Behold the Gnathostomata: gnathos (jaw) + stoma (mouth) Cephalochordata Urochordata Haikouella Myxiniformes Petromyzontiformes Chondrichthyes Osteichthyes jaws skull proto-vertebrae, true gills, two eyes, olfactory lobes many molecular characters dorsal hollow nerve cord, notochord, post-anal tail Before we get to jaws, we need to know something about hard tissues. The first type of hard tissue to evolve was cartilage. It is composed of: a matrix of collagen (a type of protein) fibers + proteoglycans (proteins coupled with long chains of sugars [e.g., chondroitin]) + ~ 75% water (bound to the sugars) Specialized cells (called chondrocytes) produce the matrix and lay it down in thin layers. lymphocytes have unique antigen receptors 1 In the basal Chordata, the chondrocytes are not part of the matrix so this type of cartilage is acellular and is therefore nonliving. In the ancestor of the Cristozoa, the chondrocytes deposit the matrix around themselves. Because these cells are now part of the matrix, cartilage is living tissue in these animals. Cartilage is flexible, but strong, and light. It has no blood vessels or nerve fibers running through it. What do you think this means in terms of cartilage s ability to repair itself?
pharyngeal slit Evolution of jaws: STEP 1 one visceral arch: tentacles around mouth acellular cartilage rod visceral arches In the ancestor of the Deuterostomia (remember, pharyngeal slits are older than the Chordata): origin of pharyngeal slits with reinforcing acellular cartilage deposited as a pair of rods, one on either side of each slit, forming an arch, holding the slit open a pharyngeal slit + its pair of solid, cartilaginous rods is called a visceral arch these animals have numerous visceral arches (sometimes up to 200). The slits are very close together, the cartilaginous rods are slender and the openings are covered in thick layers of tiny cilia function: filter feeding (remember the Cephalochordata and the Urochordata) fewer, larger tentacles STEP 2 gills rods are now made of cellular cartilage there was a dramatic reduction in the number of visceral arches the visceral arches are larger and further apart, each rod in the arch is jointed, (not solid) and larger. The cilia have been replaced by longer, thinner rays that are not packed so closely together function: respiration (true gills) In the ancestor of the Cristozoa: 3 In many extinct vertebrates without jaws (called jawless fishes ):
the first two visceral arches begin to move forward: the 1 st visceral arch (now called the mandibular arch) and the 2 nd visceral arch (the hyoid arch) separate from the remaining visceral arches (branchial arches) In the ancestor of the Gnathostomata (many extinct jawed fishes): parts of the mandibular arch are modified to form the main components of the upper and lower jaw (feeding) the upper jaw is attached to the skull by ligaments the rays on the hyoid arch provides support for the gills (respiration) the branchial arches still function as gills (respiration) tentacles gone! STEP 3 skull gills STEP 4: JAWS!!!!! skull gills 4 So let s place the preceding information on an expanded tree for the Chordata that includes many extinct lineages of jawless and jawed fishes: Cephalochordata Urochordata Haikouella Myxiniformes Petromyzontiformes [ extinct jawless fishes ] [ ------- Gnathostomata -------- ] [ extinct jawed fishes ] 3. somewhere during this time the mandibular and hyoid arches separate from the
branchial arches Note: the term visceral arch refers to a pharyngeal slit plus its pair of cartilaginous rods 2. reduction in number of visceral arches. The arches are larger and further apart. The rods are jointed and made of cellular cartilage. Cilia are replaced by long filaments. Function: respiration (true gills) 4. mandibular arch forms upper and lower jaw (function: feeding), hyoid arch supports gills (function: respiration) branchial arches remain as gills (function: respiration) 1. many closely placed pharyngeal slits each supported by a pair of acellular cartilaginous rods covered in dense cilia Function: filter feeding you do not need to know the tree, just be aware that the oldest fossil sharks are represented by scales and teeth (about 455 my). Since that time, Chondrichthyes has undergone several radiations, followed by extinctions. How do you know it is Chondrichthyes??? 1st dorsal fin 2nd dorsal fin anal fin clasper
caudal fin pectoral fin pelvic fin 6 *Claspers: male intromittent (copulatory) organ formed by a modification of some cartilaginous rays in the pelvic fin. The clasper is only unique to Chondrichthyes among living species. Some extinct jawed fishes called placoderms also had a clasper, so this structure is not an autapomorphy for Chondrichthyes. Spermatozoa travel from the testis, through a duct to the sperm sac where they are stored. During copulation the spermatozoa are ejected from the sac through the urogenital papilla into the clasper and from there into the female: testis duct sperm sac all Chondrichthyes have internal fertilization via the clasper male (with claspers) on left, female (no claspers) on right. These are two Atlantic Sharpnose sharks that were caught for tagging purposes, then released. urogenital papilla opens into clasper 7 *** cartilaginous endoskeleton stiffened with calcium salts [autapomorphy]. This can make it very difficult to age cartilaginous fishes because calcium salts tend to be scattered randomly in the skeleton (photograph on left below) rather than laid down in growth rings: On the right is a picture of bone, in which calcium salts are laid down in layers and can be used to age the animal. This cannot be done in Chondrichthyes because they do not make bone.
