Late Eocene gadiform (Teleostei) skull from Seymour Island, Antarctic Peninsula

Abstract

On the basis of a skull from the late Eocene La Meseta Formation on Seymour Island, Antarctic Peninsula, a gadiform fish is reported from the Antarctic region for the first time. This specimen, the most completely preserved fossil teleost cranium yet described from Antarctica, provides convincing evidence for the presence of Gadiformes in a far southerly location under temperate climatic conditions 40 m.y. ago. The exoccipital condyles, supraoccipital and lambdoidal crests, and post-temporal and supratemporal fossae are well preserved, as are the roofing bones on the posterior half of the skull. Comparative osteological study indicates that these features are very similar in appearance to those of merlucciids (hakes) and gadids (cods).

Full text

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Antarctic Science
3
(I):
87-95
(1991)
Late Eocene gadiform (Teleostei) skull from Seymour Island,
Antarctic Peninsula
JOSEPH
T.
EASTMAN'
and
LANCE GRANDE2
'Department
of
Zoological and Biomedical Sciences,
Ohio
University, Athens, Ohio
45701
-2979,
USA
2Departmeci
of
Geology, Field Museum
of
Natural History, Chicago,
Illinois
60605-2496,
USA
'Correspondence and reprint requests
Abstract:
On the basis of a skull from the late Eocene La Meseta Formation on Seymour Island, Antarctic
Peninsula, a gadiform fish is reported from the Antarctic region for the first time. This specimen, the most
completely preserved fossil teleost cranium yet described from Antarctica, provides convincing evidence for
the presence of Gadiformes in a far southerly location under temperate climatic conditions
40
m.y.
ago. The
exoccipital condyles, supraoccipital and lambdoidal crests, and post-temporal and supratemporal fossae are
well preserved, as are the roofing bones on the posterior half of the skull. Comparative osteological study
indicates that these features are very similar in appearance to those of merlucciids (hakes) and gadids (cods).
Received
28
June
1990,
accepted
25
September
1990
Key
Words:
La Meseta Formation, cranial osteology, gadiform evolution, fossil fish.
Introduction
With its rich fossil beds and key Gondwanan location during
the late Cretaceous and early Tertiary, Seymour Island
(Fig.
1)
has provided critical
data
for interpreting
the
role of
Antarctica in the evolution, dispersal and biogeography of
groups ranging from plants (Case 1988)
to
benthic marine
invertebrates (Zinsmeister
&
Feldmann 1984) and marsupials
(Woodbume
&
Zinsmeister 1984, Case
et
al.
1988). Seymour
Island is regarded
as
the most important terrestrial site for
late Cretaceous/early Tertiary fossils in
the
Southern
Hemisphere (Woodburne 1988).
Seymour Island is also amajor locality forpost-Palaeozoic
fossil fishes in Antarctica, and one of only two known
Antarctic sites for fossil teleosts. The late Cretaceous and
lateEocene faunas of Seymour Island include adiverse array
of chondrichthyans (sharks, saw sharks, myliobatid rays and
ratfishes), siluriform, trachichthyoid and clupeoid teleosts
and a variety of fragmentary and unidentifiable teleost jaw
bones and vertebrae (Welton
&
Zinsmeister 1980, Grande
&
Eastman 1986, Grande
&
Chatterjee 1987, Cione
&
Medina
1987, Eastman
&
Grande 1989, Doktor
et
al.
1988). The
recentreportofgadiforms from SeymourIsland(Jerzm&ska
1988) is based on isolated centra only, and is therefore
somewhat tenuous. Study of the Seymour Island faunas has
added greatly
to
our knowledge of the higher taxonomic
diversity of fossil fishes from Antarctica.
The Seymour Island faunas are not closely related to any
component of the Recent Antarctic fish fauna (Eastman
&
Grande 1989). The Recent fauna is exclusively marine and
is less diverse than might
be
expected given theconsiderable
age and large area of the ecosystem. The Notothenioidei, a
perciform suborder of about 100 species, are the dominant
element of the highly endemic Recent fauna (DeWitt 1971).
87
Since they are without representation in the fossil record and
lack an identifiable sister group, hypotheses pertaining to the
origin of the Notothenioidei are speculative (Eastman 1991).
