# Xenoxylon synecology and palaeoclimatic implications for the Mesozoic of Eurasia

Marc Philippe, Changhwan Oh, Kyungsik Kim

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Marc Philippe, Changhwan Oh, Kyungsik Kim. Xenoxylon synecology and palaeoclimatic implications for the Mesozoic of Eurasia. Acta Palaeontologica Polonica, 2015, Xenoxylon synecology and palaeoclimatic implications for the Mesozoic of Eurasia, 60, pp.245-256. 10.4202/app.2012.0132. hal-01361531

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# *Xenoxylon* synecology and palaeoclimatic implications for the Mesozoic of Eurasia

CHANGHWAN OH, MARC PHILIPPE, and KYUNGSIK KIM

<figure><img src="image_2.png" /></figure>

Oh, C., Philippe, M., and Kim, K. 2015. *Xenoxylon* synecology and palaeoclimatic implications for the Mesozoic of Eurasia. *Acta Palaeontologica Polonica* 60 (1): 245–256.

The distribution of fossil wood genera has been demonstrated to be an effective proxy for Mesozoic terrestrial climates. In this study, we investigated the phytocoenoses, which were associated with *Xenoxylon* confirmed to be a marker for a cool and/or wet climate in a boreal hemisphere (i.e., *Xenoxylon*-phytocoenoses) during the Mesozoic, using specimens of fossil wood. It was confirmed that *Xenoxylon* co-occurs more often with some wood genera than with others. For example, *Protocedroxylon*, a wood that is most likely related to the Pinaceae, is the genus most often associated with *Xenoxylon*-phytocoenoses. Although *Taxodioxylon* is also found in *Xenoxylon*-phytocoenoses, it is not found, however, as consistently as *Protocedroxylon*. The distribution and diversity of *Xenoxylon*-phytocoenoses changed throughout the Mesozoic. During the Late Triassic and Late Cretaceous, *Xenoxylon*-phytocoenoses had low diversity and were restricted to higher palaeolatitudes during the Late Cretaceous. However, during the Early to Middle Jurassic, *Xenoxylon*-phytocoenoses were distributed much farther south, while their diversity concomitantly increased sharply. From the Late Jurassic to the Early Cretaceous, the distribution of *Xenoxylon*-phytocoenoses moved northward in Europe and even more so in East Asia. The changes in the distribution of *Xenoxylon*-phytocoenoses are in agreement with changes in both global and regional climates. Our results also demonstrated that, within the *Xenoxylon* distribution range, the corresponding phytocoenoses were differentiated along a latitudinal gradient and according to the global climate change patterns during the Mesozoic.

**Key words:** Fossil wood, *Xenoxylon*-phytocoenoses, palaeoecology, palaeoclimate, Mesozoic, Eurasia.

Changhwan Oh [bluegaia@kopri.re.kr], Division of Polar Earth-System Sciences, Korea Polar Research Institute, 26 Songdomirae-ro, Yeonsu-gu, Incheon, 406-840, Republic of Korea.

Marc Philippe [marc.philippe@univ-lyon1.fr], Paléobotanique, Université Claude Bernard-Lyon 1 and UMR5276 of the CNRS, F69622, France.

Kyungsik Kim [kksik@jbnu.ac.kr], Department of Life Sciences, Chonbuk National University, 570-752, Republic of Korea.

Received 11 December 2012, accepted 3 July 2013, available online 17 July 2013.

Copyright © 2015 C. Oh et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

## Introduction

As the need to understand the long term evolution of climate has become a major scientific challenge, Mesozoic palaeoclimates have been revealed to be of great interest. With a distribution of continental masses different from the current one, during a globally warm climatic mode, and during a now well-documented and important shift in $pCO_2$, Mesozoic conditions are an interesting point of comparison for understanding the relative roles of major climate determinants. Several recent papers (e.g., Gröcke et al. 2003; Galli et al. 2005; Joral et al. 2011) focused on specific events, such as the Toarcian Oceanic Anoxic Event (OAE) (Mailliot et al. 2009; Gomez and Goy 2011), in order to decipher the determination of major Mesozoic climate shifts. Most studies, if not

all, have focused on the marine realm, as data from terrestrial environments are relatively scarce.

