Evolutionary stasis, ecophenotypy and environmental controls on ammonite morphology in the Late Cretaceous (Maastrichtian) Western Interior Seaway, USA

We test for the presence of evolutionary stasis in a species of Late Cretaceous ammonoid cephalopod, Hoploscaphites nicolletii, from the North American Western Interior Seaway. A comprehensive dataset of morphological traits was compiled across the entire spatial and temporal range of this species. These were analysed in conjunction with sedimentologically and geochemically derived palaeoenvironmental conditions hypothesized to apply selective pressures. All changes in shell shape were observed to be ephemeral and reversable, that is, no unidirectional trend could be observed in any of the morphological traits analysed. Correlations between palaeoenvironmental conditions and morphological traits suggests ecophenotypic processes were at play; however, either environmental changes were too minor and/or provided no isolating mechanism to drive speciation. These data support mechanisms of stasis such as homogenizing gene flow or stabilizing selection under a fluctuating optimum (probably reflecting spatiotemporally heterogeneous palaeoenvironmental conditions). Finally, changes in shell size were not significantly associated with changes in shell‐specific δ18O, despite a correlation between shell size and δ18O averaged across horizons. This suggests a mismatch in scales of geochemical sampling that supports caution when making broad interpretations based on averaged geochemical data.

Abstract: We test for the presence of evolutionary stasis in a species of Late Cretaceous ammonoid cephalopod, Hoploscaphites nicolletii, from the North American Western Interior Seaway. A comprehensive dataset of morphological traits was compiled across the entire spatial and temporal range of this species. These were analysed in conjunction with sedimentologically and geochemically derived palaeoenvironmental conditions hypothesized to apply selective pressures. All changes in shell shape were observed to be ephemeral and reversable, that is, no unidirectional trend could be observed in any of the morphological traits analysed. Correlations between palaeoenvironmental conditions and morphological traits suggests ecophenotypic processes were at play; however, either environmental changes were too minor and/or provided no isolating mechanism to drive speciation. These data support mechanisms of stasis such as homogenizing gene flow or stabilizing selection under a fluctuating optimum (probably reflecting spatiotemporally heterogeneous palaeoenvironmental conditions). Finally, changes in shell size were not significantly associated with changes in shellspecific d 18 O, despite a correlation between shell size and d 18 O averaged across horizons. This suggests a mismatch in scales of geochemical sampling that supports caution when making broad interpretations based on averaged geochemical data. I N V E S T I G A T I O N S of species' responses to environmental change using the fossil record are of fundamental importance for understanding the processes driving macroevolutionary changes through time. One pattern of response documented in the fossil record that continues to demand explanation is that of stasis, or the occurrence of 'little or no net accrued species-wide morphological change during a species/lineages existence up to millions of years' (Eldredge & Gould 1972;Eldredge et al. 2005;Lidgard & Hopkins 2015). Quantitative statistical analysis and modelling have confirmed the dominant relative frequency of evolutionary stasis in fossil datasets compared to sustained directional change (Hunt 2007;Hopkins & Lidgard 2012). Such studies also support complex 'mosaic' evolution where the processes underlying phenotypic change can vary among individual morphological traits, across the geographical range of a species, and/or between different environments (Grey et al. 2008;Hunt et al. 2015). However, debate continues as to the evolutionary processes that yield this pattern as well as the role of palaeoenvironmental factors such as temperature and productivity. The mechanisms behind stasis are still unclear as many potential drivers can be difficult to assess in the fossil record. In addition, such studies require large sample sizes, excellent preservation of fossils, and well-constrained stratigraphic successions (Lieberman et al. 1995;Eldredge et al. 2005).
