Biology 3315 Comparative Vertebrate Morphology Protochordates and Fishes 1. Echinodermata If fossil forms are included, echinoderms are a very diverse assemblage; several classes are now entirely extinct. One such extinct group, the carpoids, has been placed within the phylum (subphylum Calcichordata) and are thought to be ancestral to primitive vertebrates. Some representative modern echinoderms are shown here. In what way are the Echinoderms related to Chordates? 2. Hemichordata Enteropneusta Balanoglossus The picture is an adult acorn worm (genus Balanoglossus). Identify the following structures: Proboscis Collar Branchial region with pharyngeal slits 3. Hemichordata Enteropneusta Balanoglossus The slide is a longitudinal section through the anterior region of the body of an acorn worm. Locate the following structures: Large muscular proboscis with its internal cavity or coelom Stomochord within the base of the proboscis Collar region Branchial region The stomochord was at one time regarded as the homologue of the chordate notochord, but is now regarded as an unrelated structure of uncertain function. 1
4. Urochordata Tunicates: Sea Squirts/Ascidian tadpole Examine the larval tunicate and attempt to locate the following: oral adhesive disks otolith (statolith) eyespot endostyle siphons (branchial, atrial) pharynx gut (stomach, esophagus, intestine) axial complex In what ways is the tunicate larva less specialized than Amphioxus and, therefore, a potentially better candidate for the ancestry of vertebrates? Note that the swimming tadpole in the slide represents a stage intermediate between the adult and the larval forms. 5. Urochordata Tunicates: Sea Squirts Model of adult tunicate. Identify the following structures: tunic atrium siphons pharynx gut gonads Which organ systems are elaborated and which are atrophied during metamorphosis in tunicates? 6. Cephalochordata Amphioxus whole mount Amphioxus are small filter-feeding animals that live in shallow tidal waters where they burrow into the sand, usually tail-first. Examine this whole mount of Amphioxus, then identify the following structures: Oral hood Wheel organ Pharynx/pharyngeal bars Intestine Notochord Anus Pigment granules Trunk muscles (myomeres) Midgut caecum (intestinal diverticulum) Atrium Velum 2
7. Cephalochordata Amphioxus sagittal section This sagittal section is from a more mature specimen than the whole mounted Amphioxus. The segmented nature of the mature gonads (large, ventrally placed masses staining purple), quite unlike those of vertebrates, are clearly shown here, as well as in the preserved specimens within the plastic blocks. Notice that the gonads extend well into the pharyngeal region (where pharyngeal bars are present); the mature gametes are released into the chamber (i.e., atrium) surrounding the pharynx. The segmented trunk muscles and the connective tissue sheets (myosepta) between them are also evident. 8. Cephalochordata Amphioxus cross section For this cross section of Amphioxus, identify the following structures: dorsal nerve cord midgut caecum coelom dorsal fin ray gonad pharyngeal gill bars and slits endostyle atrium Some of the above structures are best seen in the midgut section (next to pharyngeal section). Observe also the partitioning of the trunk muscles by connective tissue sheets or myosepta. How do these septa relate to the notochord? 9. Vertebrata Myxiniformes Hagfish Hagfish are entirely marine and are major scavengers on sunken vertebrate carcasses. Although they are highly specialized in many respects, hagfish are thought to be the most primitive living vertebrates. The absence of bone in the group is a truly primitive condition. 3
10. Petromyzontiformes Developmental changes in Lamprey Ammocoete larva Because the adult lamprey is, in several respects, degenerate, the larval form is morphologically more similar to higher vertebrates. For example, it displays a gall bladder and hepatic duct, which will be lost at metamorphosis; the pharynx had not yet separated from the esophagus as a blind pouch; the gonads are still paired instead of fused into a single organ. In the larva the endostyle (i.e. subpharyngeal gland) has not yet become the thyroid gland and the eyes are only partly formed. 11. Petromyzontiformes Lamprey Adult lamprey are highly predaceous fishes that attach themselves to the external surface of other fishes and scrape the flesh of their prey with a modified tongue. Due to the secondary loss of bone, there are almost no fossil lamprey. Lamprey bear few obvious resemblances to their extinct armored relatives, the ostracoderms. 12. Petromyzontiformes Lamprey This specimen is the internal skeleton of a lamprey. Note the absence of bone, the complex branchial basket, or visceral skeleton, supporting the pharynx, and the presence of bony teeth in the circular mouth. Note: the branchial basket on this specimen has been broken and is on the bottom of the container. 4
13. Petromyzontiformes Lamprey This slide is a cross section of the larval lamprey (Ammocoetes). This animal exhibits some obvious similarities to the cephalochordate Amphioxus in both its burrowing and filter feeding habits and several aspects of its anatomy. For this reason, it has been used in arguments linking cephalochordates and primitive jawless fishes. Identify the following: brain nerve cord notochord eye pharynx liver (with gall bladder) intestine subpharyngeal gland 14. Pteraspidomorphi Fragments of bony armor from fossil deposits of Ordovician age (488-444 myo) have now been identified as belonging to fishes of this group, making pteraspids the oldest known vertebrates. It is possible, although presently unproven, that all later vertebrates are derived from pteraspids. Note the morphological diversity within this group, as well as the important features of pteraspids. 15. Cephalaspidomorpha Osteostraci The broad, flattened head shield and upward facing eyes are characteristic of Osteostraci. Notice also the presence of pectoral lobes, which are generally considered not to be true fins due to the lack of an internal skeleton. In contrast to other ostracoderms, Osteostraci possess a heterocercal tail. These fishes appear to have been bottom dwelling filter feeders. 5
