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SYNAPSIDS RULE
Overview
Seed plants and tetrapods had appeared by the late Devonian.!
Let's consider how they affected the organization of terrestrial ecosystems in the next
period, the Carboniferous (360 TO 285 Mya)
As you know, the positions of continents have not been constant through earth history,
and that the movements of continents may have major effects on life, climate, and ocean
circulation.
North America and Europe were near the equator; Africa, South America, Australia, and
Antarctica are at low southern latitudes.!
Parst of Asia are spread at mid-to-high northern latitudes.
Extensive coal swamps at low and high latitudes.
Massive glaciation was occurring at southern high latitudes.
All fossils of land animals that we currently know are restricted to a belt near the tropics.
Marine Life
Beginning a recovery after the Late Devonian extinction of tabulate-stromatoporoid reefs,
extinctions of many fish, floating and swimming animals, and many freshwater forms.
Ammonoids rediversified quickly
Reefs remained poorly developed after the demise of the tabulate-stromatoporoid reefs
near the end of the Devonian. Frame-building organisms were scarce.
Brachiopods continued to thrive.
Bryozoans were prominent.
Crinoids and blastoids were common in the sea.
Large foraminifera (fusulinids) appeared. First occur in Upper Mississippian; most
abundant in Pennsylvanian and Permian.
A Golden Age of Sharks
Sharks and ray-finned fishes persisted as a diverse group of predators.
Heavily-armored fish were replaced by more mobile forms.
Ability to swim more rapidly became a necessity as predators became more efficient.
Sharks have undergone two major adaptive radiations and survived at least five mass
extinctions that many other creatures did not.
The first major shark radiation occurred during the Carboniferous Period
With the exception of acanthodians, few fishes swam in early Carboniferous seas.
The fossil record indicates that more than 75% of fish groups alive during the late
Devonian died out before the beginning of the Carboniferous.
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The placoderms - a once dominant group of armored fishes - survived this extinction
event, but at greatly reduced diversity and abundance.
Stethacanthus
Perhaps in response to the ecological niches vacated by the placoderms, the
stethacanthids exploded into a riot of bizarre forms and lifestyles.
One of the most outlandish of these sharks was Stethacanthus
Best known from Carboniferous deposits in central Scotland and Montana, Stethacanthus
was a two-foot (60-centimetre) long shark that inhabited warm, shallow seas.
Female Stethacanthus were perfectly charming, graceful little sharks.
But the males can perhaps be best described as haberdashery-impaired.
Male Stethacanthus had an enormous, flat-topped dorsal fin bristling with enlarged
scales.
Basically, it looked like a fish with a brush sticking out of its back.
In addition, male Stethacanthus had similar enlarged scales on top of the head, making
the whole contraption resemble a set of large, bristle-toothed jaws.
It seems likely that the dorsal brush and cranial bristles of Stethacanthus played some
role in their courtship rituals.
Similar contests of strength are known to occur in modern bannerfishes of the genus
Heniochus.
To a lesser degree than Stethacanthus, bannerfishes have sculpted foreheads which
facilitate males locking together, eyeball to eyeball, for macho pushing matches.
Falcatus
Falcatus was also a stethacanthid, but it grew to a length of only about six inches (15
centimetres) - about the same size as the very smallest of living sharks.
Falcatus inhabited the warm, shallow seas that invaded the American mainland during
the early Carboniferous, about 325 million years ago.!
Male Falcatus had a large, sword-like appendage - apparently a modified fin spine projecting forward over its head like a sunshade.
Falcatus seems to have used this odd head ornament in a kind of piscine foreplay.
There has been a pair of fossilized Falcatus apparently preserved in the act of mating.
The slab of limestone shows the larger female grasping the male (identifiable by its
claspers) by the 'antler' projecting from its head.
In Falcatus we have clear evidence of sexual dimorphism in ancient sharks.
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The stethacanthids were only one group of sharks freed by the decline of the placoderms.
Many other shark groups also underwent massive radiations during the Carboniferous
Period.
Shark diversity during the Carboniferous Period was nothing less than astonishing.
The Carboniferous boasted about 45 families of sharks (compared with about 40 families
of modern sharks - not counting the rays, which would appear later).
