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Subphylum Vertebrata. The vertebrates are a large and diverse group including the fishes and tetrapods. Vertebrates possess the basic chordate characteristics, but also a number of novel homologous structures.
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Subphylum Vertebrata • The vertebrates are a large and diverse group including the fishes and tetrapods. • Vertebrates possess the basic chordate characteristics, but also a number of novel homologous structures. • An alternative name for the group Craniata is actually a better descriptor for the entire group because all members possess a cranium, but some jawless fishes lack vertebrae.
Important developments of the Vertebrates • Musculoskeletal system. Vertebrates possess an endoskeleton, which is much more economical in materials than the exoskeleton of invertebrates. • It forms a jointed scaffolding for the attachment of muscles. Initially the endoskeleton probably was cartilaginous (it still is in jawless fishes and sharks) and later became bony in many groups.
Important developments of the Vertebrates • Bone is stronger than cartilage, which makes it a better material to use for muscle attachment in places where mechanical stress may be high. • Bone may have evolved initially as a means of storing minerals and was later adapted for use in the skeleton.
Important developments of the Vertebrates • Various aspects of vertebrate physiology have been upgraded also to meet increased metabolic demands. • For example the pharynx, which was used for filter feeding in primitive chordates has had muscles added that make it a powerful water pumping organ. • With the conversion of the pharyngeal slits to highly vascularized gills the pharynx has become specialized for gas exchange.
Important developments of the Vertebrates • The ancestors of vertebrates switched from filter feeding to more active feeding, which required movement and the ability to sense the environment in detail. • With these changes came the need for a control center to process information. The anterior end of the nerve cord consequently became enlarged into a brain.
Important developments of the Vertebrates • The vertebrate brain in fact developed into a tripartite brain (with a forebrain, midbrain, and hindbrain) that was enclosed within a protective cranium of bone or cartilage.
Important developments of the Vertebrates • Sense organs have also become highly developed among the vertebrates. • These include complex eyes, pressure receptors, taste and smell receptors, lateral line receptors for detecting water vibrations, and electroreceptors that detect electrical currents.
Important developments of the Vertebrates • The development of the head in vertebrates with its array of sense organs appears to have been driven by the evolution of new embryonic tissues that give rise to cells that play an important role in the formation of sensory structures.
Important developments of the Vertebrates • A factor that may have played a major role in the evolution of the vertebrates is the duplication of Hox genes. • Hox genes play a major role in embryonic development and vertebrates have four sets, jawless vertebrates have two sets whereas invertebrates and amphioxus have only one.
Hox genes • Because a single hox gene influences the expression of many other structural genes a change in when and where a hox gene is turned on may lead to major morphological changes in the phenotype such as the addition or loss of legs, arms, antennae and other structures.
http://evolution.berkeley.edu/evolibrary/images/mutantfly.jpghttp://evolution.berkeley.edu/evolibrary/images/mutantfly.jpg
Induced ectopic eyes In Drosopila (arrowed) From Induction of Ectopic Eyes byTargeted Expression ofthe eyeless Gene in Drosophila Georg Halder,* Patrick Callaerts,* Walter J. Gehring. Science. Vol. 267 24 March 1995
Hox genes • The duplication of the Hox genes appears to have occurred around the time vertebrates originated and it may be that this gene duplication freed up copies of these genes, which control development, to generate more complex animals.
Hox genes • One group of animals in whose evolution hox genes are hypothesized to have played a major role is snakes. • It’s suggested that the hox genes controlling the expression of the chest region in lizard ancestors of snakes expanded their zone of control in the developing embryo.
Hox genes • As the hox genes for thoracic development increased their influence, limb development was suppressed at the same time giving the limbless condition we see in snakes today.
Early vertebrate ancestors • Fossils of early chordates are scarce, but a few are known including Pikaia from the Burgess Shale (approx 580 mya) that appears to be an early cephalochordate and has a notochord and segmented muscles.
15.8 Pikaia
Early vertebrate ancestors • Another fossil from China Haikouella lanceolata about 525mya. • This fossil has a notochord, pharynx, and a dorsal nerve cord which are chordate characters, but also pharyngeal muscles, eyes, a head, gills and a brain which are vertebrate traits.
Jawless early vertebrates • A wide variety of armored jawless fishes called ostracoderms are known from the Ordovician (approximately 490-440 mya) up to near the end of the Devonian period (about 360 mya). • These fish in many cases lack paired fins and so probably were not precision swimmers.
15.10 Ostracoderms
Jawless early vertebrates • The ostracoderms were heavily armored and jawless with narrow, fixed mouths. They appear to have been mainly filter feeders that used their pharyngeal muscles to pump water. • Ultimately, the ostracoderms were outcompeted by fish that possessed the next big evolutionary development: jaws.