As trees are long-lived sessile organisms, they are of interest for documenting terrestrial environments, and in particular, climate changes at various scales. Using Quaternary trees, growth-rings are often used as a climate proxy. However, such an approach is difficult to transpose to the Mesozoic (Brison et al. 2001). In the fossil record, trees are represented by avatars, such as fossilised leaves, pollen, and wood. Fossilised wood is of interest for palaeoecological syntheses, as it is not very mobile and is more resistant than other plant megafossils. However, due to their rather chaotic systematics and nomenclature, fossilised wood samples have not frequently been used for palaeoecological purposes (Philippe 1993). Recently, nomenclatural (Philippe and
Bamford 2008) and taxonomic (Philippe 1994; Philippe and Hayes 2010; Philippe et al. 2013) reappraisals of fossilised wood genera have been carried out. These reappraisals now allow for a reconsideration of the fossil record for wood, in search of global distribution patterns.

The distribution of fossil wood genera has been demonstrated to be an efficient proxy for Mesozoic terrestrial climates (Tsunada and Yamazaki 1984; Philippe et al. 2004; Oh et al. 2011). Specifically, the distribution of the wood of the extinct (Late Triassic to Late Cretaceous) conifer genus *Xenoxylon* Gothan, 1905 was confirmed as being a potential marker of a cool and/or wet climate in the boreal hemisphere (Philippe and Thévenard 1996; Philippe et al. 2009). Inferences from the *Xenoxylon* distribution data recently received support when they found agreement with the results obtained from analysing oxygen isotopes in vertebrate tooth enamel from the Cretaceous in Far-East Asia (Amiot et al. 2011).

In our current study, we investigated fossilised wood specimens associated with *Xenoxylon*, i.e., *Xenoxylon*-phytocoenoses. During preliminary research, we had noticed that *Xenoxylon* often co-occurs with two other wood genera, *Protocedroxylon* Gothan, 1910 and *Taxodioxylon* Hartig, 1848, in Far-East Asia. The frequent association of these genera in assemblages of fossilised wood was first noted by Gothan (1910). Since then, several researchers have noticed that *Protocedroxylon* and *Xenoxylon* often co-occurred (Shilkina 1967; Shilkina and Khudayberdyev 1971; Yamazaki and Tsunada 1981). Yamazaki and Tsunada (1982: 605) hypothesised that the corresponding trees may have inhabited similar ecologies. The co-occurrence of these genera was later hypothesised as being characteristic of cool and wet temperate climates (Philippe and Hayes 2010: 61). However, no study has yet investigated the association of *Xenoxylon* with other wood genera.

Therefore, this study addresses the synecology of *Xenoxylon*, a fossil wood genus that is restricted to the Mesozoic of Eurasia, and asks the following questions: Was *Xenoxylon* a part of diversified vegetation? When and where did this (these) phytocoenosi(e)s appear, spread, and disappear? Does the geographic distribution of this (these) phytocoenosi(e)s provide a climatic signal?

*Institutional abbreviations*.—BMNH, Natural History Museum in London; BIN, Komarov Botanical Institute of the Russian Academy of Sciences in Saint Petersburg; CBNU, Chonbuk National University, South Korea; HMB, Humboldt Museum in Berlin; NCU, Prof. Nishida's collection at Chuo University in Tokyo; NHMD, Natural History Museum of Denmark in Copenhagen; SMNH, Swedish Museum of Natural History in Stockholm; TUMS, Tohoku University Museum in Sendai.

*Other abbreviations*.—DI, diversity index; EMJ, Early to Middle Jurassic; FAD, First Appearance Date; GD, the number of genera; LJEK, Late Jurassic to Early Cretaceous; LK, Late Cretaceous; LT, Late Triassic; N, the number of occurrence; OAE, Oceanic Anoxic Event.

<figure><img src="image_3.png" /><figcaption>Fig. 1. **A.** In situ erect stump of Mesozoic *Xenoxylon* from Bayakodan, near Kuwajima in Shiraminemura, Japan. Used with the permission of Kazuo Terada (specimen no. 53901 in Suzuki and Terada 1992). **B.** Typical xenoxylean radial pitting and window-like cross-field pits in radial section of *Xenoxylon* (slide CBNU 73112).</figcaption></figure>

## Historical background

*Xenoxylon* is one among the approximately fifty Mesozoic fossil wood genera (Philippe and Bamford 2008). Its secondary xylem has distinctive anatomical features, which have no equivalents among extant wood, i.e., radial bordered pits that are exclusively, or at least partly, flattened and contiguous (xenoxylean radial pitting) and one or rarely two window-like simple cross-field pits in the early wood (Fig. 1).