But at the close of the Permian Period, about 250 million years ago, there occurred what
has been called the "Mother of All Mass Extinctions".
In a geological instant, fully 99% of marine species were wiped out - including the
stethacanthids.
But some shark lineages survived this catastrophe, one of them eventually giving rise to
modern sharks.
!
Freshwater Habitats
Ray-finned Fishes
The modern bony fishes, class Osteichthyes, appeared in the late Silurian or early
Devonian (about 395 million years ago).
The early forms were freshwater fishes from about 230 million years ago.
A subclass of the Osteichthyes, the ray-finned fishes (subclass Actinopterygii), became
and have remained the dominant group of fishes throughout the world – they were
certainly common in the Carbonifierous
Note:
It was not the ray-finned fishes, however, that led to the evolution of the land vertebrates.
The ancestors of the land vertebrates are found among another group of bony fishes
called the Choanichthyes or Sarcopterygii.
Choanate fishes are characterized by internal nostrils, fleshy fins called lobe fins, and
cosmoid scales.
The choanate fishes appeared in the late Silurian or early Devonian, more than 390
million years ago, and possibly arose from the acanthodians.
The choanate fishes include a group known as the Crossopterygii, which has one living
representative, the coelacanth Latimeria.
During the Devonian Period some crossopterygian fishes of the order Rhipidistia crawled
out of the water to become the first amphibians
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Sharks
Also in freshwater lakes swam lungfishes and xenacanth sharks - descendants of
Antarctilamna that persisted in freshwater environments until the early Triassic Period,
about 220 million years ago
Xenacanths were almost exclusively freshwater inhabitants, and had a long, rearwardpointing fin spine just behind the cranium (the name xenacanth means "strange spine"),
diplodont teeth, a slender, eel-like body, an elongate dorsal fin extending along most of
the back, and a symmetrical, tapering tail.
!
Carboniferous Land Plants
The Carboniferous is famed for extensive coal beds, which were formed in swamps.
During the Carboniferous large parts of Europe and North America were situated in an
equatorial position and extensive swamp forests developed in the wetland areas at the
continental margins.
The largest Carboniferous basin is the so-called Paralic Basin which extended from
Ireland, over England, northern France, Belgium, The Netherlands, Germany (Ruhr
District) into Poland.
Temporary sea level changes resulted in flooding of the lowland areas and marine
sediments were deposited.
Marine bands are intercalated in the mainly terrestrial sequences.
Also on the continents several, usually smaller, so-called epicontinental or intramontane
basins, in which also peat-forming swamp vegetation types occurred, developed (e.g. the
Saar Basin, Germany).
Common coal swamp plant genera:
Lycopod trees or club mosses (Lepidodendron, Sigillaria ) and ferns dominate swamps in
the early Carboniferous
Lycopods are spore-bearing plants, and were confined to swamps.
Some grew to about 100 ft tall and were 3 feet across at the base.
Ferns and fern-like plants were also common, including spore-bearing ferns and seed
ferns like Glossopteris.
Sphenopsids (like Calamites) were spore-bearing and similar to living horsetails or
scouring rushes.
They are often interpreted as living in moist areas, even perhaps standing water
Seed plants occur in drier regions.
For example, Cordaites were tall gymnosperms (up to 100 ft)
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At the end of the Carboniferous, the swamps dry out in a series of pulsed climatic events,
and gymnosperms come to dominate the new, less swampy!floras.
Note: Few lycopods or sphenopsids exist after the Permian. Cordaites became extinct by
the end of the Permian. (In the Permian, conifers took over the terrestrial environments.)
Carboniferous Terrestrial Invertebrates
Detritivores - land snail, millipedes, mites, and insects (roaches, earwigs)
Note: Insects first appeared in Devonian as wingless.
Wings appeared by the Late Carboniferous.
Types of wings:
Fixed wings (ex.: dragonflies, damselflies, mayflies)
Folding wings - many specializations develop
Predators - scorpions, arachnids (spiders, mites, etc.), centipedes, insects
Many kinds of spiders lived on the ground and from the late Carboniferous a millepede
with a length of more than 1.8 metres is known.
Gigantic dragonflies, with a wingspan of over 60 centimetres, flew among the tree tops.
