Heads and what goes in them

Enigma 1 - heads


Branchiostoma from visualphotos.com
The sudden appearance of the head is one of the major enigmas of chordate evolution. There is little in the fossil record to suggest that the head evolved gradually. More likely it appeared in one or two quick steps. It is not just heads, but the presence inside them of:

Branchiostoma has a rudimentary equivalent to the brain and pituitary gland (a patch of sensory tissue in the roof of the mouth call Haetschek's pit) but nothing resembling the rest. Comparing craniate neural tube embryology to that of Branchiostoma we note that something is missing he as well.

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Wait for it .

Neural Crest!

Remember the neural crest? Most of its cells give rise to the peripheral nervous system. Some, however, take on an outwardly mesenchyme-like form and migrate to distant parts of the body, in which they give rise to the skeleton of the gill arches and the front of the braincase, and to the cranial nerves.

Note: The special sense organs develop through the inductive interaction of neural tube ectoderm and ectodermal placodes on the body surface.


landesbioscience.com
Most of neural crest cells give rise to the peripheral nervous system. Some, however, take on an outwardly amoeba-like form and migrate to distant parts of the body, in which they give rise to:

The phylogenetic distribution of neural crest:

Wholly consistent with the molecular consensus that urochordates are the closest relatives of Craniata.

Fully appreciating these relationships requires an excursion into the realm of genomics:

Regulatory genes


Wikimedia Commons
The structure of DNA was discovered in 1953, and its role as the physical repository of genes illuminated in the following decades. A brief review of how information encoded as nucleic acid is expressed as proteins goes like this:

Link to simple animation or to a more detailed explication.

The magic of the arrangement is that DNA is only transcribed into mRNA when the protein it codes for is needed. E.G.: The bacterium Escherichia coli freely metabolizes glucose, but if glucose is lacking, and lactose is present, it can metabolize it by producing an enzyme, beta-galactosidase, that breaks lactose down in to glucose and galactose. But how does it know when to make beta-galactosidase?

The work of François Jacob and Jacques Monod established the answer (Nobel Prize in 1965): Adjacent to the gene for beta-galactosidase is a small gene to which a protein, the lac repressor binds. This blocks RNA polymerase from unzipping the DNA and making mRNA. But, lactose, itself, binds to the lac repressor causing it to fall off of the DNA strand and allowing RNA polymerase to do its thing. Once the lactose has been metabolized, the lac repressor reattaches and transcription ceases. Animation.

Of course, the lac repressor protein is coded for by a repressor gene whose activity may be regulated by other repressors. Indeed, the expression of genes as proteins is the result of complex interactions of regulatory and structural genes and their protein products.


HOX genes


The Biology Corner
Hox genes: During the late 20th century it became known that segmentation in bilaterians is governed by a special class of regulatory genes. The story:

Fruit flies are a favorite model for geneticists, with short generation spans and interesting mutations that often effect entire sections of the body (modifying or eliminating body segments and/or the appendages that grow from them). Investigation into these segmentation-altering mutations revealed that they can be caused by mutations to eight genes. What makes this interesting:

The homeobox codes for a proteins called the homeodomain that is functionally similar to regulatory proteins that block or allow transcription of other genes. It appears that the cluster of eight genes controls the identity of body segments in fruit flies. These are called Hox genes, after the homeoboxes they contain.

But it gets better: The search for homologs to fruit fly Hox genes found them in almost every animal surveyed. But sometimes there was more than one version. All craniates, for example, have two clusters of Hox gene homologs, in each of which the genes occur in the same order on the chromosome as the regions for which they code (Mammals have four!)

This is huge for two reasons:

Why do the numbers of Hox genes and Hox gene clusters differ? - Errors in transcriptions result in duplications in which paralogous copies of the genes are generated - i.e. genes coexisting within the same genome that trace their descent to a common ancestor genes. Remember:


What happens to an organism's phenotype (physical form) when a gene or gene cluster that controls body segmentation is duplicated?

The presence of neural crest seems to be another synapomorphy of Craniata - chordates with heads. Recently discovered taxa from Chengjiang that approximate the ancestral condition for Craniates:


Myllokunmingia from bgchaos.com
Myllokunmingia: Reconstruction - right, Specimen. Superficially similar to Branchiostoma except that Branchoistoma's body is no longer than its notochord. In craniates, the head is present in front of it, and contains the special sense capsules. The connection to neural crest?

Neural crest cells actually give rise to:

The appearance of the contents of the head seem connected to the appearance of neural crest. Although not specifically neural crest, its appearence is contemporaneous with:

Hox genes control segmentation and fundamental orientation of embryo. They are conservative gene clusters found throughout the animal realm. Besides controlling orientation and segmentation, each gene influences a specific region of the body. Indeed, their physical sequence on the chromosome generally matches the position of the segment of the body in which they are expressed.

Note: The gene coding for the fruit fly's mouth codes for the rear of the mouse's head.

Putting it together: In 2008, the genome of Branchiostoma was sequenced. We now know:

The upshot is that the appearance of a complex head and branchial skeleton may have been the result of Hox gene duplication and the subsequent independent evolution of the resulting gene clusters. This introduces an interesting variation on the traditional gradualist view of evolution. Quite possibly, the material that natural selection shaped into the head appeared all at once, as the result of a one-time occurrence - the duplication of a group of genes.

Gene transcription errors have profound effects. One common consequence of such errors is the gene duplication event, in which two paralogous copies of a gene are generated. (As opposed to homologous versions of physical structures. ) Once present, they may each evolve independently and ultimately code for different proteins. For example, the lamprey has a single globin molecule, coded for by a single gene. In contrast, mammals have four globin molecules, each coded for by separate genes thought to have originated in at least three duplication events.


