The oldest hexanchiform shark from the Southern Hemisphere (Neoselachii; Early Cretaceous, Antarctica)

Abstract

The oldest record of the hexanchiform sharks from the Southern Hemisphere and the second chondrichthyan report known from Carboniferous to Early Cretaceous beds in Antarctica is given. The material was collected in late Aptian rocks of the Kotick Point Formation outcropping in the western part of James Ross Island, near Antarctic Peninsula. It consists of an isolated tooth assignable to a hexanchiform different from the other described genera. The tooth shows putative plesiomorphic cusp (few cusps, no serrations) and apomorphic root characters (relatively deep, quadrangular). It could be related to a species close to the origin of Hexanchus (unknown in beds older than Cenomanian).

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Antarctic Science 21(5), 501–504 (2009) &Antarctic Science Ltd 2009 doi:10.1017/S0954102009990228
The oldest hexanchiform shark from the Southern Hemisphere
(Neoselachii; Early Cretaceous, Antarctica)
ALBERTO LUIS CIONE
1
* and FRANCISCO MEDINA
2
1
Divisio
´n Paleontologı
´a de Vertebrados, Museo de La Plata, 1900 La Plata, Argentina
2
Departamento de Ciencias Geolo
´gicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires,
Ciudad Universitaria, 1428 Buenos Aires, Argentina
*acione@museo.fcnym.unlp.edu.ar
Abstract: The oldest record of the hexanchiform sharks from the Southern Hemisphere and the second
chondrichthyan report known from Carboniferous to Early Cretaceous beds in Antarctica is given. The
material was collected in late Aptian rocks of the Kotick Point Formation outcropping in the western part of
James Ross Island, near Antarctic Peninsula. It consists of an isolated tooth assignable to a hexanchiform
different from the other described genera. The tooth shows putative plesiomorphic cusp (few cusps, no
serrations) and apomorphic root characters (relatively deep, quadrangular). It could be related to a species
close to the origin of Hexanchus (unknown in beds older than Cenomanian).
Received 6 December 2008, accepted 23 March 2009
Key words: Aptian, Hexanchiformes, James Ross Island, Neoselachii
Introduction
The chondrichthyan fossil record from Antarctica is scant
and patchy. It is restricted to some Devonian primitive
sharks (Young 1982), a palaeospinacid tooth from Early
Cretaceous beds (Kriwet 2003), some sharks, rays, and
holocephalan from late Cretaceous beds of Ross, Vega, and
Seymour Islands, and especially the diverse record of the
Eocene La Meseta Formation of Seymour Island (Cione
et al. 1977, Welton & Zinsmeister 1980, Cione & Medina
1987, Grande & Chatterjee 1987, Richter & Ward 1990,
Long 1992, 1994, Cione & Reguero 1994, 1998, Klug et al.
2005).
Hexanchiformes present a single dorsal fin without
preceding spine, more than five branchial slits, and peculiar
teeth. We here follow the definition of Carvalho (1996) which
includes Clamydoselachus in the order. Hexanchiformes
ranges from the Sinemurian (lower Jurassic) to the Recent
(Underwood & Ward 2004). Presently, they are distributed
almost worldwide but in the lower Cretaceous they are known
from very few localities in Europe and Australia (Cappetta
1987, 1990, Thies 1987, Siverson 1997). According to Thies
& Reif (1985), Hexanchiformes (excluding Clamydoselachus)
are the first true ‘‘meat-cutters’’ among the neoselachians and
its appearance could have been stimulated by the rise of the
big amphibious and marine reptiles during the Triassic. In this
paper, a peculiar hexanchiform tooth from Early Cretaceous
beds of the James Ross Island is described.
Stratigraphical and geographical provenance
James Ross Island is near the north-eastern part of the
Antarctic Peninsula. During the Mesozoic to Tertiary a
volcanic arc was located in the Antarctic Peninsula, while a
back-arc basin (the James Ross Island or Larsen basin)
developed to the east. Cretaceous strata on James Ross
Island comprise a thick succession divided into two major
lithostratigraphic units: the Gustav Group (Ineson et al.
1986) and the Marambio Group (Olivero et al. 1986).
The Cretaceous deposits of the Gustav Group are
exposed in western James Ross Island forming a belt of
about 50 km in length (Fig. 1). The group includes, from
base to top, the following stratigraphic units: Lagelius Point
Formation, Kotick Point Formation, an unnamed unit,
Whisky Bay Formation, and Hidden Lake Formation
(Ineson et al. 1986, Medina & Riccardi 2005). The group
is characterized by thick sequences of conglomerates,
breccias, sandstones, and mudstones that show marked
lateral and vertical facies variations.
The tooth here described was collected about 2.1 km
south-west of Kotick Point. In this locality two sections
(Fig. 1, A' & A) have been measured along a succession,
about 330 m thick ascribed to the Kotick Point Formation
(Fig. 1). The base is not exposed.
The lowest 170 m, studied along Section A' and the lower
part of the succession exposed in Section A, are characterized
by grey silty mudstones and conglomerates with minor fine
sandstone intercalations. Late Aptian ammonoids are present
in different levels. Sanmartinoceras sp. and Pseudosilesites
russoi (Leanza) occur between 124–160 m, ‘‘Lithancylus’’ cf .
guanacoense (Leanza) between 94–192 m, and species of
genera Australiceras and Tropaeum between 65–156 m.
The shark tooth level is located at 102 above the base of the
section (Fig. 1).
The following 75 m (170–245 m) are composed by
mudstones and siltstones, with subordinate intercalations
501

