Introducing Sharks, Skates, Rays, and Chimaeras 13
Few sharks live below 3,000 m (9,840 ft), the record holders being a Portu-
guese Dogfish (Centroscymnus coelolepis) seen at 3,690 m (12,100 ft) and an
unidentified dogfish (Squalidae) at 4,050 m (13,280 ft).
In terms of latitude and water temperature, selachian true sharks are
most diverse in warm temperate oceans, although many occupy cold, deep
water below the warmer surface. The tropics have the next highest diver-
sity, followed by cold temperate and polar seas (see “Which geographic
regions have the most species of sharks?” in chapter 5). Stingrays are more
diverse in inshore tropical regions. In contrast, more species of skates are
found in temperate oceans and in cool, deep waters than in the shallow,
warm tropics. Chimaeras are also chiefly cool-water species.
Some selachians swim near the surface, others prefer the bottom, and
other species are found at midwater depths. Skates and rays are almost en-
tirely bottom-livers, feeding on organisms that live on or in the bottom
sediments. The notable exceptions are the manta rays, which swim in mid-
water and feed on zooplankton (small invertebrates up in the water col-
umn). Chimaeras live near the bottom and also feed on bottom-dwelling
invertebrates that they crush with their tooth plates.
A few rays and even fewer sharks live and reproduce in fresh water, in-
cluding far up major rivers (see “How many shark species live in rivers and
lakes?” in chapter 5).
When did sharks evolve?
Fossil teeth that undoubtedly belonged to sharks have been found in
geological deposits that date to the early Devonian period, about 418 mil-
lion years ago. An actual sharklike fossil with the revealing name of Doliodus
problematicus is thought to be 409 million years old. (Fossils of entire sharks
are very rare because the cartilage that makes up most of a shark’s skeleton
does not fossilize well; we usually have just teeth to tell us about the entire
animal.) Some sharklike scales from the late Ordovician (455 million years
old) have been found, but scientists are hesitant to proclaim that they came
from a shark. Holocephalans arose somewhat later, the oldest fossils dating
back to at least the late Devonian, about 375 million years ago. The batoid
skates and rays appear even later, in the Jurassic (about 175 million years
ago). The earliest chondrichthyans already had internal fertilization (evi-
dent as claspers on apparent male specimens) and replacement teeth.
Sharks were much more diverse in body shape and habitat in the past.
In the late Paleozoic and Mesozoic periods (300 to 100 million years ago),
sharks were the dominant predators in both marine and freshwater habitats,
instead of being primarily marine as they are today. Some early sharks were
Cladoselache, Xenacanthus, Ctenacanthus, Hybodus, and Helicoprion. Helicoprion