There have been several arguments related to *Xenoxylon*'s systematic affinity. For example, *Podozamites* Braun, 1843, *Baiera* Braun, 1843, *Sciadopitys* Siebold and Zucc, 1842, or some extant Podocarpaceae have been suggested as its allied taxa (Nathorst 1897; Jarmolenko 1933; Arnold 1953; Bailey 1953; Shilkina and Khudayberdyev 1971). More recently, based on geochemical analysis, it appeared that *Xenoxylon* was closer to the Podocarpaceae, Cupressaceae sensu lato, and to a lesser extent, to the Cheirolepidiaceae than to the Pinaceae and Araucariaceae (Marynowski et al. 2008). The latter authors also suggested that *Xenoxylon* could be related to the Miroviaceae, which is an extinct Mesozoic conifer family. However, because *Xenoxylon* has never been found connected with foliage or any reproductive structures, it is difficult to make a decision about its systematic position.

Although the systematic position of this morphogenus is unclear, it is considered to be a monophyletic taxon (Marynowski et al. 2008). In addition, it is a bioindicator for a cool and/or wet climate (Philippe and Thévenard 1996; Philippe et al. 2009).

## Material and methods

*Time intervals and geographic subdivisions*.—The *Xenoxylon* time range covers the Late Triassic–Late Cretaceous interval. As significant palaeobiogeographic changes occurred
in the Northern Hemisphere during this time interval, we subsequently subdivided the interval into four more palaeoecologically uniform time slices: the Late Triassic (LT); the Early to Middle Jurassic (EMJ); the Late Jurassic to Early Cretaceous (LJEK); and the Late Cretaceous (LK). None of these are perfectly homogeneous, but separating the data onto more time slices would affect the significance.

Asia is here defined as the continental area east of a line running from the Ural Mountains southward to the Caucasia and then westwards to the Bosporus. Greenland is here included in Europe, as it was a part of that continent during that time. We investigated the palaeogeographic distribution of fossilised wood assemblages containing *Xenoxylon* specimens. For this purpose, we merged data, on the basis of geographic origin, into twenty-two super-localities (Table 1).

**Data sources.**—For this study, we performed a literature survey for Mesozoic fossilised wood, limiting ourselves to the assemblages that included *Xenoxylon*. We considered only fossilised wood data that were based on isolated secondary xylem (tracheidoxyls). Some unpublished data were also used, either established by samples sent by colleagues (see acknowledgments section), or obtained while revising collections at HMB, NHMD, BMNH, BIN, TUMS, SMNH, and NCU.

The process of extracting the data set considered in this paper was not an easy task. In order to decrease taxonomic biases, which are, unfortunately, all too common in the palaeoxyiological literature, we decided to perform our analysis on the generic level. We also decided not to consider any fossilised wood assemblages that included *Protocedroxylon* and/or *Taxodioxylon* (two genera commonly associated with *Xenoxylon*) but not *Xenoxylon*, or those fossilised wood floras that were limited to *Xenoxylon* alone (like e.g., in Terada et al. 2011). Not considering *Xenoxylon*-free assemblages is a problem, as it limits the validity of co-occurrence results to the *Xenoxylon* distribution area. However, considering *Xenoxylon*-free assemblages would force us to incorporate a huge number of wood floras (a number estimated at over 300) into the data set that were far from being taxonomically and nomenclaturally uniform. Moreover, incorporating the *Xenoxylon*-free assemblages would be the equivalent of using negative data, which is always a perilous task in palaeontology. Finally, it must be kept in mind that fossilised wood taxa are fossil taxa; thus, two specimens belonging to the same fossil taxon did not necessarily originate from the same biological taxon. This is a strong limitation, but taking into account only those assemblages including both *Xenoxylon* and other genera, i.e., using a palaeoecological restriction, narrows the bias induced by form taxonomy.

Another limitation results from the incompleteness of the fossil record. Indeed, some *Xenoxylon* data might have been published for locations that have not been extensively sampled, or for which all wood specimens have not been determined. Little can be done about this limitation, as it is impossible to decipher a priori which assemblages may have been under-sampled.

