The Early Part of the Paleozoic Era

Dr. Allen

Paleozoic era

Cambrian period (~ 541 - 485 million years ago)

Four major continents (after Rodinia broke up):

• Gondwanaland/ Gondwana (Southern continent)

• Laurentia (North America, Greenland)

• Baltica (Scandinavia)

• Siberia

Oceans:

• Panthalassa

• Lapetus

Late Cambrian ~ 514 million years ago

Cambrian Explosion

Appearance of major animal bauplans (body plans) in the fossil record

Metazoans:

• Multicellular and eukaryotic

• In Kingdom Animalia

Primary germ layers:

• Diplobastic vs Triploblastic

Symmetry:

• Radial vs Bilateral

• Developmental biology

• Biomineralized skeletons

• Complex food webs and ecology

• Predator-Prey arms races

Animal Body Plans

Porifera

• Sponges

Cnidaria

• Jellyfish

Platyhelminthes

• Flatworms

Rotifera

• Rotifers

Nematoda

• Round worms

Mollusca

• Mollusk

Annelida

• Segmented worms

Arthropoda

• Insects, Crustaceans

Echinodermata

• Crinoids, Sea urchins

Chordata

• Vertebrates

Burgess shale

~ 509 million years ago (Middle Cambrian) on Laurentia

This drawing depicts a recontruction of the acient reef (Cathedral Escarpment) that hosted the Burgess Shale animals

Superimposed are the quarry site, the town of Field, and the surrounding mountains we see today

The animals lived in the ocean, but today are found on a mountain ridge between Wapta Mountain and Mt. Field

Beginning ~ 175 million years ago, mountain-building forces moved the fossils from their burial grounds on the sea floor eastward on huge faults to their current positions

Burgess shale site

Named after the Burgess Pass in Yoho National Park in the Canadian Rockies, British Columbira, Canada

Black shale formation extends for many miles

Konservat Lagerstatten

Discovered in 1909 by Charles Dolittle Walcott

• Excavated ~ 65,000 specimens (now in the Smithsonian)

Significance of the finds wasn’t realized at the time of discovery

Walcott simply placed the unusual new species within the phyla known to exist during that period, a process Stephen Jay Gould dubbed “shoehorning” (lumping) in his book about the Burgess Shale, Wonderful Life (1989)

Some of the fossils represent animals with no known descendants and that are difficult to classify

• Opabinia regalis

• Anomalocaris canadensis

The Paleo Dive Game features some Cambrian life

Anomalocaris, complete specimen

Disarticulated parts of Anomalocaris were originally assigned to three different animal phyla

Frontal appendages

• Thought to be shrimp-like (Arthropoda)

Circular mouth parts

• Thought to be a jellyfish (Cnidaria)

Main body

• Thought to be a see cucumber (Echinodermata)

In 1978 Simon Conway Morris reexamined some of the specimens

In the 1980’s Harry Whittington and Derek Briggs uncovered intact specimens of Anomalocaris showing all three features connected as parts of an early stem-group arthropod (Radiodonta)

Interesting Burgess Shale Fauna

Haplophrentis carinatus

• one-inch fossil

• extinct mollusk with an operculum

Marella splended

• Ancestrial anthropod

Hallucigenia sparsa

• A panarthopod, like a velvet worm

Pikaia gracilens

• an early chordate

Metaspriggnia walcotti

• an early chordate

Punctuated Equilibrium

Stephen Jay Gould suggested that the great diversity of fossils indicate that life at the time was much more diverse than that surviving today and that many of the unique lineages were evolutionary experiments that became extict

Burgess shale fauna has been used in support of evolution by punctuated equilibrium rather then gradualism (also called phyletic gradualism)

However, much of the radiation in animal life can be traced back to the Precambrian

Radical shift in abundance of hard parts and increase in preservation potential

Chengjiang, China

Another important Lagerstatten of the Cambrian, Yunnan province, China

Maotianshan shales of the Early Cambrian (518 -520 million years ago)

Mostly soft-bodied organisms

Shares ~15% of genera and ~85% of phyla with Burgess Shale

Trilobites as Index Fossils for the Cambrian

Late Cambrain

• Monkaspis

Middle Cambrain

• Paradoxides

Early Cambrain

• Olenellus

Post-Cambrian Life

Cambrian-Ordovician extinction event at ~485 million years ago

Wiped out 10% to 50% of global species such as trilobites and brachiopods, reducing their diversity and abundance

Jawless fish (aganthans) rose in dominance, including armored types called ostracoderms and eel-life conodonts

Conodonts were known for their hard mineralized tooth-like “conodont elements”

• Archeognathus primus

• Manticolepis subrecta

Ostracoderms

Armored heads

Round or slit-like mouths

No paired fins

Horseshoe Crabs

Lunataspis is a stem xiphosuran

Limulus polypheus (extant)

Eurypterids

“Sea urchins”

Gigantism in Paleozoic arthropods is well documented

Large dragonfly-like flying forms and giant millipedes

High oxygen levels in the atmosphere

Gigantism is also seen in trilobites of the Ordovician and eurypterids of the Devonian

Decline of Eurypterids due to Competition with Vertebrates

Suborder Stylonurina

• Benthic scavenger, sweeping feeding

Suborder Eurypterina

• Active, nektonic predatory

“Mass-moult-mate” Hypothesis of Eurypterids

Proposed: Mass migrations of Eurypterus in the style of extant horseshoe crab (Limulus)

Moults vs mortalities: Disarticulation patterns support moulted appendaged

Salinity tolerance cannot be tested

Findings didn’t fit with prediction of male dominance in gender ratio (compared to Limulus)

Based on the results, a revised mass-mate-spawn-moult hypothesis was proposed:

• Following mating, males left the breeding area to return to deeper waters (where they moulted), whilst females remained behind to fertilize their eggs

Questions remain about whether gender ratio was interpreted correctly