Download Lesson 4 Lesson Outline - Department of Zoology, UBC

Lesson 4
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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
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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.
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References:
♦ Chapter 4: 45-57
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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
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
Agnatha
Uro
chordata
Cephalo
chordata
Myxinoidea
(hagfish)
Gnathostomes
Petromyzontidae
(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
Gnathostomata
Agnatha
Chondrichthyes
Elasmo
branchii
Holo
cephali
Osteichthyes
Actinop
terygii
Sarcop
terygii
Tetrapods
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 gasfilled 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
Amiiformes
Teleostei
gar
bowfin
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.)