Lucas et al., eds., 2008, Neogene Mammals. New Mexico Museum of Natural History and Science Bulletin 44. 239 MAGNETIC STRATIGRAPHY OF THE MASSACRE LAKE BEDS (LATE HEMINGFORDIAN, EARLY MIOCENE), NORTHWEST NEVADA, AND THE AGE OF THE “PROBOSCIDEAN DATUM” IN NORTH AMERICA

DONALD R. PROTHERO1, EDWARD BYRD DAVIS2 AND SAMANTHA S.B. HOPKINS2

1 Department of Geology, Occidental College, Los Angeles, CA 90041; 2 Department of Geological Sciences, University of Oregon, Eugene, OR 97403

Abstract—The “Proboscidean Datum” was proposed by Tedford et al. (1987) and Woodburne and Swisher (1995) as a marker of the late Barstovian (middle Miocene, about 14.5 Ma) in North America. Subsequently, a number of pre-late Barstovian proboscidean fossils have been reported, casting doubt on the validity of the Proboscidean Datum. The oldest of these is from the late Hemingfordian Massacre Lake local fauna of northwest Nevada, which produced a single proboscidean tooth fragment. Magnetic stratigraphy was conducted on these beds, which yielded a stable remanence held mainly in magnetite that was entirely reversed in polarity. Based on the 40Ar/39Ar date of 16.474 ±0.035 Ma at the top of the section, we correlate the Massacre Lake beds with earliest Chron C5Cr (16.4-17.3 Ma). In addition, a number of other early Barstovian magnetostratigraphic sections with Proboscidea are reviewed, and quite a few yield fossils that date between 15.8 and 16.2 Ma. Our analysis of faunal data suggests that these early occurrences are simply the first places colonized by the immigrating proboscideans and not marked by a unique ecological or taphonomic history.

INTRODUCTION Barstovian occurrences of gomphotheres in Ixtapa, Chiapas, Mexico, as In their landmark review of the late Oligocene to early Pliocene well as the North Coalinga specimen mentioned above. “At their first (Arikareean through Hemphillian) mammalian faunas of North America, occurrence, the Proboscidea are too rare to be used as a defining taxon for Tedford et al. (1987, p. 188) noted that the first appearance of Proboscidea the beginning of the late Hemingfordian, and their actual first appearance in North America seemed to mark the beginning of the late Barstovian. probably is late in the span, but it may be useful to include Zygolophodon Despite their large size and relatively easy fossilization, Proboscidea as part of the characterizing fauna of the early Barstovian and to regard were conspicuous in their absence from the richly fossiliferous early the midcontinent appearance of gomphotheriids as an event defining the Barstovian Olcott fauna of Nebraska, and the early Barstovian Pawnee beginning of the late Barstovian” (2004, p. 214). They also noted (2004, Creek fauna of Colorado. As Tedford et al. (1987, p. 168) noted, “despite p. 215) that “this proboscidean datum is reasonably isochronous in the intensive large-scale collecting in the many Olcott channel fills, no re- majority of instances and we will continue to regard it as the most useful mains of Proboscidea have been found.” At that time, no evidence of way to define late Barstovian time.” Proboscidea was known from the early Barstovian of the type Barstow Thus, the validity of the Proboscidean Datum has become less Formation in California, either. However, Tedford et al. (1987, p. 162) well supported as new data emerge. As the above list shows, early did note the presence of Proboscidea in probable early Barstovian faunas Barstovian occurrences are the norm, rather than the exception, and only from Oregon. two major regions still lack early Barstovian proboscideans: the Great The presence of Proboscidea thus became one of the key criteria Plains and New Mexico. Thus, it is worthwhile to review the for distinguishing early and late Barstovian faunas. Woodburne and Swisher chronostratigraphy of most of these early proboscidean occurrences, 40 39 (1995, p. 347) referred to this late Barstovian occurrence as the “Probos- using magnetic stratigraphy and Ar/ Ar dating wherever possible, and cidean Datum.” However, they also noted that Proboscidea were known refine the calibration of these various “Proboscidean Datums” across from the late Hemingfordian Massacre Lake fauna, as well as the early North America. For this reason, the detailed calibration of the earliest Barstovian of Florida (Bryant, 1991). Consequently, they indicated occurrence, the Massacre Lake l.f., is crucial to evaluating the time sig- (Woodburne and Swisher, 1995, fig. 4) a latest Hemingfordian Probos- nificance of the Proboscidean Datum. cidean Datum for the northwestern and southeastern United States, and GEOLOGIC SETTING a late Barstovian Proboscidean Datum for the interior of North America. They noted (p. 347) that “this event is subject to the vagaries of biofacies The most comprehensive work on the Massacre Lake l.f. is and should be used with extreme caution.” Morea’s (1981) unpublished dissertation, but some of the important Tedford et al. (2004, p. 215) updated the list of pre-late Barstovian taxa from his description of the fauna have been published (e.g., Morea proboscidean occurrences. They include not only the late Hemingfordian and Korth, 2002; Baskin, 2003). Morea (1981) presents a stratigraphic at Massacre Lake and early Barstovian of Florida mentioned earlier, but framework for the fauna, which is all preserved in a layer of yellowish also proboscidean trackways (but no bones yet) that occur in the early brown silty tuff that crops out on the opposing sides of two hills sepa- Barstovian of the type Barstow Formation (Reynolds, 1999; Reynolds rated by a drainage 10 km west of Massacre Lake in northwest Nevada and Woodburne, 2001). In addition, Tedford et al. (2004) listed the fol- (Fig. 1). Most of the specimens are preserved in silica-cemented nodules lowing early Barstovian occurrences of Proboscidea: North Coalinga that must be painstakingly prepared by hand. Careful preparation re- local fauna and Sharktooth Hill local fauna, California; Virgin Valley and veals teeth, bones, and some relatively complete skulls and jaws of small High Rock lake sites, northwestern Nevada; Mascall, Sucker Creek and animals and many fragmentary skeletal elements of larger animals. Among Skull Springs, southeast Oregon; Deep River and Madison Valley beds, the specimens collected from this locality is a single fragmentary loph of Montana; Burkeville l.f., Texas Gulf Coast; and Willacoochee Creek a proboscidean tooth, UCMP 75606, loc. V6161 (Fig. 2), collected by Fauna, Florida. Tedford et al. (2004, p. 214) noted that most of these Woodburne and a party from UC Berkeley in 1961. This tooth has been fossils, where identifiable, appeared to be of the mammutid Zygolophodon, tentatively assigned (Morea, 1981) to Zygolophodon on the basis of its and not of gomphotheres. However, they also noted apparent early small size, low crown height, and the apparent lack of conules in the 240

