Last lecture, we saw the diversity of marine vertebrates ("fish"), ending with the transition from fully aquatic forms to taxa which
could at least on occasion come up on land:
In this set of lectures, we will examine what happened to the diversity of terrestrial vertebrates.
Stegocephalia: The Fingered Vertebrates
A subclade of the sauropterygian fish, stegocephalians are first known from the Devonian. Relative to their kin, stegocephalians (tetrapods and their extinct relatives) are distinguished by:
Below is a general cladogram for the fossil vertebrates:
And here is a more detailed phylogeny of post-Devonian stegocephalians, with emphasis on the relationships of amphibian-grade taxa:
In the Mississippian many stegocephalians (such as Crassigyrinus) were still essentially "fingered fish" incapable of movement on land. Others, such as Pederpes, had well-developed arms and legs, and digit counts of five on each limb: these animals probably were fairly terrestrial as adults.
Tetrapoda: The Crown-Group Stegocephalians
Tetrapoda proper is generally restricted to the crown group of Stegocephalia: roughly, the concestor of frogs and dogs, and all of its descendants. Exact taxonomic make-up of Tetrapoda is hindered by uncertainty about the status of various clades of amphibian-grade tetrapods: are the inside the crown-group, or its sister taxon?
Living tetrapods are characterized by:
And many of the Paleozoic "amphibians" were decidedly unlike modern lissamphibians. Some were marine, whereas contact with salt water would be fatal to most living amphibians. Additionally, many of these "amphibians" retained fish-like dermal scales.
Tetrapoda divides into two main branches. Batrachomorpha includes modern frogs and salamanders (collectively Batrachia) and everything closer to them than to amniotes. Reptilomorpha comprises amniotes and everything closer to them than to batrachians.
There is considerable debate over the relationships of the living amphibians. Many studies suggest that Batrachia (contianing the Anura/Salienta (frogs & toads) and the Caudata/Urodela (salamanders & newts)), the Gymnophiona/Apoda (the caecilians), and the extinct Albanerpetontidae (a clade of superficially salamander-like tailed, four-legged amphibians that survived up into the Pliocene) form the monophyletic group Lissamphibia Although we think of lissamphibians as having naked skin, that is in fact a trait of Batrachia only, as there are subcutaneous collagenous dermal scales with a slight bit of mineralization in gymnophionans, and better developed scales in albanerpetontids. Lissamphibia is characterized by dramatic reductions of the skeleton (for example, the ribs no longer extend far from the vertebrae.
However, some recent studies contend that "Lissamphibia" is paraphyletic, with Batrachia nested among the
batrachomorphs, but with gymnophionans among the microsaur lepospondyls (a reptilomorph group):
The Temnospondyli are a clade or grade of amphibian-grade tetrapods. In some studies they are the sister group to Tetrapoda, but in others they are a grade of basal batrachomorphs including the ancestors of Lissamphibia (or just Batrachia). Temnospondyls were a highly diverse group of the late Paleozoic and Triassic, and survived (at very low diversity) up into the mid-Cretaceous. Some represented batrachomorph versions of the alligator morph; others were more narrow-snouted fish eaters. The largest were, at 4 m length, the largest of the amphibian-grade tetrapods.
Basal (amphibian-grade) reptilomorphs included Anthracosauria (a predominantly marine group of narrow-skulled piscivores); Lepospondyli (a diverse group of late Paleozoic animals including the boomerang-headed Diplocaulus and the eel-like aïstopods); Semouryiamorpha (another late Paleozoic clade, with very terrestrialized adults but fully aquatic larvae); and "Diadectomorpha" (a clade or grade representing the sister group or groups of Amniota). Diadectomorphs may have actually had an amniotic egg: at present, their form of reproduction is unknown. Diadectomorphs included both large-bodied terrestrial herbivores and nearly as large terrestrial carnvores.
Mammals, turtles, tuataras, lizards (including snakes), crocodilians, and birds collectively form the living part of Amniota, the crown-group reptilomorphs. Amniotes are characterized first and foremost by the amniotic egg:
With an egg with an external shell and a amniotic sac in which the embryo can develop, tetrapods were able to fully leave the water.
