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A new Lower Triassic ichthyopterygian
assemblage from Fossil Hill, Nevada
Neil P. Kelley
1,2
, Ryosuke Motani
2
, Patrick Embree
3
and
Michael J. Orchard
4
1
Department of Paleobiology, National Museum of Natural History, Smithsonian, Washington,
District of Columbia, United States
2
Department of Earth and Planetary Sciences, University of California, Davis, Davis, California,
United States
3
Orangevale, California, United States
4
Natural Resources Canada–Geological Survey of Canada, Vancouver, British Columbia, Canada
ABSTRACT
We report a new ichthyopterygian assemblage from Lower Triassic horizons of the
Prida Formation at Fossil Hill in central Nevada. Although fragmentary, the
specimens collected so far document a diverse fauna. One partial jaw exhibits
isodont dentition with blunt tipped, mesiodistally compressed crowns and striated
enamel. These features are shared with the Early Triassic genus Utatsusaurus known
from coeval deposits in Japan and British Columbia. An additional specimen
exhibits a different dentition characterized by relatively small, rounded posterior
teeth resembling other Early Triassic ichthyopterygians, particularly Grippia. This
Nevada assemblage marks a southward latitudinal extension for Early Triassic
ichthyopterygians along the eastern margin of Panthalassa and indicates repeated
trans-hemispheric dispersal events in Early Triassic ichthyopter ygians.
Subjects Evolutionary studies, Paleontology
Keywords Triassic, Ichthyosaur, Ichthyopterygia, Marine reptile, Nevada
INTRODUCTION
Ichthyosaurs were among the most enduring and successful Mesozoic marine reptile
groups, appearing in the Early Triassic and persisting some 150 million years until their
extinction in the Late Cretaceous (McGowan & Motani, 2003). The fossil record of early
ichthyopterygians (the clade comprising ichthyosaurs and close relatives) includes a
variety of morphologically disparate taxa from widespread localities in Asia, North
America and the Arctic. Most of these assemblages are broadly contemporaneous, all
being late Spathian (late Early Triassic) in age. Recent discoveries in China (Ji et al., 2014;
Motani et al., 2015) have extended this record earlier into the Spathian and have shed new
light on the phylogenetic and biogeographic origins of the clade. However, the rapid early
diversification and trans-hemispheric dispersal histor y of ichthyopterygians during the
Early Triassic remains poorly understood.
Nevada has been an important source of Triassic marine reptile fossils since the 19
th
Century producing abundant and well-preserved Middle Triassic (Leidy, 1868; Merriam,
1905; Merriam, 1908; Merr iam, 1910; Sander, Rieppel & Bucher, 1994; Sander, Rieppel &
Bucher, 1997; Fro
¨
bisch, Sander & Rieppel, 2006; Fro
¨
bisch et al., 2013) and Late Triassic
How to cite this article Kelley et al. (2016), A ne w Lower Triassic ichthyopterygian assemblage from Fossil Hill, Nevada. PeerJ 4:e1626;
DOI 10.7717/peerj.1626
Submitted 23 October 2015
Accepted 5 January 2016
Published 26 January 2016
Corresponding author
Neil P. Kelley, kelleynp@si.edu
Academic editor
Andrew Farke
Additional Information and
Declarations can be found on
page 12
DOI 10.7717/peerj.1626
Copyright
2016 Kelley et al.
Distributed under
Creative Commons CC-BY 4.0
(Camp, 1976; Camp, 1980) ichthyopterygian and sauropterygian fossils. Notably, Early
20
th
Century field work led by John Merriam and Annie Alexander at the Fossil Hill locality
in the Humboldt Range produced several specimens of the ichthyosaur Cymbospondylus
(Merriam, 1908)–previously described by Leidy (1868) on the basis of fragmentary remains–
as well as the type specimens of Omphalosaurus nevadanus (Merriam, 1906)andPhalarodon
fraasi (Merriam, 1910). Later work by Camp (1976); Camp (1980); Sander, Rieppel & Bucher
(1994); Sander, Rieppel & Bucher (1997) and Schmitz et al. (2004) and others illuminated rich
Middle and Late Triassic marine reptile assemblages preserved in Nevada.
In contrast, knowledge of Early Triassic marine reptile fossils in this region is scant.
The only published Early Triassic marine reptile occurrence from Nevada is based on a
partial jaw referred to the enigmatic genus Omphalosaurus and described as a second
species, O. nettarhynchus (Mazin & Bucher, 1987). This specimen was collected from the
Spathian-aged informally designated “lower member” of the Prida Formation in the
Humboldt Range, which sits immediately below the well-known Fossil Hill Member of the
Prida Formation, famous for its rich marine reptile assemblage including the
aforementioned Cymbospondylus, Phalarodon, and Omphalosaurus nevadanus.
Fragmentary, float-derived remains of Early Triassic ichthyopterygians have been reported
from Spathian horizons in southeastern Idaho (Massare & Callaway, 1994; Scheyer et al.,
2014) roughly 500 km to the northeast of the Fossil Hill locality. Even further to the east,
the early sauropterygian Corosaurus alcovensis (Case, 1936) is known from the Alcova
limestone in Wyoming whose Early Triassic age was recently confirmed (Lovelace &
Doebbert, 2015).
Here, we report a new Early Triassic ichthyopterygian assemblage from the lower
member of the Prida Formation at the Fossil Hill Locality. These fossils are Spathian
(Lower Triassic) in age based on co-occuring conodont and ammonoid faunas and sit
stratigraphically below the diverse Middle Triassic marine reptile assemblage from the
Fossil Hill Member of the Prida Formation. These occurrences extend the southward
latitudinal range of early ichthyopterygians in North America and demonstrate that early
in their evolutionary history, multiple ichthyopterygian taxa quickly dispersed around or
across wide expanses of ocean and ranged from sub-tropical to hig h temperate waters on
the eastern margin of northern Panthalassa.
Institutional Abbreviations
USNM, National Museum of Natural History, Smithsonian Institution, Washington,
D.C., U.S.A.