Ammonoid cephalopods (hereafter referred to as 'ammonites', the term commonly used by Cretaceous ammonoid workers; Wright et al. 1996) are often considered classic organisms for the study of evolution thanks to their excellent preservation, high rates of speciation and extinction, and preservation of morphological changes through ontogeny (see references in De Baets et al. 2015;Monnet et al. 2015). The record of ammonites in the Late Cretaceous Western Interior Seaway (WIS; Fig. 1) of the USA meets the criteria outlined above for a rigorous study of macroevolutionary patterns. Factors such as sea level, temperature, oxygenation, food supply, and energy levels/turbidity in the environment of deposition have all been suggested as drivers of morphological change and macroevolution in ammonites (Landman & Waage 1993a;Jacobs et al. 1994;Wilmsen & Mosavinia 2011;Monnet et al. 2012;De Baets et al. 2015;Lehmann et al. 2016;Monnet et al. 2015). This is supported by evidence from modern cephalopods, which appear especially sensitive to rapid changes in temperature and food supply (Doubleday et al. 2016), and from studies of non-cephalopod molluscs with similar life history traits that show an association between shell shape and temperature, productivity (Teusch et al. 2002;Teusch & Guralnick 2003). Here we examine morphological changes of a single species of ammonite Hoploscaphites nicolletii (Morton 1842) (Fig. 2) across its entire temporal and geographical range within the WIS, and couple morphological investigation with an analysis of the oxygen and carbon isotopic composition of well-preserved shell material to determine environmental conditions during the lifetime of this species. Using this approach, we can test for the presence of, and processes driving, evolutionary stasis, as well as establish for the first time how the morphology of an entire ammonite species tracks changes in the environment across its lifetime.
The WIS was a shallow epicontinental seaway (maximum water depths c. 100 m; Gill et al. 1966) extending from the western Canadian Arctic to the proto-Gulf of Mexico (Kauffman & Caldwell 1993) during the Cretaceous period (Fig. 1). The WIS retreated during the Maastrichtian stage (72-66 Ma) due to global eustatic changes and uplift along its western margin (Waage 1968;Cobban et al. 1994). The fauna in this seaway included giant marine squamates (e.g. mosasaurs and plesiosaurs), fish and turtles, as well as ammonite and other cephalopods. Among the most common ammonites in the WIS are those belonging to the family Scaphitidae (hereafter referred to as scaphitids). Scaphitids are well preserved, widely distributed and locally abundant, making them ideal index fossils for biostratigraphic chronology (Landman & Waage 1993b;Cobban et al. 2006;Merewether & McKinney 2015). Analysis of stable isotopes, functional morphology and faunal associations has also yielded much information about the mode of life and ecology of scaphitids (Landman et al. 2012;Sessa et al. 2015;Ferguson et al. 2019). Trends in shell size, shape and compression have been documented previously in more restricted studies of scaphitids and other ammonite groups from the WIS (Landman 1987;Landman & Waage 1993a, b;Yacobucci 2003Yacobucci , 2008Landman et al. 2010Landman et al. , 2017Landman et al. , 2019, in some cases showing correlation with changing environmental parameters (Jacobs et al. 1994;Klein & Landman 2019). Intraspecific and ecophenotypic variation are also apparently common in these ammonites (Landman et al. 2008;De Baets et al. 2015) but are rarely quantified or considered in an evolutionary framework.

MATERIAL AND METHOD
We focus on specimens of Hoploscaphites nicolletii, which is endemic to North and South Dakota (Fig. 2) (Cobban et al. 1994(Cobban et al. , 2006; grey box shows extent of the location map. specimens come from a single horizon in the upper portion of the Elk Butte Member of the Pierre Shale (AMNH loc. 3302) which is the stratigraphically lowest occurrence of Hoploscaphites nicolletii (Fig. 3). The composite stratigraphic section encompassing all five of these horizons is c. 25 m thick (Figs 4-6) (Waage 1968;Landman & Waage 1993b). The section conservatively representing c. 0.5-0.75 myr based on the duration of other WIS ammonite biozones correlated to high-resolution radiometric dates (Cobban et al. 2006;Merewether & McKinney 2015).