Herbivores - insects with piercing, sucking, tearing - no clear leaf-eaters, however.
Carboniferous Tetrapods
During the Carboniferous and the Permian the basic groups of tetrapods (amphibian-like
animals) evolved that would populate the post-paleozoic world.
Recall that during the Devonian, the Tetrapods arose.
These consisted of the four-legged animals that evolved from Sarcopterygian fish, living
members of which include the coelacanth Latimeria and the lungfish Protopterus
(Africa), Lepidosiren (South America) and Neoceratodus (Australia).
The early tetrapods were, like early land plants, tied to the water by their reproductive
mechanisms.
Like most "fish" they had external fertilization in water and laid eggs in water which
developed into aquatic larvae.
The larvae metamorphosed into land-living adults. Living "amphibians" have inherited
this primitive mode of reproduction.
Amphibians-like tetrapods diversified dramatically during the Carboniferous, and came
to dominate terrestrial and aquatic habitats (see cladogram)
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Amphibian-like tetrapods:
Temnospondyls - had a rounded, crocodile-like snout.! They were diverse fish eaters in
both water and on land.!
They gave rise to the modern amphibians (frogs, toads, salamanders, etc.).
Aistopods - Legless, snake-like animals with large eyes and light skulls.!
They ate insects and fish; probably aquatic.
Nectridians - Newt-like, aquatic, medium-sized animals.!
Many had bizarre horns on the back of their heads.! They probably ate fish.
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Microsaurs - Lizard-like bodies, short teeth perhaps for puncturing arthropod cuticles.!
Both aquatic and terrestrial forms
Reptile-like amphibians
Seymoriamorphs - Large (>2m) reptile-like animals with long snouts.!
The Carboniferous forms were aquatic, fish eaters.! In the Permian, they became more
fully terrestrial.
Diadectomorphs: Large (>2m) reptile-like animals with short, deep skulls, massive limbs,
and peg-like teeth. Fully terrestrial. These animals were the first tetrapod herbivores.
The Amniotic Egg
Early tetrapods were, like early land plants, tied to the water by their reproductive
mechanisms.
Like most "fish" they had external fertilization in water and laid eggs in water which
developed into aquatic larvae.
The larvae metamorphosed into land-living adults. Living "amphibians" have inherited
this primitive mode of reproduction.
Sometime during the Carboniferous, one group of tetrapods developed the amniotic egg.
Primitively, the amniotic egg has a shell hardened by calcium carbonate which is
impermeable to water but allows gases to be transpired. The embryo lies floating in the
amniotic fluid, which is formed by the embryo.
The shell is formed by the mother.
The amniotic egg is a major shared derived character for the taxa in the Amniota, which
includes us as well as the dinosaurs.
Soon after the amniotes appeared, two distinct group are recognizable - these are the
Synapsida (which includes us) and the Sauropsida (which includes the dinosaurs).
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So what is an amniotic egg, and why do we care?
Amphibian females (like their fishy ancestors) lay a large number of eggs in water, which
the male then fertilizes externally.!
The larval amphibian emerges from the egg, and develops as a largely aquatic organism.!
Some metamorphose into a more terrestrial form, and move onto land.! Others spend their
entire life in water.
Animals with an amniotic egg have similar stages, but they all take place inside a wellprotected egg.!
They emerge from the egg into air with an adult-like form.
The key innovation is the Amniotic Egg, which contains the following parts:
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Semipermable shell (mineral/leather): allows O in and CO out, but retains water.!
It must be tough enough to support the egg in the air or sediment.
Extra-embryonic membranes:
Amnion: encloses the embryo with a fluid very similar to environmental water.
Chorion: coats the inside of shell and the yolk (nutrient source).
Allantois: encloses a cavity for solid wastes.
These packages must be fertilized before they are coated with shells and membranes, so
amniotes must have internal fertilization.
This is the second Origin of Sexual Intercourse.
The amniotic egg allowed tetrapods to breed away from water.! It opened up a vast new
landscape that they could invade.
The boundary between Amniotes and Amphibians is hard to define because:
Eggs don't fossilize well.!
"Amphibians" aren't a clade - there are no novelities that are shared only by the animals
we call Amphibians.!
