Macroevolutionary Patterns Ch. 2 & 18 pg. 689-717 / 46-48 / 61-65
How Organic Remains Fossilize Life Through Time:
Fossils are traces left by organisms of the past… There are Geological time scale is a hierarchy divided into eons,
4 general categories of formation: eras, periods, epochs and stages.
1. Compression: Radioisotopes and accurate dating techniques allowed
Organic material buried in sediment before absolute time intervals to be assigned.
decomposition leaving an impression in material
below, from weight of the sediment. Phanerozoic era beings with the Cambrian Explosion and
ends in the present.
2. Cast & Model:
Remains decay after being buried in sediment.
3 Component Eras:
Molds – unfilled spaces
Cast – filled with new material and hardened into 1. Paleozoic (Ancient Life):
rock Begins with radiation of animals, ends with mass
These fossils preserve info about external and extinction at the end of the Permian.
internal surfaces 2. Mesozoic (Middle Life):
3. Permineralized: “Age of Reptiles” beginning at the end of the
Structures buried in sediment and dissolved Permian extinction and ends at the KT boundary
materials precipitate into the cell. with the extinction of the dinosaurs
These fossils preserve internal structure detail 3. Cenozoic (Recent Life):
4. Unaltered Remains: “Age of Mammals” consisting of Tertiary Period
Preserved in environments that discourage loss (5 epochs) and Quaternary Period (2 epochs)
from weathering. Almost all animal phyla currently recognized appeared in
These fossils represent a small fraction of the
fossil record, found in peat bogs, permafrost… etc fossils ~40 MYA during the Cambrian Period.
The fossilization process requires:
2. Burial – Water saturated sediments
3. Lack of oxygen
Strengths and Weaknesses of Fossil Record:
Fossilization results in 3 types of sampling bias:
1. Geographic – Propensity of fossils to come from
lowland and marine habitats
2. Taxonomic – Marine organisms dominate fossil
record but make up 10% of extant species
3. Temporal – Earth’s crust is being recycled, so old
rocks are rarer than new. States that the ability to
sample life forms should decline with time. Macroevolutionary Patterns Ch. 2 & 18 pg. 689-717 / 46-48 / 61-65
Ediacaran Fauna Phylogeny and Morphology
First evidence of animals in fossils from these fauna were Lineages found at the base of the tree (Fig 18.12, 699)
places around the end of the Proterozoic era (early life) predominate in the Ediacaran Fauna.
Most are impression/compression fossil types without More heavily derived groups were found in the Burgess
shells or hard parts Shale Fauna.
Generally small in size and simple morphologically. Diploblasts / Triploblasts:
Have asymmetrical or radial symmetry. Cnidaria/Ctenophora are Diploblasts, 2 embryonic tissue
types. Remaining animals are Triploblasts.
Fossilized embryos support hypothesis that bilaterians
evolved pre-Cambrian, coming from the Doushantou Tissue types in Diploblasts:
formation. 1. Ectoderm, Produce adult skin and nervous system
In Doushantou rocks, phosphate minerals replaces the 2. Endoderm, Produce gut and associated organs
soft tissue leaving fine anatomical details.
Triploblasts have a unique “Mesoderm” which develop
Some Ediacaran specimen qualifying as fossils are actually into gonads, heart, muscle, connective tissues and blood.
“Trace Fossils” … remnants of burrows, fecal pellets,
animal tracks or other traces. The origin of diplo/triploblastsa re important because
they made it possible for muscle-lined, fluid-filled cavities
Burgess Shale Fauna: to evolve.
Variety of large, complex and bilaterally symmetric forms. Protosomes/Deuterosomes:
Most species rich lineages of present day animals found. Animals evolved 2 basic methods for producing a
Primarily compression/impression fossils with great detail multicellular body with 3 embryonic tissue types.
but little overlap with Ediacaran Fauna. Bilaterians consist of proto/deuterosomes that differ in
how they undergo gastrulation; a mass movement of
Many fossils referred to as Problematica because they
represent unique phyla that under further study were cells that rearranges the embryonic cells after cleavage,
defining the three embryonic tissue types.
close relatives of living phyla.
Protosomes gastrulation forms the mouth first,
Number of phyla existant during the Cambrian is now
Deuterosomes forms the anal region first.
recognized as an observed diversity today.
Lophotrochzoans / Ecdysozoans:
Burgess Shale Fauna consist of major morphological
innovations including large body size, segmented parts, Protosomes consist of 2 major groups.
limbs, antennae, shells, external skeletons, etc. Ecdysozoans are molting animals whereas
Earliest members of all major animal lineages were found lophotrochzoans have a lophophore, a feeding structure
in this fossil record at the same time in geographically
They’re both evolutionary innovations that appeared in
the Cambrian Explosion represented by the Burgess Shale
Fauna. Macroevolutionary Patterns Ch. 2 & 18 pg. 689-717 / 46-48 / 61-65
AR is when a small group of ancestral species rapidly
Was the Cambrian Explosion Really Explosive?
diversifies into a large number of descendant species
Although many phyla and morphological innovations first occupying a variety of ecological niches.