Geological setting
The Fox Hills Formation represents the marginal marine phase of the progradational episode that marks the final closure of the WIS and contains the youngest marine faunas from the region. It crops out across a wide geographical area in Wyoming, Montana, South Dakota and North Dakota, and records the transition from an offshore setting, represented by the marine clays and shales of the Pierre Shale, to a delta plain terrestrial setting represented by the Hell Creek Formation (Fig. 3). The Fox Hills Formation displays a complicated internal stratigraphy and relationship to these surrounding units, as well as lateral variability in thickness and palaeoenvironment (Waage 1968;Speden 1970;Landman & Waage 1993b;Landman et al. 2013).
In the 'type area' (Fig. 1)   the bivalve Spyridoceramus tegulatus and coleoid gladii. The Elk Butte Member generally contains only rare isolated brachiopod fossils and a sparse foraminiferal assemblage dominated by aggluntinated benthic taxa (Mello 1969).
The transition from the Pierre Shale to the Fox Hills Formation is marked by an increase in grain size (Waage 1968). Environmental reconstruction of the Fox Hills Formation in the type area indicate that it can be divided into upper and lower parts, products of distinct depositional regimes (Trail City and Timber Lake members, Iron Lightning Member) (Waage 1968;Rhoads et al. 1972). Within each of these parts there are distinct lateral changes in facies (Fig. 3). Sediments of the lower Fox Hills were deposited in a subtidal environment on and around a submarine sand body that grew into the area from the northeast. Clayey silts that preceded the sand body and eventually flanked it constitute the Trail City Member, while the sand body itself is referred to as the Timber Lake Member. Two lithofacies are represented in the Trail City Member; in the western part of the type area, the Irish Creek lithofacies consists of thin interbeds of clay and silt which lack fossiliferous concretion layers and contain virtually no evidence of bioturbation. In the eastern part of the type area where the Trail City directly underlies the Timber Lake Member, the clayey silts of the Little Eagle lithofacies show a high degree of bioturbation and contain layers of abundantly fossiliferous calcitic concretions (Fig. 3).
These concretion layers in the lower 15 m of the Little Eagle lithofacies define four successive assemblage zones (Waage 1964(Waage , 1968. Careful mapping indicates these cover a roughly lobate area of c. 4000 km 2 possibly representing a large palaeoembayment of the WIS (Figs 1, 2). AZs are each named after their dominant ammonite or bivalve species: (1) lower nicolletii assemblage zone (LNAZ); (2) Limopsis-Gervillia assemblage zone (LGAZ); (3) upper nicolletii assemblage zone (UNAZ); and (4) Protocardia-Oxytoma assemblage zone (POAZ). Similar concretion horizons also occur in the Timber Lake Member (Sphenodiscus layer, Cucullaea AZ, Cymbophora-Tellina AZ), and at the time-transgressive contact between the Trail City and Timber Lake members ('transition F I G 4 . Box-and-whisker plots showing size (LMAX) and shape ratios of Hoploscaphites nicolletii specimens, plotted against lithostratigraphy and biostratigraphy of the Pierre Shale and Fox Hills Formation and stratigraphic distribution of fossiliferous concretion horizons. A, data from macroconchs. B, data from microconchs. Note that the metric LMAX/H S is not measured on microconchs due to the lack of a straight umbilical seam in these forms. H.birk., Hoploscaphites birkelundae.
concretions') ( Fig. 3). Homogeneity of these faunas supports AZ formation simultaneously throughout their geographical extent, and thus their utility as snapshots of in situ shallow marine communities (Rhoads et al. 1972). Analysis of fossil concentrations indicate these communities were probably affected by recurrent mass mortality events which were responsible for AZ formation, although the precise mechanisms behind these events is unknown (Waage 1964(Waage , 1968Speden 1970). Changes in benthic oxygen levels, turbidity or salinity caused by periods of rapid freshwater influx are all considered likely. The excellent preservation of fossils and occurrence of delicate elements such as in situ ammonite jaws (aptychi) in the concretions indicate rapid burial and minimal post-mortem transport (Landman & Waage 1993b).

