Vertebrate Diversity October 4, 2005
The big picture – I Systematics
The big picture – II Diversity
Highlights
The early groups
The transition to land
Non-dinosauran “reptiles”
The avian dinosaurs
The big picture – I Systematics
Jawless
fish
Jawed
fish
Anapsids
Amphibians
Dinosaurs
Lizards
Birds
Synapsid
“reptiles”
Mammals
diapsid
skull
synapsid skull
amniote egg
tetrapods
jaws
vertebrae
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Note the key defining features and their order of acquisition:
Vertebrae
Jaws
Tetrapods (four limbs- terrestrial vertebrates)
Amniote egg (permits egg-laying out of water). Hard or leathery egg that allows
passage of gases (respiration) but not water. A membrane (the amnion) encloses the
embryo, another collects waste, a yolk provides food. Freedom from limitation to aquatic
development.
Milsom and Rigby (2004)
Skull types: anapsid, diapsid, synapsid
Turtles are anapsids,
Dinos, birds and lizards are diapsids,
Mammals are synapsids
The skull types:
Anapsid – no openings behind eye
Diapsid – two openings behind eye
Synapsid – one opening behind eye
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Note that birds and dinosaurs are “sister groups”
Note that “reptiles” are a paraphyletic group (they don’t include all the descendants)
Note that synapsids include mammals and some “reptiles”
Here’s a view in geologic time:
Milsom and Rigby (2004)
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The big picture – II Diversity
Diversity of marine (lighter line) and terrestrial (darker line) families during the
Phanerozoic. From Benton and Harper, 1997.
Terrestrial (continental) diversity (includes plants and terrestrial invertebrates, in addition
to terrestrial vertebrates): Steady, one phase, exponential increase (contrast to marine).
Effects of extinction not so evident.
Terrestrial evolutionary themes:
Early: Independence from water (exploiting an increasing variety of habitats)
Later: Increased diversity. Diversification as an exponential process
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Highlights of vertebrate diversity
1. The early groups: “fish” are a paraphyletic group, consisting of several distinct
clades of swimming vertebrates. Two major groups, the Placoderms and
Acanthodians are extinct; the Agnathans (jawless fish), Chondrichthyes (sharks,
skates and rays), and Osteichthyes (the bony fish) are extant (=still living today).
Class Agnatha (Cambrian to Recent)- jawless fish. Living representatives include the
parasitic hagfish and lamprey. Cambrian through Devonian agnathans had an armored
head shield.
Early agnathan fishes (both
Devonian). Note armored
head shields. (Benton and
Harper, 1997)
Class Placodermi (Devonian). First jawed fish. Heavily armored.
Class Chondrichthyes (Devonian-Recent). Cartilaginous fishes, including the modern
sharks, skates and rays. Mostly represented in fossil record by their teeth.
Class Acanthodii (Silurian-Permian). Small spiny fish.
Class Osteichthyes (Devonian-Recent). Bony-fish. Includes ray-finned fish and
lobe-finned fish. Lobe-finned fish include the ancestors of tetrapods.
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placoderm
Acanthodian
Osteichthyes
Osteichthyes
Osteichthyes
Benton and Harper (1997)
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2. The transition to land
Problems of terrestrialization
• Physical support and locomotion - greater weight without buoyant effect of water.
Support of skeleton in animals; hydrostatic or woody support in plants
• Drying-out. Conserving water; preventing dessication; impervious skin in animals,
waxy cuticle in plants
• Nutrient supply - unlike water, where many animals can extract food suspended in the
water (bivalves, brachiopods, corals), air is much less dense and does not support a
significant "plankton”. No filter-feeders; gotta go and get the food.
Similar problem in plants – can’t absorb nutrients directly from surrounding water;
must come from roots in moist soil. Root system, vascular system for conducting water
and nutrients.
Why move to land?
Most scenarios for the transition to a terrestrial way of life involve a transition not
from the marine environment to land, but from ephemeral lakes and ponds to land.
Not only does the direct evidence suggest this, but if you think about it, you
should see that only in environments that periodically dried up - like many lakes - would
adaptations for life on dry land be useful.
In other words, many of the adaptations that we recognize as adaptations for life
on land were originally adaptations for surviving intervals of time when the lakes dried
up, or to enable movement from pond to pond.
There are two important messages about evolution here:
1. Evolution is opportunistic. There is no internal 'drive’ toward a goal of
terrestrial life, only a taking advantage of new habitats.
2. Structures that evolved for one function (resistance to dessication- lungs,
impervious skin) may be used later in evolution for another function (terrestrial
life)
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The transition to land was made independently by many groups - and at different times.
