Eurasia

8.1 Overview

The quality of published K-T boundary terrestrial data diminishes dramatically outside of North America. Of the 20 non-North American K-T boundary sections, only one scored as much as 10 points on Table 2.1. Our evaluation of these sections begins with the 14 localities known from Eurasia, which we subdivide based on the data available into Europe, Japan, China, and the Russian Far East. Each of these regions is discussed in its own section in this chapter.

Two of the first three places in the world where an iridium anomaly was found at the K-T boundary are in Europe. However, detailed paleobotanical and palynological records of the event are absent at those localities because the boundary occurs in marine rocks. The microfossils that record extinction at the K-T boundary in those places are marine foraminifera, and although they provide an excellent fossil record of the event, they reveal nothing about land plants and the boundary. There are some nonmarine rocks spanning the boundary in Europe, however, and we review the published records from those areas. Abundant nonmarine rock sequences are to be found in Asia, especially in northeast China and the Russian Far East, and extensive literature is available. We have supplemented the Asian literature with our own field work.

A comparison of palynomorph assemblages from Upper Cretaceous and lower Paleogene intervals in western Europe, northwest Africa, and southeast China by Song and Huang (1997) provides an overview of the distribution of plants in those areas at that time, based on palynological records published through 1996 (see also Chapter 5). Song and Huang's data show that during K-T boundary time, western Europe was in the Normapolles Province and at least part of China was in the Aquilapollenites Province.

As discussed in Chapter 5, at the end of the Cretaceous, western Europe was in the Normapolles palynofloristic province, which extended from eastern North America to western Asia. Floristic changes across the K-T boundary in this broad region were not pronounced. Song and Huang (1997) invoked a change in climate towards cooler conditions from Maastrichtian to early Paleocene time in the Northern Hemisphere to account for changes that did occur. As noted in Chapter 5, however, there is little or no solid evidence for this alleged climate change. Typical Normapolles pollen was common in the Cretaceous in Europe. In the Paleogene, closely related new species appeared that have simpler apertures; these pollen types are collectively known as the "Postnormapolles" group. Thus, in Europe there is a shift in diversity and abundance of Normapolles taxa below the boundary to Postnormapolles taxa above (Song and Huang 1997). This shift cannot be interpreted as an extinction event; it is a gradual change in the composition of successive floras, not an abrupt disappearance of one with replacement by another. On closer inspection, however, it is evident that few critical studies of plants at the K-T boundary have been conducted in Europe, so it is perhaps premature to interpret the record as being in strong contrast with that from North America. Several examples from the western Mediterranean region support this assertion.

Ashraf and Erben (1986) studied palynological successions near the Mediterranean coasts of northeastern Spain and southern France (Figure 8.1). Their database consisted of 224 samples from seven sections, but among these, only one in Spain (Coll de Nargo, locality 86) and two in France (Rousset, locality 89, and Albas, locality 90) span the K-T boundary. They recorded occurrences of 11 species of marine dinoflagellate cysts along with 77 species of terrestrial spores and pollen, and compiled a composite range chart too large to reproduce here. We summarize their data as follows. Almost 80% of the species listed on the chart are pteridophyte spores, and only 12% are angiosperms. A single species of angio-sperm (not a Normapolles type) occurs in proximity to the posited position of the boundary, and it is present both below and above it. Independent age control from plant fossils, vertebrates, paleomagnetism, or isotopic ages is lacking at these localities, except for Rousset, where dinosaur eggshells have been reported in the Maastrichtian part of the section. These data do not constitute a useful palynological analysis of the K-T boundary, and Cojan (1989) suggested that in fact the section studied by Ashraf and Erben (1986) did not cross the K-T boundary, but was instead within the Campanian-Maastrichtian transition.

Not far from the study area of Ashraf and Erben (1986), Medus et al. (1988) investigated three nonmarine sections in the Spanish Pyrenees, in the provinces

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Figure 8.1 Map of western Europe showing approximate positions of K-T boundary localities discussed in the text (numbers are keyed to Table 2.1 and Appendix).