Craniata - so what about heads?

To review:

Major craniate groups: Hyperotreti and Vertabrata



From disciplineorregret.com

Bones

Hagfish and lampreys, as the only living jawless vertebrates, provide an interesting glimpse of early vertebrate evolution, however they lack the proper hard tissues by which we know the vast diversity of early vertebrates - bone.

Fossil vertbrates are mostly known from hard tissues - bone and teeth. Bone is composed of:

This material is secreted and maintained by living cells: Bony tissue can be:

What is its history?

The earliest known phosphatic hard tissues were acellular, and were tooth-like in being made of:

Among living craniates, bone in any form only occurs among members of Vertebrata - craniates with vertebral elements protecting their spinal cords. What does the study of fossil organisms tell us about the distribution of bony tissue?

What is its history?

Among living craniates, bone in an form only occurs among members of Vertebrata - craniates with vertebral elements protecting their spinal cords. NOTE: although we tend to get sloppy, strictly speaking, Hyperotreti (hagfish) are considered the sister taxon of Vertebrata but not members of it. What does the study of fossil organisms tell us about the distribution of bony tissue?

Conodont animals


Conodonts on pinhead from University of Leicester
Conodonts: Since 1856, paleontologists have been aware of minute (0.1 - 0.5 mm.) fossils made of apatite (calcium phosphate), the same mineral as vertebrate bone and teeth.



Conodont apparatus from Purnell et al.
In the 1960s the situation was clarified somewhat by the discovery of articulated groups of conodonts. For the first time it became clear that these elements (or most of them) worked together as part of a conodont apparatus (right).


Clydagnathus from Conway-Morris, 1983.
In 1983, Derrick Briggs published on Clydagnathus, an Early Carboniferous age eel-shaped creature in which he noted:

Euconodonts are clearly closely related to Craniata and might be inside it.

Much work remains to be done on their anatomy and phylogenetic position.



From Veras, Rodrigo, 2013, Evolucionismo
Is our reconstruction of Euconodonts correct? Some morphological interpretations, especially the huge eyes, seem to beg for revision, especially considering that the living jawless vertebrates have modest eyes. Could they be otic capsules? If that were true, then maybe euconodonts represent a stage in which the head is not fully developed.

Could hagfish or lampreys represent euconodonts that have secondarily lost their phosphatic hard-parts?

This enigma is unresolved.


Anatolepis armor from Palaeos
Conodonts were not the only representation of craniate hard tissues in the Cambrian, however. Enigmatic, scale-like plates of bony armor called Anatolepis were also present. In this and similar creatures, histologically tooth-like denticles complete with enamel and dentin formed a superficial body armor.

Indeed, in many early vertebrates, there seems to have been little difference between teeth and scales, which took the form of little denticles with a pulp cavity, dentin, and enamel. A survey of the diversity of fossil jawless vertebrates tracks the proliferation of different bone types in different parts of the body.


Euphanerops longaevus from Christian Science Monitor
Euphanerops (right) and Jamoytius (Silurian)

The phylogeny of Sansom et al., 2010 was made possible by reexamination of Jamoytius and its sister taxon Euphaneriops, often previously cited as the ancestral vertebrate or close to Hyperoartia. Jamoytius represents the most primitive vertebrate with hard tissue elements outside the mouth: W-shaped bony acellular scales - composites of dentin and enamel. Euphanerops preserves cartilagenous internal skeletal elements, including arcualie and fin radials.


Pterygolepis from Palaeos
Anaspida (Silurian)

Morphology:

Overall, anaspids seem adapted for active swimming. Exactly how they ate is mysterious, but they lack obvious adptations to suspension feeding or to taking large prey.

Synapomorphy of Anaspida and jawed-vertebrates: Dermal skeleton of head.


Thelodonts from Wikimedia Commons
Thelodonti (Ordovician - Devonian)

Morphology:

  • In some cases, these tooth-like scales line the oral cavity and mouth.
  • Again, no paired fins, there are paired triangular "fin flaps" without skeleton or muscle.
  • Hypocercal caudal fin.
  • Paired nostrils (in contrast to the vertebrates we have considered so far)

    Living chondrichthyans preserve a similar pattern.

    Synapomorphies of Thelodonti and jawed-vertebrates:

    Issue: True Bone:

    Before proceeding, a note: The cells that secrete and maintain hard tissue may be locked within it, yielding cellular bone. Seen in larger bony elements. Cellular bone forms in two ways:

    Pteraspidomorphi (Cambrian (assuming Anatolepis) minimally Ordovician- Devonian).

    The earliest well-preserved vertebrate, the Ordovician form Sacabambaspis, ironically represents a more derived form of hard tissue, in which individual denticles are integrated into broad head-shield composite elements and joined to one another through dermal layers of aspidin, a composite of thelodont-like denticles, lamina of dentin, and cellular dermal bone. These shields are the first vertebrate elements that we can call proper bone. these elements seem to have played the roles of:

    Note: It was not an internal skeleton.

    By the Silurian, many Sacabambaspis - like creatures are known, E.G. Pteraspis (right).

    Morphology:


    Synapomorphy of Pteraspidomorphi and jawed-vertebrates:


    Galeaspida: Restricted to southern China and Indochina, then a separate continent. (Silurian - Devonian)

    Morphology:

    Synapomorphy of Galeaspida and jawed-vertebrates: Endochondral bone in braincase.


    Cephalaspis Bionet Skola
    Osteostraci (Silurian - Devonian): Resemble galeaspids but with differences:

    Morphology:

    Synapomorphies of Osteostraci and jawed-vertebrates: