Also known as "tunicates" or "sea squirts." Adults are attached to some hard surface. Anatomy: Their bodies consist of a large basket-like pharynx perforated by numerous slits. On one side, the endostyle secretes mucus which is transported across the inner surface of the pharynx by cilia, capturing food items as it goes. Entire body enclosed by a "tunic" made primarily of tunicin, a unique complex sugar. Water enters pharynx through a "mouth" and, after filtration, enters an atrium from which it is expelled through a separate opening, the atriopore. Heart and open circulatory system present. Blood flow reverses periodically. Larvae: Before settling on a hard surface, the larva is tadpole-shaped, and swims about searching for a suitable attachment place. The larva swims by means of a tail which degenerates after attachment. But note, the tail has a notochord. Furthermore, although in the adult, terms like dorsal and ventral have little meaning, in the larva it is clear that the nerve cord is dorsal.

• Ascidiacea: "Standard-issue" tunicates (because easy to study). the ones that spend their adult lives attached to an accessible to biologists substrate. May be solitary of colonial.
• Larvacea: Do not metamorphose, but remain as free swimming "tadpoles." The freaky shit - They secrete a gelatinous structure, the house (that's the technical term) which they inhabit and push around as they swim. The house gathers and concentrates food in suspension. Invariably solitary
• Thaliacea: Salps. Metamorphose into huge living pharynxes but do not attach. The tunic is invested with muscle, alowing them to swim by "jet-propulsion." Can be solitary or colonial.

Urochordates have no record to speak of, however a Chengjiang organisms published as a possible echinoderm may actually be an adult urochordate.

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• Cephalochordata: Lancelets
• Craniata: Hagfish, euconodonts, and vertebrates

Characteristics:

• Myomeres chevron-shaped
• Closed circulatory system (no heart) organized around two major vessels
  • Ventral aorta transmitting blood forward to the pharynx. Splits into smaller vessels passing up the branchial bars. These coalesce into the...
  • Dorsal aorta, which passes posteriorly taking blood to the body.
• Anterior diverticulm of the gut, in most cases developed into the liver.
• At least a slight anterior expansion of the nerve tube.

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Represented by the living Branchiostoma.

• Nomenclature: Earlier names have priority in scientific nomenclature. Unfortunately, the publication in which this animal was originally named (Branchiostoma) was largely ignored, and it was named a second time (Amphioxus). Additionally, there had been a common name (Lancelet). When the dust settled, Branchiostoma was acknowledged the proper scientific name, and amphioxus and lancelet are both considered common names. Live with it.
  
• Anatomy: In many ways, Branchiostoma is like a large sleek version of a urochordate larva. Similarities include:
  
  • Food is captured on a film of mucus during filtration of water through a large pharynx.
    
  • Both water and mucus moved through pharynx by cilia.
    
  • After filtration, water enters atrium, then expelled through atriopore.
    
  • Body propelled by lateral undulations, and supported by a notochord.
    
  • Like adult urochordates, Branchiostoma just sits and filters most of the time. Instead of attaching to a hard surface, it buries itself in mud, with only mouth and tip of shout protruding.
    

• Differences:
  
  • Rather than being a perforated basket, the pharynx has long vertical slits separated by branchial bars, on either side.
    
  • Water enters pharynx through opening in a membrane called the velum.
    
  • Propulsion is by muscles that are organized in chevron-shaped myomeres.
    
  • The notochord extends to the tip of the snout.
    
  • When disturbed, the animal swims around, fish-like, before settling down.
    
  • Although there is no heart, the circulatory system is closed. Blood flows through a ventral aorta to the branchial bars, up through them, and into a dorsal aorta through which it is taken to the body.
    
  • The dorsal nerve cord is hollow and there is a slight anterior enlargement. (Indeed, genes expressed in vertebrate brains are also expressed in this region, providing some basis for the establishment of homology.
    
  • A diverticulum of the gut occupies place of liver.