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Figure 8.1 Map of western Europe showing approximate positions of K-T boundary localities discussed in the text (numbers are keyed to Table 2.1 and Appendix).

of Barcelona and Leerida (Lleida). Their multidisciplinary study involved palyno-morphs, charophytes, ostracodes, and mollusks, and they collected samples for iridium analysis. No iridium anomalies were detected, however. They determined that ostracodes and mollusks had little biostratigraphic value, and few stratigraphic levels proved to be favorable for palynology. Medus et al. (1988) concluded that they could not locate the K-T boundary with any precision, but that the K-T "transition" could be recognized from occurrences of charophytes in one of their sections and of palynomorphs in another (Fontllonga). This study is of virtually no use in revealing the nature of the plant record across the K-T boundary in Europe, but subsequent investigations revealed more data.

The section at Fontllonga in the Spanish Pyrenees (locality 87) was the subject of subsequent studies by Medus et al. (1992), Lopez-Martinez et al. (1998), Lopez-Martinez et al. (1999), Mayr et al. (1999), and Fernandez-Marron et al. (2004). The position of the K-T boundary was determined using magnetostratigraphy (Lopez-Martinez et al. 1998). In addition to palynomorphs, the Fontllonga section includes fossil vertebrates below and above the designated position of the K-T boundary (Lopez-Martinez et al. 1999) and fossil leaves above (Medus et al. 1992, Lopez-Martinez et al. 1999). The palynoflora is characterized by Normapolles and other triporate pollen species (Medus et al. 1992), and fern spores are notably abundant (Fernandez-Marron et al. 2004). The palynomorph record does not show significant change (extinction) through this interval. Such change as is noted was attributed either to paleoclimate by Medus et al. (1992) or to paleoen-vironmental change and marine influence by Mayr et al. (1999) and Fernandez-Marron et al. (2004).

Fernandez-Marron et al. (2004) conducted the most recent and most thorough study of the Fontllonga locality and they compared it with another in the southern Pyrenees at Campo (locality 88). The Campo locality is in a paralic paleoenvironment, but pollen and spores are present. Fernandez-Marron et al. called attention to changes in abundance of fern spores from the Maastrichtian to the lower Paleocene parts of the section (although specifically not comparing it with the fern-spore spike of western North America) and recorded statistically significant changes in the spore-pollen ratio across the K-T boundary (59% to 45% in diversity, 62% to 37% in abundance). They attributed these changes to marine influence within the Campo section. Fernandez-Marron et al. (2004) also reviewed the previous studies conducted in the area by Medus et al. (1992) and Lopez-Martinez et al. (1999). Above the K-T boundary Fernandez-Marron et al. (2004) observed neither an increase in abundance of bisaccate gymnosperm pollen as recorded by Medus et al. (1992) nor a decrease in abundance of Normapolles pollen as recorded by Lopez-Martinez et al. (1999). Thus, available evidence from the western Mediterranean region is not definitive, but it suggests that there was no profound change in palynofloras across the K-T boundary.

In Germany and adjacent parts of central Europe (Figure 8.1), Knobloch et al. (1993) reviewed records of occurrence of megaspores, the mesofossils Costatheca and Spermatites, seeds, and fossil fruits in rocks of Cenomanian to Paleocene age. They found little evidence of abrupt floristic changes from the Maastrichtian to the Paleocene, although many heterosporous plants (known from their mega-spores) became extinct by the end of the Cretaceous. Knobloch et al. concluded that angiosperms evolved without notable interruption through the K-T transition and there was no significant event at the K-T boundary in central Europe. However, the coarse scale at which they grouped their data (single assemblages from the Maastrichtian and lower Paleocene, respectively) renders this study inconclusive.

Of greater interest are results from a study by Brinkhuis and Schioler (1996) at Geulhemmerberg, in the southeast part of the Netherlands (Figure 8.1). The

Diagram Fossil Layer Occurence

Figure 8.2 Diagram of occurrences of palynomorphs in the K-T boundary interval in the Geulhemmerberg caves, the Netherlands (modified from Brinkhuis and Schieler 1996). An anomalous abundance of bryophyte spores above the boundary reflects a major change in the ecosystem and may be analogous to a fern-spore spike. The K-T boundary is placed just below sample G2A, which is at the level where bryophyte spores first appear. Reprinted with kind permission of Springer Science and Business Media.

