Rise of Animals 7

Question 1

How are vertebrates seen in the Cambrian explosion?

Answer 1

There are basal chordates that are known from the Chengjiang and Burgess shale Faunas such as the lancelet like Pikaia
There was also a fossil vertebrate named Haikouichthys was found in the Chengjiang site dated at 515MYA, prior to this the oldest vertebrates known were from the Ordovician period in 475 MYA

Question 2

What are the features of Pikaia?

Answer 2

These are animals which lacked bine or cartilage, paired fins and Jaws so they are not true vertebrates however like chordates they do possess the stiff notochord, dorsal nerve cord, zig-zag myotomes and gill slits

Question 3

What are the features of Haikouichthys?

Answer 3

This appears similar to lampreys in form having eyes and myotomes
This is a crown group vertebrate, possible the sister group to all vertebrates except hagfish

Question 4

What are Lampreys?

Answer 4

These are members of the agnathans a paraphyletic group of jawless fishes that were once very diverse

Question 5

What is the relationship between lampreys and hagfish?

Answer 5

Lamprey are not the most basal vertebrates known instead this is hagfish which are believed to be the sister group to the rest of the vertebrates on the basis of morphology and some molecular phylogenies
This means there must be fossil vertebrates older than Haikouichthys waiting to be found
Hagfish and lampreys also both lack bone which first developed in the Conodonts

Question 6

What are the Conodonts?

Answer 6

These were eel-shaped predators with large eyes and a mouth with toothlike plates but they lacked true jaws

Question 7

What was involved in the basal chordates to jawless fishes and from jawless fish to vertebrates?

Answer 7

Dupilcations of the characteristic bilaterian hox cluster resulting in plampreys having paired hox clusters and vertebrates with jaws haiving 4 hox clusters

Question 8

What are fishes?

Answer 8

These are a paraphyletic assemblage as they gave rise to the tetrapods

Question 9

What re the three clades that represent jawed vertebrates or gnathostomes today?

Answer 9

Chondrichthyes (sharks, rays and chimaeras)
Actinopterygii (Ray finned fishes)
Sarcopterygii (lobefinned fishes)

Question 10

What is the relationship between lobe finned fishes and ray finned fishes?

Answer 10

They are thought to share a common ancestor (due to their shared characteristics of a bony skeleton, gill cover and fins supported by bony rays) causing them to be grouped together in the Osteichthyes

Question 11

What are ray finned fishes?

Answer 11

These are the largest group of living fishes with at least 23,000 species they have fins supported only by dermal fin rays while the fins of lobe finned fish are supported by basal bones today these are represented by only 8 species of fish

Question 12

Where did the earliest four footed animals descend from?

Answer 12

A paraphyletic group of lobe-finned fishes called the osteolepiforms with several clades of this group independently developing tetrapod-like features such as limb like fins

Question 13

What is the oldest known true tetrapod?

Answer 13

Acanthostega from the late Devonian period about 360 MYA

Question 14

Whys is it though that limbs first evolved?

Answer 14

It was initially thought that limbs evolved to allow lobe-finned fishes to move between pools of water, however an examination of Acanthostega morphology makes this appear to be untrue and it is more likely that early tetrapods were fully aquatic animals and instead they evolved shorter and stronger appendages to move around in shallow, vegetation-choked bodies of water with the change in function from exclusively paddling to a combination of paddling and walking along the bottom required many of the changes necessary to support locomotion on land

Question 15

How does an examination of Acanthostega suggest that limbs were not initially evolved to allow movement between different pools of water?

Answer 15

the morphology of the basal tetrapod Acanthostega suggests that it was adapted for life in water with the structure of the forelimb being such that it could not have flexed from the elbow to lie in a load bearing posture and thus was probably held horizontally, the pelvis was also only loosely attached to the spine and would not be able to support the hind limbs in an upright position
Acanthostega also had true caudal fins with fin rays, features which are a hinderence on land as well as fully functional internal gills

Question 16

How can the transition from fish fin to amphibian limbe be explained in terms of changes in hox gene expression?