Morphometric data collection and analysis
Using a large collection from four institutions (Yale Peabody Museum (YPM), 1137 specimens; American Museum of Natural History (AMNH), 3 specimens; Museum at the Black Hills Institute of Geological Research (MBHI), 164 specimens; and the Timber Lake Museum (TLM), 69 specimens), we measured 1373 complete specimens of Hoploscaphites nicolletii (see Witts et al. 2020, dataset S1) from the five stratigraphic horizons in the Pierre Shale (Elk Butte Member) and Fox Hills Formation (Trail City Member) described above. From each measured specimen we calculated ratios that capture the size, shape and degree of compression of these shells throughout their geographic and stratigraphic ranges (Figs 2, 4-6). Specimens from the Elk F I G 5 . Box-and-whisker plots showing shell compression ratios of Hoploscaphites nicolletii specimens, plotted against lithostratigraphy and biostratigraphy of the Pierre Shale and Fox Hills Formation and stratigraphic distribution of fossiliferous concretion horizons. A, data from macroconchs. B, data from microconchs. Note these analyses are restricted to the assemblage zones in the Fox Hills Formation, as specimens from AMNH loc. 3302 (Pierre Shale) were not suitable for these measurements. H.birk., Hoploscaphites birkelundae.
Butte Member were measured for size and shape but are embedded in concretions, making measurements of shell compression impossible. Scaphitids are dimorphic (Davis et al. 1996), with smaller microconchs (presumed to be males) and larger macronconchs (presumed to be females).
We measured examples of both dimorphs but concentrated on mature adult macroconchs for analysis because of their abundance compared to microconchs in the stratigraphic section (1340 total macroconchs vs 33 total microconchs). The vast disparity in the number of macroconchs versus micronconchs of H. nicolletii has been noted by previous authors (Landman & Waage 1993b;Landman et al. 2008) and probably reflects a palaeoecological signal (e.g. environmental segregation of sexes or mass mortality of macroconchs). The onset of maturity in scaphitids is marked by the uncoiling of the body chamber, and measuring complete specimens ensures that our measurements consistently represent a single (adult) ontogenetic stage.
All measurements were made on actual specimens using electronic calipers (Fig. 2). To assess size, the maximum length of the adult shell (LMAX) was measured from the venter of the adult phragmocone to the venter of the hook. The umbilical diameter (UD) of the adult shell was measured through the centre of the umbilicus, parallel to the line of maximum length. Several ratios capture the shape of the shell. The ratio of maximum length to whorl height of the phragmocone along the line of maximum length (LMAX/ H P ) is a measure of the degree of uncoiling. The ratio of maximum length to whorl height at midshaft (LMAX/H S ) is a measure of the degree of curvature of the body chamber in lateral view, where a value of 2 equals a perfect semicircle. This ratio only applies to macroconchs as the umbilical seam of the body chamber usually coincides with the line of maximum length in these forms, and thus the whorl height is the distance from the line of maximum length to the ventral margin of the body chamber (equivalent to the radius of a semicircle). The ratio of umbilical diameter to maximum length (UD/LMAX) provides a measure of the relative size of the umbilicus. Whorl width (W) and height (H) were measured at three points on the adult shell: (1) the adoral (close to the aperture) end of the phragmocone along the line of maximum length (W P and H P ); (2) the body chamber at midshaft (W S and H S ); (3) the hook at the point of recurvature (W H and H H ). The ratios of whorl height to whorl width were calculated at each of the three points on the shell outlined above (W P /H P , W S /H S , W H /H H ) and provide a measure of the degree of whorl compression.

Statistical analysis of evolutionary stasis and multiple linear regression modelling
The sequence over which Hoploscaphites nicolletii occurs is too short (too few time steps) to use likelihood approaches to test how this time series conforms to evolutionary models (e.g. strict stasis vs an unbiased random walk sensu Hunt 2007; Hopkins & Lidgard 2012;Hunt et al. 2015). Evolutionary changes are therefore evaluated F I G . 6 . A-G, summary of mean morphological data (size and shape) from adult macroconchs of Hoploscaphites nicolletii; black circles are mean trait values with 95% confidence intervals indicated by horizontal bars; confidence intervals are often smaller than the circle diameter; statistically significant morphological shifts are marked by double arrows, with a Bonferroni correction indicating a critical p value <0.002. H, temperature data derived from stable oxygen isotope (d 18 O) analysis of H. nicolletii shell material and assuming a d 18 O seawater value of À2&; filled grey circles are data from macroconchs; white circles are data from microconchs. H.birk., Hoploscaphites birkelundae.
using box and whisker plots to graphically examine the entire distribution of morphological parameters through time (Monnet et al. 2012) (Figs 4,5). Non-parametric Mann-Whitney U-tests with a Bonferroni correction for multiple comparisons were then used to evaluate the statistical significance of changes in mean trait values between stratigraphic horizons (Fig. 6).
