Lesson 4 ◊ Lesson Outline: ♦ Vertebrate Phylogeny Continued o Evolution of the Tetrapods o Evolution of the Amniotes o Evolution of the Sauropsids o Evolution of the Synapsids ◊ Objectives: At the end of this lesson you should be able to: ♦ Describe the major groups of Tetrapod Vertebrates ♦ Describe the major steps in the evolution of each group ◊ References: ♦ Chapter 4: 57-85 ◊ Reading for Next Lesson: ♦ Chapter 5: 86-104 Anamniotes Amniotes Osteichthyes Tetrapods Actinop Crossop Amphibia terygii terygii Dipnoi The Tetrapods include the Amphibia and the Amniotes The shift from an aquatic to a terrestrial habitat was a challenging one. Fish were well adapted to their habitat. In the late Paleozoic, an offshoot of the fishes gave rise to the tetrapods. At the time there appear to have been long periods of persistent drought and a few Crossopterygian fishes (Rhipidistians) became increasingly adapted for terrestrial life and gave rise to the Labyrinthodonts, the predecessors of the amphibians. Preadaptations were important here. Again, it is believed that this was a period of global persistent drought. This gave rise to stagnant pools which would warm up, become hypoxic, become salty and deprived of food. Two anatomical features appear to have allowed fish to escape these problems. Lungs and Legs (7, 8) Today there are five genera of fish that have true lungs (and all live in areas of seasonal drought). The fossil record indicates that in the late Paleozoic, a great majority of fish possessed lungs. If droughts were severe and ponds dried up, fish with larger fins that could "crawl" along river channels could find another pond. The limbs and girdles of early tetrapods were generally more ossified and stronger and the vertebral column tended to increase in prominence. Connection of the shoulder girdle with the skull was lost and a mobile neck region developed that allowed the head to move relative to the body. Thus, tetrapods inherited paired appendages, jaws, backbones and lungs from fishes. Note: 1) higher vertebrates are the fortuitous outcome of a climatic change and 2) highly adapted fish had to evolve through a transition form that was poorly adapted and struggling to take advantage of a new habitat. Amphibia are distinguished as tetrapods that lay eggs which lack shells and extraembryonic membranes. Their eggs must be laid in water. Class Amphibia Subclass Lissamphibia Order Gymnophiona (Apoda (no feet) = caecilians) Order Urodela (salamanders) Order Salientia (Anura = frogs and toads) The history of how the living amphibians descended from their ancestors is obscure. All are specialized and represent considerable departure in morphology, ecology and behaviour from ancient tetrapods. They retain an aquatic larval stage but, for the most part, have a semi-terrestrial or terrestrial adult stage. The real distinction of the Amniotes is their break from water. The Amniotes bear embryos enveloped in extraembryonic membranes (9). The embryo with these membranes is usually packaged in a calcareous or leathery shelled egg. Some believe this to be the most significant step in vertebrate history. Not only can the egg now be laid on land, it also eliminates the need for a larval stage. The young are born as terrestrial miniatures of their parents. So, which came first - the Reptile or the Egg? It is currently believed that the earliest reptiles developed the amniotic egg but remained semi-terrestrial for some time, living in water and eating fish. They developed the amniotic egg due to selection pressures from predation (laying caviar for others) and seasonal drought. Thus, they could lay eggs on land but still lived in water. With the rise of the insects, a food source was created that favoured the complete move to a terrestrial life style and this came later. Amniota Sauropsida (Reptilia) Synapsida Parareptilia Eureptilia Lepidosauria Archosauria Testudinata Sphenodonta Squamata Crocodilia Dinosaurs Aves (Chelonia) The amniotes are composed of two main lineages, the Synapsida (the mammals) and the Sauropsida (the reptiles, dinosaurs and birds) Traditionally, the primary distinguishing feature of these different groups was the characteristics of the temporal region of the skull (10), the area behind the eye. Differences in this region appear to be a reliable indication of evolutionary lineage - even if it reveals little about the animals themselves. In primitive amniotes (and anamniotes), the temporal region is covered completely by bone. This is referred to as an anapsid skull and is retained by the turtles and their allies. Two variations arise from this basic skull type. The synapsid skull found in mammals has a single pair of temporal openings bordered above by a temporal bar formed by squamosal and postorbital bones. In the other group that diverged from the anapsids, there are two pairs of temporal openings separated by the temporal bar. These are the diapsids and this group includes the dinosaurs and gave rise to all living reptiles except the turtles, and to the birds. The Sauropsida include the reptiles, dinosaurs and birds. These