Heads and what goes in them
Enigma 1 - heads
Branchiostoma from visualphotos.com
- Brain - Expanded anterior portion of neural tube.
- Special sense organs occupying paired capsules:
- Olfactory capsules for olfaction
- Eyes for vision
- Otic capsules for hearing and balance
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.
Wait for it .
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.
migrate to distant parts of the body, in which they give rise to:
- cranial nerves
- some skeletal and connective tissues in the front of the braincase,
- some pigment containing cells
- the brachial (pharyngeal gill) arches,
- parts of the braincase floor.
The phylogenetic distribution of neural crest:
- Only members of Craniata have unambiguous neural crest cells that behave as described above.
- although neural crest homologs (marked by gene expression only, not shape or behavior) may be present in Branchiostoma, what is absent are:
- morphologically identifiable neural crest
- as are ectodermal placodes.
- peripheral nerves (!)
- In urochordates, migratory neural crest cells are present, but only control the distribution of pigment in the skin. Although there are no special sense organs, the atrium arises through the invagination of an ectodermal placode.
Fully appreciating these relationships requires an excursion into the realm of genomics:
- DNA at rest in the nucleus is a double helix (spiral staircase) whose corresponding base pairs are connected through weak bonds.
- When a gene is translated into a protein, RNA polymerase separates the DNA helices and uses one of them as a template for assembling a single-strand messenger RNA molecule (mRNA).
- mRNA passes from the nucleus into the cytoplasm, where it encounters the two components of the ribosome.
- The ribosome grabs passing transfer RNA (tRNA) molecules. tRNA have nucleotide triplets (codons) exposed at one end. Each nucleotide triplet combination is associated with a specific amino acid, that is bound to the opposite end of the tRNA.
- The ribosome matches nucleotide triplets on the mRNA to corresponding tRNAs, moving down the mRNA strand. Amino acids connected to adjacent tRNAs bind to form a protein, an amino acid polymer, that forms part of the cell's structure or does work in it.
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.
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 genes are close to one another on a single chromosome, and are physically lined up in the same order as the segments of the body they effect.
- Each gene (of ~ 1000 base pairs) contains a nearly identical sequence of about 60 base-pairs called homeoboxes.
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:
- The fruit flies and mammals belong to the two major clades of bilaterians, and their last common ancestor lived over 600 mya, and yet they share important regions of the genome.
- The regions of the body in which paralogous genes are expressed allow us to establish their homologies.
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:
- Homologous genes: Versions of genes in different lineages of organisms sharing common evolutionary origin.
- Paralogous genes: Different versions of genes in the same genome sharing common evolutionary origins.
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
Neural crest cells actually give rise to:
- The front of the braincase.
- cranial nerves (nerves that exit the brain directly)
- some pigment-containing cells
- the brachial (pharyngeal gill) arches,
- jaws (in craniates that have them)
- The special sense capsules develop through inductive interactions between placodes - thickened plates of ectoderm in the embryo's outer layer and outpouchings of the the ectoderm of the neural tube.
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.
- What codes for the part of the mouse with the brains in it?
- Did the last common ancestor of flies and mice even have a proper head?
Putting it together: In 2008, the genome of Branchiostoma was sequenced. We now know:
- Neural crest tissues are coded for by homeobox genes in craniates.
- The homeobox genes of craniates differ from those of Branchiostoma in an important respect: Rather than having one homeobox gene cluster, craniates all have at least two, arguably the result of a gene duplication event.
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?
Major craniate groups: Hyperotreti and Vertabrata
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:
- A mineral component - made of calcium phosphate (i.e. the mineral hydroxyapatite).
- A protein component - made mostly of the fibrous protein collagen.
- Osteoblasts: Active cells that secrete bone's extracellular matrix
- Osteocytes: When osteoblasts become trapped inside the bony matrix, they become inactive osteocytes and function in maintenance. In both life stages, these cells extend long processes (extensions) through canals in the bone. Osteoblast/osteocytes may be derived from nerve cells, whose axons and dendrites are similar. (Note: The corresponding cells responsible for bony tissue in teeth are called:
- odontoblasts/odontocytes when they secrete dentin
- ameloblasts when they secrete enamel)
- Osteoclasts: play a major role in bone remodeling by removing bone. They resemble and might be derived from macrophages.
- Acellular if the bone cells migrate away as the bone is laid down and not enclosed by it. Particularly dense acellular bone is dentin, characteristic of teeth and placoid scales. Dentin is characterized by parallel dentin tubules housing processes of the retreating odontoblasts.
- Cellular if the bone cells are enclosed inside the bone.
What is its history?
The earliest known phosphatic hard tissues were acellular, and were tooth-like in being made of:
- Dentin: Dense acellular bone
- Enamel: Bony tissue made entirely of interlocking crystals of hydroxyapatite (i.e. lacking collagen).
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?
Conodonts on pinhead from University of Leicester
- (Cambrian - Triassic)
- Highly diverse and rapidly evolving, thus excellent index fossils.
- Originally proposed to be the teeth of some unknown fish, but paleontologists soon determined they were were clueless about:
- What kind of animal they were from
- What part of the animal they represented.
Conodont apparatus from Purnell et al.
Clydagnathus from Conway-Morris, 1983.
- Chordate-like V-shaped segmented muscle blocks
- Midline fins supporsted by fin rays
- The conodont apparatus in an anterior position, suitable for use in feeding.
