◊ Lesson Outline: ♦ Parade of the Craniates in Time and Taxa o Vertebrate Classification
♦ Vertebrate Phylogeny o Evolution of the Craniates o Evolution of the Vertebrates o Evolution of the Gnathostomes o Evolution of the Chondrichthyes o Evolution of the Osteichthyes
◊ Objectives: At the end of this lesson you should be able to:
♦ Describe the major groups of non-Tetrapod Vertebrates. ♦ Describe the major steps in the evolution of each group.
◊ References: ♦ Chapter 4: 45-57
◊ Reading for Next Lesson: ♦ Chapter 4: 57-85
Vertebrate Phylogenetic Relationships Who's Who Amongst the Vertebrates?
Vertebrate Classification: There are several ways of classifying vertebrates. Traditional taxonomy divides vertebrates into:
Kingdom Phylum Class Order Family Genus Species
From the last lecture, we have a good idea of the relationship of the protochordates: Phylum Echinodermata Phylum Hemichordata Phylum Chordata Subphylum Urochordata Subphylum Cephalochordata Subphylum Craniata (Vertebrata)
Our focus will be on the Vertebrata. (Note: Vertebrates are a Subphylum and, technically, vertebrates ≠ craniates)
The vertebrates can also be grouped by shared distinctive features. These include:
Agnathans versus Gnathostomes: Agnathans lack jaws while all other vertebrates possess jaws
Fishes versus Tetrapods versus Quadrapeds: Amphibians, reptiles, birds and mammals are collectively referred to as tetrapods (four footed). This group includes derived forms that no longer possess four feet such as the legless amphibians and reptiles, the birds and flippered mammals as well as the quadrapeds.
Anamniotes versus Amniotes: Reptiles birds and mammals produce a thin sac around the embryo called an amnion which encases the embryo in a protective water compartment. Fish and amphibians do not.
Vertebrata Craniata Chordata
Uro Cephalo Myxinoidea Petromyzontidae chordata chordata (hagfish) (lampreys)
With the evolution of improved sensory organs and an anterior end, came the first innovation : the evolution of a cranium (1).
With this comes the evolution of eyes, ears, nose and other sensory organs and an enlarged neural tube associated with the sense organs that forms the brain.
The cranium is a composite structure of bone or cartilage that supports the sensory organs and protects the brain. At this early stage, the evolution of the cranium is associated with the evolution of cartilage.
The next major step in vertebrate evolution was the appearance of a muscular pharyngeal pump for feeding – a muscular pump to produce a food-bearing water current (2).
Super Class Agnatha This group includes the hagfish (Myxinoidea) and the lampreys (Petromyzontida). Together they are known as the cyclostomes (round mouth). All living agnathans lack bone. The two groups are extremely different morphologically and physiologically. They are often considered as the most primitive vertebrates but really they are highly evolved and modified – adapted to very specialized lifestyles – and not at all similar in many ways to the general ancestral state.
Hagfish possess a single nostril and a vestibular apparatus (balancing organ) with a single semicircular canal. Their body fluid is isosmotic with sea water which is similar to marine invertebrates and distinct from all vertebrates.
They are bottom feeding scavengers found only in sea water. Their eyes are vestigial and they feed mainly on invertebrates and dead or weakened fish.
While it is believed that the suction pump was initially used for suspension feeding and deposit feeding by sucking, the lamprey, although jawless, have diversified to exploit the expanded pharyngeal pump with a rasp like muscular tongue for attacking vertebrate prey. They too have a single nostril but it does not connect to the pharyngeal cavity. They breathe in and out through the pharyngeal slits leaving the mouth free to form a negative pressure and suck onto their prey. They reproduce in fresh water and migrate to sea as juveniles. They possess two semicircular canals. They are hyposmotic to sea water.
Craniates versus Vertebrates
The next major step was the evolution of – the vertebral column (vertebrates) (3) – a series of separate bones or cartilage blocks that are firmly joined as a backbone defining the body axis.
