Anisian (Middle Triassic) ammonoids from North America:
quantitative biochronology and biodiversity
Claude Monnet and Hugo Bucher
Paläontologisches Institut und Museum, Universität Zürich, Karl Schmid Strasse 4, CH-8006 Zürich, Switzerland
email: claude.monnet@pim.unizh.ch; hugo.fr.bucher@pim.unizh.ch
ABSTRACT: This study synthesizes and revises the ammonoid zonations as well as their correlation with each other for western Nevada
(USA), British Columbia (Canada), and the Sverdrup Basin (Canada) by utilizing the unitary association method. Based on a standardized taxonomy, the Anisian in the studied areas contains 13, 10, and 3 zones and a total of 174, 90, and 7 species, respectively. The
zonations are correlated by means of a ‘common taxa zonation’, which includes all taxa common to the studied basins. This leads to new
and more precise correlations, which are at slight variance with those of the literature. Hence, the Buddhaites hagei Zone (Canada) correlates only with the Intornites mctaggarti Subzone (Nevada) and not with the entire Acrochordiceras hyatti Zone (Nevada). The
Tetsaoceras hayesi Zone (Canada) appears to correlate with the Unionvillites hadleyi Subzone (Nevada) of the hyatti Zone and not with
the Nevadisculites taylori Zone. The Hollandites minor Zone (Canada) more than likely correlates with the taylori Zone (Nevada) rather
than the Balatonites shoshonensis Zone as is usually acknowledged. The unitary association method enables us to quantify the
diachronism of the studied taxa, which affects about 67% of the genera and 18% of the species common to Nevada and British Columbia.
Therefore, this diachronism is significant and its value for correlation should not be overlooked. Finally, a diversity analysis based on the
revised zonations is performed. This analysis reveals that the major event occurred during the Nevada hadleyi Subzone (early Middle
Anisian), which in Nevada and British Columbia, records the highest species richness of the Anisian as well as enhanced exchanges between usually latitudinally restricted faunas. This event may reflect significant changes in climate or oceanic circulation at that time.
INTRODUCTION
Triassic ammonoid zonations were European in nature (Tozer
1984), since most Triassic stages were first defined in the Alps.
Hence, they have usually been considered as the standard, reference zonal sequences for the Triassic. However, most of these
zonations were flawed by a typological taxonomy, which resulted in far too many arbitrary species, as well as poor
superpositional control, and the condensed occurrence of several faunas (Tozer 1971, 1994b; Brack and Rieber 1993; Balini,
in Gaetani 1993). The Anisian stage (early Middle Triassic;
245-240.7 Ma, Gradstein and Ogg 2004; Mundil et al. 1996)
serves as an illustrative example of these problems. Consequently, the European sequence of Anisian ammonoid zones
still lacks consensus and even though a thorough taxonomic revision is needed, progress is being made towards resolving
these conflicts (e.g. Brack and Rieber 1986, 1993, 1996; Balini
1992a, b, in Gaetani 1993; Gaetani 1993; Mietto and Manfrin
1995; Brack et al. 1995, 2003; Mietto et al. 2003; Vörös et al.
2003).
At present, the newer, more refined North American zonation is
known to be more robust and is based on a coherent taxonomy
(Tozer 1971, 1994b). Indeed, the biochronology of the North
American Triassic is based on what is probably the world’s
most complete sequence of Anisian ammonoid faunas. The current debate on the definition of the Anisian/Ladinian boundary
in Europe (e.g. Vörös 1993; Brack and Rieber 1994; Brack et al.
2003; Mietto et al. 2003; Vörös et al. 2003; and the final 2005
decision of the Subcommission on Triassic Stratigraphy) as
well as the discovery of important new faunas in North America
during the last few decades has prompted this revision. Indeed,
some of these new faunas bring to the forefront major changes
in the standard zonation and new correlations across the North
stratigraphy, vol. 2, no. 4, pp. 281-296, 2005, text-figures 1-10, (2006)
American plate-bound series (as opposed to accreted terranes),
as well as with the European sequence.
GEOLOGIC SETTING
Marine Triassic rocks yielding rich Anisian ammonoid faunas
occur around the Pacific and Arctic oceans, as well as in the
Tethyan belt, which comprises the Alps, Turkey, Iran, southern
Tibet, southern China, and Indonesia (Tozer 1984). This study
takes into account the following three basins distributed along a
paleolatitude gradient in the Cordillera of western North America: Nevada, British Columbia, and the Sverdrup Basin. The relative positions of these three basins have remained unchanged
since Triassic times, thus providing a paleogeographical latitudinal transect (text-fig. 1).
Triassic rocks of the Sverdrup Basin provide a record of marine
facies from the Lower Griesbachian to the Upper Norian.
Anisian faunas occur in the Schei Point and Blaa Mountain formations, which are composed of shales, siltstones and sandstones. Triassic rocks of British Columbia belong to the
plate-bound series of the tectonic Eastern Belt (Rocky Mountains). Anisian faunas occur in the Toad Formation, which consists mainly of dark gray calcareous siltstone and shale. Most of
the Triassic ammonoid faunas known from Canada occur in a
similar biofacies in which the ammonoids are associated with
other fossils indicative of a pelagic habitat (e.g. radiolarians,
conodonts, thin-shelled bivalves such as Daonella). Tozer
(1994a) provided a brief geologic review of these two Canadian
areas.