Figure 8.2 Diagram of occurrences of palynomorphs in the K-T boundary interval in the Geulhemmerberg caves, the Netherlands (modified from Brinkhuis and Schieler 1996). An anomalous abundance of bryophyte spores above the boundary reflects a major change in the ecosystem and may be analogous to a fern-spore spike. The K-T boundary is placed just below sample G2A, which is at the level where bryophyte spores first appear. Reprinted with kind permission of Springer Science and Business Media.

depositional environment at Geulhemmerberg (locality 91) was marginal marine, inner neritic, with a substantial influx of terrestrial palynomorphs. Brinkhuis and Schioler determined the position of the K-T boundary from occurrences of marine dinoflagellate cysts. At the boundary they noted an anomalous increase in the relative abundance of bryophyte spores (Figure 8.2) that may indicate a major change in the terrestrial ecosystem. They surmised that the abundance of bryophyte spores might be analogous to the fern-spore spike. Their data are based on 24 samples from an interval of 125 cm in which palynomorphs are well preserved. Unfortunately, other than bryophyte spores, terrestrial palynomorphs are exceedingly scarce in these samples.

The final study of the K-T boundary in Europe that we judge to be of relevance to the record of plants is that of Herngreen et al. (1998), also in the Netherlands. These authors investigated the biostratigraphy of foraminifera, ostracodes, calcareous nannofossils, and palynomorphs (including dinoflagel-lates, spores, and pollen) in the Maastrichtian-Danian (Maastrichtian-lower

Paleocene) interval in Curfs Quarry (locality 92), in the Maastrichtian type area. Recovery of spores and pollen from marine limestone samples was low. Assemblages of pollen clearly belong to the Normapolles Province. Herngreen et al. (1998) concluded that there is no indication of drastic change in the character or composition of terrestrial vegetation at the K-T boundary in the Curfs Quarry section. They attributed the changes they did record to paleocli-mate. Their diagram of relative abundances of palynomorphs in their section (Figure 8.3) shows a sudden increase in the abundance of bryophyte spores in the Paleocene similar to that reported by Brinkhuis and Schioler (1996).

The significance of the bryophytes in the Dutch sections is unclear, and it could be attributed to changes in paleoenvironment if not paleoclimate. In summary, the record from Europe is sparse, ambiguous, and non-definitive. There seems to be little or no evidence of an extinction event among angio-sperms, but too few detailed studies have been conducted in nonmarine strata to build a reliable database.

8.3 Japan

The islands of Japan lie within the Aquilapollenites Province (see Chapter 5), and hence palynological assemblages from the Maastrichtian and Paleocene rocks in Japan have much in common with those of western North America. Whereas the history of plants at the K-T boundary in the Normapolles Province of Europe is ambiguous, we would expect the record from Japan to be easier to interpret, because of the floristic similarities with North America. However, detailed studies relating to the boundary have not been conducted. The most intriguing records are from Hokkaido, the northernmost of the Japanese islands (Figure 8.4). Palynologist Kiyoshi Takahashi conducted the studies that provide most of the data from this area, beginning in the early 1960s. Unfortunately, most of these reports are descriptions of palynofloras of Late Cretaceous age from various stratigraphic units in Japan and do not pertain to the K-T boundary.

Takahashi and Shimono (1982) described a palynoflora from west-central Japan that includes 114 species, 75 of which are angiosperm pollen. Among the angiosperms are 43 species Takahashi and Shimono assigned to the Triprojectacites group. As explained in Section 5.1, this group of morphologically distinctive pollen includes the genus Aquilapollenites and, according to Takahashi and Shimono, 10 other genera, many of which we treat as synonyms of Aquilapollenites (Figure 8.5). Our taxonomic viewpoint aside, Takahashi and Shimono described 34 species that are either in the genus Aquilapollenites or in closely related genera. They used this extraordinary assemblage to determine

Figure 8.3 Diagram of occurrences of palynomorphs in the K-T boundary interval in Curfs Quarry, the Netherlands (from Herngreen et al. 1998). The anomalous abundance of bryophyte spores (column at far right, shaded for emphasis) may be analogous to a fern-spore spike. Other columns show relative abundances of dinoflagellate cysts, acritarchs, and other pollen and spores. Reprinted by permission.

Figure 8.3 Diagram of occurrences of palynomorphs in the K-T boundary interval in Curfs Quarry, the Netherlands (from Herngreen et al. 1998). The anomalous abundance of bryophyte spores (column at far right, shaded for emphasis) may be analogous to a fern-spore spike. Other columns show relative abundances of dinoflagellate cysts, acritarchs, and other pollen and spores. Reprinted by permission.