  Because the derived characters of the:
  • Life-long retention of the notochord, hollow dorsal nerve cord, and myomeres throughout life
  • Closed circulatory system
  are shared with vertebrates, they are synapomorphies of a monophyletic group containing both cephalochordates and vertebrates, but not urochordates, hemichordates, or echinoderms.

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  Synapomorphy of Cephalochordata:
  • Notochord extends to the front end of the body.

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  Fossil record: Sparse, but well represented in the Cambrian lagerstätten. Pikaia from Burgess Shale. Cathaymyrus from Chengjiang Formation is argued to be the earliest.

  Pikaia, the Burgess Shale cephalochordate.

• Head including:
  • Anterior expansion of nerve cord into brain.
  • Special sense organs:
    • Olfactory capsules for olfaction
    • Eyes for vision
    • Otic capsules for hearing
• Neural Crest
  • New germ cell type, making craniates effectively "tetrablastic"
  • Remember the gastrula with its endoderm, mesoderm, and ectoderm. In chordates, the hollow neural tube originates as an infolding of the ectoderm above the mesodermal notochord.
  • Appears dorsal and lateral to the neural tube.
    

  • Contributes to a great variety of adult tissues and structures including:
    • sensory neurons (nerve cells)
    • some skeletal and connective tissues in the skull,
    • some pigment containing cells
    • other integumentary tissues
    • the brachial arches,
    • jaws
    • parts of the braincase floor.
  • But note the neural crest is absent in Branchiostoma.
• Mouth, with more specialized mouth parts
  • Arches of pharynx now support gills, used for both feeding and respiration
  • Branchial apparatus supported by cartiligenous brachial arches (gill bars) external to hemibranchs
  • Pharyngeal muscular pump (hypomere)
• Caudal fin stregthened by cartiligenous radials
• Larval endostyle transforms into adult thyroid gland
• Gut
  • Liver and pancreas present (formed through endoderm-mesoderm induction)
  • Digestive system invested with smooth muscular lining (rather than cilia)
• Circulatory System
  • Two chambered heart
  • Hemoglobin for oxygen transport
  • Erythrocytes to contain hemoglobin
• Increased size (an order of magnitude)
• Cartilaginous endoskeleton: including fin rays and brachial arches

In terms of metabolic rate and aerobic capacity, only cephalopod mollusks and some arthropods are comparable to craniates.

Origin of the head
What do all of these taxa have in common that the outgroup (Cephalochordata) lacks? Heads! What phenomena are implicated in the sudden appearance of this complex structure?

• Homeobox (aka Homeotic, aka 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.

• 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. 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.

• Putting it together.
  
  • The subjects of homeobox genes, gene duplication, and the origin of the craniate head come together in the "new head" model of craniate evolution
    
  • Recall that craniates possess some unique tissues, not found in Branchiostoma, that generate many structures of head
    
    1. epidermal placodes - precursor to cranial structures including the lens of eye.
    2. hypomere - precursor to pharyngeal muscles
    3. neural crest - precursor to many things, including all bone, and cartilage of anterior cranium and gill arches.
  • These special tissues are coded for by homeobox genes.
  • The homeobox genes of craniates differ from those of Branchiostoma in one important respect: Rather than having one homeobox gene cluster, craniates all have at least two, arguably the result of a gene duplication event.
    
  • 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 occurance - the duplication of a group of genes.

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Fossil record: (Cam - Rec.) Arguably begins with the Chengjiang taxon Haikouella lanceolata.

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Hyperotreti (Hagfish) (Pennsylvanian - Recent, however record consists of one Penn. age fossil, Myxinikela, from Mazon Creek lagerstätte.) Morphology: The skeleton consists of the notochord and specialized cartilages of mouth. The braincase is made primarily of connective tissue. A two-chambered heart is present. Single olfactory pouch communicates with the pharynx. Eyes are present but very small, with no extrinsic muscles. Otic (inner ear) capsules have only one semicircular canal (as opposed to three in jawed vertebrates). Feeding apparatus. Keratinous "teeth" on paired protrusible plates used to grasp small prey or rasp pieces off of carcasses of larger animals. An adult hagfish is much too big to achieve gas exchange by simple diffusion. Its gill slits are, therefore, lined with thin pleats of heavily vascularized tissue - proper gills - which serve as breathing organs. Mucus glands secrete copious amounts of mucus as defense. Yuck.