The oldest amniotes are from the Pennsylvanian. As the egg itself is not preserved, we must rely on other anatomical features to recongize fossil members of Amniota:
Synapsid phylogenetic relationships are a thing of beauty, with regards to stratigraphic ranges and orders of appearance:
Basal synapsids were once collectively refered to as Pelycosauria, but this is a paraphyletic group. During the Early Permian Epoch primitive synapsids radiated into many different forms:
The early synapsids evolved themselves into extinction: that is, they were replaced in the Middle and Late Permian Epoch by the Therapsida: the advanced synapsids. Once called the "mammal-like reptiles", they are not true reptiles. Instead, they are the advanced branch of the synapsid phylogeny. They differed from earlier synapsids by:
The Middle and Late Permian therapsids included:
Ancestrally, all vertebrates are cold-blooded (warm their bodies primarily using sunlight). However, some evidence suggests that the advanced therapsids of the Late Permian may have had elevated metabolisms (that is, were at least partially warm-blooded):
Additionally, some therapsids seem to have had parental care of the young, keeping them in burrows.
Mammals and their closest relatives (more properly Mammaliformes, or sometimes "Mammaliaformes") appear in fossil record the same time as dinosaurs, in
Mammals are very advanced therapsids synapsids.
True mammals (Mammalia) found from Middle Jurassic onward.
Like birds, many of the features that characterize modern mammals don't fossilize:
On the other hand, some mammalian features are preservable:
Many features limited to Mammalia among living amniotes were probably found in their closest non-mammalian therapsids relatives. For example, we can't say for certan when warm-blood, fur, sweat & mammary glands show up. We can determine a few of these, though:
Living mammals are divided into three clades:
However, mammal diversity in the Mesozoic was MUCH different. Many groups of Mesozoic mammals have long since died out. And some Mesozoic mammal groups survived the end of the Cretaceous, but have since died out.
Most Mesozoic mammals very small (shrew-to-house cat sized, with a few badger-sized forms in the Cretaceous); mammals only become large AFTER extinction of non-avian dinosaurs.
Oldest mammaliforms of the Late Triassic and Early Jurassic were fairly small. But by Middle and Late Jurassic, there were already some specialized mammals:
Some major groups of Jurassic and Cretaceous mammals:
Prototheria (sometimes called "Australosphenida"; monotremes and their extinct relatives):
There are other branches of early mammals (docodonts, symmetrodonts, etc.), but the most important remaining two are joined together as the clade Theria. Therians are united by various skeletal (parasaggital stance, some dental, etc.) and soft-tissue (nipples, external ears, etc.) features. Therians include the metatheres and eutheres, which diverged by the earliest Late Jurassic.
Metatheria (marsupials and their extinct relatives):
Eutheria (placentals and our extinct relatives):
Prototheres, allotheres (as multitubercultates), a few "dryolestids/cladotheres" (stem-therians), metatheres (including the first marsupials), and eutheres (including the first placentals) all survived the great extinction event at the end of the Cretaceous.
During the Cenozoic, tremendous radiation of the placental (and to a lesser degree, marsupial) mammals. Among the evolutionary patterns:
Parareptilia and the Matter of Turtles
Reptilia has traditionally been a paraphyletic assemblage (e.g., all non-bird, non-mammal amniotes). Currently the name is used for a clade that contains the living turtles, tuataras, lizards (including snakes), crocodylians, and birds, and all taxa closer to them than to mammals. This clade (also called "Sauropsida") is characterized by:
Basal reptiles all retained the anapsid skull condition. Several Permian groups (plus one group that survived through the Triassic) form a clade, sometimes called "Anapsida" but also named Parareptilia. Among the parareptiles were the aquatic Mesosauridae, the long-legged (and possibly bipedal at a full run) Bolosauridae, the large armored herbivorous Pareiasauria, and the smaller herbivorous Procolophonoidea of the Permian through Triassic.
Possibly among the parareptiles (and perhaps even among the pareiasaurs) lies the origin of Testudinata (the clade of modern turtles (Testudines) and their extinct kin). Unfortunately, turtles are extremely transformed anatomically, so that when they first show up in the Late Triassic they are already considerably changed from what ever their ancestors looked like. Although turtle phylogeny is fairly well understood, it remains uncertain if turtles are parareptiles, basal eureptiles, lepidosauromorphs, basal archosauromorphs, or even archosaurs! All these results have been found by various molecular or morphological phylogenetic analyses.
Eureptiles, including Lepidosauromorpha and basal Archosauromorpha
Basal eureptiles are generally anapsid-skulled, "lizard-like" animals (of course, true lizards are actually highly specialized animals!). Among the eureptiles, however, the quick-snapping bite associated with the diapsid skull condition evolved:
In diapsids, the hindlimbs are often longer than the forelimbs, allowing for a bipedal run for at least short distances.