MATERIALS AND METHODS
Geological and stratigraphic setting
The new fossils reported here were collected from multiple horizons within the unnamed
lower member of the Prida Formation of the Star Peak Group at Fossil Hill, on the eastern
flank of the Humboldt Range in Pershing County, Nevada (Fig. 1). The Star Peak Group
consists of a sequence of syndepositionally deformed carbonate-dominated units
deposited on what was then the western shelf of North America (Nichols & Silberling, 1977;
Kelley et al. (2016), PeerJ, DOI 10.7717/peerj.1626 2/16
10 m
FH1-1
FH1-1A
FH1-2
FH1-3
FH1-4
FH1-5
FH1-6
FH1-7
FH1-8
FH1-9
FH1-10
Koipato Group
Fossil Hill Member
Upper Member
Lower Member
Star Peak Group
Prida Formation
Lower Triassic
Middle T
riassic
Permo-Triassic
Spathian Stage
Anisian Stage
Ladinian Stage
Rotelliformis
Meeki
Occidentalis
Subasperum
Mimetus
Prohungarites
gutstadti
W
eitschati
??
Weaver
Rhyolite
Mostly covered.
Biostratigraphic b
oundar
ies approximate.
FH-14
FH-20
FH-30
FH-35
FH-40
Inferred horizon of USNM 559349
Distinctive ‘oncoid’/sponge horizon
USNM 559350
Small faults
may exaggerate
thickness
Location of conodont samples
A
Ammonite
zone
B
Bed
Figure 1 Summarized stratigraphy and regional map. (A) Stratigraphy of the Triassic Prida Formation near Fossil Hill in the Humboldt Range,
Nevada indicating horizons of specimens USNM 559349 and 559350 and conodont samples. (B) Regional map, modified from Silberling (1962).
Kelley et al. (2016), PeerJ, DOI 10.7717/peerj.1626 3/16
Wyld, 2000). In the study area, the lower member of the Prida Formation forms the base of
the Star Peak Group and sits unconformably atop the Permian/Lower Triassic aged Koipato
Group volcanics (Wyld, 2000).
The lower member of the Prida Formation transitions from siliciclastic sand and
conglomerate layers near the contact with the underlying Koipato Group to dark-grey
limestone above with intermittent microbialite, conglomerate and chert-dominated beds.
The presence of conglomerates and microbialites indicate relatively shallow conditions with
a general trend towards deeper water facies characteristicof the overlying FossilHill Member
(Wyld, 2000). Gastropods and bivalves are abundant in lower layers whereas conodonts and
ammonoids are found locally within middle and upper layers of the lower member.
Scattered vertebrate fossils occur in multiple horizons within the lower member (Fig. 1),
but are most abundant in the middle carbonate layers where they are associated with the
conodont Triassospathodus symmetricus (Orchard, 1995) and the ammonoid Prohungarites
gutstadti (Guex et al., 2010) indicating a late Spathian age (Subcolumbites ammonoid
biozone). These fossils were collected by the landowner and co-author Patrick Embree, who
donated the material to the Smithsonian National Museum of Natural History.
RESULTS
Systematic paleontology
Ichthyopterygia (Owen, 1840).
cf. Utatsusaurus (Shikama, Kamei & Murata, 1978 ).
Diagnosis. Teeth smaller than marginal dentition present on pterygoid; squamosal not
entirely eliminated from supratemporal fenestra by supratemporal; interclavicle
cruciform; dorsal margin of external naris formed entirely by nasal; prefrontal shelf
prominent; transverse flange of the pterygoid well defined and anterolaterally projecting;
supratemporal terrace present; tooth implantation subthecodont, with both dental groove
and shallow socket; tooth crowns of middle to posteriorly placed teeth distomesially
compressed; humerus as wide proximally as distally; humerus longer than wide; ulnar
facet of humerus as wide as radial facet; no more than five phalanges in any digit; posterior
dorsal vertebrae cylindrical in outline (from Cuthbertson, Russell & Anderson, 2013a; after
Motani, 1999; McGowan & Motani, 2003).
Referred specimen. USNM 559349 Partial mandible including teeth. (Fig. 2).
Locality. Fossil Hill, Humboldt Range, Pershing County, Nevada.
Horizon and age. Found as surface float within an outcrop of Lower Triassic (upper
Spathian) lower member of Prida Formation, Star Peak Group. Based on location and
matrix lithology this jaw is inferred to derive from horizon FH1–7 (Fig. 1), which is
Spathian based on the occurrence of the ammonoid Prohungar ites gutstadi (i.e.
Subcolumbites Zone of Guex et al. (2010)) and conodonts Triassospathodus symmetricus
(Orchard, 1995) and Neostrachanognathus sp. extracted from the matrix.
Description. USNM 559349 is a partial mandible measuring 82 mm long. The jaw
fragment preser ves portions of the dentary, surangular and splenial. The surfaces of the
dentary and surangular are heavily striated and the orientation of these striations differs
between the bones. The suture between the dentary and surangular is long and straight,
Kelley et al. (2016), PeerJ, DOI 10.7717/peerj.1626 4/16
extending across the entire preserved portion of the jaw. In places this suture is indistinct
but can be traced by the contrasting surface striation patterns of the dentary and
surangular. The splenial can be observed at the broken anterior edge of the fossil where it
comprises the medial and ventral portion of the jaw where a thin projection wraps
underneath the surangular. A row of irregular weathered depressions follows the
approximate course of the suture between the surangular and dentary but it is not clear if
these represent natural foramina or are simply artifacts of weathering. Judging from the
arrangement of the bones, the fragment likely represents a central-posterior portion of the
left mandibular ramus anterior to the coronoid process.
Thirteen lower teeth are present, along with an additional poorly preserved isolated
tooth between the tenth and eleventh in-place teeth, which may be either a disarticulated
upper or lower tooth. The teeth are set within alveoli along a continuous groove. No
distinct bony septa between alveoli are visible but may be present at the bottom of the
dental groove, being concealed in matrix that is very difficult to remove through
mechanical preparation. The roots of some teeth are clearly expanded at the base and
exhibit plications that are coarser than crown striation. The most anterior tooth is
completely exposed anteriorly, revealing its root structure inside the dental groove. It is
seen there that the root ceases its expansion once inside the groove, and teeth are
Figure 2 Specimen USNM 559349, partial ichthyopterygian jaw cf. Utatsusaurus. (A) Complete
specimen, in labial view, anterior to the left. Squares on scale bar equal 5 mm. (B) Magnified view of
anterior dentition, squares on scale bar equal 1 mm.