In this paper we describe the skull of
a
hake- or cod-like
teleost (order Gadiformes) from
a
late Eocene deposit on
Seymour Island. It is a significant discovery for several
reasons. This specimen is the most intact teleost cranium
known from Antarctica. Furthermore,
few
skeletal (non-
otolith)
remains
of
Eocene
gadrforms
are
known
from
anywhere
in the world (Rosen
&
Patterson 1969, Nolf
&
Steurbaut
1989), and this fossil provides convincing evidence for the
presence of this group in a far southerly location. With the
exception
of
reputed otoliths from Australia (Schwarzhans
1985) and isolated vertebrae from Seymour Island (Jerzmaliska
198S), this fossil
is
from
the
oldest and only known gadiform
locality in the Southern Hemisphere. There are
a
few
gadiforms in the Recent Antarctic fauna (Gon
&
Heemstra
1990), hence this gadiform fossil is the first established,
although not ancestral, link
between
an order of fossil and
modem teleosts
in
Antarctica. Finally,
as
Seymour Island is
distant from the suspected centre of origin of gadiforms in
the boreal Atlantic (Svetovidov 1948,
Ho
1990), the fossil
also provides a new perspective on the biogeography of the
group.
Abbreviations
Institutional;
BMNH, British Museum (Natural History),
Department of Palaeontology, London;
FMNH,
Field Museum
of Natural History, Chicago;
FSFL,
Far
Seas
Fisheries
Research Laboratory, Shimizu, Japan; MCZ, Museum of
Comparative Zoology, Harvard University, Cambridge,
MA;
USNM, National Museum of Natural History, Smithsonian

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88
J.T.
EASTMAN
and
L.
GRANDE
5
VALLEY
MBR
0
5
hm
Fig.
1.
Map
of
Seymour Island
(from
Case 1988) indicating
late Eocene locality
RV-8200
within the La Meseta
Formation. Fossil gadiform (USNM 433396) was collected at
this
site. Inset shows position of Seymour Island relative
to
South
America and the Antarctic Peninsula.
Institution, Washington.
Osteological,
in
figures:
bcsc, bony cover of supraorbital
sensory canal; bcst, bony cover of supratemporal sensory
canal;
boc,
basioccipital;
epo,
epiotic;
exo,
exoccipital;
exoc, exoccipital condyle; fr, frontal; frcr, frontal crest; fsn,
foramen for first spinal nerve: le, lateral ethmoid; mc,
mucous cavity; mes, mesethmoid;
ns,
neural spine of first
vertebra; pa,parietal; pa1,palatine; ptf, post-temporal fossa:
pto, pterotic; ptt, post-temporal; pzy, prezygapophysis
of
first vertebra:
soc,
supraoccipital; socr, supraoccipital crest;
spo,
sphenotic;
vo,
vomer.
Material examined
Dry skulls (DS), cleared and stained specimens (C&S) and
fossils examined in this study are listed below. Those
without an institutional abbreviation are privately held by
J.T.
Eastman. Names of extinct taxa are preceded by a
Gadidae
d%ger
0).
G& cullarius:
1 specimen
(FMNH
59940). DS. Georges
Bank.
Gadus
ogac:
1 specimen (FH 51362).
DS.
Holstenborg,
Greenland.
Gadus morhua:
1 specimen (FMNH CNHM 52556).
DS.
No locality
data;
4 specimens.
DS.
No locality data.
Microgadus tomcod
2 specimens (FMNH 9784
&
9785).
DS.
Atlantic coast of North America.
Pollachius virens:
1
specimen.
DS.
No
locality data.
Lota
lota:
1 specimen (FMNH 85962). DS. Lake
Lotidae
Michigan,
Lake
County, Indiana.
Molva vulgaris:
1
specimen (FMNH 34). DS. Germany.
Nezumiu aequalis:
2 specimens (FMNH 67790).
C&S.
Macrouridae
Surinam.
Melanonidae
Melanonus zugmayeri:
3 specimens (FMNH 65808).
C&S. Gulf of Mexico.
Merlucciidae
Macruronus nouaezelandiae:
2 specimens
(FSFL
uncatalogued). DS. New Zealand.
Merluccius albidus:
1
specimen (MCZ 51467). DS.
North
Sargasso Sea.
Merluccius bilinearis:
1
specimen (MCZ 90398). DS.