Triassic rocks of northwestern Nevada belong to a shallow marine shelf terrane (Speed 1978), in which sedimentation rates
were controlled by differential uplift and subsidence resulting
from an incipient extension of northwestern Nevada following
281
Claude Monnet and Hugo Bucher: Anisian ammonoid biochronology of North America
the Sonoma orogeny (Wyld 2000). The studied areas are therefore parautochthonous with respect to the North American
craton (Wyld 2000). A detailed stratigraphical analysis of the
basin was worked out by Silberling and Wallace (1969) and
Nichols and Silberling (1977) following many years of intensive work. Anisian faunas occur in the Fossil Hill Member,
which is common to the Favret and Prida formations of the Star
Peak Group. Lithologically, the Fossil Hill Member is mainly
composed of thin-bedded, dark micritic limestones alternating
with silty shales, which were deposited in an oxygen-poor basin. Ammonoids occur with radiolarians, conodonts, halobiid
bivalves, other cephalopods, fishes, and ichthyosaurs.
rotelliformis Zone (text-fig. 2). These new faunas also lead to
the recognition of 11 new genera and 15 new species, as well as
the revision of several species defined by Smith (1914), but subsequently considered as junior synonyms by Spath (1934) and
Silberling and Nichols (1982). Text-figure 3 displays the synthetic biochronological distribution of these faunas. In addition
to their taxonomic and biochronological importance for correlation with the European sequence, these faunas are also of essential significance for the phylogenetic reconstruction of the
Paraceratitinae.
TAXONOMIC NOTES
Zonation method
The data utilized for the biochronological analysis have been
compiled from multiple sources (new field data and literature)
in which taxonomic definitions may be at variance. In order to
establish reliable correlations and to avoid inconsistencies between the zonations for each basin, it becomes necessary to first
achieve a standardized taxonomy. The use of each species name
must be consistent throughout the entire data set. It must also
account for intraspecific variability and for ontogenetic
changes. Note that inherent limitations to this standardization
are obviously imposed by the quality of available taxonomic
data (i.e. plots of occurrences against lithologic columns and illustration of intraspecific variability and ontogeny). It is noteworthy that we used a population approach to identify species,
since ammonoids may display a large intraspecific variability
and covariation of characters (the 1st Buckman law of
covariation), which ranges from compressed, involute, weakly
ribbed forms to more depressed, more evolute, strongly ribbed
forms (e.g. Reeside and Cobban 1960; Callomon 1985; Dagys
and Weitschat 1993; Checa et al. 1997). For further details, see
Tozer (1971) and Monnet and Bucher (in press).
The chronological component of the ammonoid fossil record is
assessed here by utilizing the unitary association method
(UAM). This method, created by Guex (1977, 1991), is favored
since it is acknowledged to have the following invaluable advantages: 1) it is a quantitative and deterministic method based
on the coexistence of species; 2) it constructs discrete (non-continuous) biozones in agreement with the discontinuous nature of
the fossil record; 3) it preserves the integrity of the original data
set (i.e. all raw documented associations of taxa – coexistence in
space – are preserved and no reversed sequences of ranges are
created), contrary to most other biochronological methods (e.g.
probabilistic and multivariate treatments of local first and last
appearance datums; see discussion in Baumgartner 1984a and
Boulard 1993); 4) its efficiency in solving complicated
biochronological problems has been demonstrated with taxonomic groups having a much less favorable record than
ammonoids (e.g. radiolarians: Baumgartner 1984b;
Baumgartner et al. 1995; O’Dogherty 1994; Carter et al. 1998;
micromammals: Guex and Martinez 1996; dinoflagellates: Edwards and Guex 1996; nannoplankton: Boulard 1993); 5) it usually involves a two or threefold increase in biochronological
resolution, even in the case of ammonoids (for examples, see
Monnet and Bucher 1999, 2002), which are traditionally acknowledged as one of the leading groups for dating Mesozoic
marine rocks; 6) it allows one to assess a posteriori and objectively, the diachronism of the studied taxa and to choose the actual characteristic taxa of each zone; and, last but not least, 7)
Escarguel and Bucher (2004) demonstrated that the unknown
duration of discrete biochronozones produced by the unitary association method does not involve ceteris paribus, a methodological bias when inferring temporal changes in taxonomic
richness (an invaluable property for biodiversity analyses).
The standard biochronological scheme for the Anisian was produced: 1) by Silberling and Wallace (1969), Silberling and
Nichols (1982), and Bucher (1988, 1989, 1992a, b; 1994) for
Nevada, 2) by Silberling and Tozer (1968), Tozer (1967, 1982,
1994a) and Bucher (2002) for British Columbia, and 3) by
Tozer (1994a) for the Sverdrup Basin. These works, based on
bed-by-bed sampling, revealed that the Nevada sequence
should be considered as the world’s most complete sequence
for low paleolatitude Anisian ammonoid faunas. Therefore, the
data necessary for the correlation of North American Anisian
ammonoid biochronology were derived from the references
cited above. Bucher and Orchard (1995) also provided additional preliminary taxonomic and biochronological data for
ammonoids spanning the Anisian/Ladinian boundary.