Kelley et al. (2016), PeerJ, DOI 10.7717/peerj.1626 5/16
embedded to both the labial wall and the base of the groove. A narrow gap emerges
between the lingual wall and the root toward the dentigerous margin. Tooth implantation
is likely subthecodont (sensu Motani, 1997a), although histological study is necessary to
firmly establish this. The root cross-section is much wider than long, as reported for
Utatsusaurus hataii (Motani, 1996).
Teeth are isodont and conical with striated crowns. Tooth roots are extensively
exposed above the alveolar margin such that they account for half or more of the exposed
height of each tooth. Tooth crowns are distinguished from these exposed roots by a
distinct margin, with most crowns slightly constricted at their base. Some crowns exhibit
slightly higher convexity of their anterior surface relative to the posterior surface given
them a slightly recurved appearance. The teeth are also recurved lingually toward the tip,
as clearly seen in the most anterior tooth (Fig. 2B). This curvature closely resembles what
was described for Utatsusaurus hataii (Motani, 1996:Fig. 3; Cuthbertson, Russell &
Anderson, 2013a:Figs. 7C and 7D). The tips of the teeth are relatively blunt. Tooth crowns
are approximately 3.6 mm tall and 2 mm in mesiodistal diameter. Spacing between teeth
ranges from 3 mm to 6 mm, more widely spaced teeth may have replacement teeth
between them. One 10 mm gap along the tooth row likely represents at least one missing
tooth. Several teeth are broken, either at the root or the crown, revealing a pulp cavity
without evidence of infolding of the dentine.
Remarks. The tooth morphology observed in this specimen closely resembles that
described for Utatsusaurus hataii from the Lower Triassic Osawa For mation of
Kitakami, Japan (Sh ikama, Kamei & Murata, 1978). Most no tably, the t eet h curve
lingually and slig htly posteriorly toward the tip, which is a feature that is uniquely
known in Utatsusaur us among basal ichthyopterygians. Other shared features include:
isodont dentition, tal l exposed roots, blunt conical striated crow ns, a slightly constricted
base of some crowns, and an absence of infolding in the pulp cavity. Each of these
features is observed in the holotyp e of Utatsusaurus hataii (IGPS 959 41) (Mot ani, 1996)
and mos t are reported in a refe rred specimen (UH R 30691 ) (Cuthbertson, Russell &
Anderson, 2013a).NootherEarlyTriassicorlaterichthyopterygian exhibits this suite
of dental characters.
The teeth of this specimen do diverge from Utatsusaurus hataii in their much larger
size. The maximum tooth exposed height in this specimen is 11 mm, whereas the
maximum crown height and width are 4.5 mm and 2.3 mm respectively (Table 1),
compared with 3.3 mm, 1.7 mm and 0.9 mm for the same measurements of teeth in
the holotype of Utatsusaurus hataii (Motani, 1996). However, the holotype represents
a juvenile (Motani, 1997c), so the size difference may partly be explained as
ontogenetic variation. A referred specimen (UHR 30691) is somewhat larger than the
holotype, however, the teeth of this specimen are still considerably smaller than those
of USNM 559349. Despite this difference in size, the overall shape of the teeth in each
of these specimens is very similar. Motani (1996) reported “crown shape index”
values–calculated as crown height divided by average basal diameter of the tooth crown
(Massare, 1987)–ranging between 0.9 and 3.1 with an average value of 1.9 in the type
specimen of Utatsusaurus hataii, while these values range from 1.3 to 2.3 with an
Kelley et al. (2016), PeerJ, DOI 10.7717/peerj.1626 6/16
average of 1.8 in USNM 559349. Likewise, the “crown ratio”–calculated as crown
height divided by total exposed tooth height–averages 0.51 in IGPS 95941 and 0.43 in
USNM 559349.
The overall arrangement of bones in the fragmentary mandible is con sistent with
that observed in the Utatsusaurus holotype (IGPS 94941) and the referred specimen
(UHR 30691), notably the long, strai g ht contact between the dentar y and surangul ar.
The suture between the surangular and dentary is well developed in IGPS 94941 but
indistinct in the larger UHR 30691 , possibly a consequence of differin g ontogenetic
stages, or differential weathe ring , between these two specimens. USNM 559349
approaches UHR 30691 in that the suture between the dentary and surangular is
somewha t obscured, but its a pproximate course can be traced by a contrast in bone
texture and an irregular row of weathered pits. This mi ght suggest a relatively mature
indivi dual, as has been proposed for IGPS 9494, but t his is highly speculative given the
incomplete nature of the materi al.
A partial skull from the Lower Triassic Vega Phroso Member of the Sulphur Mountain
Formation from British Columbia, Canada was referred to Utatsusaurus sp. by Nicholls &
Brinkman (1993), based largely on the presence of the same dental features detailed above.
The teeth of the British Columbia specimen are similar in size to those of the Nevada
specimen described here (Nicholls & Brinkman, 1993) and distinctly larger than those
found in the holotype of Utatsusaurus hataii from Japan (Motani, 1996). It is therefore
possible that these larger-toothed specimens from the eastern margin of Panthalassa
(Nevada, British Columbia) represent a form allied with but distinct from Utatsusaurus
hataii; however more complete material is needed before this can be confirmed. Recently,
Cuthbertson, Russell & Anderson (2014) described another partial skull from the Vega
Phroso Member, which they also referred to Utatsusaurus sp., although they concluded
that the material originally referred to this genus by Nicholls & Brinkman (1993) was
Table 1 Summarized tooth measurements from USNM 559349 and USNM 559350. All measurements
in mm except for shape index and crown ratio, which are ratios.
