Chordates Part 1: anatomy,
diversity & evolution
Intro: anatomy - what is a chordate?
Amphioxus, the lancelet (Cephalochordata) is the most primitive living
chordate. There are fundamental similarities between Amphioxus and all
other chordates – these 5 key features are the chordate body plan, which
is evidence of common ancestry.
These features are homologous – same feature in different animals,
sometimes in very different forms. The concepts of homology were
worked out by British anatomist Richard Owen (identified notochord),
also a creationist. Darwin figured out what these features meant –
common ancestry.
1. NOTOCHORD:
A stiffened rod of tissue between the nerve chord and digestive tract.
Provides rigid support for the back – gives muscles something to pull
against whilst swimming
Lost in adult vertebrates and tunicates, but present in all chordate
embryos (including humans)
Vertebral skeleton of vertebrates is formed by bone segments developed
around the notochord – it is the scaffold for the assembly of backbone
2. NERVE CORD:
Nervous system has a hollow, central cord, formed when the ectoderm
rolls up to form a tube (neurulation)
In vertebrates, nerve cord was elaborated to form the spinal cord
The front end of the nerve cord is even more elaborated and enlarged:
the brain. Hollow space (vesicle) inside the brain corresponds to the
hollow inside of the nerve tube.
3. MUSCLE BANDS
Muscles are arranged in overlapping segments in Amphioxus
This is retained in vertebrates – vertebrate and ribs correspond to muscle
bands: one vertebra/pair of ribs per muscle band
Chordate bands are homologous and evidence of common ancestery,
similar segments of worms and arthropods are analogous and convergent
(primitive relatives of chordates, worms and arthropods lack segments –
they all invented them independently)
Muscle segments in Homo sapiens: segmental muscles (serratus) down
the back correspond to the segments seen in lancelet and fish – although
highly modified, we retain the basic segmental structure seen in primitive
chordates.
4. PHARYNGEAL SLITS
Slits that create an opening between the throat (pharynx) and the outside
of the body, allowing water to pass through.
Retained in vertebrates – best seen in animals like lampreys and sharks,
which have many well-developed gill slits. Gill slits are present in
embryonic humans, but lost in the adult.
5. POST-ANAL TAIL
, In most animals (worms, arthropods, echinoderms, hemichordates) the
anus is located at the end of the body. In chordates, the body projects
past the anus – a tail (used for swimming).
Retained in more advanced vertebrates, even in humans and other apes –
but reduced to a tiny set of fused vertebrae that don’t project outside of
the body: the coccyx, or tailbone.
Diversity: a brief survey of the group
Most specialised and complex group – gnathostomes – are the most
diverse. Parallels other groups, like arthropods, where specialised,
complex members are more diverse than less complex forms.
Lancelets – 22 species, Lampreys – 40 species, Hagfish – 60 species,
Tunicates – 2000 species, Jawed fish – 60,000 species
LARGE SIZE:
Chordates are relatively large – smallest fish and frogs are a little under a
centimetre in length. Largest wales grow to 100 feet and 200 tonnes in
weight, and some dinosaurs approached this size.
As big things usually eat smaller things, chordates often sit at the top of
the food chain.
DISPARITY:
Diversity = number of species, Disparity = number of forms
Chordate diversity (60,000 species) is low compared to arthropods
(millions), but disparity is high – many body forms, feeding and
locomotion strategies, ecological niches
DESCENT WITH MODIFICATION
To produce this diversity and disparity, chordates have modified the basic
chordate body plan by gradually adding new features (e.g. brains, eyes,
jaws, limbs, lungs) onto a lancelet-like body plan and by eliminating old
ones (reducing gill slits and tail humans)
But the basic chordate features are always retained in the embryo – this
is because evolution works by slowly tweaking existing designs:
incremental.
Origins: relationships of chordates
DEUTEROSTOMES:
Bilaterally symmetrical animals (Bilateria) can be divided into 2 major
groups: Protostomia (‘mouth first’) and Deuterostomia (‘mouth second’)
In both groups, the embryo starts as a hollow ball of cells (blastula) and
then one end gets pushed in (gastrulation)
In Protostomes, this pocket becomes a mouth and an anus is formed
after. In Deuterostomes, this pocket becomes the anus and then a mouth
forms.
Protostomes include Annelida, Mollusca, Platyhlminthes, Tardigrada,
Nematoda, Arthropoda, Brachipoda, Chaetognatha, Rotifera
diversity & evolution
Intro: anatomy - what is a chordate?