500 1.000 1.600 Miles

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Figure 8.4 Map of East Asia including Japan, China, and the Russian Far East showing approximate positions of K-T boundary localities discussed in the text (numbers are keyed to Table 2.1 and Appendix).

500 1.000 1.600 Miles

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Figure 8.4 Map of East Asia including Japan, China, and the Russian Far East showing approximate positions of K-T boundary localities discussed in the text (numbers are keyed to Table 2.1 and Appendix).

the age of the rocks from which it came as Maastrichtian; previously it was of uncertain age and had been thought to be Paleogene or even Neogene in age. Takahashi and Shimono did not discuss the K-T boundary or the age of any overlying rocks. The importance of their study for our purposes is that it clearly establishes the presence of numerous species of Aquilapollenites in rocks of Maastrichtian age in Japan. Species of the genus Wodehouseia were also present in their assemblage. Takahashi and Shimono noted that their assemblage had nine species in common with Maastrichtian assemblages in Hokkaido (northern Japan), Siberia, and North America.

Directly relevant to the K-T boundary in Japan, Saito et al. (1986) reported a fern-spore spike in a marine section, Kawaruppu in eastern Hokkaido (locality 93); Figure 8.6. The position of the boundary was determined on the basis of foraminifera. A claystone layer in the section was identified as the K-T boundary claystone. This report has great significance because it suggests that the

Figure 8.5 Diagrams illustrating triprojectate pollen including Aquiiapoiienites and others considered synonymous with that genus (1-7) from the Aquiiapoiienites paly-nofloral province (from Takahashi and Shimono 1982). Other genera illustrated are Fibuiapoiiis (8a, 8b), Pentapoiienites (9), Cranweiiia (10a, 10b), and Orbicuiapoiiis (11a, 11b). Reprinted by permission.

Figure 8.5 Diagrams illustrating triprojectate pollen including Aquiiapoiienites and others considered synonymous with that genus (1-7) from the Aquiiapoiienites paly-nofloral province (from Takahashi and Shimono 1982). Other genera illustrated are Fibuiapoiiis (8a, 8b), Pentapoiienites (9), Cranweiiia (10a, 10b), and Orbicuiapoiiis (11a, 11b). Reprinted by permission.

Figure 8.6 K-T boundary interval at a marine locality in Hokkaido, Japan (modified from Saito et al. 1986). A possible fern-spore spike is present, as indicated by the dashed line in the column at far right. Reprinted by permission.

fern-spore spike, well known in western North America, maybe present in Japan, as well. With regard to the putative K-T boundary claystone, no evidence verifying its identification, such as the presence of an iridium anomaly or shocked quartz, was presented. Other than the fern-spore abundance, there is little paly-nological data from the section. Only the relative abundance of gymnosperm pollen is discussed (it increases above the boundary); there are no data on the stratigraphic distribution of pollen taxa, such as Aquilapollenites or associated genera. We do not question that the position of the K-T boundary was correctly identified on the basis of foraminifera; the details of the foraminiferal record were published the same year by Kaiho and Saito (1986). We regard this locality as one that might benefit from further palynological study, but in fact a later study in the same area by Takahashi and Yamanoi (1992) produced ambiguous results.

Takahashi and Yamanoi (1992) analyzed the palynofloristic changes through the Cretaceous-Paleogene transition in the Katsuhira Formation of eastern Hokkaido. According to Saito et al. (1986), this interval spans the K-T boundary. No significant palynofloristic change was observed within the interval, however. Takahashi and Yamanoi acknowledged and reviewed the North American record, but they did not recognize a parallel change within the Katsuhira Formation.

Thus, the plant microfossil record of the K-T boundary in Japan, although intriguing, remains unclear. A major obstacle to clarification may be the lack of nonmarine rocks of latest Maastrichtian and earliest Paleocene age, which is further complicated by discontinuity of outcrop in the heavily forested terrain.