Specimen Proxmial
width (mm)
a
Exposed
height (mm)
b
Crown
width (mm)
c
Crown
height (mm)
d
Crown shape
index
e
Crown
ratio
f
USNM
559349
Max. 4.2 11.1 2.3 4.5 2.3 0.51
Min. 2.5 6.4 1.9 2.7 1.3 0.39
Mean 3.2 8.5 2.1 3.6 1.8 0.43
USNM
559350
Max. 1.6 2.5 2.1 1.7 2.1 0.88
Min. 1.0 1.8 1.2 1.1 1.0 0.61
Mean 1.4 2.2 1.6 1.5 1.4 0.69
Notes:
a
Measured as mesio-distal width of the root at the jawline, following Motani (1996).
b
Measured as distance from tip of crown to jaw margin.
c
Measured as mesio-distal width of the crown at its widest point.
d
Base of crown is distinctive in USNM 559349 due to crown ornamentation; base of crown in USNM 559350 is less
distinct but can be approximated by slight basal constriction.
e
Calculated as crown height/crown width, following Massare (1987).
f
Calculated as crown height/exposed tooth height, following Motani (1996).
Kelley et al. (2016), PeerJ, DOI 10.7717/peerj.1626 7/16
non-diagnostic at the genus level. Unfortunately this recently described material lacks a
lower jaw or teeth and cannot be compared to USNM 559349.
Ichthyopterygia (Owen, 1840).
cf. Grippiidae (Wiman, 1929).
Definition. The last common ancestor of Grippia longirostris and Gulosaurus helmi, and
all its descendants (Ji et al., 2015).
Diagnosis. Maxilla with multiple tooth rows; posterior tooth crown rounded;
supratemporal-postorbital contact present; proximal manual phalanges not closely
packed proximo-distally (from Ji et al. (2015)).
Referred specimen. USNM 559350 Partial maxilla including teeth. (Fig. 3).
Locality. Fossil Hill, Humboldt Range, Pershing County, Nevada.
Horizon and Age. Collected from FH1-5 (Fig. 1) horizon which is Spathian in age
based on the occurrence of the ammonoid Prohungarites gutstadi (Guex et al., 2010) and
conodonts including Triassospathodus and Neostrachanognathus. This horizon is also
characterized by distinctive spherical structures originally interpreted as microbial (i.e.
‘oncoids’) but more recently suggested to represent sponge “reefs” (Brayard et al., 2011;
see further discussion below).
Description. USNM 559350 (Fig. 3) is a partial maxilla measuring 20 mm and bearing
five teeth exposed in medial view. The teeth are robust cones exhibiting a trend of
posteriorly increasing basal diameter, whereas the crown height remains constant giving
the posteriormost preserved tooth a distinctly rounded shape. There is a distinct
Figure 3 USNM 559350 Partial ichthyopterygian maxilla cf. Gr ippidia. (A) Partial maxilla in lingual
view, anterior to the left. Squares on scale bar equal 1 mm. White arrow indicates possible attachment
facet for tooth in second lingual tooth row. (B) Magnified view of dentition.
Kelley et al. (2016), PeerJ, DOI 10.7717/peerj.1626 8/16
constriction below the crown separating it from the root below, however the constriction
is very slight in the anteriormost tooth. The anteriormost crown height and width are
1.7 mm and 1.2 mm respectively; in the second posteriormost tooth, which is better
preserved than the posteriormost tooth, crown height and width are 1.3 mm and 2.1 mm.
The tooth enamel appears smooth and polished with little indication of striation;
however, this could be attributed to tooth wear. Faint plication is visible on some roots.
Although some teeth are abraded, none expose the pulp cavity clearly enough to
determine presence or absence of infolded dentine.
In medial view the teeth are attached to the lingual wall of the maxilla, representing
pleurodont tooth attachment. An expanded bone of attachment conceals the bases of the
two posteriormost teeth, suggesting subleurodont attachment, a modified form of
pleurodont attachment (Motani, 1997a), in at least the posterior region of the maxillary
tooth row. While only a single row of teeth is observed, a shallow depression on the lingual
margin of the tooth row immediately anterior to the second posteriormost tooth could
represent the attachment facet of a missing tooth. If this were the case it might possibly
represent a second row immediately lingual to the preserved teeth. Wide spacing between
the four anteriormost teeth would easily accommodate an additional offset tooth row as
observed in the maxillary dentition of Grippia (Motani, 1997b) and Gulosaurus
(Cuthbertson, Russell & Anderson, 2013b).
Remarks. Among Early Triassic ichthyopterygians, small, robust teeth, similar to those
reported here, are typical of the posterior dentition of Grippia (Motani, 1997b). Rounded
teeth are also observed in the Early Triassic genus Chaohusaurus (Motani & You, 1998)
and, to a lesser extent, Gulosaurus (Cuthbertson, Russell & Anderson, 2013b). Grippia was
previously reported from the Lower Triassic Vega Phroso Member of the Sulphur
Mountain Formation in British Columbia (Brinkman, Xijin & Nicholls, 1992 ). This
specimen was later redescribed as a distinct taxon, Gulosaurus helmi (Cuthbertson, Russell &
Anderson, 2013b) and found to be sister taxon to Grippia longirostris.Similarly,recentwork
by Ji et al. (2015), established a new clade Grippioidea including Grippia, Gulosaurus,
Utatsusaurus, and Parvinatator, Nicholls & Brinkman (1995) although the precise
relationships among these taxa varied somewhat depending on taxon and character
inclusion. The enigmatic marine reptile Omphalosaurus nettarhynchus (Mazin & Bucher,
1987), previously reported from Spathian lower member of the Prida Formation, also
possesses rounded dentition, but is distinct from this specimen by its much larger size and
in exhibiting a broad pavement of rounded teeth on the mandible.