Gonick, Cartoon History of the Universe, Part I.
Plants
1. The fossil record of the first terrestrial plants
Now, note that things like cyanobacteria (blue-green algae) are common on wet
surfaces and can survive many episodes of drying out. These aren't usually counted as
"true" terrestrial plants, though I'm sure they have been present since sometime in the
Archean.
The earliest true land plants date from the Late Silurian. They are termed true
land plants because they clearly have what is called a vascular system: elongated dead
cells that serve to conduct water up through narrow tubes.
Terrestrial animals
2. Arthropods. Earliest record is from Late Silurian; though the best documented record
is Early Devonian. Aquatic arthropods well-equipped for life on land: a hardened and
waterproof exoskeleton limits drying out and walking legs provide mobility.
Spiders, mites, centipedes, insects.
Food chain probably based on decaying plant debris along shores of lakes and
deltas.
3. Terrestrial vertebrates - Oldest amphibian fossil is Late Devonian in age.
derived from a group of fish called lobe-finned fish in that their fins are
attached to their vertebral column with bones and muscles.
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Ichthyostega and Acanthostega the no-so “missing links”: The first tetrpods: four
limbs with pelvic and pectoral girdle (hips and shoulders) Note seven and eight digits.
Class Amphibia (Carboniferous to Recent). A paraphyletic group because they do
not include all their descendants, the reptiles. Includes a variety of fossil forms quite
unlike modern amphibians. Have pelvic and shoulder girdles.
Benton and Harper (1997)
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A new look at locomotion in Ichthyostega, a early amphibian. From Ahlberg, P.E., Clack,
J.A. and Henning Blom, H., 2005. The axial skeleton of the Devonian tetrapod Ichthyostega. Nature 437,
137-140.
a, New reconstruction of Ichthyostega, most anterior cervical vertebrae (obscured by skull in lateral view)
in grey outline. The reconstruction shows a maximally extended presacral column. Note the revised
dentition, orientation of the limb elements and proportional differences (larger limb girdles and shorter tail)
compared with b (ref. 5). The forearm is shown close to the beginning of the powerstroke; the manus
remains unknown. Neural spines of the cervical and anterior caudal regions are also unknown. Regions are
as follows: cervical (1), thoracic (2), lumbar (3) and sacral (4); unlabelled, caudal. Hindlimb based on ref. 7
and new observations. b, Previous reconstruction of Ichthyostega from ref. 5. c, Lateral view reconstruction
of Acanthostega based on unpublished illustrations by Coates and refs 4 and 16. Grey shading in a and c
shows what are considered to be the sacral neural spines. Scale bars, 100 mm.
Ahlberg et al, point to the fact that, unlike fish, the vertebrae differ along the spinal
column. The ones closest to the shoulder lean back, those in the center are upright, and
those near the hips lean forward. Muscle scars suggest that this arrangement allowed the
trunk of the animal to be lifted off the ground, but restricted lateral, snake-like
movement. The vertebrae in the tail, however, allowed a snake-like motion.
--They interpret movement with limbs on land, with movement by the tail in the water. A
“compromise” set of adaptations that may not have been very efficient, but given the lack
of other terrestrial organisms, adequate for the time.
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Reconstructions of the habitats and movements of Ichthyostega. From Nature 437, 38-39
(2005).
3.
Non-dinosauran “reptiles”. A paraphyletic group (a group that doesn’t include all
the descendants) that includes:
Anapsids: turtles and relatives, dating back to the Carboniferous – the oldest
amniotes
Non-dinosaur diapsids: snakes, lizards, crocodiles, pterosaurs and Mesozoic
marine reptiles (ichthyosaurs, plesiosaurs)
Non-mammalian synapsids: the pelycosaurs (“sailed backed lizards” of the
Permian, and “mammal-like” reptiles of the Triassic. The ancestors of
mammals.
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Anapsid example:
Prothero (2004)
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Non-dinosauran diapsid example:
Non-mammalian synapsid example
Benton and Harper (1997)
Functional morphology of the “sail”?
Protection from predators (appearing bigger than you are/ spiny)
Sexual display
Thermoregulation: the “sail” as a radiator
All/some/or none of the above
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4. The avian dinosaurs
Class Aves (Jurassic - Recent). poor fossils; lightly constructed, often live in habitats not
conducive to fossilization of their remains.
Archaeopteryx, from the Jurassic, shows a mix of reptilian (long bony tail, teeth, claws on
forelimbs) and avian features (beak, feathers, wings). Derived from dinosaurs.
Prothero (1998)
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