of fine sandstones and conglomerates. The overlaying 85 m
(245–330 m) comprise massive dark grey siltstones and
mudstones with intercalations of fine-grained sandstones
and occasional thin conglomerates. The upper fauna
(197–330 m) of the Kotick Point Formation is dominated
by molluscs. They are locally abundant but occur in low
diversity. Some of the molluscs, figured and described
by Medina et al. (1983, 2003), Medina & Riccardi
(2006), and Medina (2007), are characteristic of the
Lower Albian.
Age
The shark tooth described here from Kotick Point is
certainly of Late Aptian age. Species of the genus
Australiceras and Tropaeum are characteristic of the
Aptian. All of these taxa occur in the section where the
tooth was found. This interval is securely dated as
Late Aptian by the occurrence of the heteromorph ammonite
‘‘Lithancylus’’ cf. guanacoense which is particularly
important because it is restricted to the Upper Aptian
Australiceras hallei Assemblage Zone and Peltocrioceras
deeckei Assemblage Zone in Patagonia (Medina & Riccardi
2005).
Systematics
Neoselachii
Hexanchiformes
Family indeterminated
Fig. 2
Material and repository: MLP 98-XI-20-1. An almost complete
lower right lateral tooth. Departamento Paleontologı´a de
Vertebrados, Museo de La Plata, 1900 La Plata, Argentina.
Geographic and stratigraphic provenance: James Ross
Island, Antarctic Peninsula; Kotick Point Formation, Upper
Aptian.
Collector: The material was collected by Francisco
Medina in 1997.
Description: Total length (TL): 13.8 mm, maximum height
(MH): 7.3 mm, maximum root height (MRH): 3.8 mm (TL/
MRH 53.6, TL/MH 51.9).
The crown is labiolingually compressed and the
enameloid is smooth. There is a large distally inclined
main cusp followed by three (perhaps four) distally inclined
cusplets. Main cusp and distal cusplets present the same
angle what indicates that it is a lower tooth. The main cusp
has a long mesial smooth cutting edge. The upper part of
Fig. 1. Maps showing the position of
the section measured (A, A') through
the Kotick Point Formation. The
arrow indicates the level where the
shark tooth was found.
502 ALBERTO L. CIONE & FRANCISCO MEDINA