8.4 China

According to Song and Huang (1997), China was in the Aquilapollenites Province in Maastrichtian time. China encompasses a vast area (Figure 8.4) that can be subdivided palynofloristically, however, and only the northeastern region was within the Aquilapollenites Province in the Late Cretaceous. Palynological assemblages from the northwestern part of China include many species of pollen of the Normapolles group. Zhao et al. (1981) had earlier documented that the region was within the Normapolles Province in both latest Cretaceous and early Paleogene time. Wang et al. (1990) stated that, in both the Late Cretaceous and early Paleogene, three palynofloral provinces or regions existed in China: northeastern, western, and southern (they assigned the area that might be considered central China to the southern region). According to Song et al. (1983), the dominant pollen types present indicate that northeastern China had a semi-humid, subtropical to temperate climate in the Late Cretaceous, and a humid warm-temperate climate in the Paleocene; the rest of

China (western, central, and southern) had an arid, subtropical climate in both Late Cretaceous and Paleocene time.

Several summaries of Upper Cretaceous nonmarine stratigraphy within various basins and provinces of China have been published that included some Paleocene units (Chen 1983, Hao and Guan 1984, Chen 1996, Chen et al. 2006). Most of the biostratigraphic determinations are based not on plant fossils but on conchostrachans, ostracodes, and mollusks. Disparities in indicated ages and correlations among these summaries suggest considerable uncertainty in the exact ages and stratigraphic relations of the units. Mateer and Chen (1992) reviewed potential K-T boundary intervals in nonmarine rocks of China. They reported that the paleontological records of most of the ten basins they investigated primarily involve charophytes and ostracodes rather than plant fossils. They concluded that, based on palynomorphs, potential K-T boundary localities may exist in the Minghe Basin in Gansu and Qinghai provinces (western region), the Jiangsu Basin in Jiangsu Province (southern region), and the Songliao Basin in Heilongjiang and Liaoning provinces (northeastern region). Here we discuss the data available from each of these regions. The southern and northeastern regions have yielded the most data.

Wang et al. (1990) briefly summarized data on palynological assemblages of Cenomanian to Miocene age from Xinjiang and Qinghai provinces, which are, respectively, in the western and southern palynofloral regions they defined. They were unable to locate the K-T boundary in these regions; they found mixed assemblages in both of their study areas.

Wang and Zhao (1980) described four palynological assemblages from the Jianghan Basin in Hubei Province, among which the third encompasses the upper part of the Cretaceous (Senonian) through the lower part of the Paleocene. Hubei Province evidently lies within the southern palynofloral region of Wang et al. (1990). The palynofloras of this region are characterized by an abundance of ephedralean gymnosperm pollen species, and Wang and Zhao (1980) concluded that the flora indicates an arid climate. Their paper included a range chart of selected species grouped into the four assemblages; it shows no break within the stratigraphic distribution of the third assemblage, and hence the K-T boundary is not distinguishable.

A Maastrichtian and Paleocene section in the Nanxiong Basin, Guangdong Province (locality 94), has been the subject of extensive studies by several authors, most notably Zhao et al. (1991), Erben et al. (1995), Stets et al. (1996), Buck et al. (2004), and Taylor et al. (2006). Guangdong Province is in the southern palynofloral region of China. The stratigraphic units of interest are the Pingling Formation of the Nanxiong Group and the Shanghu Formation of the Luofozhai Group. These are primarily red beds, which are notoriously poor for palynomorph recovery. Stets et al. (1996) discussed collecting and preparing 425 samples, but without being specific they noted that the recovery of palyno-morphs was less than they had anticipated, which is probably an understatement. Nonetheless, they were able to identify 41 palynomorph species, most of which are pteridophyte spores. They identified seven species of gymnosperm pollen, none of which included the ephedralean types that according to Wang and Zhao (1980) are characteristic of the southern palynofloral region of China. On the basis of their palynostratigraphy, Stets et al. claimed to have bracketed the boundary within an interval 21 m thick, 80 to 101 m below the top of the Pingling Formation. A previous investigation (Zhao et al. 1991) had placed the K-T boundary at the contact of the Pingling and the overlying Shanghu formations, primarily on a lithostratigraphic basis. Stets et al. (1996) reported extinction of the majority of taxa they regarded as indicative of Late Cretaceous age. Neither a K-T boundary claystone layer nor an iridium anomaly was found, but Stets et al. noted changes in the ratios of the stable isotopes 18O and 13C. They interpreted that their stratigraphic section is within an interval of reversed geomagnetic polarity, which they assumed to be subchron C29r. Stets et al. reported fragments of dinosaur eggshells above their bracketed position of the K-T boundary, in the uppermost part of the Pingling Formation. They concluded that these eggshell fragments are evidence that dinosaurs had survived into the Paleocene. Although this book is not about dinosaurs and the K-T boundary, this report of dinosaur fossils (eggshell fragments) above a putative K-T boundary established by palynology requires further comment.