Alternatively, this specimen may have some affinity with Chaohusaurus, an Early
Triassic ichthyopterygian from China in which some specimens also show distinctly
rounded posterior dentition (Motani & You, 1998). However the posterior teeth of
Chaohusaurus are generally smaller and more tightly packed than in USNM 559350,
averaging approximately ten teeth over 20 mm rather than the five teeth observed over the
same distance in this specimen. Chaohusaurus was previously regarded as a grippidian
partly on the basis of possessing multiple maxillary tooth rows and rounded posterior
dentition (Motani, 1999). However, more recent analyses (Cuthbertson, Russell &
Anderson, 2013b; Ji et al., 2015) do not support this placement.
Kelley et al. (2016), PeerJ, DOI 10.7717/peerj.1626 9/16
The crown shape index in USNM 559350 ranges from 1.0–2.1 and averages 1.4
( Table 1), similar to the average shape index reported for the maxillary dentition of
Grippia, 1.4 (Motani, 1997b) and Gulosaurus 1.64 (Cuthbertson, Russell & Anderson,
2013b). Thus, the rounded dentition observed in USNM 559350 is strongly suggestive of
an affinity with some other Early Triassic ichthyopterygians but more precise placement
will require more complete skeletal material.
DISCUSSION
Despite the fragmentary nature of the remains described here, their resemblance with the
distinctive dentitions of other Early Triassic ichthyopterygians allows tentative
interpretations to be made. The presence of Utatsusaurus-like and Grippia or
Chaohusaurus-like forms suggests similarity with the Lower Triassic Vega-Phroso
assemblage from the Wapiti Lake region of British Columbia, from which Utatsusaurus
(Nicholls & Brinkman, 1993; Cuthbertson, Russell & Anderson, 2014) and grippidians
(Brinkman, Xijin & Nicholls, 1992; Cuthbertson, Russell & Anderson, 2013b) have also been
reported. However, the type locality of Utatsusaurus is in the Osawa Formation of Japan
(Shikama, Kamei & Murata, 1978), whereas the type localities of Grippia and
Chaohusaurus are in the Vikinghøgda Formation (= “Sticky Keep Formation” of older
references) of Spitsbergen (Wiman, 1929; Wiman, 1933; Hounslow et al., 2008) and the
Nanlinghu Formation of Anhui Province, China (Young & Dong, 1972), respectively.
Thus, Early Triassic ichthyopterygian taxa were widely distributed around the margins of
northern Panthalassa (Cuthbertson, Russell & Anderson, 2013b).
This broad distribution early in their evolutionary history, from numerous Late
Spathian (Subcolumbites Zone) localities of broadly coeval age (Scheyer et al., 2014 ), has
made it difficult to pinpoint the biogeographic origins of the group. However, recent work
in China has extended the biostratigraphic range of ichthyopterygians to the underlying
Procolumbites Zone (Motani et al., 2014; Ji et al., 2014). Furthermore, the occurrence of
diverse and endemic hupehsuchians, widely regarded as the ichthyopterygian sister-
group, and the plesiomorphic ichthyosauromorph Cartorhynchus (Motani et al., 2015)are
consistent with an origin of ichthyopterygians near the south China block in equatorial
western Panthalassa.
The inferred nearshore lifestyle of most Early Triassic ichthyopterygians has led others
to propose that these early marine reptiles dispersed along coastlines or across transient
epicontinental corridors (Cuthbertson, Russell & Anderson, 2013b). However, there is little
geological evidence for such corridors in the Early Triassic, which was a time of relatively
low global sea level (Miller et al., 2005). Furthermore the absence of Early Triassic
ichthyopterygian fossils in Western Tethys is surprising under this scenario. Conversely,
the biogeographic histories of other aquatic–and even terrestrial–reptile groups are
marked by occasional transoceanic dispersal events (Rocha et al., 2006; Ve
´
lez-Juarbe,
Brochu & Santos, 2007), and such events could explain the distribution of Early Triassic
ichthyopteryians on opposite shores of Panthalassa.
Brayard et al. (2009) identified trans-Panthalassan distribution patterns in Spathian
ammonoids, identifying similar ammonoid faunas in Nevada, Kitakami and British
Kelley et al. (2016), PeerJ, DOI 10.7717/peerj.1626 10/16
Columbia, which they attributed to oceanographic currents. The occurrence of some Early
Triassic marine reptile taxa (e.g. Utatsusaurus) on both the eastern and western margins of
of Panthalassa might reflect sporadic crossing of deep ocean basins by these lineages,
potentially facilitated by the same ocean currents that mediated transoceanic dispersals of
contemporaneous marine invertebrates (Fig. 4). The w ide distribution of Early Triassic
sauropterygians, including the South China Block (Jiang et al., 2014) and western margin
of North America (Storrs, 1991; Lovelace & Doebbert, 2015) on opposite shores of
Panthalassa indicates to a similar dispersal history in the early members of that marine
reptile clade. Isolated terranes such as South Kitakami, South Primoyre and Chulitna could
have served as stepping-stones for shallow marine taxa. Dispersal along coastlines around
the northern margins of Panthalassa remains an alternative scenario that could explain the
broad distribution of some Early Triassic ichthyopterygians, with pronounced global
150 200 250 30050 100
-40
-20
0
20
40
60
350
K
PANTHALASSA
TETHYS
S
B
I
N
C
T
PANGEA PANGEA
Grippidae
Utatsusaurus
Other ichthyopterygia
Figure 4 Distribution of Early Triassic ichthyopterygians. Paleogeographic distribution of Early
Triassic ichthyopterygians, map modified from Brayard et al. (2009). Locality abbreviations as follows:
(B) British Columbia; (C) South China; (I) Idaho; (K) South Kitakami; (N) Nevada (present study,
highlighted in yellow); (S) Spitsbergen; (T) Timor. Arrows indicate inferred ammonoid dispersal routes
(Brayard et al., 2009).
Figure 5 Distinctive sedimentary structures associated with horizon of USNM 559350. Spherical
structures in FH1-5 that may represent microbial structures or sponges. This appears to be a widespread
and distinctive regional Lower Triassic facies associated with recovery from the end-Permian mass-
extinction. Vertebrate fossils also occur in this horizon including USNM 559350 described here.
Hammer for scale is approximately 30 cm in length.