This study also includes recent, unpublished data from British
Columbia (occurrence of the Parafrechites meeki Zone near the
Alaska Highway) and northwestern Nevada (occurrence of the
Paracrochordiceras americanum Zone in the northern New
Pass Range), which add significantly to the correlations. Recent
investigations in the Augusta Mountains (northwest Nevada)
bring to the forefront additional new faunas whose taxonomy
have been analyzed and described by Monnet and Bucher (in
press) in a thorough monographic treatment. These new faunas
enable the recognition of: 1) two new zones at the base of the
Late Anisian, namely the Gymnotoceras weitschati and G.
mimetus zones, bracketed between the Balatonites
shoshonensis and Gymnotoceras rotelliformis zones, 2) a new
subzone in the latest Middle Anisian (Bulogites mojsvari
Subzone), and 3) a revision of the subdivisions of the
282
BIOCHRONOLOGY
The unitary association method constructs zonations composed
of a sequence of discrete, association zones, called UA-zones,
which are maximal sets of intersecting ranges of taxa and the
finest possible subdivisions from the association concept. Fundamentally, Oppel Zones, Concurrent Range Zones, Assemblage Zones, and Unitary Associations Zones are closely related
because they are all based on the coexistence of species. The
UAM differs from other association methods in that it parsimoniously exploits conflicting biostratigraphic relationships that
commonly occur among first occurrences (FOs) and last occurrences (LOs) of taxa, to infer virtual associations (i.e. coexistences in time but not in space). The reader should be aware that
a strict association zone (such as those produced by the UAM) is
characterized either by the taxa occurring only within this zone,
or by the intersecting ranges of taxa observed within the zone.
The UAM has been automated by the BioGraph computer program (Savary and Guex 1991, 1999). Edwards and Guex (1996)
and Monnet and Bucher (2002) summarized the major princi-
Stratigraphy, vol. 2, no. 4, 2005 (2006)
TEXT-FIGURE 1
Location of studied areas (northwestern Nevada, British Columbia, and Sverdrup Basin). Middle Triassic paleogeographical map modified after
Golonka and Ford (2000) and Stampfli and Borel (2002).
ples of this deterministic method. See Guex (1991), Angiolini
and Bucher (1999) and Monnet and Bucher (2002) for an exhaustive methodological use of the UAM. Monnet and Bucher
(1999, 2002) also developed an additional optimization procedure that automatically select for the best legal input data set in
order to obtain more accurrate results (a minimum number of
remaining biostratigraphic contradictions and a maximum
number of UA-zones). This latter approach is the method used
throughout this study.
Indeed, common taxa zonation allows for the correlation of local, basin-scaled zonations with greater confidence, because it
is based on all of their common taxa rather than just a few taxa
arbitrarily chosen as index guides. Hence, this method determines if a local zone is an exact correlative of another local
zone, if it is correlative with a group of zones, or if it has no correlative at all in the other basin. Secondly, it makes it possible to
assess the diachronism of the taxa common to the studied basins
since it is usually not negligible and must be taken into account.
Correlation method
RESULTS
In order to avoid the pitfalls of diachronism and endemic taxa,
the revised zonal sequences are correlated by creating a
zonation, which considers all of the taxa common to the studied
basins. This ‘common taxa zonation’ is achieved by utilizing
the UAM in conjunction with the previously cited optimization
procedure at both the species and genus levels. These ‘common
taxa zonations’ make it possible to date each local zone (i.e. restricted to a basin) by determining which local zone documents
the characteristic or age-diagnostic species and/or genera (singletons or pairs of intersecting ranges) of each global zone (at
the scale of North America) of the common taxa zonation.
The zonal sequences are separately revised for each studied area
by utilizing the unitary association method. The resulting data
set contains 174, 90, and 7 species for the entire Anisian from
Nevada, British Columbia and the Sverdrup Basin, respectively.
Likewise, the three studied areas contain 13, 10, and 3 zones, respectively. The faunal content of each zone is listed in text-figure 4.
The interest of this approach is twofold. First, it enables one to
be more objective and precise in correlating the studied areas.
The results of this revision are congruent with the previous empirical zonations with the exception of the Nevadites hyatti
Subzone of the Nevada Frechites occidentalis Zone, which is
herein rejected because it does not have diagnostic taxa or association of taxa. Indeed, its entire faunal content is included
within the more diverse Nevadites humboldtensis Subzone.
283
Claude Monnet and Hugo Bucher: Anisian ammonoid biochronology of North America
TEXT-FIGURE 2
Revised ammonoid zonation of the Fossil Hill Member (Nevada) around the Middle/Late Anisian boundary (after Monnet and Bucher, in press).
Hence, the two subzones of the occidentalis Zone as defined by
Silberling and Nichols (1982) are here merged.
The zonal sequences of Nevada, British Columbia and the
Sverdrup Basin have been correlated by utilizing the ‘common
taxa zonation’ as explained above. For example, the sequences
for Nevada and British Columbia have been processed together
as a two-section data set after taking into account all taxa (first
species and then genera) common to the two basins. This yields
a zonation consisting of six zones from 11 common species
(text-fig. 5A), and 14 zones from 30 common genera (text-fig.
5B). For instance, in Nevada and British Columbia the Para-
284
crochordiceras americanum Zone contains the restricted occurrence of the genus Columbisculites, which constitutes a characteristic taxon of the ‘common-zone‘ 4 (text-fig. 5B).