Dinosaur eggshell fragments and nests containing complete eggs were first reported in the Nanyung [Nanxiong] Group by Young [Yang] and Chow [Zhou] (1963). Zhao et al. (1991) also discussed the presence of eggshell fragments and used their highest stratigraphic occurrence to support placing the K-T boundary well above the position designated by Stets et al. (1996). The associated paleomagnetic data led Zhao et al. (1991) to conclude that dinosaur extinction in southern China took place 200 000-300 000 years before the terminal Cretaceous event. This deduction did not go unchallenged, however. Russell et al. (1993) published a reinterpretation of the paleomagnetic data, concluding that a hiatus representing more than six million years exists at the contact between the Pingling Formation and the Shanghu Formation, where Zhao et al. (1991) placed the K-T boundary. Russell et al. also noted that no iridium anomaly that could verify the position of the boundary had been found in the section studied by Zhao et al.

Zhao et al. (2002) later reported minor iridium anomalies at three different levels within the Pingling Formation, based on analyses of the dinosaur eggshell fragments (none of these "anomalies" is associated with a boundary claystone layer). Stets et al. (1996), who placed the K-T boundary 80 to 101 m below the top of the Pingling Formation, had concluded that dinosaur eggs and eggshell fragments above that level were Paleocene in age.

Interpretations of the stratigraphic occurrence and biostratigraphic significance of dinosaur eggshells and the placement of the K-T boundary in the Nanxiong Basin were challenged by Buck et al. (2004). Buck et al. restudied the "Nanxiong" [Pingling] Formation and the overlying Shanghu Formation. Utilizing the palynologic data of Stets et al. (1996), Buck et al. (2004) concurred with Stets et al. in placing the K-T boundary in the upper part of the Pingling Formation, well below its contact with the overlying Shanghu Formation. Based on their interpretations of lithofacies, Buck et al. (2004) concluded that the uppermost 101 m of the Pingling Formation and the lowermost part of the Shanghu Formation were deposited as debris flows that reworked Cretaceous fossils, including the dinosaur eggshell fragments, above the level of the K-T boundary. Buck et al. concluded that the assertions of Zhao et al. (1991) and Stets et al. (1996), that dinosaur fossils occur above the K-T boundary in the Nanxiong Basin, is based on a misunderstanding of the sedimentological origin of the deposits in the upper part of the Pingling Formation and the lower part of the Shanghu Formation.

The most recent and most comprehensive study of the controversial Pingling-Shanghu red-bed section in the Nanxiong Basin is that of Taylor et al. (2006). These authors reviewed all of the pertinent literature and revisited the site. Their results are presented in Figure 8.7. On the basis of the data from vertebrate paleontology including dinosaur eggs and Paleocene mammals, they reasserted that the placement of the K-T boundary at the contact between the Pingling and Shanghu formations established by Zhao et al. (1991) is correct. As for the claim by Buck et al. (2004) that the uppermost part of the Nanxiong [Pingling] Formation is composed of debris-flow sediments containing reworked fossils, Taylor et al. (2006) noted the presence of in situ dinosaur eggs in nests, which could not have been reworked, right below the formational contact. Taylor et al. did not review the palynological data of Erben et al. (1995), Stets et al. (1996), and Zhao et al. (2002), which Buck et al. (2004) had accepted. Our review of those data leaves us skeptical that any of the species of fossil pollen said to be indicative of Tertiary age are exclusively Paleogene. We conclude that the position of the K-T boundary in the Nanxiong Basin remains unresolved, but if the well-reasoned interpretations of Taylor et al. (2006) are correct, the evidence for Paleocene dinosaurs is unconvincing.

Returning to the data on plants and the K-T boundary in China, we move to the northeastern part of that country. In the earliest published report on the K-T boundary in China, Hao et al. (1979) studied the Mingshui Formation in the Songliao Basin, a large geologic feature in the northeastern part of the country

Magnetostratigraphy

Magnetostratigraphy

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