<table><caption>Table 1. List of *Xenoxylon* super-localities.</caption><thead><tr><th>No.</th><th>Super-localities</th><th>Continent</th></tr></thead><tbody><tr><td>1</td><td>Sakhalin</td><td>Asia</td></tr><tr><td>2</td><td>Primorye</td><td>Asia</td></tr><tr><td>3</td><td>Japan</td><td>Asia</td></tr><tr><td>4</td><td>Korea (South and North Korea)</td><td>Asia</td></tr><tr><td>5</td><td>Northeastern China (Beijing, Liaoning, Jilin, Heilongjiang, and Inner Mongolia)</td><td>Asia</td></tr><tr><td>6</td><td>Central China (Henan)</td><td>Asia</td></tr><tr><td>7</td><td>Southern China (Guangdong)</td><td>Asia</td></tr><tr><td>8</td><td>Western China (Xinjiang)</td><td>Asia</td></tr><tr><td>9</td><td>Vietnam</td><td>Asia</td></tr><tr><td>10</td><td>Iran</td><td>Asia</td></tr><tr><td>11</td><td>Georgia</td><td>Asia</td></tr><tr><td>12</td><td>Uzbekistan</td><td>Asia</td></tr><tr><td>13</td><td>Southeastern Mongolia</td><td>Asia</td></tr><tr><td>14</td><td>Arctic (Franz Josef Land, Svalbard)</td><td>Europe</td></tr><tr><td>15</td><td>Poland</td><td>Europe</td></tr><tr><td>16</td><td>Germany</td><td>Europe</td></tr><tr><td>17</td><td>Lithuania</td><td>Europe</td></tr><tr><td>18</td><td>UK</td><td>Europe</td></tr><tr><td>19</td><td>Greenland</td><td>Europe</td></tr><tr><td>20</td><td>Alaska (Central North Slope)</td><td>North America</td></tr><tr><td>21</td><td>Canadian Arctic Archipelago (CAA)</td><td>North America</td></tr><tr><td>22</td><td>Canada (Alberta)</td><td>North America</td></tr></tbody></table>

To avoid redundancy, data in the database are understood to represent the occurrence of a fossil genus in a geological formation, not taking into account if this genus (or a synonym) had been reported several times from that formation, in one or several localities. Stratigraphically uncertain data were not considered for our study, specifically, the reworked fossilised wood assemblages from the Mekong River terraces (Vozenin-Serra and Privé-Gill 1991), or the loose wood assemblage from the Choshi area (Nishida et al. 1993). Several Mesozoic fossilised wood data points, particularly from China, are poorly age constrained. Geological formations were sometimes renamed after the original publication, or their age was reappraised. For Chinese data, we generally followed the recent review of Zheng et al. (2008).

A wood assemblage was reported from Korea by Kim et al. (2005) as being Late Triassic in age. However, the age of the corresponding Jogyeri Formation was subsequently revised to be Middle Jurassic by Egawa and Lee (2011).

Oh (2010), in his thesis, reported some fossilised wood assemblages, which included *Xenoxylon* from the Chengzhe and/or the Muling formations of the Jixi Group from the Early Cretaceous in Heilongjiang Province, China. Some of these fossilised wood specimens clearly originated from the Muling Formation, but the origin of some of the others was less clear; therefore, we grouped the two formations into one.

Parrish and Spicer (1988) reported two unidentified fossil wood taxa, taxon A and taxon B, with *Xenoxylon latiporosum* from the Nanushuk Group, Central North Slope, Alaska. The Nanushuk Group was then considered to be from the mid-Cretaceous (Albian to Cenomanian), based on Foramin-
ifera (Sliter 1979) and palaeobotanical evidence (Spicer and Parrish 1986). Recently, the Nanushuk Group was renamed as the Nanushuk Formation, and its age was revised to be middle Albian to Cenomanian (Mull et al. 2003). According to Mull et al. (2003: 14), most of the formation is Albian; therefore, we assigned these fossilised wood assemblages to the Albian.

**Nomenclature and taxonomy.**—The nomenclature here follows Philippe (1993, 1995), Philippe and Bamford (2008), and Philippe and Hayes (2010).

In 2008, Philippe and Bamford suggested that the genera with Abietineentüpfelung (i.e., rounded pits occurring on the transverse wall of ray cells) *Metacedroxylon* Holden, 1913, *Protocedroxylon* and pro parte *Cedroxylon* Kraus in Schimmer, 1870, might be synonyms of *Araucariopitys* Hollick and Jeffrey, 1909. However, examination by Philippe and Hayes (2010: 59–60) of a topotype for *Araucariopitys* revealed that this latter name should be used only for short shoots with preserved pith and araucarioid cross-fields, while *Protocedroxylon* should be used for tracheidoxyls with a clearly mixed type of radial pitting and few spaced taxodioid oculi-pores in the cross-fields. Some *Araucariopitys* species have been mentioned in our database, but all can be reassigned to *Protocedroxylon*. Consequently, we used *Protocedroxylon* here as the correct name for the data originally published for the study area and interval as *Cedroxylon*, *Metacedroxylon*, *Protocedroxylon*, and *Araucariopitys*.