Based on these ‘common taxa zonations’, the resulting proposed correlation of the zonal sequences for Nevada, British
Columbia and the Sverdrup Basin is displayed in text-figure 6.
It is immediately apparent that the low-paleolatitude Nevada sequence contains by far the most complete succession of
ammonoid faunas. Based on the present state of available data,
all demonstrated correlations as well as all remaining uncertainties are graphically represented by the boxes in text-figure 6.
Stratigraphy, vol. 2, no. 4, 2005 (2006)
TEXT-FIGURE 3
Synthetic range chart showing the biostratigraphic distribution of ammonoids around the Middle/Late Anisian boundary in the Fossil Hill Member (after
Monnet and Bucher, in press).
Note that uncertainties in the correlations are portrayed by thick
vertical black bars in text-figure 6. For instance, the weitschati
Zone is documented neither in British Columbia, nor in the
Sverdrup Basin. It is noteworthy that several local, basin-scaled
zones do not contain the diagnostic faunas of the ‘common taxa
zonations’. This uncertainty indicates either that the correlation
remains indeterminate or that equivalent zones have not been
documented in the other basins. As an example, the correlation
of the Tetsaoceras hayesi Zone of British Columbia with the
Nevada sequence is relatively uncertain. Nevertheless, this
zone probably correlates with the Unionvillites hadleyi
Subzone of Nevada, because among their common, nondiachronous datums both zones contain the index species
Tetsaoceras hayesi and document the last occurrence of Isculites tozeri (see text-fig. 4). A zone of a particular basin may
also correlate with several zones in the other basin. For instance, the Lenotropites caurus Zone of Nevada may equally
correlate with the Azarianites bufonis and Grambergia nahwisi
subzones of British Columbia because the characteristic species
or pairs of species of the caurus Zone are found in these two
subzones.
The correlations proposed here are at slight variance with those
already published by other workers (e.g. Tozer 1994b: text-fig.
7). For instance, Tozer broadly correlates the Buddhaites hagei
Zone with the Acrochordiceras hyatti Zone, whereas our work
indicates that the hagei Zone correlates more precisely with the
Intornites mctaggarti Subzone of the hyatti Zone because of the
common and exclusive occurrence of Hollandites pelletieri.
Our correlations also show two additional discrepancies with
Tozer’s scheme. First, Tozer correlated the hayesi Zone with
the Nevadisculites taylori Zone of Nevada, whereas it is herein
correlated with the hadleyi Subzone of the hyatti Zone. This
correlation is prompted by the coexistence of Tetsaoceras
hayesi and Gymnites perplanus with the genus Alanites. Second, Tozer correlated the Hollandites minor Zone with the
shoshonensis Zone of Nevada. We find that the minor Zone is in
fact difficult to correlate with any degree of precision. The only
species of the minor Zone in common with the Nevada sequence is Amphipopanoceras selwyni. This species is found in
the Favreticeras rieberi Subzone of Nevada and in the hayesi
Zone of British Columbia, which in turn correlates with the
hadleyi Subzone of Nevada. Unfortunately, the genera common
to Nevada and British Columbia in this zone are of no help because their ranges extend well beyond the zone. Therefore, the
minor Zone is roughly equivalent to the Pseudodanubites
nicholsi-F. rieberi interval of Nevada, which indicates that the
minor Zone is more likely correlative with a part of the taylori
Zone and not of the shoshonensis Zone as hypothesized by
Tozer (1994b).
285
Claude Monnet and Hugo Bucher: Anisian ammonoid biochronology of North America
TEXT-FIGURE 4
Biostratigraphic ranges of Anisian ammonoid species from the Sverdrup Basin [A], British Columbia [B], and Nevada [C]. Numbers refer to the successive zones and subzones of each studied area. Zonal and subzonal name giving taxa are reported in text-figure 6.
286
Stratigraphy, vol. 2, no. 4, 2005 (2006)
TEXT-FIGURE 4
Continued.
287
Claude Monnet and Hugo Bucher: Anisian ammonoid biochronology of North America
TEXT-FIGURE 5
Common taxa zonations of Anisian ammonoids from Nevada and British Columbia at the species level [A] and genus level [B].
288
Stratigraphy, vol. 2, no. 4, 2005 (2006)
TEXT-FIGURE 6
New correlations of the ammonoid zonations of Nevada, British Columbia, and the Sverdrup Basin. Numbers on right refer to those of text-figure 4.
Numbers on left refer to those of text-figure 8. Thick vertical black bars indicate poorly constrained correlations. The length of these black bars shows the
maximal amount of uncertainty.
289
Claude Monnet and Hugo Bucher: Anisian ammonoid biochronology of North America
tween two successive zones) have narrow fluctuations (text-fig.
9B, E) and are significant only at the Early/Middle Anisian
boundary (peak of origination) and within the Middle Anisian at
the hyatti/taylori zone boundary (peak of extinction).
TEXT-FIGURE 7
Correlation of Nevada and British Columbia ammonoid zones after
Tozer (1994b). Note the position of the minor and hayesi Zones as compared to text-figure 6.