Amphioxus, the lancelet (Cephalochordata) is the most primitive living
chordate. There are fundamental similarities between Amphioxus and all
other chordates – these 5 key features are the chordate body plan, which
is evidence of common ancestry.
These features are homologous – same feature in different animals,
sometimes in very different forms. The concepts of homology were
worked out by British anatomist Richard Owen (identified notochord),
also a creationist. Darwin figured out what these features meant –
common ancestry.
1. NOTOCHORD:
A stiffened rod of tissue between the nerve chord and digestive tract.
Provides rigid support for the back – gives muscles something to pull
against whilst swimming
Lost in adult vertebrates and tunicates, but present in all chordate
embryos (including humans)
Vertebral skeleton of vertebrates is formed by bone segments developed
around the notochord – it is the scaffold for the assembly of backbone
2. NERVE CORD:
Nervous system has a hollow, central cord, formed when the ectoderm
rolls up to form a tube (neurulation)
In vertebrates, nerve cord was elaborated to form the spinal cord
The front end of the nerve cord is even more elaborated and enlarged:
the brain. Hollow space (vesicle) inside the brain corresponds to the
hollow inside of the nerve tube.
3. MUSCLE BANDS
Muscles are arranged in overlapping segments in Amphioxus
This is retained in vertebrates – vertebrate and ribs correspond to muscle
bands: one vertebra/pair of ribs per muscle band
Chordate bands are homologous and evidence of common ancestery,
similar segments of worms and arthropods are analogous and convergent
(primitive relatives of chordates, worms and arthropods lack segments –
they all invented them independently)
Muscle segments in Homo sapiens: segmental muscles (serratus) down
the back correspond to the segments seen in lancelet and fish – although
highly modified, we retain the basic segmental structure seen in primitive
chordates.
4. PHARYNGEAL SLITS
Slits that create an opening between the throat (pharynx) and the outside
of the body, allowing water to pass through.
Retained in vertebrates – best seen in animals like lampreys and sharks,
which have many well-developed gill slits. Gill slits are present in
embryonic humans, but lost in the adult.
5. POST-ANAL TAIL
, In most animals (worms, arthropods, echinoderms, hemichordates) the
anus is located at the end of the body. In chordates, the body projects
past the anus – a tail (used for swimming).
Retained in more advanced vertebrates, even in humans and other apes –
but reduced to a tiny set of fused vertebrae that don’t project outside of
the body: the coccyx, or tailbone.
Diversity: a brief survey of the group
Most specialised and complex group – gnathostomes – are the most
diverse. Parallels other groups, like arthropods, where specialised,
complex members are more diverse than less complex forms.
Lancelets – 22 species, Lampreys – 40 species, Hagfish – 60 species,
Tunicates – 2000 species, Jawed fish – 60,000 species
LARGE SIZE:
Chordates are relatively large – smallest fish and frogs are a little under a
centimetre in length. Largest wales grow to 100 feet and 200 tonnes in
weight, and some dinosaurs approached this size.
As big things usually eat smaller things, chordates often sit at the top of
the food chain.
DISPARITY:
Diversity = number of species, Disparity = number of forms
Chordate diversity (60,000 species) is low compared to arthropods
(millions), but disparity is high – many body forms, feeding and
locomotion strategies, ecological niches
DESCENT WITH MODIFICATION
To produce this diversity and disparity, chordates have modified the basic
chordate body plan by gradually adding new features (e.g. brains, eyes,
jaws, limbs, lungs) onto a lancelet-like body plan and by eliminating old
ones (reducing gill slits and tail humans)
But the basic chordate features are always retained in the embryo – this
is because evolution works by slowly tweaking existing designs:
incremental.
Origins: relationships of chordates
DEUTEROSTOMES:
Bilaterally symmetrical animals (Bilateria) can be divided into 2 major
groups: Protostomia (‘mouth first’) and Deuterostomia (‘mouth second’)
In both groups, the embryo starts as a hollow ball of cells (blastula) and
then one end gets pushed in (gastrulation)
In Protostomes, this pocket becomes a mouth and an anus is formed
after. In Deuterostomes, this pocket becomes the anus and then a mouth
forms.
Protostomes include Annelida, Mollusca, Platyhlminthes, Tardigrada,
Nematoda, Arthropoda, Brachipoda, Chaetognatha, Rotifera