Atlantic Ocean; 2 specimens
(FMNH
45398). C&S.
Gulf of Mexico.
Merlucciusproductus:
1
specimen (USNM 289520). DS.
Puget Sound, Washington:
1
specimen (MCZ 90399).
DS.
Monterey Bay, California.
tRhinocephalus planiceps:
2 specimens (BMNH
P.
1738f
&
P.9429). From the
early
Eocene
London
Clay,
Sheppey, England.
Steindachneria argentea:
3
specimens (FMNH 67856).
C&S. Gulf of Mexico.
Muraenolepidae
Percopsidae
Muraenolepis microps:
1 specimen.
DS.
South Georgia.
Percopsis guttatus:
1
specimen (FMNH 155).
DS.
No locality data.
Ophidiidae
Ophidion holbrookii:
2
specimens (FMNH 69282). C&S.
Campeche, Mexico.
Carapidae
Carapus bermudensis:
1
specimen (FMNH 79881).
C&S.
Gulf of Mexico.
Batrac hoididae
Opsanus beta:
2 specimens (FMNH 40287). C&S.
Corpus Christi, Texas;
1
specimen.
DS.
Florida.
Lophiodes reticulatus:
1
specimen (FMNH 77261). C&S.
Lophiomus setigerus:
1 specimen (FMNH 57144).
C&S.
Lophius americanus:
1
specimen.
DS.
New Jersey.
Histrio
sp.: 4 specimens (FMNH46140). C&S. Angelfish
Lophiidae
No locality data.
Misaki, Japan.
An tennariidae
Creek, Miami, Florida.
Ogcocephalidae
Ogcocephalus
mutus:
1
specimen
(FMNH
64894). C&S.
off Guianan coast.

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LATE EOCENE GADIFORM
FROM
SEYMOUR
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89
Systematic palaeontology
Class OSTEICHTHYES (Huxley 1880)
Subclass ACTINOPTERYGII (Cope 1887)
Subdivision TELEOSTEI
(sensu
Patterson
&
Rosen 1977)
Superorder PARACANTHOPTERYGII
(sensu
Patterson
&
Rosen 1989)
Unranked subgroup ANACANTHINI
(sensu
Patterson
&
Rosen 1989)
Order
GADIFOW
(sensu
Cohen 1984)
Figs 2-4
Material.
USNM
4333% dorsal portion of a skull with
an
exceptionally
wellpreservedright posteriorregion. This is the only known
specimen.
Locality, age and horizon.
RV-8200 (University of California, Riverside fossil mammal
locality), La Meseta Formation, north-western Seymour
Island, northern Antarctic Peninsula
(64”
13’s;
56’40
W);
late Eocene, approximately 40 Ma.
Woodburne
&
Zinsmeister (1984) thoroughly reviewed
the age and palaeontological correlations of La Meseta
Formation and Case (1988) and Case
et
al.
(1988) gave a
detailed stratigraphy
with
diagrams for locality RV-8200
(Fig. 1). Zinsmeister
&
Feldmann (1984) concluded that
many Seymour Island fossils were deposited in a beach
facies, indicating that the fauna was adapted to shallow
conditions.
Reasons
for
inclusion
in
Gadiformes.
Character 11 in Patterson
&
Rosen’s (1989, p. 33) proposed
cladogram of paracanthopterygians is the only observable
synapomorphic (diagnostic) feature for the Seymour Island
fossil: “Exoccipital facets ‘cod-like’, the exoccipital condyles
widely separated, cartilage-filled and tube-like, articulating
with
comparably modified ‘prezygapophyses’ on the first
centrum” (Figs 2
&
3). This character diagnoses the fossil
as a member of the Gadiformes, Pediculati or Bythitoidei.
We believe it is a gadiform because the shape and pattern of
thin, sharp crests on the dorsal surface of the skull closely
resembles that of a hake or cod and not that of any member
of Pediculati or Bythitoidei that we have
seen.
The dorsal
and posterior aspects of the cranium of the fossil do not
exhibit any other synapomorphic characters that allow a
more precise diagnosis
(see
comparison below).
Description.
The specimen
consists
of a
well
preserved
partd
neurocranium.
Matrix removal provided a dorsal exposure, with detail best
preserved in the posterior half of the skull.
