines have evolved into large animals, flourishing in the warm, rain-soaked equatorial forests of the Carboniferous Period. From the arthropods have emerged true insects, speciating and reproducing at an astounding rate, occupying a wide range of habitats on the ever-changing Earth. We are now on the verge of another enormous change in the history of life—the evolution of the reptiles and the synapsids. Class Reptilia will enjoy tremendous biological success, and will eventually come to dominate the Earth’s landmasses—until the conditions that make such dominance possible are destroyed by the unfolding of blind, random chance. The end of reptilian dominance will open the door for an unusual kind of animal, descended from a line that appeared at about the same time as the reptiles—the synapsids—allowing it to assume pride of place. This unusual kind of animal will be a mammal. The world will never be the same because of this.
It may seem like an odd place to start, but the story of the reptiles and synapsids begins with the story of eggs. The vast majority of amphibians, however well-adapted to the land, had to keep their fertilized eggs in an aquatic environment of some sort to keep them from drying out. However, in some of those amphibia that resembled reptiles—the reptilomorpha—a different kind of egg evolved, one that did not have to be deposited in water. This kind of egg possessed a special membrane called an amnion. This membrane, in conjunction with other structures, kept the embryo encased in moisture. The evolution of the amniote egg from an anamniote egg (of the kind laid by amphibians) was the result of several evolutionary innovations. Two biologists who have studied this matter closely have hypothesized that one of the major changes was the evolution of a fibrous shell membrane, which provided greater physical support for the egg’s contents and allowed for eggs of a larger size to be laid. [We must suppose, in my view, that the earliest manifestations of shells were simply particularly thick or leathery gelatinous membranes.] Additionally, the gelatinous layers which surround a typical amphibian egg began to be replaced by layers with a lower water content, water content which was transferred to a yolk that nourished the embryo. The reduction of water content in the egg’s jelly-like layers facilitated the exchange of gases between the outside environment and the interior of the egg, and helped a specialized structure important to this exchange of gases (the chorioallantois) to develop more fully. Further, these researchers hypothesize, the shell membrane served as a source of calcium for the embryo, helping it generate a skeleton. Taken together, these developments allowed eggs to resist desiccation and preserve embryos.1 What were the selection pressures that may have brought these changes about? Several decades ago two zoologists considering the evolution of the avian egg (which was descended from the reptilian variety) said the tendency toward more durable shells was probably a response to predation by microbes and invertebrates found in the soil. This hardening would have made the absorption of water from the environment more difficult, so selection also probably favored increased quantities of water in the egg at the time of its laying.2
This was a biological revolution of the first order: it meant that eggs could be laid on dry land, and that the animals capable of producing such eggs could occupy a vastly greater range of environments. Because of this new kind of egg, the evolution of true reptiles and synapsids was possible.3 The animals hatched from those eggs would have faced a far greater number and variety of selection pressures. Qualities which facilitated inland survival, such as dry, water-impermeable skin, a rib cage adapted to help inflate the lungs, and a more advanced heart would have been adaptive advantages.4 The amniotes—those animals which produce such eggs or their modified varieties—have grown in number and scope since the amniotic egg first evolved, an event which occurred no later than 320 million ybp and perhaps much earlier, and now include all reptiles (including the reptile-derived birds) and mammals, as well as a number of extinct species. It is the amniotes that compete with the viruses, bacteria, insects, and each other for domination of the land areas of the world.
The Divergence of the Reptiles and the Synapsids
There has been a great deal of controversy and debate among paleontologists regarding the evolutionary relationships and classification of the various amniotes. Of special significance has been the search for the basal amniotes, the animal group from which all of the extant vertebrate land animals ultimately derived. There have been many cladograms (diagrams showing phylogenetic relationships) constructed over the years to try to trace the evolutionary history of the reptiles and synapsids, and new discoveries, which are the life-blood of paleontology, alter these cladograms on a regular basis. As of this writing, a true consensus has yet to emerge, and the use of somewhat outdated terms by some researchers can make the picture less clear.