16. Cephalaspidomorpha Anaspida There is considerable variation in body proportions and the development of the dorsal spines. Compare the external gill openings with those of the Osteostraci and the Pteraspidomorphi. The row of lateral port-hole-like gill openings in anaspids is one of several features that have been used to link these ostracoderms to the Petromyzontiformes. 17. Placodermi Arthrodira Some arthrodires attained extremely large size, with certain Devonian species from near Cleveland, Ohio exceeding 20 feet in length. These fish were clearly active predators and have strong, often serrated jaws. Note the distinctive hinge between the head and thoracic armor. Movement at this hinge may have facilitated respiratory activity, but may also have been advantageous in feeding. Some workers have suggested that arthrodires may be ancestral to chondrichthyan fishes. The superficial similarities of some arthrodires to the modern ratfishes are especially strong (compare this model to figures and specimen at station 23). 18. Placodermi Antiarchi The long, jointed pectoral appendages of this group are particularly distinctive. They are a fresh water group and are thought to have been adapted for life in fast moving streams. What is it about their morphology that supports this thought? How do antiarchs compare with cephalaspids with regard to body armor, eye position and tail shape? Do you think antiarchs were active swimmers? Why or why not? The jaws in these fish are very small and were originally though not to be present. 6
19. Chondrichthyes Elasmobranchii Cladoselachimorpha Cladodonts This extinct elasmobranchii group contains the most primitive of the known chondrichthyan fishes. The name cladodont refers to the distinct form of the teeth, often the only portion of these early shark-like fishes preserved as fossils. The broad based pectoral fins, strongly developed lower lobe of the heterocercal tail, and relatively large eyes are all distinctive features of this group. How do these compare with the modern sharks? Cladodonts are further distinguished by the apparent absence of claspers on the pelvic fins of males. 20. Chondrichthyes Elasmobranchii Xenacanthimorpha Pleurocanths These shark-like fishes were unusual, not only for several morphological features, but also because they are the only group of cartilaginous fishes to have regularly inhabited fresh water. The specialized form of the pectoral fin and tail, and the spine at the back of the head readily distinguish pleurocanths from other elasmobranchs. Clasping organs are present in the males. 21. Chondrichthyes Elasmobranchii Selachimorpha (sharks) Batidoidimorpha (skates and rays) Locate the general features of this group, using those cited in the handout and the textbook. Compare the following features between the selachimorpha and the batidoidimorpha: general body form position and size of spiracle and eyes development and placement of fins location of gill slits 7
22. Chondrichthyes Elasmobranchii Selachimorpha (sharks) This skeleton of a dogfish shark (Squalus) shows a complete lack of bone, a characteristic feature of the class. Once thought to be primitive, the absence of bone is now regarded as a secondary condition in these fishes. 23. Chondrichthyes Holocephali Chimaera/Ratfishes Refer to your textbook and the illustration for the principle external characters of Chimaera. What are some of the obvious differences between the male and female specimens shown here? The peculiar teeth of these fish are the basis for their common name, ratfishes; the teeth are used to pick up and crush shellfish from the ocean floor. 24. Acanthodii Refer to the survey of vertebrates handout and the textbook for characteristics of this group of primitive bony fishes. Acanthodians were originally classified as placoderms and later thought to be related to the chondrichthyes. Details of the head skeleton and the structure of the scales now show that the acanthodians are best regarded as a distinct class a sister group of the bony fishes. Notice the numerous ventrolateral fin spines, each supporting a web of softer skin behind. 8
25. Actinopterygii Paleonisciformes Polypteriformes Bichirs Of the African bichirs, the peculiar genus, Polypterus, is best known. The fleshy pectoral fins of Polypterus, together with its possession of a lung, originally led to its classification as a sarcopterygian. Polypterus retains the enamel covered ganoid (another name for enamel) scales of primitive bony fishes. 26. Actinopterygii Paleonisciformes Ascipenseriformes Sturgeon and Paddle Fish Specimens represented at this station are the sturgeon (genus Acipenser) and the paddle fish (genus Polydon). Both have cartilaginous skeletons and can grow to great lengths. The large, persistent notochord of the sturgeon has sufficient internal pressure to function as a hydrostatic axial skeleton. It uses suction feeding through a ventrally located mouth to feed on other fish, worms and crustaceans. The paddle fish filters food from the water while slowly swim forward with their large mouth open. 27. Actinopterygii Neopterygii Lepisosteiformes (gars) Amiiformes (bowfins) Teleostei (most fish) Gars, Bowfins, and Teleosts The gars and bowfins, which were formerly classified as Holostei, are now classified as Neopterygii, along with the Teleostei. Gars and bowfins are the most primitive surviving neopterygians. Teleosts are the most evolutionarily advanced of the actinopterygian fishes and dominate the fish faunas of both fresh and marine waters. The specimens at this help to illustrate the impressive degree of structural diversity found in modern neopterygians. 9
28. Sarcopterygii Coelacanthaformes Coelacanths Locate the major external characters of the sarcopterygians using the taxonomy handout and your textbook. Provided here is an illustration of the sole surviving coelacanthiform, Latimeria the coelacanth (The Fishes, p. 74). 29. Sarcopterygii Rhipidistia Eusthenopteron and Panderichthyes Such extinct rhipidistian fishes gave rise to the early tetrapods such as Acanthostega during the Devonian. The group includes the porolepiformes, the osteolepiformes (e.g., Eusthenopteron, Osteolepis) and the panderichthyids (e.g., Panderichthyes). 30. Sarcopterygii Dipnoi Lungfish Compare the general body form and the development of the fins (especially pectoral) between the primitive, extinct lungfish (upper figure) and the modern lungfishes. How are the latter more specialized and degenerate? How do the Dipnoi differ from the Crossopterygii? 10