8 * high concentrations of urea in their tissues, which is unusual but not unique to Chondrichthyes (makes them taste bad, at least to us). Urea is a byproduct of breaking down (digesting) protein: protein catabolism excess nitrogen ions (N -3 ) ammonia (NH 3 ) is either excreted directly or converted to either: urea (stored or excreted) or uric acid (excreted) We will eventually return to the importance of these byproducts!!! Who are the chondrichthyans? The Holocephali (ratfish, chimaeras, spookfish) [holos (entire) + kephale (head)] There are few of the marine animals that on account of structure and relationships to other forms living and extinct have as great interest for zoologists and palaeontologists as the Chimaeroids. (Garman 1904) ~ 43 extant species (and more being discovered every year), cold deep marine waters along continental shelves (500-2,600 m) 9 most species have a long, whip like caudal fin (hence the common name ratfish ) swim using pectoral fins like wings, the caudal fin is of little use for swimming because it is so thin obvious pattern of lines on head and running down the sides formed by pores to canals under the skin containing cells specialized for detecting movement in the water. These canals are collectively called the lateral line. The canals make it look like the fish was taken apart into bits and sewed together again (badly... sort of like Frankenstein).. lateral line pores ampullae of Lorenzini pores... and clusters of pores from cells specialized to detect electrical fields produced by prey (ampullae of Lorenzini [black dots on the diagram above]) 10
Jaws again... naming parts remember, each cartilaginous rod is jointed epi cerato (4 parts) The prefix epi or cerato refers to the position of the particular section (think of it this way: epi is the top section, cerato is the bottom section) epimandibula ceratomandibula epihyoid skull ceratohyoid The two parts of the mandibular arch (that we will study) are called the epimandibula and the ceratomandibula, while the two parts of the hyoid arch that are important in this class are the epihyoid and ceratohyoid The ancestral condition for Gnathostomata: lower jaw ceratohyoid Autodiastyly: (i) the upper jaw is suspended from the skull by ligaments (orange lines on the diagram) and (ii) the hyoid arch (in red) has thin projections called gills 11 rays (in green) that help support the gill openings (so the hyoid arch is involved in respiration). This type of jaw structure is found in basal extinct jawed fishes. The holocephalan jaw is modified thus: lower jaw ceratohyoid Holostyly: the upper jaw is fused to the braincase (notice that the ligaments are gone). This jaw type is unusual and is why these fishes are called the Holocephali. Some of the rays (in green) on the hyoid arch enlarge and fuse to form a covering over the gills called the operculum (so the hyoid arch is still involved in respiration - note: look for the operculum on ratfish you see in the lab!). 12
*** three pairs of continuously growing, heavily mineralized, tooth-plates [autapomorphy] produces a rigid surface for grinding hard prey items (predators primarily on molluscs, echinoderms, cephalopods, small fish) Reproduction like all cartilaginous fishes, males have claspers (begin developing in juveniles) The clasper on each pelvic fin is subdivided into two or 3 branches depending upon the species. The picture above is from two different species of ratfish. 13 Sexually mature males also have a frontal tenaculum (a small club-like structure on the head just above the eye) and prepelvic tenaculae (a pair of structures that sit in pouches just in front of the pelvic fin that are extruded during courtship. Note the word tenaculae is just the plural form of tenaculum!) ***Both of these characters (the frontal tenaculum and the prepelvic tenaculae) are autapomorphies for the Holocephali 14 Photograph of a frontal tenaculum from two different ratfish showing the sharp spines (modified scales) on the bulb-like end Photograph of a prepelvic tenaculum from two different ratfish also with scales modified to form sharp spines (Luchetti et al., 2011. Journal of Fish Biology 79:399-417) Females are oviparous (ovum [egg] + parere [to bring forth] = egg laying) and lecithotrophic (lekithos [yolk] + trophe [food] = embryo gets nutrition only from yolk deposited by the mother in the egg) female supplies fertilized eggs with lots of yolk, covers them with a tough egg case, then deposits the egg case in the environment eggs and babies (called pups) fend for themselves