The specimen measures 112
mm
from the anteriormost
point on the mesethmoid
to
the posterior tip of
the
supraoccipital
crest. The length of
the
orbitorostral portion of the skull,
as
measured from the anteriormost point on the mesethmoid to
the frontal-sphenotic notch,
is
greater than the length of the
posterior portion of
the
skull.
Parts
of the vomer, parasphenoid and basioccipital are
evident on the ventral side of the skull, but they provide little
information. The intercalar, a
key
diagnostic feature of the
gadiform skull (Regan 1903, Svetovidov 1948), is not
preserved.
Radiography (Fig. 4) indicates a large flat bone, possibly
an opercle, lateral to the left posterior part of the skull. This
bone is not evident in the prepared skull. The bone
has
a
rounded lateral margin and a sharply pointed posterior
margin.
A
bony
ridge extends to the tip of the posterior
margin.
Although incomplete
on
the left, the lateral ethmoid is
visible on the right and consists
of
anterior and dorsal
processes (Mujib 1967). The dorsal process forms the
anterior quarter of the
orbital
margin and
is
the most prominent
lateral projection in the rostra1 region of the skull. The
anterior
process
and
its
medial projection
are
pamally
preserved
on the right side.
The mesethmoid is moderately
high
with a long, thin
posterior crest. The angle with a vertex between the dorsal
andanterior marginsoftheboneis 110-1 15’.Theknobarea,
whichispartially
missing,isthewidestpartofthebone.
The
ventralmost portion of the mesethmoid is obscured by the
compressed vomer and lateral ethmoid.
Portions of the vomer
are
visible both dorsally and ventrally,
with the ventral part most extensive. Since the ventralmost
portion of the vomer is missing, it
is
not known if teeth were
present. Although other bones of the suspensorium are not
evident, a portion of what may be
the
palatine is disarticulated
and preserved beneath the neurocranium on the right side.
The preserved piece may be the long, anteriorly directed
maxillary process of the palatine (Howes 1987). The nature
of the palatine-ethmoid contact is not evident.
Although the entire length of the frontals is preserved, the
most dorsal portion
of
the anterior half of the bones is
missing. Hence some superficial features are not evident
and it cannot be determined
if
the frontals are paired or fused
(Table
I).
The frontals are distinctly narrowed
in
the
interorbital region. There
is
a prominent lateral notch at the
frontal-sphenotic junction on each side.
The frontals possess a well-developed V-shaped crest
which is continuous posteriorly with the midline supraoccipital
crest. Together they form the Y-shaped cranial crest
characteristic of many gadifoms.
A
ridge on the left frontal

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90
J.T. EASTMAN and
L.
GRANDE
Fig.
2.
Seymour Island gadifonn
(USNM
433396).
a.
dorsal view
of
neurocranium.
x
1.5.
b.
drawing
of
same.
represents the remains of the frontal crest on this bone. The
frontal crests converge posteromedially on the supraoccipital
crest.
A
wide, deep mucous cavity occupies the area
between the frontal crests.
The supraorbital crests diverge from
the
frontal crests and
run posterolaterally on the frontal. They continue as the
supratemporal crests on the sphenotic and pterotic as far
as
the pterotic wing. The flange-like nature of the crest is
clearly evident at the sphenotic-pterotic junction
on
both left
and right sides. Pieces
of
the supratemporal crest are intact
in these areas and are open laterally.
The supraoccipital is complete and dominated by a high
crest. The crest is very long,
as
in other gadiforms, extending
over one-third the length of the entire skull.
A
small piece

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LATE EOCENE GADIFORM FROM SEYMOUR ISLAND
91
of the most posterior aspect and some of the middle part of
the crest are missing. When viewed laterally (Fig.
3),
the
free margin of the crest is slightly rounded and is several
times thicker than the base. The supraoccipital forms
the
dorsal boundary of the foramen magnum.
Most of the basioccipital
is
missing, although parts of the
ventralmost portion are visible.
A
portion of the concavity
of the condyle may also
be
present ventrally.
The exoccipitals
are
complete and the right
is
well exposed.
It
is
intact
to
its ventral margin where
it
would have articulated
with the missing intercalar. The dorsomedial part of the
exoccipital forms the lateral border of the foramen magnum
and contains a wide cleft for the first spinal nerve (Figs
2&3).
