Stratigraphy
The stratigraphic distribution of the valid ichthyosaurian
species is summarized in Table 2. Below is a brief review of
the stratigraphic positions of the major ichthyosaur-producing
localities (not exhaustive).
Lower Triassic
Osawa Formation (Miyagi, Japan): Most ichthyosaurian
fossils are from the so-called Subcolombites Zone (Shtkama
et al-, 1978), which probably corresponds to Keyserlingites
subrobustus Zone (Ehiro, 1993). Utatsusaurus is the only
known ichthyosaur from the formation.
Qinglong Formation (Anhui, China): The ichthyosaurian
fossils occur in the Nanlinghu Member, which corresponds
to the Neospathodus triangularis Zone and the Subcolombites
Zone (Li & Jiang, 1997). Chaohusaurus is the only
known ichthyosaur from this formation.
Sticky Keep Formation (Spitsbergen, Svalbard): Ichthyosaurian
fossils are known from two horizons: the
Lower Saurian Level (“Unteres Saurier Niveau”) at the top
of the Lower Triassic and the slightly lower Grippia Level
(“Niveau”). Both levels belong to the upper part of the
Formation, and are positioned in the Keyserlingites subrobustus
Zone, which represents the top of the Olenekian
(Harland, 1997).
Sulphur Mountain Formation (British Columbia, Canada):
The reptilian fossils are usually found in loose slabs,
which makes it difficult to determine their precise stratigraphic
derivation (Callaway & Brinkman, 1989; Brink-
man et al., 1992; Nicholls & Brinkman, 1995). Therefore,
ichthyosaurs from this formation may be from any horizon
of Early to Mid-Triassic age.
Middle Triassic
Grenzbitumen horizon (Grenzbitumen-Zone) (Switzerland
and Italy): These beds, as the name implies, straddle
the boundary of the Anisian and Ladinian. The beds correspond
to the lower upper Muschelkalk (mol) in the Germanic
Basin (Rieppel & Hagdorn, 1994: fig. 12).
Muschelkalk (Germany): The age of the Muschelkalk in
the Germanic Basin ranges from the Anisian to the Ladinian.
Mixosaurus atavus, which seems to occur in the lower
part of the beds, is of Anisian age, according to Maisch &
Matzke (1998B).
Prida Formation (Nevada, USA): Merriam (1908) reported
that most of the reptilian remains had been found in the
Fossil Hill and Saurian Hill of the West Humboldt Range,
which corresponds to the Fossil Hill Member of Nichols &
SlLBERUNG (1977). Only the upper part of this member (the
uppermost shononensis to the lower meeki zones) is exposed
at the Fossil Hill (Nichols & Silberling, 1977), so the reptilian
fossils are probably late Anisian in age.
Sulphur Mountain Formation (British Columbia, Canada):
See Lower Triassic.
Upper Triassic
Wayao Formation (Guizhou, People’s Republic of China):
According to Y in et al. (2000), the Wayao Formation is
early Camian in age, based on conodonts such as Meta-
polygnathus polygnathiformis as well as ammonites such as
Trachyceras. However, some conodonts reported from the
same formation pose puzzling contradictions: Paragon-
dolella foliata suggests a Ladinian age; P. inclinata occurs
in both Ladinian- and Camian-age strata in the Western
Canada Sedimentary Basin; and Neogondolella navicula indicates
a Norian date (Orchard & Tozer, 1997). As Rieppel
et al. (2000) suggested, further review of the available data
is necessary. It should be noted that Li (1999), Rieppel et al.
(2000), and the literature cited therein, treated this unit as
the Wayao Member of the Falang Formation, rather than as
an independent formation. Y in et al. (2000) is followed here
simply because the senior author is on the staff of the
Geological Survey of Guizhou Province and is expected to
know the local geology.
Hosselkus Limestone (California, USA): Merriam (1908)
reported that the majority of ichthyosaurs from the Hosselkus
Limestone are from the beds containing the ammonites
Juvavites and Tropites subbullatus, located near the top
of the Lower Division. This corresponds to the welleri Zone
(late Camian) of Silberling & Tozer (1968). Shonisaurus sp.