Michael Benton, in his standard work on paleontology and in a more recent collaborative volume, assigns the position of basal amniotes to Hylonomus (found in the Joggins rock formation of Nova Scotia) and Paleothyris, lizard-like animals descended from a group known as the anthracosaurs from the period 300-310 million ybp. Benton classifies the synapsids as a subclass of Class Reptilia 5—a view that is not, to say the least, universally shared. A much different approach is taken by Donald Prothero. He agrees that the anthracosaurs did show increasingly amniote-like characteristics (as exemplified by such animals as Seymouria and Limnoscelis) and he also notes the contribution of a reptile-like animal called Westlothiana lizziae (a specimen found in Scotland) dating from 330 million ybp, along with probable true reptile Hylonomus, which he places at 315 million ybp. However, and most significantly, Prothero argues that the earliest synapsids, Protoclepsydrops and Archaeothyris, are just as old as Westlothiana and Hylonomus, and therefore the synapsids and reptiles must have diverged from a common ancestor before these animals appeared. He is firm in his contention that the synapsids have never been reptiles and that the term “mammal-like reptile” is, in his words, “obsolete and misleading”. He states:
Although the earliest forms are almost indistinguishable from the earliest true reptiles in most features, creatures such as Protoclepsydrops and Archaeothyris still show a number of unique synapsid specializations, including a hole in the side of the skull (temporal opening) beneath the postorbital [behind the eye socket] and squamosal [on the side of the skull] bones, the beginnings of true canine-like teeth, and a number of other subtle features in the skull and palate.6
T. S. Kemp of Oxford University, one of the world’s foremost authorities on the evolution of mammals, has entitled a chapter in the most recent edition of his study on mammalian evolution, “Evolution of the mammal-like reptiles”. He believes the evidence points to the synapsids as being a group derived from the reptile stem. Kemp explains the use of the term “mammal-like reptiles” by saying:
By definition, a mammal-like reptile possesses some, but not all the characters that define living mammals…The earliest, most primitive ones have very few mammalian characters, just a small temporal fenestra [small hole] in the skull and an enlarged canine tooth in the jaw… If, on the other hand, a mammal is defined as an animal that possesses any of the modern mammal characters, then some extremely non-mammalian forms, primitive, sprawling-limbed, and no doubt scaly, ectothermic [“cold-blooded”] creatures must be included.7
Kemp thinks that Westlothiana is a very possible stem-amniote. Kemp begins the story of the mammals with Archaeothyris, an animal he characterizes as reptile-like, having retained many reptile-like features in its skull and teeth. He argues that mammal-like reptiles radiated over a period of 100 million years. In Kemp’s view, the radiation of the synapsids truly begins with a group known as the pelycosaurs. It was a member of this group (the remains of which do, indeed, seem reptile-like) that Kemp hypothesizes gave rise to the therapsids, the synapsid group out of which true mammals ultimately evolved.8
Kenneth Rose, of Johns Hopkins University and the Smithsonian National Museum of Natural History has given his view of the matter in his study of the origins of mammals:
The ancestors of mammals, Synapsida, diverged from basal amniotes—protothyrid captorhinomorphs—at least 300 million years ago, in the Pennsylvanian Period. As the oldest and most primitive amniotes, protothyrids were also ultimately ancestral to reptiles (including lizards, snakes, and turtles) and archosaurs (crocodilians, dinosaurs, and birds). Synapsids include two successive radiations, the Pennsylvanian-Permian Pelycosauria, and the largely Permo-Triassic Therapsida …Although synapsids were long classified as reptiles, it is now accepted that they shared a more recent ancestry with mammals. Therapsids arose in the Permian from sphenacodontid pelycosaurs (which include the carnivorous “sail-backed” Dimetrodon from Texas). The Cynodontia of the late Permian-Triassic were the most mammal-like therapsids.
Rose points out that the cynodonts acquired mammal-like traits, such as changes in dentition, the palate, jaw, vertebrae, and thoracic girdle, only over a period of tens of millions of years, and that the general emergence of mammals was fairly gradual.9 So in this perspective, one might say that out of a common amniote ancestor arose two distinct lines of animal, which nonetheless bore striking anatomical similarities to each other. The synapsids probably did look like reptiles to begin with, but over a period of perhaps 70-80 million years became more and more differentiated from them in appearance.