Answer 16

In the first bony fishes hox genes controlled the formation of a limb axis along which skeletal bones formed, at the end of these bones was a ridge from which fin rays developed which was controlled by a separate set of regulatory genes, bony fishes diverged into two clades in the ray finned fish the hox genes controlling the limb axis switched off earlier while the genes in the ridge turned on earlier producing a short fin axis and elongated fin rays
In the lobe finned fish hox gene expression was prolonged producing a longer limb axis and a bigger set of skeletal bones while gene expression it the ridge was delayed so that the fin rays became a fringe surrounding the internal fin skeleton
While in the osteolepiform fishes the ridge stopped growth all together so that no fin rays developed at all while the limb axis bent around the ridge at the end of the limb so that the developing skeletal bones became fingers

Question 17

What were thought to be the unique characteristics found in birds and why are these no longer seen as characteristic?

Answer 17

Feathers, hollow bones, a wishbone, a three fingered hand, a sideways flaxing wirst and back-turned pubic bone
Whilethese were previously thought to be unique many fo these characterisitcs have been identifiec in fossils of certain therapod dinosaurs

Question 18

What is the oldest known bird?

Answer 18

Archaeopteryx form the late Jurrasic (150 MYA) found in Limestome in Germany

Question 19

What is the bird clade defined as?

Answer 19

The common ancestor Archaeopteryx and living birds plus all of its descendants
This common ancestor retains ancestral features lost in later birds such as a hand with three clawed fingers, a long tail and teeth it was recognized as a bird mainly because it had feathers on the wing and tail featehrs have since been seen as a quintessential bird character

Question 20

What is Sinosauropteryx?

Answer 20

This is a small Chinese dinosaur that had afringe of hair-like or down-like structures along its neck and back
It is closely related to another small dinosaur Compsognathus which is found it the same deposits as archaeopteryx
These animals are both therapods aclade of bipedal, predatory dinosaurs that share a unique joint between the skull and neck

Question 21

What was Protoarchaeopteryx?

Answer 21

A dinosaur which has down like feathers on its body and tail and vaned, barbed, symmetrical feathers at the end of its tail

Question 22

What are the features of Caudipteryx?

Answer 22

This dinosaur has vaned, barbed feathers attached to the hand, although its arms are much shorter than birds and therefore it could not fly, it also had a tail fan composed of symmetrical feathers and down like feathers

Question 23

What were the features of Beipiaosaurus?

Answer 23

This was a larger animals with a length of just over 2m and had a covering of downy filaments up to 70mm ling over its arms and legs

Question 24

What were the features of Sinornithosaurus?

Answer 24

This dinosaur was covered by a layer of down-like filament but seems to have lacked vaned, barbed feathers
This dinosaur belongs to the dromaeosaurids a group that includes velociraptor

Question 25

What does a phylogenetic analysis based on morphological characteristics reveal about the relationship between birds and therapod dinosaurs?

Answer 25

This nests the birds inside the therapod dinosaurs showing that therapod dinosaurs and dinosaurs as a whole are paraphyletic unless they include the birds making birds a clade of dinosaurs

Question 26

What characteristics of birds evolved in dinosaurs?

Answer 26

Like all dinosaurs birds hold their hind limbs vertically under the body
birds share hollow bones with the therapods
the three fingered hand is shared by the clade known as the tetanurans
the wishbone is shared with the Coelurosaurs which included the giant T rex
the sideways-flexing wrist and back turned pubic bone are shared by the maniraptorans a group which comprises the common ancestor of dromaeosaurids
feathers appear to have evolved well back in the therapod tree as they occur in therapods that are not closely related to birds

Question 27

Why did feathers evolve?

Answer 27

These did not evolve for flight but instead perhaps evolved to keep small therapod dinosaurs warm or as display structures
There is also fossil evidence to suggest that the lineage that led to T rex had fetahers making it quite possible that juvenile tyrannosaurs had down feathers (no evidence has been found in adults)
Three fossils of the ostrich dinosaur Ornithomimus edmontonicus from the upper cretaceous show a filamentous covering and the presence of a wing like structure in adults suggesting that the elongated feathers in adults were display structures