This is because we have, rather arbitrarily, decided to call some descendents of
amphibians something else.!
As a consequence, the Amphibian-Amniote boudary is "fuzzy".
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Westlothiana lizziae - the first amniote, is known from the early Carboniferous.
Despite this early appearance, Amniotes don't diversify until end Carboniferous.
What are the main groups of amniotes?
The primary subdivisions of amniotes are based on "holes in the head".
Early tetrapods had a braincase enclosed by bone, as well as outer skull armor that was
separated from this braincase.!
This interior space between the braincase and outer skull is filled with musculature to
open and close the jaws.!
Some amnniotes begin to open windows in this outer skull armor and begin to expand
jaw muscles onto the outside of the head.!
It's thought that this expansion of jaw musculature allows greater diversity in chewing
and food processing.
Anapsids - no holes in head behind the eye.!
This is the primitive state for tetrapods.!
It occurs in the earliest tetrapods and is also seen in turtles, the most primitive reptiles.
Synapsids - one hole in head behind the eye.!
This is one of the evolutionary novelties in head holes.!
It occurs in pelycosaurs, therapsids, and mammals.
Diapsids - two holes in head behind the eye.!
This is another evolutionary novelty in head holes.! It occurs in nearly all reptiles except
turtles (i.e., lizards, snakes, crocodiles, pterosaurs, dinosaurs and birds).
Westlothiana lizziae had an anapsid skull.!
By the end of the Carboniferous, both synapsid and diapsid amniotes were present as
well.
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Terrestrial Ecosystems of the Permian (285 to 245 Mya)
All the continents had crashed together to form the supercontinent, Pangea.
The swamps of tropical Europe and America were drying out.
Evaporites (salt deposits) and red beds (indicating dry soils) were more common.
As in the Carboniferous, early Permian tetrapods are only known from the equatorial
zone.
Permian Plants
Drying of the swamps took a long time; spanning from Late Carboniferous through the
late Permian.
Seed plants came to dominate lowlands.
Wet areas with spore-bearing plants persisted, but were increasingly rare.
Q. How were vertebrates affected by these changes?
Permian Tetrapods
Amphibian diversity drops, but some groups persist and become better adapted for life in
the air.
The decline is tied to:
Drying of swamps. Amphibian reproduction is tied to water.
Competition and/or Predation by amniotes.
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Amniotes were free to breed and feed away from water. Drying wasn't a problem.
Synapsid amniotes (one hole) dominate landscapes in the Permian.
There were two main synapsid groups in the Permian:
Pelycosaurs, who dominated in the early Permian, and
Therapsids, descendents of pelycosaurs, who dominated in the late Permian.
Pelycosaur communities of the early Permian only occur in the tropical zone.
Carnivorous pelycosaurs: up to 2 meters and 200 kg, big heads & eyes, long snouts,
strong jaws, pointy teeth with some differentiation from front-to-back.
They ate fish & amphibians.
Some had sails.
Sprawling front & hind legs, but vertebral spines prevented lateral undulation during
locomotion.
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Herbivorous pelycosaurs: medium to large (3 m, 300 kg), small heads, blunt
undifferentiated teeth, large gut cavity, some had sails.!
The same sprawling posture as in carnivorous pelycosaurs.
Herbivory among the Tetrapods
Plant eating arose many times in tetrapods.
How does it evolve? Animals can't break down cellulose or lignin, the structural
components of plants.!
Plant feeding animals use two different strategies for dealing with this problem.
1. Be small and picky
Avoid cellulose-rich plant parts; only eat nutrient-rich plant parts (fruits & nuts).
There is only enough high-nutrient food to support small animals with this strategy.
Example: Squirrels or fruit bats.
2. Be big and indiscriminate
Eat everything that you can.
Pack your gut with microbes and let them ferment this low quality food.
Eat the microbes and the waste products they exude.
Must be large to fit the massive, fermenting-chamber gut.
Examples: Rhinos and elephants
Herbivorous pelycosaurs took the big, indiscriminate route.!
They were large, and probably evolved from large carnivorous ancestors.!
They had small heads, so cellulose digestion must have come from microbes.!
They did, in fact, have big guts.