FIGURE 2. Proboscidean tooth fragment from Massacre Lake, UCMP 75606, loc. V6161. Scale = 1 cm. resistant agglomeratic ledges that marked the base of Morea’s (1981) unit 4, and correspond to UCMP V6160 and RV7043, the main fossil mam- mal localities. Another 10 m of section was covered and could not be sampled. The section is capped by the light gray rhyolitic volcanic ash- flow tuff that yields the 40Ar/39Ar date of 16.474 Ma (Swisher, 1992; but given as 16.2 Ma by Woodburne and Swisher, 1995). This same ash was originally was K-Ar dated at 15.6 Ma (Evernden et al., 1964). Samples were collected as oriented block samples in the field and subsampled with a drill press in the lab to produce a core. A minimum of FIGURE 1. Index map showing the location of the Massacre Lake fossil three samples was collected at each site, although site 8 had 5 samples localities and magnetic section in northwestern Nevada. (Table 2). Poorly consolidated samples were hardened with sodium sili- cate in the field. Samples that were too fragile to drill were molded into valleys between lophs. This is consistent with most of the early Barstovian cylinders using Zircar aluminum ceramic and dried in the magnetically faunas that contained Proboscidea. The remainder of the Massacre Lake shielded room. Samples were measured on the 2G cryogenic magnetom- l.f. is late Hemingfordian in character (Table 1). A number of previously eter at Occidental College, using a Caltech-style automatic sample changer. unrecorded species are present in the fauna, and currently appear to be After measuring NRM (natural remanent magnetization), each sample endemic to this area. However, Massacre Lake is the only late was treated with alternating field (AF) demagnetization at 25, 50, 75, and Hemingfordian mammal fauna known from the Great Basin, so these 100 Gauss to determine the coercivity behavior of the sample, and to endemics may not be truly site-specific, but instead reflect the faunal demagnetize any multi-domain grains before their remanence is baked in. character of the whole Great Basin at that time. A comprehensive re- Every sample was then thermally demagnetized at 50°C steps from 200° evaluation of the fauna reflecting the broader perspective gained in the 25 to 630°C to examine the demagnetization behavior in detail. This process years since its initial description would be an important contribution to removes any chemical remanent overprints due to iron hydroxides such the paleontology literature. as goethite (which dehydrates at 200°C), and shows how the samples In his unpublished dissertation, Swisher (1992) 40Ar/39Ar dated behaved as the Curie temperatures of magnetite (578°C) and hematite an ash flow tuff, referred to by Morea (1981) as the “light gray rhyolitic (630°C) were approached. ash flow tuff,” or “Unit 5” and by Swisher as the “Tuff of Big Basin,” at Results were graphed on orthogonal demagnetization (“Zijderveld”) 16.474 ± 0.035 Ma. This ash layer lies stratigraphically just above the plots (Fig. 3), and average directions of each sample were determined by beds that produce the Massacre Lake fauna. The fossiliferous beds are the least-squares method of Kirschvink (1980). Mean directions for each also underlain by a thin layer of basalt, Morea’s “Unit 2” and Swisher’s site were then analyzed using Fisher (1953) statistics, and classified “Massacre Lake Basalt,” 40Ar/39Ar dated by Swisher at 16.328 ± 0.030 according to the scheme of Opdyke et al. (1977). Ma. These works by Swisher and Morea provide a context for more detailed analysis of the age of this biostratigraphically critical mid-Mi- RESULTS ocene fauna. Representative orthogonal demagnetization plots of the Massa- METHODS cre Lake samples are shown in Figure 3. All sites showed a clear reversed polarity during demagnetization, with rapid decreases in intensity during The stratigraphic section closely followed the section of Morea AF demagnetization, suggesting that a low-coercivity mineral such as (1981, fig. 4). The lower sites (unit 3 of Morea, 1981) were taken in a magnetite is the primary carrier of the remanence. In addition, nearly all traverse in the south rim of the main east-west trending wash samples were completely demagnetized by 500-550°C, and had no re- (N48°39’06.7” W119°44’56.3”) from the basal, finely-laminated tuf- manence left at 600°C, also consistent with the idea that magnetite is the faceous siltstones (site 1) through the hard lapilli tuff (site 2, RV 7317), primary carrier of the remanence. This is not surprising, given that basal- and concluding with the white tuff near the top of the ravine (site 3). The tic lapilli with high magnetite content (Morea, 1981) are common through- tuff at the top of the local bench was then traced north across the wash out the sequence. Most samples (Fig. 3A) showed a single stable compo- to the upward continuation of the sequence, where the section was nent of remanence that was reversed at NRM, although some (Fig.3B) resumed at N41°39’10.1” W119°44’49.4”. Sites 4 and 5 were located in had some slight overprinted component that was removed by 300°C. A the coarse gray sandy tuff at the base of the sequence, just below the few samples (Fig. 3C) had a normal overprint that was also removed by layer with the basaltic lapilli (site 6). Sites 7 and 8 were collected in the 300°C. 241 TABLE 1. Mammal taxa from the Massacre Lake local fauna. TABLE 2. Paleomagnetic data. N = number of samples; D = declination; I =

inclination; K = precision parameter; 95 = ellipse of 95% confidence around mean.

DISCUSSION Dating of Massacre Lake The lack of stratigraphic concordance of Swisher’s (1992) two dates merits further consideration, especially given that they are signifi- cantly different from one another given the reported error in the dates. This difference indicates that the underlying basalt is genuinely younger than the ash flow tuff above. Reexamination of the stratigraphic column of Morea (1981) indicates that the layer from which the Massacre Lake l.f. was collected is separated from the overlying ash by a covered inter- val of roughly 20 meters. Observations of the rock layers made during the collection of magnetostratigraphic samples suggest that dip of the beds changes noticeably from the faunal layers to the overlying ash. It is possible that the covered interval conceals a fault, which would place the layers from which the fauna is drawn simply younger than the date on the underlying basalt, or less than 16.3 Ma. This finding would be con- sistent with the fauna, which contains both Prototomarctus, a late Hemingfordian taxon, and Zygolophodon, a taxon previously thought to be Barstovian. This is only one possible explanation for the conflicting stratigraphic data, which we plan to address with future fieldwork. The Proboscidean Datum A summary of the magnetostratigraphy of major localities with early occurrences of proboscideans is shown in Figure 5, with a map of these occurrences in Figure 6. The oldest occurrence in the High Plains is in the upper Pawnee Creek Formation of northeastern Colorado, magnetostratigraphically correlated with Chron C5AC (about 14.2 Ma), according to Prothero and Dold (this volume). Early Barstovian probos- cideans also occur in the Mascall Fauna of central Oregon, correlated with Chron C5Br (up to 15.8 Ma) (Prothero et al., 2006). As noted above, proboscidean trackways but not body fossils are known from the Averaging all the stable reversed directions produced a mean of D early Barstovian interval of the Barstow Formation, possibly as old as