Kelley et al. (2016), PeerJ, DOI 10.7717/peerj.1626 11/16
warmth in the Early Triassic mediating limiting climatic conditions at high latitudes (Sun
et al., 2012). However, the apparent absence of ichthyopterygian fossils from high latitudes
on the western margin of northern Panthalassa remains a puzzle under this scenario.
Intriguingly, the oldest marine reptile bearing horizons at Fossil Hill are associated with
a prominent limestone marker bed bearing distinctive spherical structures ~1–2 cm in
diameter (Fig. 5). We initially interpreted these structures as microbial ‘oncoids.’
Widespread microbialite-dominated facies are characteristic of Lower Triassic strata
globally, including in the western United States (Pruss & Bottjer, 2004; Baud, Richoz &
Pruss, 2007), and are interpreted as a byproduct of the end-Permian mass extinction and
subsequent delayed biotic recovery of metazoan reefs (Pruss & Bottjer, 2004). A similar
association between the basal sauropterygian Corosaurus and stromatolites in the Lower
Triassic Alcova limestone has been reported previously (Storrs, 1991). More recently,
similar spheroidal structures from the Humboldt Range and other localities in western
North America have been interpreted as ‘transient sponge reefs’ (Brayard et al., 2011).
Thus, the diversification and dispersal of Early Triassic marine reptiles was apparently well
underway at the end of the Early Triassic (Scheyer et al., 2014 ) despite some lingering signs
of continued environmental stress preserved in the same strata. Future work at this new
locality, and elsewhere, may help to clarify the role that large-scale environmental changes
played in shaping the early evolutionary history of Mesozoic marine reptiles.
ACKNOWLEDGEMENTS
We thank Tetsuya Sato for assistance with preparation of USNM 559349. Torsten Scheyer,
Lars Schmitz, and Cheng Ji all provided helpful discussion. Reviewers Robin Cuthbertson
and Nadia Fro
¨
bisch and the handling editor also provided critiques and suggestions that
greatly improved this paper.
ADDITIONAL INFORMATION AND DECLARATIONS
Funding
The authors received no funding for this work.
Competing Interests
The authors declare no competing interests.
Author Contributions
Neil P. Kelley conceived and designed the experiments, performed the experiments,
analyzed the data, contributed reagents/materials/analysis tools, wrote the paper,
prepared figures and/or tables, reviewed drafts of the paper.
Ryosuke Motani conceived and designed the experiments, performed the experiments,
analyzed the data, contributed reagents/materials/analysis tools, reviewed drafts of the
paper.
Patrick Embree conceived and designed the experiments, performed the experiments,
analyzed the data, contributed reagents/materials/analysis tools, prepared figures and/or
tables, reviewed drafts of the paper.
Kelley et al. (2016), PeerJ, DOI 10.7717/peerj.1626 12/16
Michael J. Orchard performed the experiments, analyzed the data, contributed reagents/
materials/analysis tools, w rote the paper, reviewed drafts of the paper.
Data Deposition
The following information was supplied regarding data availability:
All data associated w ith this paper is included in the manuscript and the Supplemental
Information.
Supplemental Information
Supplemental information for this article can be found online at http://dx.doi.org/
10.7717/peerj.1626#supplemental-information.
REFERENCES
Baud A, Richoz S, Pruss S. 2007. The Lower Triassic anachronistic carbonate facies in space and
time. Global and Planetary Change 55(1–3):81–89 DOI 10.1016/j.gloplacha.2006.06.008.
Brayard A, Escarguel G, Buche r H, Bru
¨
hwiler T. 2009. Smithian and spathian (Early Triassic)
ammonoid assemblages from terranes: paleoceanographic and paleogeographic implications.
Journal of Asian Earth Sciences 36(6):420–433 DOI 10.1016/j.jseaes.2008.05.004.
Brayard A, Vennin E, Olivier N, Bylund KG, Jenks J, Stephen DA, Bucher H, Hofmann R,
Goudemand N, Escarguel G. 2011. Transient metazoan reefs in the afterm ath of the
end-Permian mass extinctio n. Nature Geoscience 4:693–697 DOI 10.1038/ngeo1264.
Brinkma n DB, Xijin ZHAO, Nicholls EL. 1992. A primitive ichthyosaur from the Lower Triassic
of British Columbia, Canada. Palaeontology 35:465–474.
Camp CL. 1976. Vorla
¨
ufige mitteilung u
¨
ber große ichthyosaurier aus der oberen trias von Nevada.
Sitzungsberichte der O
¨
sterreichischen Akademie der Wissenschaften, Mathematisch-
naturwissenschaftliche Klasse, Abteilung I 185:125–134.
Camp CL. 1980. Large ichthyosaurs from the Upper Triassic of Nevada. Palaeontographica
Abteilung A 170:139–200.
Case EC. 1936. A nothosaur from the Triassic of wyoming. University of Michigan Contributions
from the Museum of Paleontology 5(1):1–36.
Cuthbertson RS, Russell AP, Anderson JS. 2013a. Reinterpretation of the cranial morphology of
Utatsusaurus hataii (Ichthyopter ygia) (Osawa Formation, Lower Triassic, Miyagi, Japan) and its
systematic implications. Journal of Vertebrate Paleontology 33(4):817–830
DOI 10.1080/02724634.2013.756495.
Cuthbertson RS, Russell AP, Anderson JS. 2013b. Cranial morphology and relationships of a new
grippidian (Ichthyopterygia) from the Vega-Ph roso Siltstone Member (Lower Triassic) of
British Columbia, Canada. Journal of Vertebrate Paleontology 33(4):831–847
DOI 10.1080/02724634.2013.755989.
Cuthbertson RS, Russell A, Anderson J. 2014. The first substantive evidence of Utatsusaurus
(Ichthyopterygia) from the Sulphur Mountain Formation (Lower–Middle Triassic ) of British
Columbia, Canada: a skull roof description with comparison to other early taxa. Canadian
Journal of Earth Sciences 51(2):180–185 DOI 10.1139/cjes-2013-0185.
Fro
¨
bisch NB, Sander P, Rieppel O. 2006. A new species of Cymbospondylus (Diapsida, Ichthyosauria)
from the Middle Triassic of Nevada and a re-evaluation of the skull osteology of the genus.