Among the basal diapsids are some highly specialized forms, such as the gliding coelurosauravids and the arboreal drepanosaurids.
Crown-group diapsids (Sauria) includes two branches: Lepidosauromorpha and Archosauromorpha. Lepidosauromorphs include the extinct gliding Kuehneosauridae, the Rhynchocephalia (a clade containing the living tuarara as well as extinct relatives such as the aquatic pleurosaurs), and the Squamata (lizards).
In the Cretaceous, two major highly specialized groups of squamates evolved: the marine Mosasauroidea and the long-bodied Serpentes (aka the snakes). While the former perished in the K/Pg extinction, the snakes survived and (during the Neogene) underwent a major radiation to become a dominant group of small-bodied predators in meadow environments.
The archosauromorphs include numerous branches. One highly successful clade is the Euryapsida, the premier group of marine reptiles of the Mesozoic. Members of this group possessed ovoviparity or even true vivipary (the ability to develop the young inside the body until they were capable of swimming on their own). Thus freed from the land, some groups of euryapsid became highly specialized swimmers. Basal forms of the Triassic tended to be nearshore animals, but two groups of pelagic euryapsids arose:
Other primitive archosauromorphs were the broad-skulled herbivorous Rhynchosauria, the freshwater Choristodera (one branch of which, the long-snouted Champsosauridae, survived well into the Paleogene), the sometimes extraordinarly-long-necked Prolacertiformes, and the large herbivorous Trilophosauridae.
By far the most successful group of archosauromorphs (and indeed one of the most successful groups of tetrapods) are the Archosauriformes: crocodilians, birds, and
their extinct kin:
First appearing at the tail-end of the Permian, early archosauriforms contained both flesh- and fish-eating forms. Archosauriforms are distinguished by the presence of additional openings in the skull:
as well as:
It is not certain if these early archosauriforms had the behavioral traits found in both the living groups of Archosauria (that is, crocodylians and birds). Because both crocodylians and birds share the following derived traits, however, it is fairly certain that at least the concestor of all Archosauria had them, and passed them on to its descendants:
Crown-group archosauriforms (Archosauria) contains two clades: Crurotarsi (also called Pseudosucha), crocodilians and their extinct kin; and Ornithodira (also known as Avemetatarsalia), birds and their extinct kin.
Crurotarsans were a major group of Triassic animals, including:
The crurotarsans include some of the first terrestrial animals to exceed the size of oxen and hippos. Most of them could stand with a semi-erect posture of the limbs, and a few had the fully-erect (that is, parasagittal gait). All crurotarsans except for Crocodylomorpha perished in the Triassic-Jurassic extinction. During the Mesozoic, crocodyliforms radiated to produce numerous forms, including the marine Thalattosuchia, snub-nosed plant-eaters, giant semi-aquatic ambush predators, and terrestrial carnivores. Cenozoic Crocodylia remains a major component of the freshwater ecosystems.
For all their diversity, however, crurotarsans are dwarfed by their ornithodiran sister taxa. Basal ornithodirans differed from typical diapsids by having elongate lower limb elements, suggesting that they were even faster than typical reptiles. All (except perhaps for the most primitive) had cervical vertebrae greatly modified so that they look very different from dorsal vertebrae, but instead could allow the neck to form an S-shaped curve (hence the name of the group).
One branch of the ornithodirans was the
The last group of tetrapods to consider is Dinosauromorpha, which first appeared in the Middle Triassic of Argentina. The taxa modified the hindlimbs further to evolve:
The combination of these traits allowed the little dinosauromorphs to run and accelerate in a bipedal mode all the time, not just at top speeds like typical diapsids. (It is not yet certain if the earliest dinosauromorphs such as Lagerpeton and Marasuchus were primarily quadrupeds or bipedal.
The parasagittal gait and hinge-like ankle also allowed dinosauromorphs the ability to move more actively and constantly rather than only in short bursts of speeds; thus, they were striders. Additionally, although early dinosauromorphs were small (~30 cm long for Marasuchus, 1-2 m long for "silesaurs" and basal dinosaurs, etc.), the presence of limbs directly underneath the body meant that this lineage to grow to much larger size than any previous clade while still remaining terrestrial and mobile (sprawlers relagated to a semi-aquatic life if they became too big to support their weight).
By the Late Triassic, the true Dinosauria had evolved. Within this diverse lineage were:
Accomplishments of the Amniotes
In their history, the amniotes:
Tetrapod paleontology has some particular difficulties: