Ediacaran and Cambriam Biota

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Ediacaran and Cambriam Biota
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• Late Proterozoic Buildup (about 570 mya)
• For all of the 19th century and the first half
  of the 20th century, the Proterozoic eon had
  failed to yield any metazoan fossils.
• Then in 1946, R.C. Sprigg, a government
  geologist assessing abandoned lead mines in
  southern Australia discovered the first remains
  of a remarkable biota that has taken its name
  from the region, the Ediacara Hills.
• Ediacaran animals are now known from many other
  regions (Mexico, California, Canada, Ireland,
  Russia, Namibia) and the story seems to be the
  same wherever they are discovered.

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Ediacaran Fauna

• They range in age from 600 to 545 Mya.
• They are preserved as impressions, typically in
  sandstone.
• With two exceptions, all the metazoans from this
  time lacked mineralized skeletons.
• Ediacaran animals could be quite large, up to 50
  cm long, but many were rather flat.

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Ediacaria
Ediacaran Fauna cont.

• They exhibit a range of symmetries some are
  radial (e.g., Ediacaria), others are bilateral
  (Spriggina).

(A), a radially symmetrical cast of Ediacaria
preserved on the underside of a sandstone bed,
Rawnsley Quartzite, South Australia.
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Spriggina
Ediacaran Fauna cont.

• Spriggina is part of a clade of soft-bodied
  organisms that are restricted to the Precambrian.
  
• Spriggina is known largely from the Ediacara
  Hills of south Australia, near Adelaide. The
  organism had a crescent-shaped head and numerous
  segments tapering to the posterior end it is
  only about three centimeters long.
• Spriggina was described as an annelid but it now
  appears to be related to the arthropods, although
  Spriggina had no hard parts, and it is unclear
  exactly what kind of appendages it had.

Spriggina
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Dickinsonia
Ediacaran Fauna cont.

• Dickinsonia is known from Vendian rocks of south
  Australia and north Russia.
• It is often considered to be an annelid worm
  because of its apparent similarity to one genus
  of extant polychaete, Spinther.
• However, in the opinion of some, it may in fact
  be a cnidarian polyp.
• This specimen is an adult one from the Ediacara
  Hills of southern Australia

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Eoporpita
Ediacaran Fauna cont.

• Eoporpita is one of the most striking Vendian
  fossils, noted for its thick tentacles
  surrounding a central body.
• This specimen is nearly 6 cm across and was found
  at the Winter Coast of the White Sea other
  specimens have come from south Australia.
• Until recently, Eoporpita was thought to be a
  chondrophorine, but some researchers now doubt
  this interpretation of Eoporpita and consider it
  to have been a benthic polyp rather like a sea
  anemone.

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Cloudina
Ediacaran Fauna cont.

• Examples of groups with mineralized skeletons
  include Cloudina and sponges

• Cloudina had a skeleton composed of a simple
  calcite tube and was probably some sort of polyp
  organism
• Sponges produce internal skeletal supports
  called spicules that have now been found in late
  Proterozoic rocks.

Cloudina
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Ediacaran Fauna cont.

• Next of Kin?
• Some of them are clearly related to jellyfish
  and other cnidarians.
• Some of the bilateral forms could be related to
  flatworms.
• The lack of any evidence for burrows that
  penetrate far into the sediments suggests that
  none of the Ediacaran animals had coeloms, so
  they probably are not related closely related to
  the other fossil-forming metazoans (mollusks,
  arthropods, etc.).

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Ediacaran Fauna cont.

• Energy harvesting?
• The forms similar to jellyfish and sea anemones
  were probably carnivores.
• Some specimens of Cloudina appear to have been
  attacked by a boring organism, so at least one
  advanced predator must have been on the scene.
• The bilateral "wormy" forms probably consumed
  organic-rich mud.
• The extremely flat organisms that appear to lack
  mouths may have had photosynthetic endosymbionts
  living in their tissues.

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Ediacaran Fauna cont.

• Summary
• Ediacaran animals disappear from the fossil
  record about 545 mya.  Many of these species may
  well have suffered at the hands of the wave of
  organisms that appeared in the Cambrian. 
• Some seem to have survived, giving rise to
  modern cnidarians and their kin. 
• Stromatolites also begin to decline after this
  point possibly due to intensified predation and
  grazing by "new" metazoans.