In the case of araucaria-like fossilised woods (i.e., tra-cheidoxyls with araucarian radial pitting and araucarioid cross-field pitting, lacking Abietineentüpfelung and resin cations), many different names have been used until now (e.g., *Araucarioxylon* Kraus, 1870, *Dadoxylon* Endlicher, 1847, *Agathoxylon* Hartig, 1848, *Dammaroxylon* Schultze-Motel, 1966). Recently, the wood anatomist community was polled to clarify which name should be retained for araucaria-like fossilised wood (Rößler et al. 2014). In accordance with the conclusions from this poll, we used *Agathoxylon* here.

Despite these preliminary taxonomic reappraisals, the discrepancies between the systematic approaches of the various authors still obscured our database. Indeed, the same wood specimen can be assigned to different genera by different authors according to their taxonomic choices, e.g., a typical *Podocarpoxylon* Gothan, 1906, could be assigned to *Taxodioxylon* or to *Circoporoxylon* Kräusel, 1949 (see e.g., Müller-Stoll and Schultze-Motel 1989). It was considered to be too risky to revise all of the systematic attributions in the database only on the basis of the publications, without checking the types. Thus, to limit the taxonomical bias, during a second stage of data management, we chose to merge the data into seven informal taxonomical groups (Table 2), uniting similar-appearing genera. Group A (or the *Protocedroxylon* group) was for wood with Abietineentüpfelung, most likely related to the Pinaceae. Wood that resembled the extant Cupressaceae sensu lato was assigned here to group B (or the *Taxodioxylon* group). Group C (or the *Phyllocladoxylon* group) was for wood specimens with unbordered (or slightly

<table><caption>Table 2. Informal taxonomical groups of Mesozoic woods discussed in this study.</caption><thead><tr><th>Group</th><th>Included morphotaxa</th></tr></thead><tbody><tr><td>A (Protocedroxylon-group)</td><td><i>Keteleerioxylon</i>, <i>Laricioxylon</i>, <i>Palaeopiceoxylon</i>, <i>Piceoxylon</i>, <i>Pinoxylon</i>, <i>Protocedroxylon</i>, <i>Protopiceoxylon</i>, <i>Protojuniperoxylon</i></td></tr><tr><td>B (Taxodioxylon-group)</td><td><i>Cupressinoxylon</i>, <i>Juniperoxylon</i>, <i>Taxodioxylon</i></td></tr><tr><td>C (Phyllocladoxylon-group)</td><td><i>Paraphyllocladoxylon</i>, <i>Phyllocladoxylon</i>, <i>Protophyllocladoxylon</i>, <i>Protosciadopityoxylon</i>, <i>Sciadopityoxylon</i></td></tr><tr><td>D (Podocarpoxylon-group)</td><td><i>Podocarpoxylon</i>, <i>Protopodocarpoxylon</i>, <i>Prototaxodioxylon</i>, <i>Protaxodioxylon</i></td></tr><tr><td>E (Agathoxylon)</td><td><i>Agathoxylon</i></td></tr><tr><td>F (Brachyoxylon-group)</td><td><i>Brachyoxylon</i>, <i>Protobrachyoxylon</i>, <i>Protocupressinoxylon</i></td></tr><tr><td>G (others)</td><td>all the data which are not in groups A to F</td></tr></tbody></table>

bordered) oopores, which have unclear systematic affinities. Group D (the *Podocarpoxylon* group) contains wood specimens with taxodioid to podocarpoid oopores in the early wood cross-fields, and these mostly also have unclear systematic positions. All of the wood specimens having araucarian radial pitting and araucarioid cross-fields, group E, limited to *Agathoxylon*, appear more uniform, but certainly do not accommodate only the Araucariaceae and their relatives. Group F (*Brachyoxylon* group) is perhaps the most heteroclite, with wood specimens featuring a mixed type of radial pitting and araucarioid cross-fields. Group G is for all of the data that are not assigned to any of the previous groups. This partitioning does not mean that we consider that the various genera within each group are taxonomical synonyms or that they originate from the same botanical group, but only that the different genera in each group were often substituted one for the other in the literature, inconsistently, for the time interval we studied. The few wood specimen data that were nomenclaturally reassigned are listed in Table 3.