Finally, the revised zonations as well as their correlation with
each other, reveal a significant amount of diachronous taxa for
both first and last occurrences (~18% and ~67% of the common
species and genera, respectively). Text-figure 8 clearly illustrates that few species and genera are actually synchronous.
Whatever the cause(s) of this diachronism (e.g. preservation,
paleobiogeographical, or paleoecological biases), this bias is
still overlooked by a large number of ammonoid workers who
persist in using the first appearance of index taxa for correlation.
BIODIVERSITY
Several metrics are utilized to extract and analyze ammonoid
biodiversity patterns. These include species richness, origination and extinction, turnover, and poly-cohort analysis. Monnet
et al. (2003) described and discussed these biodiversity indices.
The Sverdrup Basin is not included because of its highly discontinuous fossiliferous record and its highly depauperate faunas.
The turnover (sum of originations and extinctions) is relatively
important, and indicates high evolutionary rates for the
ammonoid faunas at that time (text-fig. 9C, F). Due to a lower
resolution record, the Canadian turnover has higher values than
does Nevada, but both basins have their highest turnover values
at the Early/Middle Anisian boundary, at the hyatti/taylori zone
boundary within the Middle Anisian, and at the Middle/Late
Anisian boundary. The Early/Middle Anisian boundary is
marked by the diversification of Beyrichitinae, whereas the
Middle/Late Anisian boundary is marked by the diversification
of Paraceratitinae. Note that the Early Anisian of Nevada records the recovery of ammonoid faunas from the lowest values
of species richness at the Spathian/Anisian boundary, and
shows an increasing trend of species richness as well as the
highest turnover percentages.
However, the biodiversity fluctuations described above are relatively weak. There are no statistically abnormal values in either
basin that would indicate a particular significant event departing
from background fluctuations. This is corroborated by the absence of significantly non log-linear poly-cohorts (results not illustrated here), which suggest the absence of a statistically
significant extinction phase. More generally, a poly-cohort
which fits a log-linear regression indicates relatively stable extinction rates through time (see detailed example in Monnet et
al. 2003).
In conclusion, the Anisian ammonoid faunas of British Columbia and northwestern Nevada have no significant (departing
from background) values of biodiversity (either common or
not), which may indicate a period of relative stability for
ammonoids. Nevertheless, the hadleyi/hayesi subzones (early
Middle Anisian) record a peculiar paleobiogeographical event
in addition to their very high species richness and turnover values as seen above. Indeed, it corresponds to a brief time interval
during which exchanges did occur between latitudinally restricted faunas. For example, low paleolatitude genera such as
Pseudodanubites and Isculites occurred briefly in British Columbia, while mid- and high paleolatitude genera such as
Arctohungarites, Amphipopanoceras, and Czekanowskites expanded their ranges southward. This event may have resulted
from brief but significant changes in climate or oceanic
circulation.
The species richness (number of species occurring within each
zone) of both basins fluctuates greatly, starting with low values
in the Early Anisian (text-fig. 9A, D). It reaches its highest values in the early Middle Anisian (hadleyi/hayesi subzones), and
declines from then on to moderate values until the Anisian/
Ladinian boundary. Text-figure 10 illustrates the percentage for
each family for the entire stage in terms of its number of species, for each basin. British Columbia is dominated by
Beyrichitinae among the Ceratitidae, Longobarditidae, and to a
lesser extent by Gymnitidae and Parapopanoceratidae, while
Nevada is almost completely dominated by Beyrichitinae and
Paraceratitinae among the Ceratitidae, and to a lesser extent by
Acrochordiceratidae, Balatonitidae, Gymnitidae, and Longobarditidae. It is noteworthy that the species richness of the Nevada subzones is nearly identical on average, to that of the
British Columbia zones.
Another noteworthy event occurred during the caurus Zone,
which records the spread of typical Boreal faunas in Nevada. As
already noted by Dagys (1988), during the Early Anisian
longobarditids (Groenlandites, Grambergia), parapopanoceratids (Stenopopanoceras), and danubitids predominated in
the Boreal realm, whereas ussuritids (Ussurites, Monophyllites), japonitids (Japonites, Caucasites), acrochordiceratids (Paracrochordiceras), and sturiids (Sturia) characterized the Tethyan realm. Text-figure 5B clearly shows that most
of these Boreal genera also occurred in Nevada during the
caurus Zone. This southward migration of Boreal ammonoids
may reflect a cooling event.
Originations (number of species appearing between two successive zones) and extinctions (number of species disappearing be-
This study synthesizes and revises the zonal sequences as well
as their correlation with each other for three basins situated
290
CONCLUSIONS
Stratigraphy, vol. 2, no. 4, 2005 (2006)
TEXT-FIGURE 8
Biostratigraphic ranges and diachronism (at the zonal level and in terms of both first and last occurrences) of Anisian ammonoids from Nevada and British Columbia at the species level [A] and genus level [B]. See text-figure 6 for zone numbers. Shaded areas indicate documentation gaps in British Columbia.
291
Claude Monnet and Hugo Bucher: Anisian ammonoid biochronology of North America
TEXT-FIGURE 9
Diversity changes of ammonoids during the Anisian stage in British Columbia and Nevada. [A, D]: species richness values. [B, E]: extinction/origination values (bars) and percentages (shaded areas). [C, F]: turnover values (bars) and percentages (shaded area).