Finally, the paleontologist Kenneth D. Angielczyk, currently of the Field Museum in Chicago (which contains an extensive collection of non-mammalian synapsids) points outthat the synapsid lineage is more than 300 million years old, and while it did start with animals that were lizard-like in appearance, the term “mammal-like reptiles”, when applied to non-mammalian synapsids is something of a misnomer. All synapsids, he maintains, are more closely related to mammals than to any member of Reptilia.10
Therefore, in my view, the best evidence we have seems to indicate that by around the middle of the Carboniferous Period, approximately 315 million ybp, the sauropsids (the group which includes the reptiles) and the synapsids—one line of which ultimately led to the mammals—had evolved from a common amniote ancestor. The sauropsids, over a period of tens of millions of years, branched out into the anapsids, which include the turtles (and some extinct groups), the euryapsids, which included four extinct groups of marine reptiles, the lepidosaurians, which include snakes and lizards (as well as an extinct group of marine reptiles) and the archosaurians, which includes the crocodiles, the extinct flying reptiles known as pterodactyls, (ultimately) the birds, and the most famous reptiles of all, the dinosaurs.11 The synapsids, as we will see in greater detail in the next chapter, evolved a long line of non-mammalian types, many members of which showed more distinctly mammalian traits than the previous types, and then, with the evolution of the therapsid synapsids, animals we recognize as clearly mammalian in anatomy began to evolve.
In the millions of years following the sauropsid-synapsid divergence, animal and plant life proliferated. The planet Earth was vibrant with many varieties of organisms, living in almost every kind of environment. The nature of life on the planet seemed to have been set firmly.
And then utter catastrophe struck.
The Great Dying: The Permian Extinction
The greatest disaster to ever strike life on this planet began approximately 250 million ybp (around 24 December in our compressed chronology of the Universe). Known as the Permian Extinction, or more precisely, the Permian-Triassic Extinction, it wiped out about 70% of all the vertebrate species on land, and perhaps 90% of all marine species. The extinction devastated the world’s trees as well. Over a period of between 3 million and 8 million years, according to Douglas Erwin, a paleobiologist who has done extensive research on this event, about as many species died out as would naturally die out in 85 million years.12
Why had this happened? Erwin has surveyed and evaluated hypotheses ranging from a catastrophic reduction of primary producers (basically the lowest level of the food chain), such as marine phytoplankton, to changes in cosmic radiation, from severe climatic changes to ocean anoxia, from volcanism to extraterrestrial causes.13 It is his view that a complex of causes was responsible. Based on his research, be believes that there was a major lowering of the sea level, a regression, during this time. The regression destroyed habitats, affected the severity of seasonal climatic variations, and increased the concentration of CO2 in the atmosphere. Further increases in CO2, triggered by an enormous volcanic eruption known as the Siberian Traps, combined with the effects of the regression, disrupted the general ecological system, caused ocean anoxia and with it, the terrible loss of life that was the Permian Extinction.14
Not all scientists agree with Erwin’s contention that the extinction was stretched out over a period of several million years. A team of scientists from NASA in the United States, led by Luann Becker, a geologist at the University of California Santa Barbara, contends that an asteroid strike may have exacerbated what was already becoming a desperate situation for life on Earth. There is indeed evidence that there was severe and widespread volcanism in what is now Siberia. (Some idea of the magnitude of this eruption may be gained when it is noted that it poured out approximately 1.5 million times as much material as was expelled by the Mount St. Helens eruption in 1980.) An asteroid strike, for which convincing evidence has been uncovered, coupled with a disastrous episode of volcanism on this scale would have been a devastating one-two punch to the Earth’s life forms. Based on the most advanced methods of dating, the NASA team believes that the huge loss of life took place in a period ranging from 8,000 to 100,000 years, which in geological terms is a moment’s time.15 (On the scale of time I am using, the mass dying would have occurred over a period of between 18 seconds and 3.8 minutes.)
Robert A. Berner, a geologist from Yale University, having taken into consideration the major hypotheses discussed above, contends that the great dying was the result of the following combination of factors:
1. The release of methane gas trapped in ocean sediments, causing a major disruption of the carbon cycle.
2. Some sort of violent impact or blast of radiation which occurred at about the same time.
3. A major release of volcanic CO2 that also occurred in the same period.
Berner contends that these events occurred within a period of 10,000 to 30,000 years, and that their effects were felt over a period of millions of years. He also believes these events caused the level of atmospheric oxygen in the Triassic Period to drop, and the rate of coal deposition to be drastically reduced.16
So yet again a mass extinction altered the direction of evolution on the Earth. A combination of events, operating in a synergistic fashion, appears to have brought about this enormous wave of death. Huge numbers of animals well-adapted to conditions on the pre-extinction planet were wiped out, and new opportunities were created for those life forms which, for whatever reasons, had survived. Extinctions are the greatest selection pressure of them all. We are fortunate that enough of the synapsids survived this catastrophe to ultimately give rise to the mammalians.