= 180.8, I = -41.2, k = 7.9, 95 = 11.0, n = 25. Although there were no 16.2 Ma (Reynolds, 1999; Reynolds and Woodburne, 2001). Probos- normal directions in this section, the fact that the reversed mean is an- cidean remains also occur in the Sharktooth Hill local fauna near Bakers- tipodal for the expected normal pole for this latitude gives us a positive field, California, correlated with magnetic Chron C5Br (about 15.8 Ma) reversal test and suggests that the overprinting has been removed and the (Prothero et al., this volume a, b). No magnetic stratigraphy yet exists for directions are primary. the Virgin Valley Fauna of Nevada (although it is bracketed by 40Ar/39Ar Magnetostratigraphic results of the Massacre Lake section are dates of 15.18 and 15.84 Ma, Tedford et al., 2004), nor for the High Rock shown in Figure 4. All but one site were Class I sites of Opdyke et al. fauna of Nevada (although it lies below an 40Ar/39Ar date of 14.49 Ma, (1977), meaning that they were significantly different from a random Tedford et al. 2004), nor for the Stewart Springs Fauna of Nevada (al- distribution at the 95% confidence level. One site (site 6) had a sample though it lies below 40Ar/39Ar dates of 14.96 [biotite] and 14.89 Ma crumble, leaving only two samples for analysis, so it was a Class II site [plagioclase], Tedford et al., 2004) nor for the Sucker Creek-Skull Springs of Opdyke et al. (1977), and site statistics could not be calculated. faunas of southeastern Oregon (although they lie below an 40Ar/39Ar Correlation of the Massacre Lake section with the magnetic polar- date of ~15 Ma, Tedford et al., 2004), nor for the Deep River beds of ity time scale of Lourens et al. (2004) is shown in Figure 5. As can be seen Montana, the North Coalinga Fauna of California (although constrained from the figure, the date of 16.474 at the top of the Massacre Lake by marine biostratigraphy), the Burkeville Fauna of Texas, the reversed magnetozone correlates best with Chron C5Cr (16.4-17.3 Ma). Willacoochee Creek Fauna of Florida, or the fauna of Ixtapa, Mexico This puts the Massacre Lake fauna in the very latest part of the (although they are below a tuff dated 15-16 Ma, Ferrusquia-Villafranca, Hemingfordian as calibrated by Woodburne and Swisher (1995) and 1996). Tedford et al. (2004). 242