groups are distinguished by having anapsid, and the latter derived diapsid skulls. Living reptiles have scales (but no hair or feathers) composed partly of surface epidermis. They are ectothermic and respiration is primarily pulmonary and not cutaneous. Class Reptilia Subclass Parareptilia (Anapsida) Order Testudinata (Chelonia) The Parareptilia have a distinctive ear region wherein the eardrum is supported by the squamosal bone rather than the quadrate bone and by the retroarticular process of the lower jaw. Furthermore the foot is unique in the way the digits articulate with the ankle bone. The only living members are the turtles. The terms tortoise and terrapin are sometimes applied to terrestrial and brackish water species but there is no formal taxonomic distinction between them. The Eureptilia are diapsids with two temporal fenestrae as well as a palatine fenestra within the roof of the mouth. The two major groups are the Lepidosauria and the Archosauria Subclass Eureptilia (Diapsida) Superorder Lepidosauria Order Sphenodonta Sphenodon (tuatara) Order Squamata snakes, lizards and amphisbaenids (The Sphenodon has a complete diapsid skull. In lizards the lower temporal bar is absent and in snakes both upper and lower temporal bars are now gone. These are secondarily derived features that give the jaws greater mobility- this is especially true of snakes where it allows them to swallow prey whole) (Amphisbaenids are limbless burrowers that feed on arthropods. Many species of lizards are also secondarily limbless. Thus presence or absence of limbs does not distinguish snakes from other reptiles. Differences in internal anatomy, especially of the skull are needed to distinguish the two groups. Also, lizards have moveable eyelids and most have an external auditory meatus (opening) which snakes do not. [Thus we have finless fish (eels and symbranchus), limbless amphibians (caecilians) and reptiles (amphisbaenids, snakes and lizards) as well as some urodele amphibians that are almost limbless (Siren and Amphiuma)] Superorder Archosauria Order Crocodilia crocodiles, caimans and alligators Note: the Archosauria also include the dinosaurs and the birds. The Archosaurs are distinguished by teeth set in individual sockets (thecodont) rather than a common groove, a large mandibular foramen in the skull in front of the eye and a unique ankle design in the hindlimb with a tendency to bipedal upright posture (not seen in the crocodilia) The dinosaurs include two groups distinguished by the arrangement of the three bones that make up the pelvic girdle - the ornithischians and the saurischians. While the birds have traditionally been placed in a Class of their own (Aves), technically, they are diapsids and are included in the Eureptilia and Archosauria. They are a natural but specialized derivative of earlier reptiles. Birds outnumber all vertebrates except fishes (3D vs 2D?). They are most closely related to the crocodiles and share many of the same basic features. (similar eggs, bones and muscles) Class Aves (also Diapsida) Superorder Palaeognathae (Ratites = ostriches, rheas, emus and cassowaries) all are flightless Superorder Neognathae (Carinates) Size, flight and anatomy alone do not distinguish birds from other vertebrates. What makes birds unique is the presence of feathers and endothermy (11). Sauropsids Synapsids (mammals) Prototheria Theria Metatheria Eutheria monotremes marsupials placentals The Synapsids gave rise to the mammals. The two primary characteristics of the mammals are hair and mammary glands (12). They are endothermic furry animals nourished from birth with milk secreted by their mothers. In present day mammals, the thick coat of hair that normally distinguishes a mammal primarily acts as an insulating layer. Sebaceous or sweat glands arise in the skin of mammals in association with the hair to condition the skin and contribute to evaporative heat loss. It is believed that the other mammalian trait, mammary glands were derived from specialized skin glands. Note: they were one of the earliest groups to evolve from the stem reptiles The stem group of ancient reptiles giving rise to the mammals (therapsids) were quadrapedal, their feet had five digits, their limb position was less sprawled, their legs were positioned more directly under the weight of the body. This gives rise to a more efficient mode of locomotion. They also had teeth specialized for slicing and muscular cheeks. The bones and muscles of the skull changed substantially giving them a specialized ear and modified jaw mechanics. They also are the only group to have an inner ear composed of three bones (incus, maleus, stapes) (13), and a lower jaw composed of only one bone. The Monotremes include the duck-billed platypus and the spiny anteaters. Their distinguishing features are that their embryos develop in a shelled egg (oviparous) and they have a cloaca The Theria (or placental mammals) give rise to live young (viviparous) and are distinguished into groups that have a yolk sac placenta (the marsupials) and those that have a chorioallantoic placenta (14).