(Brown, 1900; Koken, 1907; Woodward,
1916) approach neoselachians in some ap-
parently derived characters not found in
the Mississippian ctenacanth Ctenacanthus
costellatus Traquair, 1884 (Moy-Thomas,
1936). These include fusion of the right
and left halves of the pelvic girdle (separ-
ate in C. costellatus, many other Paleozoic
elasmobranchs, and in chimaeras), orbits
more posterior on the neurocranium,
metapterygium or most posterior cartilage
of the three pectoral fin basals with a few
short posterior segments (many in C. costel-
latus), radial cartilages not extending into
distal webs of fins, and caudal fin not
crescentic (secondarily so in a few living
sharks and rays). Maisey (1975) noted that
neoselachian and ctenacanth dorsal fin
spines are similar in structure but that
hybodont spines are strikingly different.
He proposed that hybodonts appeared in
the Mississippian, after the first Upper
Devonian ctenacanths, and persisted
through the Paleozoic and Mesozoic to be
finally displaced by neoselachians, which
evolved from the last ctenacanths in the
Triassic. If correct, this hypothesis elimi-
nates the difficulty of tracing the common
ancestor of neoselachians and hybodonts
far back in the Paleozoic (where neosela-
chians are unknown) or of deriving
ctenacanth-like neoselachian spines from
hybodont spines in the Mesozoic. Maisey's
hypothesis is tentatively accepted here with
the cautionary note that hybodonts and
especially ctenacanths are relatively poorly
known (despite an abundant fossil record
of mostly fragments), and need investiga-
tion on several crucial character systems
(especially the neurocranium).
Compagno (1973) suggested a common
origin for living sharks and rays within
Schaeffer's (1967) "hybodont level"
(ctenacanths and hybodonts). Narrowing
their ancestry to ctenacanths simplifies the
compilation of derived characters separat-
ing the neoselachians from ctenacanths
and various non-euselachian sharks of the
Paleozoic, and also clarifies relationship of
neoselachians to one another and to
hybodonts, which only parallel them in
some derived characters. Combining
Maisey's (1975) data with my own com-
parative work on living sharks and rays
suggests the following set of hypotheses: 1)
Living sharks and rays stem from a com-
mon ancestral group of neoselachian
sharks. 2) This group has many derived
characters relative to well-known Missis-
sippian ctenacanths (C. costellatus and
Goodrichthyes) and non-euselachian sharks
that are widespread among living
neoselachians. 3) This group has many
primitive characters found in ctenacanths
and non-euselachians, some of which ap-
pear in mosaic distribution among living
neoselachians. 4) Living groups show a
mixture of primitive and derived charac-
ters relative to the ancestral group, with
hexanchoids and squaloids perhaps most
primitive, batoids least so. Derived charac-
ters of living groups indicate a higher state
of derivedness away from the ctenacanth
condition.
As a conceptual framework and a basis
for comparison and conjecture I propose a
set of primitive and derived characters for
the ancestral neoselachian group, from
comparisons of living neoselachians with
fossil sharks. This amounts to the cir-
cumscription of an ancestral neoselachian
"morphotype" (Fig. 1) like Zangerl's (1973)
"morphotypic design of [a] modern elas-
mobranch" or Maisey's (1975) "Euselachi-
form." Primitive characters include an anal
fin; two dorsal fins with ornamented
ctenacanth-like spines and large basal car-
tilages; pectoral fins with three basal cartil-
ages (propterygium, mesopterygium and
metapterygium); long jaws and a long
mouth gape; upper jaw (palatoquadrate)
with two articulations with the neuro-
cranium, an anterior one between a low
orbital process and the front of the orbit,
and a posterior one between the quadrate
process of the jaw and the rear surface of
the postorbital process of the cranium; a
deep groove with overhanging ridge
(quadrate groove) on outer posterior face
of palatoquadrate; suborbital shelves, sup-
raorbital crests, and complete postorbital
walls on the neurocranium; teeth with a
large median cusp, small side cusps, ridges
or sculpture on enameloid, low, flat roots