- A head, a brain and two capsules for special senses, usually thought to be very large eyes and smaller otic capsules. (?)
Euconodonts are clearly closely related to Craniata and might be inside it.
- No undisputed vertebrate makes any element that is homologous to conodonts. Indeed, a 2013 study by Murdock et al. demonstrates that conodont hard tissues evolved independently from vertebrate bone and teeth. (See also this.)
- But on a deeper level, the ability to make apatite skeletal elements places them inside Vertebrata. Are lampreys and hagfish secondarily derived in their inability to do so?
From Veras, Rodrigo, 2013, Evolucionismo
Could hagfish or lampreys represent euconodonts that have secondarily lost their phosphatic hard-parts?
This enigma is unresolved.
Anatolepis armor from Palaeos
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
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
- Hard tissue in the form of acellular plate-like scales, reminiscent of those of Jaymoutius, however these extend onto the head.
- Body is cylindrical with notochord supporting lower lobe of caudal fin.
- Pharyngeal (gill) openings are small and form a short, posteriorly slanting row.
- Retain single median nostril (reminiscent of lampreys).
- While there are no paired fins, there are paired triangular spines in a vaguely "pectoral" position.
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
- Hard tissue Entirely consists of small scales that usually disarticulate when the animal dies. These scales are distinctive, consisting of enamel and dentin layers around a pulp cavity, like a vertebrate tooth. Note: from this point on the tree onward, aquatic vertebrates generally retain scales of this sort or their derivatives, regardless of any other kind of skeletal ossification they may have.
Living chondrichthyans preserve a similar pattern.
Synapomorphies of Thelodonti and jawed-vertebrates:
- Dermal denticles with distinct root, crown, and pulp-cavity.
- Paired nostrils and nasal capsules.
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:
- Dermal bone: Laid down as a two-dimensional membrane. (E.g. human cranial bones). Ancestrally these formed near the body surface, but in derived vertebrates, their derivatives may invade deeper parts of the body.
Endochondral bone cross-section from University of Oklahoma Health Sciences Center
Interactive Histology Atlas
- Cartilage bone: Three-dimensional bone that is preformed in cartilage. (E.g. human limb bones). The cartilage, in turn, is preformed by condensations of mesenchyme cells, amoeboid mesodermal connective tissue of the embryo. (Remember them?) Cartilage bone makes its evolutionary debut in the skeleton of the braincase but is widespread in vertebral columns and the skeleton of the limbs.
- Perichondral ossification: Bone formation begins at the perichondrium - the membrane surrounding the cartilage precursor.
- Endochondral ossification: Bone formation begins in the interior of the cartilage precursor.
Note: Among living vertebrates, both types of ossification occur in cartilage bone, so neontologists and human anatomists call cartilage bone "endochondral bone." Paleontologists must distinguish them, however, because perichondral ossification evolved first and many fossil vertebrates have perichondrally ossified bone without the endochondral component.
Dermal bone cross-section from Histology at Southern Illinois University
Sacabambaspis janvieri Centre Nationale de la Recherche Scientifique - Evolution
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:
- A reservoir of calcium and phosphate for use by the animal's metabolism.
Pteraspis stensioei History of Life, Richard Cowen, University of California, Davis
in the form of acellular dermal armor. We see two armor shields, one dorsal and one ventral. Each gill opening is protected by an individual bone plate. All of this bone is dermal and forms external armor over the creature. It has no internal bone.
- The mouth is opened and closed by row of narrow parallel plates in the lower "lip."
- A head with small eyes and otic capsules above the mouth.
- Impressions of otic capsules on the underside of the dorsal shield indicate the presence of two sets of semicircular canals - similar to lampreys.
- Paired nostrils
- Notochord invades lower lobe of tail.
- The mouth is opened and closed by row of narrow parallel plates in the lower "lip."
Synapomorphy of Pteraspidomorphi and jawed-vertebrates:
- Plate-like cellular dermal bone.
Shuyu zhejiangensis Institute of
Vertebrate Paleontology and Paleoanthropology
- large flat head shield enclosing an endochondral bony braincase. The head shield is a composite of the endochondral braincase and composite dermal bone like that of pteraspidomorphs.
- large opening in upper front of head shield (maybe a large median nostril) leads to paired olfactory capsules and pharynx
- mouth is ventral
- Pharynx is large with many gill openings, suggesting suspension feeding.
- No suggestion of paired fins.
- Overall, although the equipment is different, the life-style looks like that of pteraspidomorphs.
Synapomorphy of Galeaspida and jawed-vertebrates: Endochondral bone in braincase.
Cephalaspis Bionet Skola
- large head shield enclosing an endochondral bony braincase. As in Galeaspida, the head shield is a composite of the endochondral braincase and composite dermal bone like that of pteraspidomorphs.
- Head shield contains large sensory fields.
- Head shield fused with endochondrally ossified braincase.
- Single median nostril leads to a single olfactory pouch (like in lampreys) and doesn't communicate with the pharynx.
- Notochord invades upper lobe of tail yielding heterocercal tail.
- Dorsal fin
- But the big thing: Paired pectoral fins (complete with endochondral skeleton elements and muscle) present.
Synapomorphies of Osteostraci and jawed-vertebrates:
- Paired pectoral appendages
- Heterocercal tail
- Distinct dorsal fin