Chondrichthyans diff er from most other vertebrates in having a cartilaginous skeleton, a lipid-fi lled liver, high blood urea concentrations, and an unusual pattern of en-ergy metabolism. Th ese characters may be functionally related to each other.
seen today in the hagfi shes and lampreys, but all ver-tebrates more derived than the living agnathans (in-cluding the Paleozoic radiation of jawless fi shes, the ostracoderms) have a bony exoskeleton. Although Cartilaginous Skeleton Th e absence of bone in the
endo-skeleton of chondrichthyans is the character that gives the cartilaginous fi shes their name. A cartilaginous endoskeleton is the ancestral condition in vertebrates,
Figure 5–1 Phylogenetic relationships of cartilaginous fishes. Th is diagram shows the gen-eral diversifi cation of jawed vertebrates, focusing on the probable relationships among the major groups of chondrichthyans. Th e black lines show interrelationships only; they do not indicate times of divergence or the unrecorded presence of taxa in the fossil record. Only the best-corroborated relationships are shown. Th e numbers at the branch points indicate derived characters that distinguish the lineages—see the Appendix for a list of these characters.
TriassicPaleogene
CENOZOICMESOZOIC NeogeneJurassicCretaceousPermianCarboniferousDevonian
PALEOZOIC SilurianOrdovician
12.
23.0
65.5
145.5
201.6
251
359
416
444
488
7.
? 4.
6.
11. Selachii
9. Euselachii
10. Neoselachii
8. Elasmobranchii
3. Chondrichthyes
2. Eugnathostomata 1. Gnathostomata
Millions of years
13. Osteichthyes 5. Holocephali
PLACODERMI HOLOCEPHALIMORPHA (chimaerans) PARASELACHIMORPHA Stem Elasmobranchs HYBODONTA GALEOMORPHI SQUALOMORPHI BATOIDEA (skates and rays) ACANTHODII
Stem Chondrichthyans ACTINOPTERYGII (ray-finned fishes) SARCOPTERYGII (lobed-finned fishes and terapods)
“Sharks”
299
chondrichthyans have evidently lost the extensive plates of dermal bones seen in other jawed fi shes, they retain dentine, enameloid, and even traces of bone in their teeth and their scales. Th ese unique scales are called placoid scales , or dermal denticles. Some degree of perichondral (= around the edge) ossifi cation of the endoskeleton is seen in some derived ostracoderms as well, and occasionally among chondrichthyans; it may be a general gnathostome feature.
Whether the ancestors of the chondrichthyans ever possessed the fully ossifi ed bony endoskeleton of the osteichthyans is unclear. Mineralization does occur in the axial and appendicular skeletons of chondrich-thyans as globular or stellate deposits of crystalline calcium in the superfi cial layers of cartilage matrix.
Th is condition, known as tesserate or prismatic endo-skeletal calcifi cation, is a unique derived character of Chondrichthyes.
Cartilage is lighter than bone, and the increased buoyancy that cartilaginous fi shes gained from the absence of a fully mineralized skeleton is believed to be the functional signifi cance of this character. Cartilage is not necessarily weaker than bone; calcifi ed cartilage can be made extremely strong when there are multiple lay-ers and internal struts, as in the jaws of rays that crunch mollusks.
CHONDRICHTHYES (cartilaginous fishes)
HOLOCEPHALI ELASMOBRANCHII
EUSELACHII NEOSELACHII
Hyostylic jaw Tooth plates
Gill openings covered by soft tissue
Prismatic endoskeletal calcification Pelvic claspers
Placoid scales
Gill openings separate and uncovered Tribasal fin
Underslung mouth
Dorsoventrally flattened Enlarged pectoral fins
Sharks Skates and rays
Hybodonts†
“Stem elasmobranchs”†
Paraselachians†
Chimaeras OSTHEICHTYES
(bony fishes)
Figure 5–2 Simplified cladogram of cartilaginous fishes. Quotation marks indicate para-phyletic groups. A dagger (†) indicates an extinct taxon.
Table 5–1 Classification of Chondrichthyes, the cartilaginous fishes
Neoselachii (sharks, skates, and rays)
Galeomorphi: About 394 species of sharklike fishes with an anal fin, including ragged toothed sharks, mackerel sharks, megamouth sharks, thresher sharks, requiem sharks, hornsharks, wobbegongs, nurse sharks, and whale sharks. Galeomorphs range in length from less than 1 m to 12 m and possibly to 18 m.
Squalomorphi: About 162 species of sharklike fishes without an anal fin, including most deep-sea sharks, dogfish sharks, angel sharks, and saw sharks. Squalomorphs range in length from 15 cm to more than 7 m. (The “Squalomorphi” lineage is not monophyletic, but the rela-tionships of the six lineages it contains are not yet understood. Some systematists combine squalomorphs with skates and rays, which also lack an anal fin.)
Batoidea (skates and rays): At least 638 species of electric rays, stingrays, manta rays, a large number of skates, and a few entirely freshwater species. They range from less than 1 m to more than 6 m long and more than 6 m wide.
Holocephali (ratfishes)
Chimaeriformes: Three families with about 49 species of mostly deepwater fishes: the plownose chimaerans (Callorhynchidae), shortnose chimaerans (Chimaeridae), and longnose chimaerans (Rhinochimeridae). Chimaerans range in length from 60 cm to nearly 1.5 m.
•
Urea leaks out through the gills and must be replaced continuously. When elasmobranchs are fasting, the catabolism of muscle proteins leads to increased releases of the amino acid alanine from the muscles.
•
The formation in the liver of urea from alanine results in metabolic by-products that can be turned into ketone bodies. Ben Speers-Roesch and Jason Treberg have suggested that high levels of protein breakdown and alanine release by the muscles of elasmobranchs could have favored a shift from fatty acids to ketone bodies as the primary metabolic substrate of elasmobranchs.
•
The lipids stored in the liver serve two functions:they can be converted to ketone bodies that are transported in the blood plasma to the cardiac and skeletal muscles and, in combination with the loss of mineralization in the skeleton, they make the animal buoyant. The large size of the liver contrib-utes to both of these functions.
5.2 Evolutionary Diversification
of Chondrichthyes
Early chondrichthyans, like the extant species, were di-verse in form and habitats. In the Late Devonian, many of them were found in freshwater habitats, in contrast to their primarily marine distribution today. Th rough time, diff erent lineages of Chondrichthyes developed similar but not identical modifi cations in feeding and locomotor structures. In the following sections, we will trace the radiations of chondrichthyans through the Paleozoic, Mesozoic, and Cenozoic.