The relationship between environmental variables and morphology was explored via multiple linear regression analysis (see Witts et al. 2020, dataset S2). The variables considered were: latitude, longitude (a proxy for the geographical distribution of samples), lithology (defined using published literature; e.g. Waage 1968) and average d 18 O calculated for each horizon based on data from macroconchs only. Most YPM and AMNH specimens are associated with detailed locality information including geographic coordinates (latitude/longitude), but some specimens from these collections, as well as those from MBHI are simply associated with information on the stratigraphic horizon they were derived from. Thus, these were excluded from the multiple linear regression analysis. Those from the Timber Lake Museum come from a single locality (AMNH loc. 3302), so were included. Six models were tested for each of the seven morphological characters (total of 56 models) (Table 1): two including/excluding the single horizon from the Elk Butte Member (A and C), two including/excluding the average d 18 O value calculated for each stratigraphic horizon (B and D), and two for the small sub-set of the data (n = 28) for which individual d 18 O data were available (E and F). All analyses were conducted in R (R Core Team 2018).  Fig. 7). All specimens were initially inspected for visible alteration of the shell material and any remnants of concretionary matrix were removed with deionized water and mechanical preparation. Samples were taken from either the phragmocone, body chamber or hook, and always from the mid-flanks of the outer shell layer.

Stable oxygen and carbon isotope data
Shell pieces were mounted on stubs using carbon circles at two orientations showing the lateral surface of the shell and cross section. Samples were coated with gold, then analysed using a Hitachi S-4700 Field Emission SEM in the Museum Imaging Facility (MIF) lab at AMNH. Samples were viewed under 15 kV with an aperture at 20 lm using a secondary electron detector. The surfaces were viewed and photographed at 2000 9 and cross-sectional fragments were viewed and photographed at 5000 and 15 000 9. The nacreous shell microstructure was evaluated for the degree of diagenetic alteration on a scale from 1 to 5 based on the Cochran et al. (2010) preservation index (PI) to assess the validity of isotopic results. As recommended by those authors, only specimens with a PI > 3 were used for analysis and to calculate palaeotemperature estimates. The carbon and oxygen isotope ratios of shell samples were analysed at the University of California Santa Cruz Stable Isotope Laboratory (UCSC SIL) using the methodology outlined in Landman et al. Neither of these studies included material from the concretionary horizons of the Little Eagle Lithofacies, focusing on nearshore samples from the Irish Creek Lithofacies (Fig. 3) and overlying Timber Lake Member. We chose a d 18 O water value of À2& to calculate water temperatures as a compromise based on the likelihood that the Little Eagle Lithofacies represents a more offshore environment than the Irish Creek Lithofacies or Timber Lake Member but acknowledge this requires further study.

RESULTS
The total distribution of size, shape, and shell compression measurements for specimens of Hoploscaphites nicolletii are illustrated in Figures 4 and 5 as box-and-whisker plots and summarized in Figure 6 for macroconchs. We focus on macroconchs for statistical analysis and to interpret trends (Figs 4A, 5A) due to the small number of microconchs. Statistically significant changes in size at maturity (LMAX) and all shape ratios (LMAX/H P , LMAX/H S , UD/LMAX) in macroconchs are present between the Elk Butte Member and the basal AZ of the Fox Hills Formation (LNAZ) (Table 2; Fig. 6). In addition, a statistically significant increase in LMAX is recorded between UNAZ and POAZ (Figs 4A, 6A); despite a limited sample size this increase is also visually apparent in the microconch dataset (Fig. 4B), suggesting a phenomenon that affected both dimorphs. The transition from UNAZ to POAZ also coincides with a significant increase in the degree of compression at the mid-point of the shaft (W S /H S ). A significant change in the curvature of the body chamber (LMAX/H S ) also occurs between LGAZ and UNAZ. All other macroconch characters show no statistically significant changes through time (Table 2). Importantly, in nearly all cases (and irrespective of statistical significance), directional trait change was reversed at a later point in the history of the species (Figs 4,  6).