A
large, round posteriorly directed foramen for
the
vagus
nerve is located in
the
ventrolateral part of the bone. There
is
no fontanelle for passage of an extension of the swim
bladder into the auditory region of the skull as
in
morids.
The right exoccipital condyle is ventrolateral to the cleft
for the first spinal nerve. It is round, tube-like and its centre
is
filled with matrix. This area may have been acartilaginous
in life (Rosen
&
Patterson
1969,
Rosen
1985,
Patterson
&
Rosen
1989).
The round, matrix-filled facet of the condyle
faces posteroventrally. The condyle does not project beyond
the posterior margin of the supraoccipital crest, but it does
project beyond the posterior margin of the basioccipital
3-
Seymour Island gadifom
WSNM
433396).
a.
postero-lateral view of right rear area of neurocranium.
x
(ahhough the posterior part of this bone
is
missing).
..?
1.3.
The posterolaterally positioned post-temporal fossa is a
prominent feature of
the
skull
(Figs
2&3).
It is a wide,
b.
drawing
of
same.
rectangular depression partially defined medially by the
posteriorly projecting epiotic flange and laterally by
the
Fig.
4.
Seymour Island
gadiform
(USNM
ventral soft
radiograph with
anterior
to
the
right.
The left opercle
(arrows) is not
evident in the
prepared skull.
40
kVp,
2.15
mA
and
55
min exposure on
Kodak
Industrex
M5
film. x
1.0.
433396),
dorso-

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92
J.T.
EASTMAN
and
L.
GRANDE
pterotic flange. The ventral surface (floor) of the fossa is
formed by the pterotic and possibly by
the
parietal.
A
low
parietal crest continues anteriorly from the epiotic flange
and together they form the dorsomedial margin of the fossa.
The fossa
is
in open communication with the dorsal surface
of the sphenotic anteriorly and is completely open posteriorly.
The medial wall of the fossa is formed in part by a thin,
osseous membrane adjacent to the vestibular region inside
the cranial cavity. On the left side of the fossil
this
region is
indicated by a triangular area of matrix indicating the union
of the epiotic, parietal and pterotic
in
this membranous area.
A
supratemporal fossa, well-developed on the dorsal skull
roof
in
many acanthopterygians (Patterson 1964),
is
present
on either side
of
the posterior half
of
the supraoccipital crest
at the extreme posterodorsal margin of
the
skull. Its depth
(anterior extent) cannot
be
determined
as
it
is
completely
filled with matrix on the left and partially filled on the right.
It is open posteriorly and dorsally
so
as to allow attachment
of the anteriormost fibres of the epaxial muscIes to the bones
of
the
fossa
The fossa is bounded
medially
by the supraoccipital
and medial aspect of the exoccipital; laterally by the epiotic
and
lateral
aspect
of
the
exoccipital; venay by the exoccipital
and anteriorly by the anterior aspect of the exoccipital and
possibly by a small portion of the supraoccipital.
A
wide lambdoidal crest (Mujib 1967, Inada 1981) forms
the dorsal margin of the supratemporal fossa. The crest
includes parts of the supraoccipital and epiotic. The posterior
marginof theepiotic flangeprojects laterally andencroaches
on the medial wall of the post-temporal fossa. The epiotic
flange is angled 50-60" relative to the supraoccipital crest.
Comparison
Comparative osteological study suggests that within the
Paracanthopterygii the Seymour Island specimen
is
most
similar to the Gadiformes. The specimen is not a
batrachoidiform because, unlike toadfishes, it possesses
epiotics and a mesethmoid (Regan 1912). It is not a
lophiiform because, unlike anglerfishes, the epiotics are not
separated from the parietals and do not
meet
in the midline
posterior to the supraoccipital (Regan 1912, Pietsch 1984).
Gadiformes
are
characterized by four synapomorphies
(Patterson
&
Rosen 1989). These are either features
of
the
postcranial skeleton or biochemical characteristics not
applicable to our fossil specimen. There are no cranial
characters unique
to
Gadiformes (Marshall
&
Cohen 1973).