FIGURE 3. Orthogonal demagnetization (“Zijderveld”) plots of representative pilot samples. Solid squares indicate declination (horizontal component); open squares indicate inclination (vertical component). First step is NRM, followed by AF steps of 20, 40, 60, 80 and 100 Gauss, then thermal steps from 200° to 630°C in 50°C increments. Each division equals 10-6 emu. 243 endemic rhinos, and abundant tapirs. The Texas Gulf Coast faunas yield a Barstovian proboscidean in the Burkeville l.f., but not in the much better sampled Trinity River l.f. If Tedford’s hypothesis that the first arrivals were the mammutid Zygolophodon, which preferred forested habitats, one would expect these occurrences to be common in the Florida and Texas Gulf Coasts and rare in eastern Oregon, Montana, or Nevada. As a first step in a more rigorous exploration of this possibility, we have compared the Massacre Lake l.f. to late Hemingfordian and early Barstovian faunas from the western USA. In order to ascertain whether the Massacre Lake fauna shares more faunal characteristics with the early Barstovian proboscidean faunas than might be expected by chance, we compiled genus-level faunal data for the late Hemingfordian and early Barstovian from the MIOMAP database (Carrasco et al. 2005; http://miomap.berkeley.edu). This database records taxonomic and speci- men-abundance data, derived from the published literature, for Miocene mammal faunas from the western USA. Additional information concern- FIGURE 4. Magnetic stratigraphy of the Massacre Lake beds. Stratigraphy ing the MIOMAP database can be found online or in Barnosky and after Morea (1981). Short dashed lines = volcaniclastic mudstones; random black squares = coarse agglomerates; diagonal ruled area = covered interval. Carrasco (2002), Davis (2005), or Barnosky et al. (2005). Solid circles are Class I sites of Opdyke et al. (1977), which are statistically We grouped these genus-level data by geologic formation and distinct from a random distribution at the 95% confidence level. Hachured NALMA, producing 30 faunal units (10 late Hemingfordian and 20 early circle is a Class II site of Opdyke et al. (1977) which was missing a sample Barstovian). Nine of these units contain proboscidean fossils; eight of so site statistics could not be calculated. these are placed in the early Barstovian (Barstow, Deep River, Esmeralda, Mascall, Round Mountain, Sucker Creek, Temblor, and Virgin Valley Thus, we have the earliest North American Proboscidea in Chron formations) and one in the late Hemingfordian (Massacre Lake). An C5Cr (latest Hemingfordian) of Massacre Lake (16.4-16.5 Ma), fol- additional putative early Barstovian proboscidean occurrence from the lowed by early Barstovian occurrences in Oregon (Mascall fauna, younger MIOMAP database, from the Madison Valley Beds, was eliminated, as than 15.8 Ma), California (Sharktooth Hill, about 15.8 Ma; Barstow it is from an unknown locality in the Madison Valley Beds, and could Formation, possibly as old as 16.2 Ma). The Willacoochee Creek fauna have an age ranging from early Barstovian to early Clarendonian. For is dated around 15.8 Ma, and the Sucker Creek-Skull Springs faunas lie each pair of sites we calculated the Morisita (1959) index of faunal beneath a date of 14.93 Ma. Thus, the early dates are converging on the similarity. This index has the useful paleoecological property of being interval between 15.8 and 16.2 Ma, which is remarkably good agreement, relatively immune to sample-size differences (Wolda, 1981). Values range considering how rare these early proboscidean fossils are, and how sparse from zero (no shared taxa) to slightly more than one; the higher the value, the late Hemingfordian-early Barstovian record is in most of these re- the more similar the faunas in both richness and evenness. gions. Pair-wise comparison of the 30 faunal units produces 435 indi- This then raises the question as to why such conspicuous and vidual comparisons. These are not all independent, and the properties of easily fossilized animals did not appear until the late Barstovian in Ne- the Morisita index make it difficult to perform standard statistical infer- braska or Colorado or New Mexico, which are some of the largest and ence on them. We cannot simply compare the mean similarity between best sampled faunas in all of the North American record. Tedford (pers. Massacre Lake and the nine early Barstovian faunas to the overall mean commun., 2004; Tedford et al., 2004) suggested that most of the pre-late and standard error of the 435 comparisons; instead, we have used a Barstovian occurrences are of the mammutid Zygolophodon, which may Monte-Carlo approach. We generated 1000 random samples of eight have favored more coastal, forested regions, but avoided the Great Plains. values from the 435 total comparisons and compared the means of these By contrast, Tedford postulated that gomphotheriids, which seem to random samples to the mean of the eight values for Massacre Lake. have been very successful in the Great Plains, do not show up until the The overall mean similarity for all 435 comparisons is 0.127. The late Barstovian. However, Tedford et al. (2004, p. 214) pointed out that median value for the 1000 random replicates is 0.115, and the 97.5th and the specimens from Ixtapa, Mexico, and also from North Coalinga may 2.5th percentiles are 0.258 and 0.037. The mean similarity between Mas- in fact be early Barstovian gomphotheriids, and the trackways described sacre Lake and the eight early Barstovian proboscidean faunas is 0.079, by Reynolds (1999) from the early Barstovian of the Barstow Forma- in the 22nd percentile of the Monte Carlo distribution. Because the mean tion were identified as gomphotheriid as well. Likewise, the specimens value for Massacre Lake is within the middle 95% of the Monte Carlo from Sharktooth Hill (Prothero et al., this volume b) appear to be from a distribution, we cannot reject the hypothesis that Massacre Lake is no gomphothere and not a mammutid. The Stewart Spring site in the more or less similar to the proboscidean faunas than we would expect by Esmeralda Fm. of Nevada is suggested to have a gomphothere as well, random chance. It is interesting, however, that the similarity between but this is an unverified identification from Firby’s (1966) work on the Massacre Lake and the proboscidean faunas is on the low end of the local mollusk fauna. range; we expected it to be on the high end of the range if ecological or These occurrence data suggest that there might be some environ- environmental factors were controlling the earliest occurrences of pro- mental or ecological difference between early Barstovian sites that con- boscideans. Our result suggests that there is no faunal signal differentiat- tain proboscideans and those that do not. However, there are no obvious ing proboscidean sites from non-proboscidean sites in this interval. patterns when the biogeographic data are examined closely. Although the As a final caveat about the reliability of supposedly easily fossil- High Plains and New Mexico still lack early Barstovian Proboscidea, ized and easily recognized and collected proboscideans, we point to the other regions that could be considered “interior” (such as Nevada, east- peculiarity of the Barstow Formation. Although it has been studied for ern Oregon, Montana, and Barstow and Sharktooth Hill, California) over a century and collected by hundreds of paleontologists over many yield Proboscidea. A visual search of the faunal lists yields no obvious decades, there are still no early Barstovian proboscidean body fossils— correlations with environmental factors. For example, there are probos- but there are trackways from the early Barstovian. This reminds us that cideans in both “interior” regions and also the more forested regions, we cannot count on fossilization, even in the case of large robust, thick- such as Florida and the famous Gulf Coast endemic faunas of the Mi- boned animals like proboscideans that have the best chance of preserva- ocene (Tedford et al., 1987), with their peculiar camels, protoceratids, tion in the fossil record. Absence of body fossils may not mean that the taxon was not present! 244