Multiple linear regression analysis also indicates that latitude, longitude and lithology show statistically significant relationships with macroconch LMAX (Table 3). When the unusually large specimens in the Elk Butte Member are excluded from the analysis, only latitude and lithology remain significant. Average d 18 O also shows a significant relationship with LMAX in models with and without the Elk Butte Member. However, in the two models where shell shape was compared to isotope values derived from the same shell, d 18 O is not significantly correlated with LMAX or any other morphological variable. Shape traits (LMAX/H S , LMAX/H P , UD/LMAX) each show significant relationships with variables latitude (LMAX/H P ), longitude (LMAX/H P , LMAX/H S ) and lithology (LMAX/H P , UD/LMAX), but these relationships are not maintained when the Elk Butte Member is F I G . 7 . Full stable isotope dataset (d 18 O, d 13 C) derived from Hoploscaphites nicolletii macroconchs (grey circles) and microconchs (white circles). excluded, and/or show very low r 2 values (Table 4). Shell compression ratios (W P /H P , W S /H S , W H /H H ) also show some significant covariance with environmental proxies, with very low r 2 values, which is consistent with the idea that these traits remained relatively invariant through time.
Temperature values derived from isotopic analysis

DISCUSSION
Morphometric data reveal evolutionary stasis of this ammonite species throughout its entire temporal and geographical range, concurrent with a fluctuating palaeotemperature regime around a relatively stable mean (Figs 4-6). Despite a varying degree of intraspecific variation (Figs 4, 5), morphological traits do not show any kind of unidirectional trend in terms of size and shape from first to last occurrence (Fig. 6). Whatever changes occur are  -) indicates no models showed statistically significant relationship between trait and environmental variable.  (1996). We suggest two hypothesized mechanisms are more plausible: (1) those related to differential selection across palaeoenvironments in aggregate (with or without homogenizing gene flow) (Lieberman et al. 1995;Eldredge et al. 2005;Estes & Arnold 2007), where the amount of morphological change within a palaeoenvironment may outpace the amount of change across environments; or (2) stabilizing selection with a fluctuating optimum associated with shifting environments (Hunt 2007;Hunt & Rabosky 2014). Distinguishing between these two explanations would require observing greater morphological variability within versus between environments (supporting 1 above), or the return to similar morphological traits when returning to similar environment (supporting 2 above) (Lieberman & Dudgeon 1996;Estes & Arnold 2007;Turner 2017;Voje et al. 2018). Unfortunately, the data needed to rigorously test these conditions (particularly strong environmental gradients across time and space) are unavailable, due to geographical restriction in outcrop area and lack of sufficiently detailed data for every horizon within the Trail City Member (but see discussion below and Landman et al. 2008). Regardless of the degree of completeness of the morphological and environmental data presented here, we cannot discriminate between these two processes in this species.
Evidence from functional morphology, analysis of in situ jaws, predation marks (Landman et al. 2012) and stable isotope analyses (Cochran et al. 2003;Sessa et al. 2015;Ferguson et al. 2019), imply that scaphitids were nektobenthic planktivores and poor swimmers with limited migration ability. Dispersal and gene flow between WIS populations was probably achieved via a passive planktonic larval stage distributed by currents (Landman et al. 2012;Linzmeier et al. 2018). Thus, it may be that planktonic larvae combined with a more sedentary adult phase provide the ideal conditions to promote oscillatory morphological change in response to dynamic environments across the lifetime of these species.