The Formation of Pangea
By about 250 million ybp, one of the most important processes in geological history had been largely completed—the formation of the biggest supercontinent in the Earth’s history, Pangea. Laurentia (North America and Laurussia), South America, Africa, Baltica, India, Antarctica, Australia, Arabia, and several other smaller territories were either completely joined or in close proximity to each other.17 Often, scientists refer to Pangea as a combination of two smaller landmasses, continents simply called Laurasia in the north and Gondwanaland in the south. It would appear that the northern regions of Pangea saw extensive tectonic activity at about 250 million ybp18 (which would be congruent with the massive volcanic eruptions of the time period). Pangea had extensive deserts, and was populated by significant numbers of reptiles and synapsids.19 Some 50 million years prior to the period of Pangea’s greatest extent, the collisions of continental landmasses had driven the Appalachian-Ouachita mountain chain (the growth of which we noted in the last chapter) to heights comparable to those of the modern Himalayas.20 In fact, the coming together of so many landmasses caused a great deal of mountain-building (technically known as orogeny), driven by the massive tectonic processes that caused the assembly of the supercontinent itself.21
The southern region of Pangea, Gondwanaland, had essentially been assembled by around 500 million ybp, and thus was already more than 200 million years old when it was incorporated into the supercontinent. In contrast, much of Laurasia was comparatively new, having formed during the assembly of Pangea itself.22 It was the inhabitants of Pangea, therefore, that were hit by the terrible extinction. Interestingly, after the extinction, the renewed expansion of life forms saw an increase in the diversification of reptiles and synapsids.23 Pangea was the greatest extent of land yet seen on the Earth. Its breakup, as we will note in the next chapter, had tremendous consequences.
Changes in Marine Life; the Mesozoic Radiation of Synapsids Begins
In the aftermath of the Permian-Triassic extinction, a new war for survival erupted in the oceans. Perhaps driven by competition for food sources, deadly new predators evolved out of the surviving marine species, the numbers of which had been so terribly reduced in the mass die-off. An oceanic “arms race” ensued, as predators and prey pushed each other in a deadly co-evolutionary game (although there may have been other environmental factors at work). New forms of shells and other “defensive” features appeared on a great many animals. (There were of course, many regional variations among the marine life forms involved in this competition.) So major was this change that it has been called the Mesozoic Marine Revolution, and its consequences stretched out over millions of years.24
On land, the competition took different forms. The fossil record of these eras, the late Paleozoic and the Mesozoic, the latter of which began in the aftermath of the great dying at around 250 million ybp, is not nearly as complete as we would like it to be, of course. We do have evidence of a synapsid of uncertain classification, Tetraceratops, at around 265 million ybp. This specimen was apparently a therapsid. From the Upper Permian to the Lower Triassic, roughly 260 to 245 million ybp, some 51 different therapsid families appeared (as far as we can tell), but we don’t have a good record of their earlier development.25 (We will discuss the physical features of the therapsids in some detail in the next chapter.) In general, however, it appears that the reptiles were gaining the upper hand over the synapsids. The evolution of the more gargantuan reptiles was at hand; the mammals that evolved out of the therapsid line were generally small and relatively vulnerable.
The story of life on this planet moved in fits and starts, interrupted by biological disasters that reshuffled the evolutionary deck repeatedly. But we can say in general that the single-celled life forms brought forth the multicelled aquatic life forms. The first aquatic life gave rise to the fishes. A particular kind of fish with unusually-structured fins gave rise to the tetrapods, the earliest of which ultimately gave rise to genuine amphibians. An unusual kind of amphibian began laying eggs that didn’t need to be immersed in water, broadening the scope of biological possibility. From those animals that laid such eggs, the amniotes, emerged two lines, the reptiles and synapsids. The synapsids evolved over the tens of millions of years to look less and less reptile-like in appearance. And they evolved a complex of traits that were to set them apart from all other classes of animal life. The time of the mammals was about to begin. Their original position in the web of life was an inauspicious one, and if one had seen their original members, one might be inclined to think them the losers of evolution’s story.
But that story was again headed in an unpredictable direction.