Mixosaurus(?) nordenskioeldii; Merriam, 1911: 318
Phalarodon nordenskioeldii; Nicholls et al, 1999: 6 [partim]
Phalarodon nordenskioeldii; Maisch & Matzke, 2000B: 63
[partim]
Lectotype: Nicholls et al. (1999: 6) identified a diagnostic
specimen (MNHS R33) among the original material of
Hulke (1873). This specimen should be considered the
lectotype.
Diagnosis: Large Mixosaurus, mandibular length approaching
40 cm in large adults; posterior teeth mound-shaped;
multiple maxillary tooth rows; teeth laterally compressed, at
least posteriorly; posterior tooth crowns constricted at bottom;
mandible deep; mandibular dental roots long.
Occurrence: Sassenbay and Middlehook of Isfjord, Spitsbergen,
Svalbard; Wapiti Lake, British Columbia, Canada.
Stratigraphic occurrence: Upper Sticky Keep Formation
(Spitsbergen) and Sulphur Mountain Formation (British
Columbia); Middle Triassic.
Remarks: Posterior mound-shaped teeth occur in this
species, but the frequency of such teeth varies considerably
among known individuals (Nicholls et al., 1999). The
exact shape of the mound is also quite variable (Nicholls
et al., 1999), and further studies are needed before a better
diagnosis of the species is given. Mixosaurus nordenskioeldii
shares the presence of mound-shaped posterior teeth with
two other species of Mixosaurus, M. fraasi and M. kuhn-
schnyderi, and one extreme view is to consider them as its
subjective junior synonyms. However, there are subtle
morphological differences among the three species, and
thus they are provisionally retained here as valid species.
W iman (1910: pi. 5, fig. 1) depicted a sixth digit for the
forefin of Mixosaurus nordenskioeldii, and all subsequent
authors, including RM, followed this reconstruction (e.g.,
Motani, 1999A; N icholls et al., 1999). However, his figure,
which is obviously a composite reconstruction based on
the slab and counter slab of PMU R186, is inaccurate in the
posterior part of the fin; there are only two phalanges that
belong to the supposed sixth digit in the original specimen.
Moreover, these phalanges are not located on the same
plane as the rest of the fin, with which they actually overlap.
These phalanges were most likely derived from the
other forefin. Since this is the only specimen to show the
sixth digit, it is most likely that the forefin was pentadactyl
in this species.
Many specimens of Mixosaurus nordenskioeldii show
well-spaced anterior teeth, and this is usually considered a
typical character of the species (e.g., N icholls et al., 1999).
However, most of these specimens are poorly preserved,
whereas some well-preserved specimens show closely
packed anterior teeth (e.g., PMU R102). Therefore, it is
possible that the anterior teeth were actually closely packed
in this species, or at least that this character is highly
variable.
Nicholls e t al. (1 9 9 9 ) assigned this species to the genus
Phalarodon Merriam, 1910, while synonymizing the type
species of the genus, P. fraasi, with M. nordenskioeldii. Although
their intention to preserve this historical genus is
commendable, such a change of the type species is only
possible through a formal decision by the ICZN (see Article
70b). Moreover, it would be difficult to establish a valid
case for such an appeal because the specific name nordenskioeldii
has more often been associated with Mixosaurus in
the literature than with Phalarodon. Maisch & M atzke
(2000B, 2001) repeated the same mistake.
Mixosaurus fraasi (Merriam, 1910)
Phalarodon fraasi Merriam, 1910: 382
Mixosaurus fraasi; McGowan, 1972B: 4
Phalarodon nordenskioeldii; Nicholls et al., 1999: 6 [partim]
Phalarodon nordenskioeldii; Maisch & Matzke, 2000B: 63
[partim]
Holotype: UCMP 9863, middle part of the skull, from the
external naris to the mid-orbital region.
Diagnosis: Small Mixosaurus similar to M. cornalianus in
size; large, mound-shaped teeth, exceeding 1 cm in disto-
mesial length, in both upper and lower jaws; posterior tooth
crowns constricted at bottom; dentary with wide labial shelf;
mandible shallow; mandibular dental roots probably short; posterior
tooth crowns with distomesial ridge; multiple maxillary
tooth rows; teeth laterally compressed, at least posteriorly.
Occurrence: West Humboldt Range, northern Nevada,
USA.
Stratigraphic range: Prida Formation; Middle Triassic
(Anisian).
Remarks: Nicholls et al. (1999) considered Mixosaurus
fraasi a subjective junior synonym of M. nordenskioefdii,
which was followed by Motani (1999B). However, a more
conservative-view is héld here to retain M. fraasi tentatively
as a valid species. Nicholls et al. (1999) pointed out an
ontogenetic increase in the frequency of mound-shaped
teeth in M. nordenskioeldii. The holotype of M. fraasi has
well developed mound-shaped teeth in both upper and
lower jaws, exceeding 1 cm in distomesial length, so this
individual could represent a mature individual of M. nordenskioeldii,
judging from this single character. However,
the holotype of M. fraasi is smaller than most specimens of
M. nordenskioeldii: for example, a maxilla of M. nordenskioeldii
(PMU R103), which is about 10 percent larger than that
of the type of M. fraasi, does not have any mound-shaped
teeth. Also, the roots of the mandibular teeth are short in
M. fraasi but long in M. nordenskioeldii, probably reflecting
the fact that the former has a shallow mandible. Despite
these differences, it is still possible that M. fraasi is a subjective
junior synonym of M. nordenskioeldii. Only a comprehensive
and thorough comparison of Mixosaurus spp.
may clarify this taxonomic issue.