Links between palaeoenvironment and morphology in H. nicolletii
Based on the geographical distribution of fossil-bearing concretions (Waage 1968), the AZs in the Trail City Member of the Fox Hills Formation represent a narrow range of habitable, shifting shallow marine environments around the margins of the growing Timber Lake Member submarine sand body, under the influence of a north-east trending current. AZ formation appears to occur during 'slack' periods in growth of the sand body which promoted hospitable benthic conditions (Waage 1968;Rhoads et al. 1972). Potentially multiple, semi-isolated populations of scaphitids developed within these dynamic environments, subject to different selection pressures. Landman et al. (2008) used a smaller morphological dataset to argue that north-eastsouth-west geographical differences in mean adult size (LMAX) of Hoploscaphites nicolletii in LNAZ and UNAZ, corresponding to changes in fossil distribution recognized by Waage (1968), reflected distinct populations with differences in size related to the proximity to the palaeoshoreline and variation in nutrient supply and temperature. The significance of latitude in our multiple regression analysis of LMAX is entirely consistent with this hypothesis, but our expanded dataset suggests the same is not true for shape traits such as whorl compression, which do not correlate with latitude or longitude. Traits that describe shape using LMAX as one of the terms of the ratio only show significant correlation with latitude or longitude when the Elk Butte Member is included; further changes in these traits through time are only significant between the Elk Butte Member and LNAZ (Fig. 6). These results suggest that these shifts reflect allometric variation associated with differences in body size rather than evolutionary change in shape. Previous compilations of evolutionary mode in species lineages suggest that body size (captured here by the LMAX trait and its derivatives) may be a particularly labile trait (i.e. less likely to show stasis compared to non-body size traits in response to environmental change; Hunt 2007; Hopkins & Lidgard 2012) but that size can show dynamic stasis if influencing environmental parameters regularly fluctuate over time (Hunt et al. 2015).
The unusually large specimens in the Elk Butte Member of the Pierre Shale represent a distinct population, with statistically significant differences in all traits between this horizon and the Fox Hills AZs (Fig. 6; Table 2) that cannot be related to differential preservation alone. We suggest this population lived in an offshore environment characterized by low oxygen conditions on and below the seafloor (reflected in the lack of shelly benthos; Rhoads et al. 1972;Landman et al. 2008) below a highly productive and nutrient-rich water column in which planktivorous ammonites thrived. However, scaphitids are generally absent in other low oxygen settings in the WIS, thought to reflect a nektobenthic lifestyle and reliance on well-oxygenated benthic conditions (Slattery et al. 2018). A multitude of factors can affect size at maturity in molluscs (Bucher et al. 1996;Teusch et al. 2002;Monnet et al. 2012) and clearly the specific environmental conditions supporting gigantism, as well as mechanistic explanations for the overall increase in size in both dimorphs of Hoploscaphites nicolletii in POAZ, require further investigation.
Despite low statistical support, other morphological trends in Hoploscaphites nicolletii macroconchs can be qualitatively linked to functional changes related to palaeoenvironmental shifts. The increase in the degree of compression values (W S /H S ) in POAZ coincides with a grain size increase in this interval (Waage 1968) due to initial encroachment of the submarine sand body into the area. This morphological change conforms to an expectation for selection towards increased compression due to greater hydrodynamic efficiency in a higher energy depositional environment, reflected by substrate changes (Jacobs et al. 1994;Klein & Landman 2019). This trend persists into the overlying sandy Timber Lake Member with the appearance of the morphologically similar, but more compressed species Hoploscaphites comprimus (Owen 1852). Changes to the bivalve fauna are also observed in POAZ, with an increase in the relative abundance of suspension feeding taxa, and in species more adept at dealing with increased sedimentation and turbidity (Speden 1970;Rhoads et al. 1972). The statistically significant shifts in W S /H S and LMAX/H S through time were apparently not associated with the specific environmental factors tested using the multiple regression analysis given the lack of correlation and low r 2 values (Tables 3, 4). This could reflect a relationship with other, untested environmental variables (e.g. nutrient flux, turbidity, salinity), hitch-hiking of these traits with one or multiple other morphological changes, and/or sensitivity to biotic interactions not considered here.