(previously identified as Shastasaurus careyi) occurs in the
Upper Division (Merriam, 1908), which probably corresponds
to the Klamathites macrolobatus Zone (latest Carman).
Luning Formation (Nevada, USA): Camp (1980) reported
that the main quarries for ichthyosaurian remains from the
Luning Formation of Nevada are located in the Klamathites
macrolobatus zone (latest Camian), but scattered specimens
do occur in the zone below.
Pardonet Formation (British Columbia, Canada): Ichthyosaurian
remains occur throughout the Pardonet Formation
of NW British Columbia, which represents almost
the entire Norian stage (Orchard & Tozer, 1997).
Lower Jurassic
Lower Lias: The majority of ichthyosaur species collected
from England are from this horizon, which is Hettangian
to Sinemurian in age and is often referred to as the Blue
Lias. Primary localities are Lyme Regis and Charmouth, on
the Dorset coast, and Street, Somerset. Much of the Street
material was collected from the pre-Planorbis Beds (Ar-
KELL, 1933), and thus is Rhaetian in age. Ichthyosaurs are
scarce in Pliensbachian-age strata, the primary locality
being near Seatown, on the Dorset coast, where their remains
are found in the Belemnite Marls. Other Lower
Liassic localities include the vicinity of Arlon, southern
Belgium.
Upper Lias: Most of the ichthyosaurs collected from Germany
are from this interval (Posidonienschiefer; Toarcian),
the primary locality complex being the Holzmaden area of
Württemberg in southern Germany. Other localities in this
stratigraphic interval include Whitby, Yorkshire, and Ilm-
inster, Somerset in England as well as sites in Normandy
(northern France) and Luxembourg.
Middle Jurassic:
Most of the material (Ophthalmosaurus) from the Oxford
Clay of England is Callovian in age, extending up into the
Oxfordian. The primary occurrence is the vicinity of Peterborough,
Northamptonshire, and in Oxfordshire, with
additional fossil-bearing outcrops in Cambridgeshire, Dorset,
and Wiltshire. The bones are generally very well preserved,
but skeletons are usually disarticulated and often
scattered. Other important Middle Jurassic localities for
ichthyosaurs are in Neuquen Province, Argentina, and are
Bajocian and Bathonian in age.
Upper Jurassic
The upper portion of the Oxford Clay extends into the
lowermost Oxfordian, with localities mostly in Cambridgeshire.
Much of the English Late Jurassic material is from
the Kimmeridge Clay (Kimmeridgian), with many localities
in various counties including Weymouth and Kimmeridge
(Dorsetshire), Swindon (Wiltshire), Shotover (Oxfordshire),
Ely (Cambridgeshire), and Stowbridge (Nor-
folkshire). The North American material is primarily from
the Sundance Formation of Wyoming, the age of which
ranges from Callovian to Oxfordian. Ichthyosaurs are also
known from the Tithonian-age lithographic limestones of
the Solnhofen region in the Altmühl Valley of Bavaria,
Germany, as well as from Tithonian-age strata in the Volga
region of Russia and in Neuquén Province, Argentina.
Lower Cretaceous
Isolated elements, mostly humeri and basioccipitals, occur
in the Upper Greensand (Albian) of Cambridgeshire, England.
Material is also known from the Volga region of
Russia (Hauterivian), northern Germany (Aptian), France
(Albian), northern Queensland (Albian), the Loon River
Formation of northwestern Canada (Albian), and Texas
and Wyoming (both Albian). The Wyoming material is
from the uppermost portion of the Mowry Shale.
Upper Cretaceous
In Wyoming, ichthyosaurs are known from the basal portion
of the Belle Forche Shale Member of the Colorado
Group (earliest Cenomanian). Material is plentiful in the
English Lower Chalk in Kent, and in Cambridgeshire
(Cenomanian), but it is largely fragmentary, comprising
only rostral segments, teeth, and fin elements. Ichthyosaurs
are not known to range beyond the Cenomanian.