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Cambrian Fauna

• The modern phyla of multicellular organisms show
  up in a "flash" at the beginning of the
  Phanerozoic Eon (the start of the Paleozoic Era
  and the Cambrian Period). 
• 545 Mya marks the first appearance of complex,
  sediment-penetrating trace fossils
• Implication big animals with coeloms are on the
  scene. 
• Yet they must have been soft-bodied, as we don't
  have a good body fossil record from this
  interval.

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Cambrian Trace Fossils

• The Cambrian Tapeats Sandstone occurs in the
  Grand canyon in Arizona.
• The half a billion year old sediments represent
  tidal flat and near shore deposits during a time
  when primitive metazoans ruled the earth.
• The Tapeats contains a few trace fossils,
  include Corophioides

Corophioides. These are the dwelling traces of a
large U shaped annelid worm which lived beneath
the sediment surface. Erosion has removed the top
layer, leaving the bottom of the burrow as a dish
shaped notch. This was the single most prominent
fossil found in the Tapeats in the Grand Canyon.
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Trace Fossils cont.
Simple trace fossils of bilaterian animals,
Rawnsley Quartzite (2 cm).
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• 530 Mya small-shelly fossils begin to appear in
  the record
• The shells exhibit a range of mineralogies
  (calcite, phosphate, hard organics, etc.).
• Coiled snail shells were present, indicating
  that the Phylum Mollusca had appeared.
• Sponges spicules and the extinct sponge-like
  Phylum Archaeocyatha were present.
• Many of the shelly fossils are difficult to
  relate to any modern group.
• Recently, fossilized embryos have been reported
  from this interval as well, opening the
  possibility that it may be possible to study the
  evolution of development directly from the fossil
  record.

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Archaeocyaths

• Archaeocyaths are an extinct group of sponges
  that had a very brief history.
• The first appear roughly 530 million years ago
• Diversified into hundreds of species during this
  time period, with some species contributing to
  the creation of the first reefs.
• Despite their great success in terms of numbers,
  the archaeocyaths were a short-lived group
  completely non-existent by the middle Cambrian,
  some 10 to 15 million years after their first
  appearance.

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Fossilized embryos

• Eggs of presumed jellyfish (rear) and segmented
  worm (front). About 0.5 mm in diameter.

Phospatized animal egg and early cleavage-stage
embryo, Doushantuo Formation. (250 µm)
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Trilobita

• Trilobites were among the first of the
  arthropods probably descended from segmented
  worms, developing hard exoskeletons, eyes and the
  ability to grow by shedding their old shells. 
• Trilobites were a dominant life form during much
  of the 325 million years of the Paleozoic.

• Constitute an extinct class of arthropods, the
  Trilobita, made up of eight orders, over 150
  families, about 5000 genera, and over 15,000
  described species.
• Speculations on the ecological role of
  trilobites includes planktonic, swimming, and
  crawling forms, and we can presume they filled a
  varied set of trophic (feeding) niches, although
  perhaps mostly as detritivores, predators, or
  scavengers.

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Trilobites cont.

• Trilobites are the single most diverse group of
  extinct organisms, and within the generalized
  body plan of trilobites there was a great deal of
  diversity of size and form.
• The smallest known trilobite species is just
  under a millimeter long, while the largest
  include species from 30 to 70 cm in length

Phacops
Acanthopyge
Tricrepicephalus
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Brachiopods

• Brachiopods appeared as well about 525 mya
• Brachiopods are marine lophophorates related to
  the Bryozoa and Phoronida.
• They are filter feeders, collecting food
  particles on a ciliated organ called the
  lophophore
• Common in very cold water, either in polar
  regions or at great depths in the ocean
• There are about 300 living species of
  brachiopods.

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Brachiopods cont.

• Divided into two major groups
• Class Inarticulata (including lingulids), and
  Class Articulata based on the presence or absence
  of hinge teeth and sockets.

• Articulate brachiopod

• A lingulate brachiopod
• Morphologically conservative, having lasted
  since the Cambrian with very little change in
  shape.
• This specimen is of Lingula,a living brachiopod

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Echinoderms

• Eocrinoidea
• Eocrinoids are among the earliest groups of
  echinoderms to appear, ranging from the Early
  Cambrian to the Silurian.