292
Stratigraphy, vol. 2, no. 4, 2005 (2006)
TEXT-FIGURE 10
Percentage of each Anisian family (number of species) for British Columbia [A] and Nevada [B].
293
Claude Monnet and Hugo Bucher: Anisian ammonoid biochronology of North America
along a paleolatitude gradient in the North American Cordillera
(Nevada, British Columbia, and the Sverdrup Basin). The
zonations are constructed by utilizing the unitary association
method, which constructs discrete, association zones. Based on
a standardized taxonomy, the studied areas contain 13, 10, and
3 zones and 174, 90, and 7 species, respectively. The construction of a global zonation based on all taxa common to the studied basins (common taxa zonation) leads to more precise
correlations, which are at slight variance with the ‘standard’
correlations of the literature, such as those of Tozer (1994b). Indeed, the hagei Zone (Canada) correlates only with the
mctaggarti Subzone (Nevada) rather than the entire hyatti Zone
(Nevada), and the hayesi Zone (Canada) appears to correlate
with the hadleyi Subzone (Nevada) of the hyatti Zone rather
than the taylori Zone. The minor Zone (Canada) more than
likely correlates with the taylori Zone (Nevada) rather than the
shoshonensis Zone, as is usually acknowledged. The unitary association method, utilized for this biochronological revision,
enables one to objectively and a posteriori quantify the amount
of diachronism of the studied taxa. Consequently, this study reveals that 67% of the 30 genera common to Nevada and British
Columbia are diachronous for both first and last occurrences.
Finally, these revised zonations provide a robust time frame for
diversity analyses. It demonstrates that species richness peaked
during the Nevada hadleyi Subzone (early Middle Anisian),
which obviously represents short-term migrations between normally latitudinally restricted faunas. This event may reflect
significant changes in climate or in oceanic circulation at that
time.
It is also noteworthy that the species richness values for the Nevada subzones are nearly identical on average to those of the
British Columbia zones. Although this pattern may partly reflect a preservation bias resulting from the reduced carbonate
content of the higher latitude record, it nevertheless does not
exclude lower speciation rates among higher latitude taxa. Species counts derived from single horizons of early diagenetic carbonate nodules in the high latitude record (British Columbia,
Sverdrup Basin) do actually provide a reliable measure of
species richness.
ACKNOWLEDGMENTS
BAUMGARTNER, P.O., 1984a. Comparison of unitary associations
and probabilistic ranking and scaling as applied to Mesozoic radiolarians. Computers & Geosciences 10 (1): 167-183.
–––––, 1984b. A Middle Jurassic-Early Cretaceous low latitude
radiolarian zonation based on unitary associations and age of
Tethyan radiolarites. Eclogae geologicae Helvetiae 77 (3): 729-841.
BAUMGARTNER, P.O., O’DOGHERTY, L., GORICAN, S.,
URQUHART, E., PILLEVUIT, A., and DE WEVER, P., 1995. Middle Jurassic to Lower Cretaceous radiolaria of Tethys: occurrences,
systematics, biochronology. Mémoires de Géologie (Lausanne) 23:
1172 pp.
BOULARD, C., 1993. Biochronologie quantitative: concepts, méthodes
et validité. Document des Laboratoires de Géologie de Lyon 128:
259 pp.
BRACK, P., and RIEBER, H., 1986. Stratigraphy and ammonoids from
the lower Buchenstein Beds in the Brescian Prealps and Giudicarie
and their significance for the Anisian/Ladinian boundary. Eclogae
geologicae Helvetiae 79 (1): 181-225.
–––––, 1993. Towards a better definition of the Anisian/Ladinian boundary: new biostratigraphic data and correlations of boundary sections
from Southern Alps. Eclogae geologicae Helvetiae 86 (2): 415-527.
–––––, 1994. The Anisian/Ladinian boundary: retrospective and new
constraints. Albertiana 13: 25-36.
–––––, 1996. The new “High resolution Middle Triassic ammonoid standard scale” proposed by Triassic researchers from Padova – A discussion of the Anisian/Ladinian boundary interval. Albertiana 17:
42-50.
BRACK, P., RIEBER, H ., and MUNDIL, R., 1995. The
Anisian/Ladinian boundary interval at Bagolino (Southern Alps, Italy): I. Summary and new results on ammonoid horizons and radiometric age dating. Albertiana 15: 45-56.
BRACK, P., RIEBER, H., and NICORA, A., 2003. The global stratigraphic section and point (GSSP) of the base of the Ladinian Stage
(Middle Triassic). Albertiana 28: 13-25.
BUCHER, H., 1988. A new Middle Anisian (Middle Triassic)
ammonoid zone from northwestern Nevada (USA). Eclogae
geologicae Helvetiae 81 (3): 723-762.
Jim Jenks is thanked for improving the English version of an
earlier version of this work. Careful and constructive pre-reviews by Spencer Lucas and Lucy Edwards are greatly acknowledged. Peter Brack and an anonymous reviewer also
contributed to improve the final version of the manuscript. This
work was partly supported by the Swiss NSF project
200020-105090/1 (to H.B.)
–––––, 1989: Lower Anisian ammonoids from the northern Humboldt
Range (northwestern Nevada, USA) and their bearing upon the
Lower-Middle Triassic boundary. Eclogae geologicae Helvetiae 82
(3): 945-1002.