FIGURE 5. Magnetostratigraphic correlation of major early Proboscidea-bearing localities in North America. Time scale after Lourens et al. (2004) and Woodburne and Swisher (1995). PROB. = lowest occurrence of Proboscidea in each section. Black bars are zones of normal polarity; white bars indicate reversed polarity; diagonally patterned bars are of unknown magnetic polarity (not yet studied). Colorado section after Prothero and Dold (this volume). Mascall section after Prothero et al. (2006). Barstow section after MacFadden et al. (1990). Sharktooth Hill section after Prothero et al. (this volume). diagnostic of the late Hemingfordian (Prototomarctus in particular) coex- ist here with the remains of a proboscidean (Fig. 2), known elsewhere only from the early Barstovian. An analysis of overall faunal similarity between Massacre Lake and these other proboscidean sites reveals that Massacre Lake is not especially similar to other proboscidean sites, so the occurrence of proboscideans in Hemingfordian-Barstovian boundary sites does not appear to be dictated by solely ecological or taphonomic factors; rather, this seems to be a simple immigration into existing envi- ronments. This biostratigraphically important site provides an opportu- nity for future detailed taxonomic and high-resolution stratigraphic stud- ies. ACKNOWLEDGMENTS

FIGURE 6. Map showing location of the major late Hemingfordian-early Prothero thanks A. Buller, K. Fjeld, and D. Weiser for help with Barstovian faunas in North America mentioned in text. Abbreviations of sampling in northwest Nevada, and S. Bogue and J. Kirschvink for help locality names: Ba = Barstow Fm., Bu = Burkeville fauna, DR = Deep River building and maintaining the Occidental paleomagnetics laboratory. Davis fauna, Ma = Mascall fauna, ML = Massacre Lake fauna, NC = North Coalinga and Hopkins thank the UC Berkeley vertebrate paleontology group for fauna, SC = Sucker Creek Fm., SS = Stewart Springs fauna, STH = Shark commentary on the biogeographic analysis. We all thank A. Barnosky for Tooth Hill fauna, VV = Virgin Valley fauna, WC = Willacoochee Creek access to GIS software and computational resources. We thank Spencer fauna. G. Lucas and an anonymous reviewer for helpful reviews of this paper. Prothero’s research was supported by NSF grant EAR03-09538, and by CONCLUSIONS a grant from the Donors of the Petroleum Research Fund, administered The Massacre Lake local fauna is probably about 16.4-16.5 Ma in by the American Chemical Society. Davis’ and Hopkins’ research was age, right at the end of the Hemingfordian, in magnetic Chron C5Cr. supported by NSF grant EAR-0310221, which funded the MIOMAP Consideration of the faunal data suggests a similar interpretation; taxa project. This is UCMP publication no. 1951. 245 REFERENCES

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