• Most eocrinoids were sessile and fed with their
  long brachioles (the arm-like structures, which
  in this specimen are spirally twisted).
• The body was covered by plates in early
  eocrinoids the holdfast was also covered by
  plates, but later eocrinoids evolved a stalk with
  columnals, like crinoids and blastoids.

Gogia from the Middle Cambrian House Range of
Utah.
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Echinoderms cont.

• Helicoplacoids
• The Helicoplacoidea is a small group of fossil
  echinoderms known only from the Lower Cambrian.
• In life, they were shaped somwhat like a slender
  football or a fat cigar, and were able to extend
  or contract the length of their bodies.
• Their "skin" was covered in spirals of
  overlapping ossicles that functioned like armor
  their "mouth" was a long groove that also
  spiralled around their body.
• It is thought that helicoplacoids lived in
  burrows, extending their bodies outward to feed.
  
• The helicoplacoids are among the oldest groups
  of echinoderms to appear in the fossil record,
  along with eocrinoids

Fossil of Helicoplacus from the Lower Cambrian
strata of the White Mountains in California
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• Reefs
• The reefs of the time period (525 mya) were
  composed of archaeocyathids (extinct animals
  related to sponges)

Archaeocyatha
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• 525 to 515 Mya Burgess Shale and Similar
  Deposits
• Discovered by Walcott in 1909
• The Burgess shale formation, located high in the
  Canadian Rockies, has been producing astonishing
  array of soft-bodied and hard-bodied fossils from
  the Cambrian since its discovery. 
• Fossils from the site have been collected and
  studied by a number of groups, and new
  Burgess-type localities have been discovered
  along the west coast of North America, and in
  eastern North America, Greenland, and China. 

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• Some of the Burgess animals fit nicely into
  extant phyla of soft-bodied (and shelly) animals
  such as
• Sponges
• Annelid worms
• Priapulid worms
• Arthropods (though some of the arthropods, such
  as Marella, don't look like any living arthropod)

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Cambrian Sponges

• The branching sponge Vauxia was one of the most
  common sponges encountered by Wolcott in the
  Burgess Shale

Vauxia spp.
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Cambrian Annelids

• Canadia spinosa, a polychaete annelid about 1 to
  2 inches in length
• The head bore a pair of slender tentacles while
  the body was covered with innumerable setae
  (short bristles).
• The gut could be everted anteriorly to form a
  feeding proboscis.

• Canadia could use its limbs to walk on the
  substrate or swim just above it.
• Sediment has never been found in the gut,
  suggesting that this worm may have been a
  carnivore or scavenger.

Canadia
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• Burgessochaeta
• A polychaete worm related to Canadia

Burgessochaeta
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Cambrian Priapulids

• Ottoia prolifica, probably lived in a U-shaped
  burrow that was constructed in the substrate.

• Note the anterior proboscis (on the left) and
  the dark trace of the interior digestive tract.
• Ottoia was probably carnivorous.

Ottoia
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Cambrian Arthropods

• A small "arthropod" somewhat reminiscent of a
  trilobite, but with several distinctive features
• Called the "lace crab" by Walcott, Marrella is
  the most abundant type of aminal in the Burgess
  Shale.
• More than 15,000 have been collected.

Marrella splendens
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Uncertain Fossils? - Wiwaxia

• Wiwaxia is believed to be closely related to
  either polychaetes, but is not thought to be a
  member of the group
• Longer spines project in two rows along the back,
  and evidently provided some protection from
  predators.
• The rest of the dorsal surface is covered with
  small, flat, overlapping hard plates, termed
  sclerites.
• Each of these little scales was attached with a
  root-like base and we assume Wiwaxia grew by
  molting these plates from time to time.
• It did have an anterior jaw with two rows of
  teeth on the ventral surface, suggesting it was
  another bottom feeder.

Wiwaxia
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Uncertain Phyla? - Anomalocaris

• Large creatures (60 cm long)
• Long, oval-shaped head, large eyes, feeding
  appendages at the front that look like combs, and
  a circular mouth underneath.
• Behind the headis a "trunk" withlobes
  underneath it in pairs for swimming.
• No legs or walking appendages probably swam all
  the time.
• The mouth was located on the front end of the
  head, underneath round and cylindrical, with
  many tiny teeth facing inward.