REFERENCES
–––––, 1992b: Ammonoids of the Shoshonensis Zone (Middle Anisian,
Middle Triassic) from northwestern Nevada (USA). Jahrbuch der
Geologischen Bundesanstalt Wien 135 (2): 425-465.
ANGIOLINI, L., and BUCHER, H., 1999. Taxonomy and quantitative
biochronology of Guadalupian brachiopods from the Khuff Formation, southeastern Oman. Geobios 32 (5): 665-699.
BALINI, M., 1992a. Lardaroceras gen. n., a new late Anisian
ammonoid genus from the Prezzo Limestone (Southern Alps).
Rivista Italiana di Paleontologia e Stratigrafia 98 (1): 3-28.
BALINI, M., 1992b. New genera of Anisian ammonoids from the
Prezzo Limestone (Southern Alps). Atti Ticinensi di Scienze della
Terra (Pavia) 35: 179-198.
294
–––––, 1992a: Ammonoids of the Hyatti Zone and the Anisian transgression in the Triassic Star Peak Group, northwestern Nevada, USA.
Palaeontographica Abteilung A 223: 137-166.
–––––, 1994: New ammonoids from the Taylori Zone (Middle Anisian,
Middle Triassic) from northwestern Nevada (USA). Mémoires de
Géologie (Lausanne) 22: 1-8.
———, 2002: Early Anisian (Middle Triassic) ammonoid biostratigraphy of northeastern British Columbia. Eclogae geologicae
Helvetiae 95: 277-287.
BUCHER, H., and ORCHARD, M.J., 1995. Intercalibrated ammonoid
and conodont succession, Upper Anisian-Lower Ladinian of Nevada. Albertiana 15: 66-71.
Stratigraphy, vol. 2, no. 4, 2005 (2006)
CALLOMON, J.H., 1985. The evolution of the Jurassic ammonite family Cardioceratidae. Special Papers in Palaeontology 33: 49-90.
CARTER, E.S., WHALEN, P.A., and GUEX, J., 1998: Biochronology
and paleontology of Lower Jurassic (Hettangian and Sinemurian)
radiolarians, Queen Charlotte Islands, British Columbia. Bulletin of
the Geological Survey of Canada 496: 162 pp.
CHECA, A., COMPANY, M., SANDOVAL, J., and WEITSCHAT,
W., 1997. Covariation of morphological characters in the Triassic
ammonoid Czekanowskites rieberi. Lethaia 29: 225-235.
DAGYS, A.S., 1988. Major features of the geographic differentiation of
Triassic ammonoids. In: Wiedmann, J., and Kullmann, J., Eds.
Cephalopods - Present and Past. Stuttgart, Schweizerbart’sche
Verlagsbuchhandlung: 341-349.
MONNET, C., BUCHER, H., ESCARGUEL, G., and GUEX, J., 2003.
Cenomanian (early Late Cretaceous) ammonoid faunas of Western
Europe. Part II: diversity patterns and the end-Cenomanian anoxic
event. Eclogae geologicae Helvetiae 96: 381-398.
MUNDIL, R., BRACK, P., MEIER, M., RIEBER, H., and OBERLI, F.,
1996. High resolution U-Pb dating of Middle Triassic
volcaniclastics: time-scale calibration and verification of tuning parameters for carbonate sedimentation. Earth Planetary Science Letters 141, 137-151.
NICHOLS, K.M., and SILBERLING, N.J., 1977. Stratigraphy and
depositional history of the Star Peak Group (Triassic), northwestern
Nevada. Geological Society of America Special Paper 178, 73 pp.
DAGYS, A.S., and WEITSCHAT, W., 1993. Extensive intraspecific
variation in a Triassic ammonoid from Siberia. Lethaia 26: 113-121.
O’DOGHERTY, L., 1994. Biochronology and paleontology of
mid-Cretaceous radiolarians from northern Apeninnes (Italy) and
Betic Cordillera (Spain). Mémoires de Géologie (Lausanne) 21, 415
pp.
EDWARDS, L.E., and GUEX, J., 1996. Analytical biostratigraphy and
correlation. In: Jansonius, J., and McGregor, D.C., Eds. Palynology:
principles and applications, 3. American Association of Stratigraphic Palynologists: 985-1009.
REESIDE, J.B., and COBBAN, W.A., 1960: Studies of the Mowry
Shale (Cretaceous) and contemporary formations in the United States
and Canada. U.S. Geological Survey Professional Paper 355, 1-126.
ESCARGUEL, G., and BUCHER, H., 2004: Counting taxonomic richness from discrete biochronozones of unknown duration: a simulation. Palaeogeography Palaeoclimatology Palaeoecology 202:
181-208.
SAVARY, J., and GUEX, J., 1991. BioGraph: un nouveau programme
de construction des corrélations biochronologiques basées sur les associations unitaires. Bulletin de la Société vaudoise de Sciences
naturelles 80 (3): 317-340.
GAETANI, M., 1993. Anisian/Ladinian boundary field workshop
Southern Alps - Balaton Highlands, 27 June - 4 July; Field-guide
book. I.U.G.S. Subcommission of Triassic stratigraphy: 118 pp.
–––––, 1999. Discrete biochronological scales and unitary associations:
description of the BioGraph computer program. Mémoires de
Géologie (Lausanne) 34, 281 pp.