From Brandon Cole Marine Photography

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Vertebrate synapomorphies:

• Arcualia: Sequentially arranged cartilagenous endoskeletal elements flanking the spinal cord (the beginnings of proper vertebrae) Link to illustration - Scroll down.
• Extrinsic eye muscles, allowing eye movements
• Radial muscles in fins, allowing active control of fin position.
• At least two vertical semicircular canals in the otic capsule.
• Lateral -line system with neuromasts - sensory capsules enervated by the accoustic nerve.

Hyperoartia (aka Petromyzontida, aka lampreys) - (Devonian - Recent): Characterized by: a large sucker surrounding the mouth armed with keratin "teeth.", strengthened by annular cartilage Piston cartilage supporting a protrusible "tongue" armed with more keratinous denticles unique among extant vertebrates in having a median dorsal "nostril" but some other fossil vertebrates also display the same structure. Undergo metamorphosis from suspension-feeding ammocoetes larva to parasitic adult.

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Fossil record: The oldest proper fossil lamprey is Priscomyzon riniensis of Devonian age, however suspiciously lamprey-like forms from much earlier include the Silurian Jamoytius . (Specimen.) Some researchers feel they can spot the annular cartilage in less lamprey-like critters.

the Devonian lamprey Priscomyzon (above) and recent lamprey Lampreta.

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Euconodonts: (Cam. - Tri.) 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. (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. Thus, the word "conodont" was used to refer to the elements, themselves. The unknown creatures that made them were called "conodont animals."

Four general varieties of morphology: Single cones Blades Bars Platforms These were always found as disarticulated clasts in marine sediment. This situation led to the very reasonable but misleading simplifying assumption that each type of element represented a different taxon. At this point, speculation raged about: Which major taxon the elements belonged to How were they used? Originally proposed to be dental elements of fish-like chordates by Pander (1856), but soon, more erudite-sounding hypotheses held sway. To some they seemed likely to be used in prey capture, as in similar sturctures in chaetognaths (arrow-worms) To others, they seemed like mineralizations of the suspension feeding apparatus of cephalochordates. Or perhaps they were for internal support and not for feeding at all.

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. Moreover, different elements began to be distinguished by the manner in which they were formed developmentally: Protoconodonts (Cambrian) - internal addition only. Paraconodonts (Cambrian) - both external & internal addition. Euconodonts (Cambrian - Triassic) - external addition. Since these seemed to appear in a stratigraphic (chronological) sequence, speculation was that these growth forms represented evolutionary stages. Not quite.

In 1988, single cone elements were eliminated from the roster of euconodonts by the discovery that they were the fossils of early chaetognaths. In living chaetognaths, similar elements are made of calcium carbonate and are used in prey capture.

The conodont animal: In 1983, Simon Conway-Morris (of Burgess Shale fame) published on Cladygnathus, a Mississippian age eel-shaped creature in which he noted: Chordate-like V-shaped segmented muscle blocks Midline fins supported by fin rays The conodont apparatus in an anterior position, suitable for use in feeding. Notochord A head a brain and two capsules for special senses, thought to be very large eyes and smaller otic capsules.

We now have an emerging consensus on what the "conodont animal," now the monophyletic group Euconodonta, looked like - a small, eel-shaped chordate.

But where does it go on the chordate cladogram? The presence of a phosphatic skeleton arguably places it within vertebrata, closer to the jawed vertebrates than lampreys are, but there are concerns:

• Lampreys might have secondarily lost hard tissues as a reversal.
• Some morphological interpretations of euconodonts, especially the huge eyes, seem to beg for revision.

Haikouichthys: Again, Chengjiang gives us a picture of what the ancestral vertebrate might have looked like with Haikouichthys ercaicunensis, which seems to preserve arcualia.

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