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Uncertain Fossils? - Dinomischus

• Dinomischus was a wine glass shaped animal,
  measuring about one inch long. It had a bulb at
  the base of its stem to secure it in the mud.
• On its circular upper surface, surrounded by
  petal-like bracts, was a mouth and an anus.

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Uncertain Phyla? - Opabinia

• A creature with five eyes and a long flexible
  proboscis tipped with grasping spines
• It also possesses paddle-like projections at the
  posterior end of the body.
• Opabinia is thought to have lived in the soft
  sediment on the seabed, although it presumably
  could have swum after prey using its side lobes.
• Superficially, Opabinia resembles a crustacean,
  but lacks important, distinguishing details.
• It remains unassigned to any other extinct or
  currently living, major group.

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Uncertain Phyla? - Aysheaia

• Possesses an unusual assembly of spines and
  grasping arms at the head end. Its mouth lies in
  the center of a ring of six finger-like
  projections.
• The limbs of this animal are not jointed
  instead, they are tapered, lobe-like appendages,
  ten pairs in all
• Aysheaia may have been a parasite living on
  sponges since it is commonly found in association
  with their remains (spicules).
• Presumably, the spiny parts at its head were
  designed for grasping and feeding on its prey.

Aysheaia
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Uncertain Phyla? - Hallucigenia
Old interpretation
New interpretation

• Note the paired spines (now interpreted to stick
  up on the dorsal side - or back), and the
  slightly curved legs caterpillar-like.
• Thought to related to the onychophorans or
  "velvet worms

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Cambrian Chordates - Pikaia
It is believed to be one of the earliest known
representatives of the phylum Chordata It has a
well defined notochord near the dorsal
surface. Also possesses rib-like features which
are believed to be muscles. Pikaia probably swam
above the seafloor using its body and an expanded
tail fin.
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Explanations for the Cambrian Explosion A.
Environmental Explanations Ocean
Chemistry Change in ocean chemistry allowing
shells. Soft-bodied Bias An apparent explosion,
due to the strong bias against finding
soft-bodied forms Change in Oxygen Sudden
oxygen buildup that allows big bodies and perhaps
skeletons.  Maybe.  There is strong geochemical
evidence for higher oxygen levels at the
Proterozoic-Cambrian boundary. 
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• A. Environmental Explanations cont.
• Positions of Continents
• Between 750 and 570 mya, the continents were
  grouped toward the South Pole and there were
  several episodes of continental glaciation
  effecting many areas of the world
• However, this was soon followed by movement of
  the continents away from one another

• This resulted in higher temperatures and an
  increase in coastline and continental shelf,
  augmenting the right conditions for marine life

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• B. Biotic Explanations
• Arms Race
• The rapid diversification may relate to a
  predator/prey arms race. 
• Predation would certainly favor animals with
  skeletons.
• Hox Genes
• Evidence from development biology indicates that
  the rapid development of complex body plans, with
  many distinct cell types and anatomical
  structures can occur through the action of Hox
  genes

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Hox Genes Hox genes evolved from a more inclusive
group of genes - the homeobox genes - coding for
specific proteins that activate other genes Hox
genes are uniquely arranged in a linear sequence
along the chromosome, which coresponds with both
the linear and the temporal sequence of their
activation along the anterio-posterior axis of
the embryo
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Hox Genes cont.

• The number of Hox genes arranged in a cluster
  along a chromosome is broadly comparable to the
  degree of complexity of the organism

• One in sponges, 4 to 5 in cnidarians, 6-10 in
  most of the higher metazoa, and up to 39 arrayed
  in 4 Hox clusters on different chromosomes in
  mammals

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Hox Genes cont.

• These genes control the position and the
  expression of major structural features of the
  body, including the elements of the head and the
  sequence and nature of the appendages
• Hox genes act as switches to control the
  expression of a variety of genes, which in turn
  control different structures and cell types
• The origin of multicellularity and complex body
  plans among animals was a unique phenomenon,
  dependent on the evolution of Hox genes near the
  end of the Precambrian
• Once evolved, their subsequent duplication and
  divergent change in adaptively distinct lineages
  established the basis for the radiation of the
  many metazoan phyla

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Hox Genes cont.

• Most phyla have apparently retained a relatively
  constant number of Hox genes since the Cambrian
• Also, we can recognize a hierarchy of change
  associated with Hox genes between and within
  phyla

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Cambrian Fauna and Ecosystem Function