the main cusp mesial cutting edge is convex, the middle
part of the mesial cutting edge and the distal cutting edge
are concave. The anteriormost part of the tooth is missing
but there are not mesial cusplets or serrations of any kind.
Distal cusps decrease gradually in size and present convex
mesial and distal cutting edges.
The analaucorhize root is labiolingually compressed,
tabular and deep. The lingual longitudinal protuberance is
straight and well developed. Root-crown junction occurs in
the thickest part of the tooth and it is gently convex.
Discussion
The following combination of characters allows us to
distinguish the present material from teeth of other recent and
fossil Hexanchiformes such as Notidanodon,Pachyhyexanchus,
Heptranchias,Notorynchus,Clamydoselachus,Weltonia,
Welcommia,‘‘Eonotidanus’’ serratus (Kriwet & Klug 2004),
Paranotidanus,Notidanoides pockrandti, and typical
Hexanchus (see Cappetta 1987, Ward & Thies 1987, Cione
& Reguero 1994, Cione 1996, Underwood & Ward 2004):
reduced mesiodistal length; crown labiolingually compressed;
absence of mesial cusplets or serrations; moderately developed
main cusp; three or four distal cusps, gradually decreasing in
size and with convex mesial and distal cutting edges; deep,
tabular, and compressed root; lingual longitudinal protuberance
straight and well developed, close to the root-crown junction. It
also differs from the Jurassic–Cretaceous Pseudonotidanus,
which combines characters of Hexanchiformes and
Synechodontiformes (Underwood & Ward 2004, Kriwet
& Klug 2004).
MLP 98-XI-20-1 shows putative apomorphic characters
such as a deep, quadrangular (TL/MRH 53.6) and
compressed root with primitive characters such as few
cusps and no serrations. The crown is relatively similar to
that of Notidanoides muensteri figured by Kriwet & Klug
(2004) from the Tithonian of Germany. However, the root
is not similar to coeval sharks such as Notidanodon
lanceolatus and Notidanoides pockrandti and Jurassic
species. On the contrary, the root is similar to that of
some teeth of Notorynchus aptiensis, and other younger
genera. Actually, except for the small number of cusps, it
resembles Hexanchus lower teeth. Hexanchus has not been
certainly recognized in beds older than Cenomanian
(Underwood & Mitchell 1999). Previous Jurassic records
appear to have been misidentifications.
The material is the oldest record of Hexanchiformes
from the Southern Hemisphere. There has been some
argument as to whether the Devonian shark Mcmurdodus
from Antarctica and Australia belongs in Hexanchiformes
(Young 1982, Turner & Young 1987). However, this
assignment was discarded by Burrow et al. (2008). The
oldest certain record of Hexanchiformes is early Jurassic
(de Beaumont 1960). The oldest reliable record of a recent
genus is represented by the species Notorynchus aptiensis
from the lower Cretaceous. Siverson (1997) described the
occurrence of Notidanodon lanceolatus from the Albian of
Australia. This is the only other hexanchiform (although
younger) report from southern continents from the lower
Cretaceous. Besides, the only other record of a shark from
Early Cretaceous beds of Antarctica is a tooth from the
Spartan Glacier Formation of the Fossil Bluff Group of
Alexander Island, Antarctic Peninsula which was assigned
to Palaeospinacidae, an extinct group of neoselachian
sharks (Kriwet 2003). This tooth was previously supposed
to have been recovered from Middle Jurassic strata.
However, sedimentation of the Spartan Glacier Formation
was from the Valangian to Aptian. Consequently, an Early
Cretaceous age was assigned to the specimen although its
exact stratigraphical horizon is unknown (Kriwet 2003).
Thies (1987) suggested that the Early Cretaceous cow
sharks were distributed according to temperature: Notidanoides
pockrandti and Notorynchus aptiensis were presumably
Tethyan faunal elements whereas Notidanodon lanceolatus
would be a Boreal faunal form. Jurassic hexanchiforms were
found also in these areas. Notwithstanding that the Jurassic and
Early Cretaceous elasmobranch record for the Southern
Hemisphere is extremely reduced (e.g. Siverson 1997, Cione
1999, Perea et al. 2001, Cione et al. 2002, Kriwet 2003),
the occurrence of N. lanceolatus in Australia (Siverson
1997) would confirm the bipolar biogeographical pattern
for Notidanodon hypothesized by Cione (1996).
Acknowledgements
We would like to thank the Agencia Nacional de
Promocio´n Cientı´fica y Tecnolo´gica, Consejo Nacional de
Investigaciones Cientı´ficas y Te´cnicas, and Universidad
Nacional de La Plata, for permanent financial support. To
Dr C.A. Rinaldi, former Director of the Instituto Anta´rtico
Argentino, and Fuerza Ae´rea Argentina, for field support.
Fig. 2. MLP 98-XI-20-1. Lower right lateral tooth. The line is
2 mm.
APTIAN HEXANCHIFORM FROM ANTARCTICA 503

We also thank the reviewers John Long and Ju
¨rgen Kriwet
for valuable comments.
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