GOLONKA, J., and FORD, D., 2000. Pangean (Late Carboniferous-Middle Jurassic) paleo-environment and lithofacies. Palaeogeography Palaeoclimatology Palaeoecology 161: 1-34.
SILBERLING, N.J., and NICHOLS, K.M., 1982. Middle Triassic molluscan fossils of biostratigraphic significance from the Humboldt
Range, northwestern Nevada. U.S. Geological Survey Professional
Paper 1207, 77 pp.
GRADSTEIN, F.M., and OGG, J.G., 2004. Geologic Time Scale 2004 –
why, how, and where next! Lethaia 37: 175-181.
GUEX, J., 1977. Une nouvelle méthode d’analyse biochronologique.
Bulletin de Géologie (Lausanne) 224: 309-322.
–––––, 1991. Biochronological correlations. Springer: 250 pp.
GUEX, J., and MARTINEZ, J.N., 1996. Application of the Unitary Associations to the biochronological scales based on mammals. Acta
Zoologica Cracoviensia 39: 329-341.
MIETTO, P., and MANFRIN, S., 1995. A high resolution Middle Triassic ammonoid standard scale in the Tethys Realm. A preliminary report. Bulletin de la Société géologique de France 166 (5): 539-563.
MIETTO, P., MANFRIN, S., PRETO, N., GIANOLLA, P.,
KRYSTYN, L., and ROGHI, G., 2003. GSSP at the base of the
Avisianum Subzone (FAD of Aplococeras avisianum) in the
Bagolino section (Southern Alps, NE Italy). Albertiana 28: 26-34.
MONNET, C., and BUCHER, H., 1999. Biochronologie quantitative
(associations unitaires) des faunes d’ammonites du Cénomanien du
Sud-Est de la France. Bulletin de la Société géologique de France
170: 599-610.
–––––, 2002. Cenomanian (early Late Cretaceous) ammonoid faunas of
Western Europe. Part I: biochronology (unitary associations) and
diachronism of datums. Eclogae geologicae Helvetiae 95: 57-73.
–––––, in press. New Middle and Late Anisian (Middle Triassic)
ammonoid faunas from northwestern Nevada (USA): taxonomy and
biochronology. Fossils and Strata.
SILBERLING, N.J., and TOZER, E.T., 1968. Biostratigraphic classification of the marine Triassic in North America. Geological Society of
America Special Paper 110, 63 pp.
SILBERLING, N.J., and WALLACE, R.E., 1969. Stratigraphy of the
Star Peak Group (Triassic) and overlying lower Mesozoic rocks,
Humboldt Range, Nevada. U.S. Geological Survey Professional Paper 592, 50 pp.
SMITH, J.P., 1914. The Middle Triassic marine invertebrate faunas of
North America. U.S. Geological Survey Professional Paper 83: 254
pp.
SPATH, L.F., 1934. Catalogue of the fossil cephalopoda in the British
Museum (National History). Part 4, the Ammonoidea of the Trias.
London, 521 pp.
SPEED, R.C., 1978. Paleogeographic and plate tectonic evolution of the
early Mesozoic marine province of the western Great Basin. Society
of Economic Paleontologists and Mineralogists, Pacific Coast
Paleogeography Symposium 2, 253-270.
STAMPFLI, G.M., and BOREL, G.D., 2002. A plate tectonic model for
the Paleozoic and Mesozoic constrained by dynamic plate boundaries and restored synthetic oceanic isochrons. Earth Planetary Science Letters 196 (1-2): 17-33.
TOZER, E.T., 1967. A standard for Triassic time. Bulletin of the Geological Survey of Canada 156, 93 pp.
–––––, 1971. Triassic time and ammonoids: problems and proposals.
Canadian Journal of Earth Sciences 8 (8): 989-1031.
295
Claude Monnet and Hugo Bucher: Anisian ammonoid biochronology of North America
–––––, 1982. Marine Triassic faunas of North America: their significance for assessing plate and terrane movements. Geologische
Rundschau 71 (3): 1077-1104.
VÖRÖS, A., 1993. Redefinition of the Reitzi zone at its type region
(Balaton area, Hungary) as the basal zone of the Ladinian. Acta
Geologica Hungarica 36 (1): 15-38.
–––––, 1984. The Triassic and its ammonoids: the history of a time scale.
Miscellaneous Reports of the Geological Survey of Canada 35: 171
pp.
VÖRÖS, A., BUDAI, T., HAAS, J., KOVÁCS, S., KOZUR, H., and
PÁLFY, J., 2003. A proposal for the GSSP at the base of the Reitzi
Zone (sensu stricto) at Bed 105 in the Felsöörs section, Balaton Highland, Hungary. Albertiana 28, 35-47.
–––––, 1994a. Canadian Triassic ammonoid faunas. Bulletin of the Geological Survey of Canada 467, 663 pp.
WYLD, S.J., 2000. Triassic evolution of the arc and backarc of northwestern Nevada, and evidence for extensional tectonism. Geological
Society of America Special Paper 347, 185-207.
–––––, 1994b. Significance of Triassic stage boundaries defined in
North America. Mémoires de Géologie (Lausanne) 22: 155-171.
Manuscript received
Manuscript accepted
296