Evolution -- Taxonomy -- August 20, 2024

To be added to Sophia's "15-minute binder."

Taxonomy.

I was curious, again, on the evolution and taxonomy of reptiles and dinosaurs.

We'll sort that out later but it got me to thinking about how to "teach" kids about evolution, how to help them remember how things happened. Or something along that line.

A reminder: phylum vs clade.

• phylum: taxonomy based on body plan
• clade: on tree of life based on common ancestor

Occasionally one will run into the word, paraphylum.

• Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic with respect to the excluded subgroups. In contrast, a monophyletic grouping (a clade) includes a common ancestor and all of its descendants.

What is important to animals? What is important to animals may be one way to "track" animals.

So, what's important to animals?

• breathing: obtaining oxygen:
• first in water; then on land
• homeostasis:
• maintain temperature
• maintain pH for optimization of enzymes
• movement:
• necessary to evade predators
• not necessary to obtain food, but very helpful
• pass on genes / reproduction / fertilized eggs can't dry out
• water environment --> dry land
  
• waste removal 
• seems not to be a big deal

So, with chordata -- jaws -- feeding: link here.

Most vertebrates, including humans, evolved from jawless fish which roamed the oceans 420–390 million years ago. Acquiring jaws allowed our ancestors to bite and chew, expanding the range of food they could eat and where they could live. Understanding how this mouth structure arose is therefore a central question in evolution.

Studies in lampreys and hagfish, the only species of jawless fish that still exist today, suggest that the jaw evolved from a pre-existing skeletal system surrounding the mouth and throat that was used for filtering food and breathing. A key step in this process was the acquisition of a mobile joint, essentially a skeletal hinge that can open and close the mouth. 

For this to happen, cells within the jaw skeleton – most likely cartilage cells – had to alter their gene expression to become more flexible. Such changes often involve enhancers, regions of DNA that control when a nearby gene is expressed, and in which part of the body.

Evolution of fish: wiki.

During the Devonian period a great increase in fish variety occurred, especially among the ostracoderms and placoderms, and also among the lobe-finned fish and early sharks.

This has led to the Devonian being known as the age of fishes. It was from the lobe-finned fish that the tetrapods evolved, the four-limbed vertebrates, represented today by amphibians, reptiles, mammals, and birds.

Transitional tetrapods first appeared during the early Devonian, and by the late Devonian the first tetrapods appeared.

The diversity of jawed vertebrates may indicate the evolutionary advantage of a jawed mouth; but it is unclear if the advantage of a hinged jaw is greater biting force, improved respiration, or a combination of factors. 

Chordates -- phylum Chordata

Three subphyla:

• vertebrates: fish, amphibians, reptiles, birds, and mammals
• chordates with a skull
• tunicata or urochordata: sea squirts, salps, larvaceans
• cephalochordata: lancelets -- resemble fish but no gills

Fish: jawless vs jawed

• benefit: survival among competitors

Amphibian: movement to land

• benefit: not so much survival among competitors, but survival period as water biomes dried up
• the earliest amphibians evolved in the Devonian period from tetrapodomorph sarcopterygians (lobe-finned fish with articulated limb-like fins) that evolved primitive lungs, which were helpful in adapting to dry land. 
• but then a problem: eggs dried out on land

reptiles: solved the "egg-problem"

• reptiles have an extremely diverse evolutionary history that has led to biological successes, such as dinosaurs, pterosaurs, plesiosaurs, mosasaurs, and ichthyosaurs.
• but interestingly enough, once reptiles appeared, they quickly divided into two groups: synapsids (mammals) and sauropsids (everything else)
• discussion of anapsids, synapsids, diapsids, sauropsids -- below
  
• at this point, it was best example of Darwin's origin of species
  
• to simply survive; survive / excel in a given environmental niche
• competing for food; competing for survival from predators
• what made them so successful; what allowed them to diversify so rapidly? 
• method of reproduction -- eggs: 
• maintained the advantage of two parents; mutations
• could leave large broods
• little energy used to care for eggs
• little energy involved in rearing offspring
• big question: why two groups -- synapsids vs sauropsids?
• need to explore anatomic differences
• once one answers that question, how did it happen that they differentiated into two groups and once that happened, why did both groups survive; advantages of both?
• once that one question and its derivative question are answered, then the rest is relatively easy.
• again, like the sauropsids (reptiles, birds), the synapsids (the mammals) diversified for the same two reasons:
• to simply survive; survive / excel in a given environmental niche
• competing for food; competing for survival from predators