Zoological Journal of the Linnean Society 147(4):515–538 DOI 10.1111/j.1096-3642.2006.00225.x.
Kelley et al. (2016), PeerJ, DOI 10.7717/peerj.1626 13/16
Fro
¨
bisch NB, Fro
¨
bisch J, Sander PM, Schmitz L, Rieppel O. 2013. Macropredatory ichthyosaur
from the Middle Triassic and the origin of modern trophic networks. Proceedings of the National
Academy of Sciences 110(4):1393–1397 DOI 10.1073/pnas.1216750110.
Guex J, Hungerbu
¨
hler A, Jenks JF, O’Dogherty L, Atudorei V, Taylor DG, Bucher H, Bartolini A.
2010. Spathian (Lower Triassic) ammonoids from western USA (Idaho, California, Utah and
Nevada). Me
´
moires de Ge
´
ologie 49:1–92.
Hounslow MW, Peters C, Mørk A, Weitschat W, Vigran JO. 2008. Biomagnetostratigraphy of the
Vikinghøgda Formation, Svalbard (Arctic Norway), and the geomagnetic polarity timescale for
the Lower Triassic. Geological Society of America Bulletin 120(9–10):1305–1325
DOI 10.1130/B26103.1.
Ji C, Jiang D, Motani R, Rieppel O, Hao W, Sun Z. 2015. Phylogeny of the Ichthyopterygia
incorporating the recent discoveries from South China. Journal of Vertebrate Paleontology
e1025956 Epub ahead of print 10 Nov 2015 DOI 10.1080/02724634.2015.1025956.
Ji C, Zhang C, Jiang D, Bucher H, Motani R, Tintori A. 2014. Ammonoid age control of the Early
Triassic marine reptile from Chaohu. Paleoworld 24(3 Septem ber 2015):277–282
DOI 10.1016/j.palwor.2014.11.009.
Jiang D, Motani R, Tintori A, Rieppel O, Chen G, Huang J, Zhang R, Sun Z, Ji C. 2014. The Early
Triassic Eosauropterygian Majiashanosaurus discocoracoidis, Gen. Et Sp. Nov. (Reptilia,
Sauropterygia), from Chaohu, Anhui Province, People’s Republic of China. Journal of Vertebrate
Paleontology 34(5):1044–1052 DOI 10.1080/02724634.2014.846264.
Leidy J. 1868. Notice of some reptilian remains from Nevada. Proceedings of the Academy of
Natural Sciences of Philadelphia 109:177–178.
Lovelace DM, Doebbert AC. 2015. A new age constraint for the Early Triassic Alcova Limestone
(Chugwater Group), Wyoming. Palaeogeography, Palaeoclimatology, Palaeoecology 424:1–5
DOI 10.1016/j.palaeo.2015.02.009.
Massare JA. 1987. Tooth morphology and prey preference of Mesozoic marine reptiles. Journal of
Vertebrate Paleontology 7(2):121–137 DOI 10.1080/02724634.1987.10011647.
Massare JA, Callaway JM. 1994. Cymbospondylus (Ichthyosauria: Shastasau ridae) from the
Lower Triassic Thaynes Formation of southeastern Idaho. Journal of Vertebrate Paleontology
14(1):139–141 DOI 10.1080/02724634.1994.10011545.
Mazin JM, Bucher H. 1987. Omphalosaurus nettarhynchus, une nouvelle espe
`
ce d’Omphalosauri de
´
(Reptilia, Ichthyopterygia) du Spathien de la Humboldt Range (Nevada, USA). Comptes rendus
de l’Acade
´
mie des sciences. Se
´
rie 2, Me
´
canique, Physique, Chimie, Sciences de l’univers, Sciences de
la Terre 305:823–828 French with English Abstract
McGowan C, Motani R. 2003. Part 8. Ichthyopterygia; In: Sues H-D, ed. Handbook of
Paleoherpetology. Munich: Verlag Dr. Friedrich Pfeil.
Merriam JC. 1905. A primitive ichthyosaurian limb from the Middle Triassic of Nevada. University
of California Publications. Bulletin of the Department of Geology 4(2):33–38.
Merriam J. 1906. Preliminary note on a new marine reptile from the Middle Triassic of Nevada.
University of California Publications. Bulletin of the Department of Geology 5(5):75–79.
Merriam JC. 1908. Triassic Ichthyosauria, with special reference to the American forms. Memoirs
of the University of California 1(1):1–196 DOI 10.5962/bhl.title.20827.
Merriam JC. 1910. The skull and dentition of a primitive ichthyosaurian from the Middle
Triassic. University of California Publications. Bulletin of the Department of Geology
5(24):381–390.
Kelley et al. (2016), PeerJ, DOI 10.7717/peerj.1626 14/16
Miller KG, Kominz MA, Browning JV, Wright JD, Mountain GS, Katz ME, Sugarman PJ,
Cramer BS, Christie-Blick N, Pekar SF. 2005. The Phanerozoic record of global sea-level
change. Science 31(5752):1293–1298.
Motani R. 1996. Redescription of the dental features of an Early Triassic ichthyosaur,
Utatsusaurus hataii. Journal of Vertebrate Paleontology 16(3):396–402
DOI 10.1080/02724634.1996.10011329.
Motani R. 1997a. Tempo ral and spatial distribution of tooth implantation in ichthyosaurs.
In: Callaway JM, Nicholls EL, eds. Ancient Marine Reptiles. London and New York: Academic
Press, 81–103 DOI 10.1126/science.1116412.
Motani R. 1997b. Redescription of the dentition of Grippia longirostris (Ichthyosauria) with a
comparison with Utatsusaurus hataii. Journal of Vertebrate Paleontology 17(1):39–44
DOI 10.1080/02724634.1997.10010951.
Motani R. 1997c. New information on the forefin of Utatsusaurus hataii (Ichthyosauria). Journal of
Paleontology 71(3):475–479.
Motani R. 1999. Phylogeny of the Ichthyopterygia. Journal of Vertebrate Paleontology
19(3):473–496 DOI 10.1080/02724634.1999.10011160.