Early jawed vertebrates • The origin of jaws was a hugely significant event in the evolution of the vertebrates and the success of the Gnathostomes [the jawed vertebrates, “jaw mouth”] is obvious. • The first jawed vertebrates were the placoderms heavily armored fish which arose in the late Silurian (about 410mya) and possessed not only jaws, but paired pelvic and pectoral fins that gave them much better control while swimming.
Dunkleosteus Skull http://en.wikipedia.org/wiki/Placodermi An 8-11 meter long super predator of the Devonian period Coccosteus http://en.wikipedia.org/wiki/File:Coccosteus_BW.jpg
15.13 Early jawed fishes of the Devonian (400 mya).
Evolution of Jaws • Vertebrate jaws are made of cartilage derived from the neural crest, the same material as the gill arches (which support the gills). • Jaws appear to have arisen by modification of the first cartilaginous gill arches, which aid in gill support and ventilation.
Evolution of Jaws • The advantages of possessing jaws are obvious. • However, structures must benefit the organism at all times or they will not be selected for. • What use would a proto-jaw have been before being fully transformed?
Evolution of Jaws • Mallatt (1996,1998) has suggested that jaws were originally important for gill ventilation, not grasping prey. • Gnathostomes have much higher energy demands than agnathans. They also possess a series of powerful muscles in the pharynx. These muscles allow them to both pump water across the gills and suck water into the pharynx.
Evolution of Jaws • It is likely that selection initially favored enlargement of the gill arches and the development of new muscles that enabled them to be moved and so pump water more efficiently. • Once enlarged and equipped with muscles it would have been relatively easy for the arches to have been modified into jaws.
Evolution of Jaws • Being able to close the mouth would have enabled the muscles of the pharynx to squeeze water forcefully across the gills. • Selection would have favored any change in gill arches and musculature that enhanced water movement over the gills. • Thus, Mallatt suggested that the mandibular branchial arch enlarged into protojaws because it allowed the entrance to the pharynx to be rapidly opened and closed.
Evolution of Jaws • Selection would have favored enlargement and strengthening of the mandibular arch to tolerate the forces exerted on it by the strong pharyngeal muscles. • Once the proto-jaws can be rapidly closed they can also take on a grasping function and new selective forces would quickly have driven jaw elaboration.
15.12 Note resemblance between upper jaw (palatoquadrate cartilage) and lower jaw (Meckel’s cartilage) and gill supports immediately behind in this Carboniferous shark
Evolution of Jaws • Equipped with jaws for grabbing and holding prey and powerful pharyngeal muscles that could suck in prey gnathostomes could attack moving prey. • An enormous diversification of gnathostomes followed.
Living fishes • The living fishes (not a monophyletic group) include: • the jawless fishes (e.g. lampeys), • cartilaginous fishes (e.g. sharks and rays), • bony, ray-finned fishes (most of the bony fishes such as trout, perch, pike, carp, etc) and • the bony, lobe-finned fishes (e.g. lungfishes, coelacanth).
Living jawless fishes • There are a little more than 100 species of living jawless fishes or Agnathans (the term agnathan does not represent a monophyletic group). • These belong to two classes the Myxini (hagfishes) and the Cephalaspidomorphi (lampreys).
Characteristics of agnathans • Lack jaws (duh!) • Keratinized plates and teeth used for rasping • Vertebrae absent or reduced • Notochord present • Dorsal nerve cord and brain • Sense organs include taste, smell, hearing, vision.
Hagfishes: class Myxini • Hagfishes are a marine group of primarily scavengers. • They use their keen sense of smell to find dead or dying fish and invertebrates and rasp off flesh using their toothed tongue. • As they lack jaws, they gain leverage by knotting themselves and bracing themselves against whatever they’re pulling.
Hagfishes • Hagfishes are unusual in that they have body fluids, which are in osmotic equilibrium with the surrounding sea. This is unknown in other vertebrates, but common in invertebrates. • They are also unusual in having a low pressure circulatory system that has three accessory hearts in addition to a main heart.
Hagfishes • Hagfishes have a remarkable (and revolting) ability to generate enormous quantities of slime, which they do to defend themselves from predators. • A single individual can fill a bucket with slime.
Hagfish clip • Eddie and the Hagfish • http://www.youtube.com/watch?v=NYRr_MrjebA
Lampreys: Class Cephalaspidomorphi • Lampreys occur in both marine and fresh waters and about half of all species are ectoparasites of fish (the others are non-feeding as adults and live only a few months). • Lampreys spawn in streams and the larvae (ammocoetes) live and grow as filter feeders in the stream for 3-7 years before maturing into an adult. Feeding adults live a year or so before spawning and dying.
Lampreys • Parasitic lampreys have a sucker-like mouth with which they attach to fish and rasp away at them with their keratinized teeth. • The lamprey produces an anticoagulant as it feeds to maintain blood flow. When it is full the lamprey detaches, but the open wound on the fish may kill it. At best the wound is unsightly and largely destroys the fish’s commercial value.