Whereas the following individual characters are neither
unique to nor ubiquitous within the order, they are common
to both the Seymour Island fossil and to gadiforms:
V-shaped frontal crests converging posteromedially on the
supraoccipital crest (Regan 1903, Cohen 1984), cephalic
lateral line system at least partially covered
by
a crest of the
bone on which the canals are located (Svetovidov 1948);
orbitorostral portion of skull longer than or equal to
the
posterior portion of skull (Svetovidov 1948, Fedotov 1976),
and presence of a supratemporal fossa (Rosen
&
Patterson
1969).
Familial placement is not possible because the Seymour
Island specimen possesses a mosaic of merlucciid and gadid
characters (Table
I,
Fig.
5).
The anterior region of the skull
is similar to that of a merlucciid whereas the posterior
portion appears more like a gadid skull. Within
the
Merlucciidae the specimen resembles
the
early Eocene
tRhinocephaius pianiceps
(Fig. 5a) from the London Clay
(Casier 1966) with which it sharesthesecharacters (Rosen
&
Patterson 1969): V-shaped frontal crests that converge
posteromedially on the supraoccipital crest, large crests
(outward flanges) covering parts of the supraorbital and
Table
I.
Dorsal cranial characters distinguishing merlucciids from gadids
(Mujib,
1967,
Inada,
1981).
Asterisk
(*)
indicates character state in
the
Seymour Island gadiform when this can
be
determined.
Characters Merlucciidae Gadidae
Frontal bones Paired Fused
Frontal crest V-shaped* Single, midline
Mucous cavity Single, wide, deep* Paired, narrow,
Lateral ethmoid Two parts*
Three
parts
(anterior
&
dorsal
processes) dorsal proceses)
(parietals do not
contribute)
shallow
(lateral, caudal
&
Lambdoidal crest Narrow Wide*
(parietals contribute in
gadids but not in
the
Seymour
Is.
gadiform)
Dorsal
margin of Formed by exoccipitals Formed by
foramen magnum supraoccipital*
Fig.
5.
Dorsal
photos (left) and drawings (right) of neurocrania of one fossil and two Recent gadiforms.
a.
tRhinocephuZusplaniceps
Casier
(BMNH
P.17380, early Eocene merlucciid from London Clay. Although
both
the V-shaped frontal and the midline
supraoccipital crests are broken off at their bases, remnants of the frontal crest
(arrows)
can
be
seen converging
on
the
supraoccipital crest.
x
1.6. Drawing is of the holotype
(BMNH
47985) modified from Casier (1966,
fig.
74A, p. 319).
x
1
.O.
b.
MerZuccius productus (Ayres)
(USNM
289520). a Recent merlucciid. As is common
in
many merlucciids, the first vertebra is
attached
to
the
skull
through fusion of neural spine
to
the supraoccipital crest, hence the basicraniurn appears longer than in the
other specimens in this figure. The V-shaped frontal crest (arrows) converges on the supraoccipital crest, and the supraorbital
and
supratemporal crests are also evident.
x
1.3.
supratemporal crests are also visible.
x
0.8.
e.
Gadus morhw Linnaeus, a Recent gadid. This species lacks a frontal crest but has a high supraoccipital crest. Supraorbital and

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J.T. EASTMAN
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supratemporal sensory
canals,
well-developed rostral region,
long crested mesethmoid, long supraoccipital crest, projecting
epiotics and pterotics, and exoccipital condyles well separated
from
the
basioccipital condyle. These
are
primitive characters,
possibly present in the Cretaceous ancestor of gadiforms
which may have possessed a skull roof similar to
tRhinocephalus
(Rosen
&
Patterson 1969). The V-shaped
frontal crest, for example, is present in theoldestknown non-
otolith gadiform, the unnamed t”Protocodus” from the
Palaeocene of Greenland (Cohen 1984).
The specimen from Seymour Island differs from
tRhinocephalus
in having: heavier bones with more distinct
crests; narrower skull in the lateral ethmoid and frontal
regions; a higher, more curved supraoccipital crest with a
thicker free margin; a wider lambdoidal crest; larger and
more laterally flared epiotic flange; and a more sharply
pointed posterior margin of the opercle.
Discussion
Disregarding otoliths and isolated vertebrae, there are few
Tertiary fossil gadiforms known from outside the Holarctic
Region (Rosen &Patterson 1969) and none from the Southern
Hemisphere. This Eocene gadiform from Seymour Island
is
therefore noteworthy because of its location and age, and
because it adds an additional teleostean order to the known
taxonomic diversity of the Antarctic fossil fish fauna.