The hagfish lack vertebrae but do have a cranium. Thus, in most modern systems of classification, all chordates from the Agnathans on are considered as craniates while only those from the Petromyzontida on are considered vertebrates. This is a relatively new and fine distinction.
Teleostomi Agnatha Gnathostomata
Chondrichthyes Osteichthyes Tetrapods
Elasmo Holo Actinop Sarcop branchii cephali terygii terygii Super Class Gnathostomata
The distinguishing feature of all Gnathostomes is the presence of jaws (4) - biting devices derived from the pharyngeal arches that evolved to support the gill slits.
With this there is the simultaneous appearance of paired pectoral and pelvic fins along with supportive bony or cartilagenous girdles and specialized musculature. This gave these animals stability and control for maneuverability while swimming
With this, fish were released from filter feeding, gain increased mobility, and a variety of potential life styles opened up.
There is a transition from animals with gills for feeding and skin for respiration to fish with jaws for feeding and gills for respiration.
Early Gnathostomes are believed to have fed on larger food items with a muscularized mouth that rapidly snatched prey from the water. Active predation becomes a common lifestyle in subsequent vertebrate radiation.
The Gnathostomes give rise to two major lines: the Chondrichthyes (cartilagenous fish) and the Teleostomi (Osteichthyes (bony fish) and Tetrapods)
The Chondrichthyes (cartilagenous fish) consist of two groups. All are cartilagenous which is not an ancestral trait but represents a secondary loss (i.e. bone evolved before the Chondrichthyes) (5). Thus, they have a vertebral column composed mostly of cartilage that largely replaces the notocord as the functional support for the body.
Class Chondrichthyes Subclass Elasmobranchii Subclass Holocephali The elasmobranchs include the sharks and the rays while the holocephali includes the chimaeras or rat fish.
Class Osteichthyes: These fishes have bony skeletons. Also, whereas cartilagenous fishes address problems of buoyancy with oily livers and hydrofoil fins, most bony fish possess an adjustable gas- filled swimbladder that provides neutral buoyancy and moveable fins.
Subclass Actinopterygii Ray-Finned Fishes These fish have distinctive fins supported internally with slender endoskeletal rays with muscles to control fin movement located within the body wall Palaeonisciformes Acipenseriformes paddlefish, sturgeon Polypteriformes birchirs
Neopterygii Lepisosteiformes gar Amiiformes bowfin Teleostei modern fishes
[The true fishes represent perhaps the most successful of all vertebrate lines giving rise to a tremendous diversity of species that have existed in large numbers for a long time. The teleostei encompass close to 20,000 living species spread from pole to pole and ranging from alpine lakes to deep ocean trenches. They outnumber all other vertebrates combined!]
The Teleostomi include the bony fishes (Osteichthyes) and all of the tetrapods.
It seems probable that bone first appeared as dermal armour for protection. It was laid down in membrane fashion in plates within the skin. Thus, the earliest vertebrates were without well-developed vertebrae and relied upon a strengthened notochord to meet the mechanical demands of body support and locomotion. When vertebrae first appeared in latter fishes, the vertebral elements initially rode upon or surrounded a notochord that continued to serve as the major structural component of the animal’s body. In latter fishes and terrestrial vertebrates the role of the vertebral column grows while that of the notochord declines. In adults of most derived vertebrates, the embryonic notochord disappears and is replaced by the vertebral column.
Subclass Sarcopterygii Lobe-Finned Fish These fish have fleshy fins (6) that rest at the ends of short projecting appendages with internal bony elements powered by muscles located outside the body wall along the projecting fin
The tetrapod limb evolved from the sarcopterygian fin. It is believed that these fins initially evolved for pivoting or maneuvering in shallow water - or for working bottom habitats in deeper waters.
Crossopeterygii coelocanth (deep living with a swim bladder that is filled with fat and used for buoyancy and not respiration)
Dipnoi lung fish (Have paired lungs and breathe air when water becomes hypoxic or when burrowing during periods of seasonal drought.)