Stable isotope data, WIS palaeoenvironments and speciation
Unsurprisingly, a statistically significant correlation is observed between average d 18 O for each stratigraphic horizon and morphology within several shell characters (Table 3), which supports an ecophenotypic link to palaeotemperature that has been observed in other work (Hunt et al. 2015). In contrast, none of the morphological changes in Hoploscaphites nicolletii macroconchs can be directly linked to any significant changes in temperature when comparing morphology and geochemical measurements from the same individuals (Fig. 6). This finding suggests a mismatch in scales of geochemical sampling and morphology that warrants further, refined investigation, and caution when making blanket interpretations based on averaged geochemical data.
Despite the lack of significant excursions in mean values, the range in isotopic values at any given horizon (Figs 6, 7) indicate either a large seasonal range in temperature in the WIS, and/or development of spatial gradients in d 18 O and d 13 C related to proximity to shoreline and freshwater input on a regional scale. Although there is no strong relationship between isotope values and sample position on individual shells (Witts et al. 2020, appendix fig. S2), the presence of the most negative d 13 C values in samples from the body chamber could indicate a vital effect associated with growth rate. Landman & Waage (1993b) documented a reduction in the thickness of the shell wall and weakening of ribbing at midshaft in macroconchs of H. nicolletii (Fig. 2)  Heterogeneous conditions in terms of temperature, salinity and terrestrial input were probably typical of shallow water environments in the WIS and all epeiric seaways (Cochran et al. 2003;Dennis et al. 2013;Petersen et al. 2016). However, localized environmental fluctuation clearly did not promote sustained directional changes in morphology in Hoploscaphites nicolletii throughout the duration of the Trail City Member. The combination of morphological and isotopic data suggests that for speciation to take place, ammonite populations must have either become more fully isolated and/or subject to a larger (regional) shift in palaeoenvironmental conditions. The full encroachment of the submarine sand body into the study area to form the Timber Lake Member (Fig. 2) represents such an event; probably leading to changes in water depth, substrate, local currents and disruption to factors such as productivity. Other studies have documented that widely fluctuating environments common in epeiric seaways can act to promote phenotypic plasticity in ammonites (Reyment & Kennedy 1991; Wilmsen & Mosavinia 2011) and promote punctuated evolutionary patterns (Parsons 1993) via stress-induced evolutionary 'jumps' in morphology (Monnet et al. 2013). The Timber Lake Member contains a somewhat different ammonite fauna at the species level compared to the Trail City Member AZs and coincides with the disappearance of Hoploscaphites nicolletii (Landman & Waage 1993b) indicating that such a shift in conditions was of sufficient magnitude to promote speciation and/or extinction.

CONCLUSION
We have demonstrated that the fossil record of Hoploscaphites nicolletii in the WIS is consistent with dynamic evolutionary stasis through time, and therefore corroborates findings on living (Lavou e et al. 2010) and fossil (Hunt et al. 2015;Voje 2016) lineages. Uniquely, here we track the entire temporal and geographical range of a species with high-resolution spatiotemporal data. Localized intraspecific variability and morphological oscillations occurred throughout the history of this ammonite species, but an overall static morphology was maintained across the entire range and duration of the species, despite evidence for heterogeneous conditions and variable palaeoenvironmental change. In terms of the mechanisms behind such stasis, we propose that two hypotheses most closely match these data: (1) differential selection across palaeoenvironments in aggregate (Lieberman et al. 1995;Eldredge et al. 2005;Estes & Arnold 2007); and (2) stabilizing selection with a fluctuating optimum (Hunt 2007;Hunt & Rabosky 2014). Additional data from other species lineages studied at similarly high spatiotemporal resolution are still needed to confirm the dominant, relative frequency of potential mechanisms at work behind dynamic evolutionary stasis. Notably, the commonly debated mechanisms producing stasis operate on very different taxonomic levels (Turner 2017), therefore none are likely to be mutually exclusive. Consequently, the biggest challenge in explaining stasis in case-studies from the fossil record, as demonstrated in this study, is constraining how mechanisms may operate in concert to produce these patterns.