Motani R, Jiang DY, Chen GB, Tintori A, Rieppel O, Ji C, Huang JD. 2015. A basal
ichthyosauriform with a short snout from the Lower Triassic of China. Nature
517(7535):485–488 DOI 10.1038/nature13866.
Motani R, You H. 1998. Taxonomy and limb ontogeny of Chaohusaurus geishanensis
(Ichthyosauria), with a note on the allometric equation. Journal of Vertebrate
Paleontology 18(3):533–540 DOI 10.1080/02724634.1998.10011080.
Motani R, Jiang A, Tintori A, Rieppel O, Chen GB. 2014. Terrestrial origin of viviparity in
Mesozoic marine reptiles indicated by early Triassic embryonic fossils. PLoS ONE 9(2):e88640
DOI 10.1371/journal.pone.0088640.
Nicholls EL, Brinkman D. 1993. A new specimen of Utatsusaurus (Reptilia: Ichthyosauria) from
the Lower Triassic Sulphur Mountain Formation of British Columbia. Canadian Journal of
Earth Sciences 30(3):486–490 DOI 10.1139/e93-037.
Nicholls EL, Br inkman DB. 1995. A new ichthyosaur from the Triassic Sulphur Mountain
formation of British Columbia; In: Sarjeant WAS, ed. Vertebrate Fossils and the Evolution of
Scientific concepts. Switzerland: Gordon and Breach 521–535.
Nichols KM, Silberling NJ. 1977. Stratigraphy and depositional history of the Star Peak Group
(Triassic), northwestern Nevada. Geological Society of America 178:1–74.
Orchard MJ. 1995.
Taxonomy and correlation of Lower Triassic (Spathian) segminate conodonts
from Oman and revision of some species of Neospa thodus. Journal Paleontology 69(1):110–122
DOI 10.1017/S0022336000026962.
Owen R. 1840. Report on British fossil reptiles. Part I. Report of the British Association for the
Advancement of Science. Plymouth 9:43–126 .
Rocha S, Carretero MA, Vences M, Glaw F, James Harris D. 2006. Deciphering patterns of
transoceanic dispersal: the evolutionary origin and biogeography of coastal lizards
(Cryptoblepharus) in the Western Indian Ocean region. Journal of Biogeography 33(1):13–22
DOI 10.1111/j.1365-2699.2005.01375.x.
Pruss SB, Bottjer DJ. 2004. Late Early Triassic microbial reefs of the western United States:
a description and model for their deposition in the aftermath of the end-Permian mass
extinction. Palaeogeography, Palaeoclimatology, Palaeoecology 211(1–2):127–137
DOI 10.1016/j.palaeo.2004.05.002.
Kelley et al. (2016), PeerJ, DOI 10.7717/peerj.1626 15/16
Sander PM, Rieppel OC, Bucher H. 1994. New marine vertebrate fauna from the Middle Triassic
of Nevada. Journal of Paleontology 68(3):676–680 DOI 10.1017/S00223360000 26020.
Sander PM, Rieppel OC, Bucher H. 1997. A new pistosaurid (Reptilia: Sauropterygia) from the
Middle Triassic of Nevada and its implications for the origin of the plesiosaurs. Journal of
Vertebrate Paleontology 17(3):526–533 DOI 10.1080/02724634.1997.10010999.
Scheyer TM, Romano C, Jenks J, Bucher H. 2014. Early Triassic marine biotic recovery: the
predators’ pers pective. PLoS ONE 9(3):e88987 DOI 10.1371/journal.pone.0088987.
Schmitz L, Sander PM, Storrs GW, Rieppel O. 2004. New Mixosauridae (Ichthyosauria) from the
Middle Triassic of the Augusta Mountains (Nevada, USA) and their implications for mixosaur
taxonomy. Palaeontographica, Abteilung A 270(4):133–162 DOI 10.1073/pnas.1216750110.
Shikama T, Kamei T, Murata M. 1978. Early Triassic ichthyosaurus, Utatsusaurus hataii gen. et sp.
nov., from the Kitakami massif, Northeast Japan. Science Reports of the Tohoku University,
Sendai, Second Series (Geology) 48(2):77–97.
Silberling NJ. 1962. Stratigraphic distribution of Middle Triassic Ammonites at Fossil Hill,
Humboldt Range, Nevada. Journal of Paleontology 36:153–160.
Storrs GW. 1991. Anatomy and relationships of Corosaurus alco vensis (Diapsida: Sauropterygia)
and the Triassic Alcova Limestone of Wyoming. Bulletin of the Peabody Museum of Natural
History 44:1–151.
Sun Y, Joachimski MM, Wignall PB, Yan C, Chen Y, Jiang H, Wang L, Lai X. 2012. Lethally hot
temperatures during the Early Triassic greenhouse. Science 338(6105):366–370
DOI 10.1126/science.1224126.
Ve
´
lez-Juarbe J, Brochu CA, Santos H. 2007. A gharial from the Oligocene of Puerto Rico:
transoceanic dispersal in the history of a non-marine reptile. Proceedings of the Royal Society B:
Biological Sciences 274(1615):1245–1254 DOI 10.1098/rspb.2006.0455.
Wiman C. 1929. Eine neue Reptilien-Ordnung aus der Trias Spitzbergens. Bulletin of the Geological
Institution of the University of Upsala 22:183–196.
Wiman C. 1933. Uber Grippia longirostris. Nova Acta Regiae Societatis Scientiarum Upsaliensis, ser. 4
9(4):1–19.
Wyld SJ. 2000. Triassic evolution of the arc and backarc of northwestern Nevada, and evidence for
extensional tectonism. Special Papers Geological Society of America 347:185–208
DOI 10.1130/0-8137-2347-7.185.
Young CC, Dong ZM. 1972. Chaohusaurus geishanensis from Anhui province. Academia Sinica,
Institute of Vertebrate Paleontology and Palaeonanthropology, Memoir
9:11–14.
Kelley et al. (2016), PeerJ, DOI 10.7717/peerj.1626 16/16