Mixosaurus kuhnschnyderi (Brinkmann, 1998A)
Phalarodon sp. Brinkmann, 1997: 72
Sangiorgiosaurus kuhnschnyderi Brinkmann, 1998A: 132
Mixosaurus kuhnschnyderi Brinkmann, 1998B: 170
Mixosaurus kuhnschnyderi; Motani, 1999B: 480
Holotype: PIMUZ T1324, a nearly complete skeleton.
Diagnosis: Small Mixosaurus similar to M. cornalianus in
size; alternation of conical and mound-shaped teeth in
posterior jaw; mandible deep.
Occurrence: Monte San Giorgio, Tessin, Switzerland.
Stratigraphic range: Grenzbitumen horizon; Middle Triassic
(Anisian-Ladinian boundary).
Remarks: This is another species that may be based only
on an ontogenetic stage of Mixosaurus nordenskioeldii; see
earlier sections;- Motani (1999B) was unaware of the paper
by Brinkmann (1998B), in which Sangiogiosaurus was first
synonymized with Mixosaurus.
Genus Tholodus Meyer, 1851
Tholodus Meyer, 1848: 4 6 7 [nomen nudum]
Tholodus Meyer, 1 8 5 4 :1 9 9
Type species: Tholodus schmidi Meyer, 1848.
Diagnosis: Posterior tooth crowns large (well over 2 cm in
distomesial diameter), rounded and laterally compressed;
constriction of tooth crown proximally; anterior teeth
probably pointed; multiple tooth rows; long dental roots
with clear plicidentine.
Tholodus schmidi Meyer, 1851
Tholodus schmidi Meyer, 1848: 467 [nomen nudum]
Tholodus schmidi Meyer, 1851: 199
Syntypes: A possible maxilla fragment with four teeth,
and two isolated teeth.
Diagnosis: As for the genus.
Occurrence: Zwetzen, Freyburg a. U., and Schmerbitz,
Germany; Strzelce Opolskie, Poland.
Stratigraphic range: Lower Muschelkalk; Middle Triassic
(Anisian).
Remarks: Meyer (1848) proposed the binomen Tholodus
schmidi and briefly mentioned that it represents a possibly
large fish with tooth crowns shaped like a cathedral dome.
No specification of material was given, so the name was a
nomen nudum at that point. Meyer (1851) described the
material properly with a more detailed description and
figures.
Tholodus schmidi is a poorly known form with ambiguous
affinities. It is known mostly from teeth (Meyer, 1851;
Peyer, 1939; Sander & Mazin, 1993), with some associated
jaw fragments. The posterior teeth most closely resemble
those of Mixosaurus nordenskioeldii and M. fraasi; the crowns
are rounded, laterally compressed, and constricted at the
bottom, the roots are rather straight and infolded, and
there are multiple maxillary tooth rows. The teeth of
Tholodus differ from those of the two species of Mixosaurus
in that they are more than twice as large, that they have
clearer crown striation, and that they lack distomesial ridging
on the crown. Also, multiple tooth rows in the dentary,
which is said to exist in Tholodus, have not been explicitly
described for Mixosaurus. It is possible that the species
represent a large unknown form of Mixosaurus, but additional
specimens are required for further discussions.
Affinity with Omphalosaurus has been suggested for
Tholodus (see Sander & Mazin [1993] for a review). However,
Tholodus has much more in common with Mixosaurus
than with Omphalosaurus. The only similarity between
Tholodus and Omphalosaurus is the rounded crown shape,
which is also shared with many other durophagous vertebrates.
As P eyer (1939) warned, adaptations for duro-
phagy may obscure the affinity of Tholodus.
Suborder Merriamosauriformes nov.
Definition: All hueneosaurian more closely related to
Ophthalmosaurus icenicus than Mixosaurus cornalianus.
Family incertae sedis
Genus Phantomasaurus Maisch et Matzke, 2000B
Phantomasaurus Masich et Matzke, 2000B: 65
Type species: Phantomasaurus neubigi (Sander, 1997).
Diagnosis: Slender retroarticular process (Sander, 1997);
saddle-shaped basiocdpital condyle; supernumerary paired
ossifications dorsal to supraoccipital; ventrolateral keels
on anterior vertebrae (Maisch & Matzke, 2000B).
Phantomasaurus neubigi (Sander, 1 9 9 7 )
Shastasaurus neubigi Sander, 1997: 22
Shastasaurus (?) neubigi; Motani, 1999B: 478
Phantomasaurus neubigi; Masich & Matzke, 2000B: 65
Holotype: BSP 1992 I 39, a partial skeleton.
Diagnosis: As for the genus.
Occurrence: Karlstadt am Main, Franconia, Germany.
Stratigraphic range: Lower upper Muschelkalk; Middle
Triassic (upper Anisian).