Furthermore, the specimen slightly predates the known
diversification of gadiforms in the Tethyan area.
Gadiforms are thought by some authors to have originated
in boreal Atlantic waters during the Cretaceous (Svetovidov
1948). Given the warmth and continuity of the Cretaceous
Ocean as well as the absence of ice at the poles (Kennett
1982), widespread movement of the group was theoretically
possible.
The
oldest credible gadiform fossil
is
the undescribed
t
”Protocodus” from the Palaeocene of Greenland (Cohen
1984). It may
be
near the stem
of
both
the
merlucciid-gadid
and morid-macrourid lineages (Rosen
&
Patterson 1969).
According to Fedotov (1976),
tRhinocephalus,
the earliest
known merlucciid, was restricted to the temperate waters of
the North Atlantic during the early Eocene.
tPalaeogadus,
another merlucciid presumably descended from
tRhinocephalus,
moved south into the Tethyan region via
cold bottom currents later in the Eocene.
By
the early to
middle Oligocene,
tPalaeogadus
was beginning to diversify
and was distributed from France and Switzerland
to
the
Caucasus region of the USSR (Danil’ chenko 1947, Fedotov
1976, Fedotov
&
Bannikov 1989).
Merluccius,
with 12
extant species,
also
appeared in the middle Oligocene (Fedotov
&
Bannikov 1989). Studying
Merluccius
with cladistic
methodology, Ho (1990) presented a phylogenetic hypothesis
and a vicariance model of biogeography for this genus.
The tip
of
the Antarctic Peninsula has been in a high
latitude position, approximately
6O”S,
since the late Cretaceous
(Zinsmeister 1982, Richter
&
Thomson 1989). Although
marked seasonality has
been
present at least this long,
oxygen isotopic
data
indicate that surface water temperatures
during the Palaeocene were nearly
20°C
(Woodburne
&
Zinsmeister
1984).
By
late Eocene, water temperatures near
Seymour Island had declined slowly to
10°C,
however
marine life was still abundant and diverse. In fact shallow
water marine habitats, where most species and biomass are
concentrated, were more extensive during the early Tertiary
than at present (Clarke
&
Crame 1989). Plant fossils and
palynomorph floras suggest that the late Eocene climate on
Seymour Island was similar to the modern cool temperate
climates in Tasmania, New Zealand and southern South
America (Woodburne
&
Zinsmeister 1984, Case 1988).
By
the early Oligocene water temperatures had dropped
to
5°C
(Woodburne
&
Zinsmeister 1984), still relatively moderate
compared to present Antarctic conditions.
This fossil gadifom, along with those fossil fuhes previously
reported from Seymour Island (Welton
&
Zinsmeister 1980,
Grande
&
Eastman
1986, Grande
&
Chatte@
1987),
indicates
that
the
late Cretaceous and late Eocene faunas consisted
of
representatives of cosmopolitan groups. Some, like the
Seymour Island gadiform, may have had Tethyan affinities.
These fishes lived in temperate seas and were antecedent
to
the notothenioids that later dominated the
fish
fauna under
the cold conditions of the Neogene.
There are relatively few gadiforms living in the Southern
Ocean around Antarctica (Gon
&
Heemstra 1990) and none
in the subzero shelf waters near the continent. The fossil
gadiform from Seymour Island and Recent gadiforms
inhabiting
the
Southern Hemisphere are separated by a gap
of 40 m.y. We have no knowledge of a transition fauna, if
any, and no evidence that the fossil species was directly
ancestral to any modem Southern Hemisphere gadiforms.
Other Recent components of
the
Antarctic fish fauna
are
different from and without representation in the various
fossil faunas that preceded
them
in geological time
(Eastman
&
Grande 1989).
Acknowledgments
We thank Michael
0.
Woodburne for making this specimen
available to
us
for study. The specimen was prepared by Bill
Simpson (FMNH). We are grateful to William Winn for
photography (Figs 4
&
5)
and to Danette Pratt for artwork
(Fig.
5).
For
the loan or gift of specimens we thank Peter
Forey (BMNH), Barry Chernoff (FMNH), Yuji Uozumi
(FSFL),
Karsten Hartel (MCZ) and Susan Jewett (USNM).
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