## Results

Our bibliographical review and unpublished observations allowed to assemble 148 data of co-occurrence (Supplementary Online Material, SOM available at http://app.pan.pl/app60-Oh_etal_SOM.pdf). On this basis, we prepared a table and a graph of the fossilised wood genera associated with *Xenoxylon* (Table 4 and Fig. 2), summarising the raw information for the entire time range for *Xenoxylon*, i.e., from the Late Triassic to the Late Cretaceous. Because the different nomenclatural approaches had induced an artificial scattering of data, the graphs of Fig. 2, were then redrawn to Fig. 3, using the taxonomic grouping (Table 5). The circle charts were drawn on the corresponding palaeomaps, illustrating the shares of the seven
<table><caption>Table 3. The data of nomenclatural and taxonomically modified taxa in this study.</caption><thead><tr><th>Original taxon name</th><th>Modifed taxon name or group</th><th>Reasons of modification</th><th>Reference</th></tr></thead><tbody><tr><td>Prototaxoxylon persicum</td><td>Prototelicoxylon</td><td>invalidly published name</td><td rowspan="3">Nadjafi (1982)</td></tr><tr><td>Metataxodioxylon iranicum</td><td>Protaxodioxylon</td><td>invalidly published name</td></tr><tr><td>Protopinoxylon fakhri</td><td>Protocedroxylon</td><td>invalidly published name</td></tr><tr><td>taxon A</td><td>Xenoxylon</td><td>similar anatomical features</td><td rowspan="3">Parrish and Spicer (1988)</td></tr><tr><td>taxon B</td><td>Protocedroxylon-group</td><td>presence of pitted rays</td></tr><tr><td>taxa C, D</td><td>Protocedroxylon-group</td><td>presence of fusiform rays</td></tr><tr><td>taxa B, E</td><td>Protocedroxylon-group</td><td>presence of ray tracheids</td><td>Spicer and Parrish (1990)</td></tr><tr><td>Scotoxylon yanqingense</td><td>Protojuniperoxylon</td><td>Scotoxylon was named for a fossil wood having the secondary xylem of *Protojuniperoxylon* with primarily xylem and pith by Vogellehner (1968)</td><td>Zhang et al. (2000)</td></tr><tr><td>Planoxylon sp.</td><td>Protocedroxylon</td><td>Planoxylon is limited to the Cretaceous of southernmost Gondwana</td><td>Oh (2010)</td></tr><tr><td>Dacrydioxylon sp.</td><td>Xenoxylon</td><td>very similar anatomical features between both taxa and for logical explanation of *Dacrydioxylon*'s occurrence in Canadian Arctic</td><td>Selmeier and Groser (2011)</td></tr></tbody></table>

recognised taxonomic groups (A–G) in the geographically merged data (Figs. 5A, B, 6A, B). All of them have the same diameters, although they do not represent the same quantities of data. They merely reflect the relative frequencies of each taxonomic wood group at a specific super-locality.

**Co-occurrence of *Xenoxylon* with other genera.**—Fig. 2 shows that *Protocedroxylon* is the fossil genus most commonly associated with *Xenoxylon*. The co-occurrence of *Xenoxylon* with other genera is much more infrequent (at least 50% less common). Interestingly, *Agathoxylon* is more commonly associated with *Xenoxylon* than we expected based on previous results from Asia (Oh et al. 2011).

The association of the *Protocedroxylon* group with *Xenoxylon* in the fossil strata is reported for the entire time range of

the latter, from the Late Triassic to the Late Cretaceous, shortly before *Xenoxylon* disappeared (Yamazaki et al. 1980; Yamazaki and Tsunada 1981; Nishida and Nishida 1986; Spicer and Parrish 1990). The *Protocedroxylon* group is usually dominant in the *Xenoxylon* wood assemblages, more markedly so during the Late Triassic and Late Cretaceous. The oldest *Xenoxylon*–*Protocedroxylon* co-occurrence is reported from the Late Triassic (Carnian to Norian) of the Nariwa and Mine groups in Japan (Yamazaki et al. 1980; Yamazaki and Tsunada 1981). In the Russian Arctic, Shilkina (1967) reported *Xenoxylon* with *Araucariopitys* (in this case, a taxonomic synonym of *Protocedroxylon*) in the Late Triassic of the Heiss Island formation in Franz Josef Land. The youngest *Xenoxylon* wood assemblage with the *Protocedroxylon* group is

<figure><img src="image_4.png" /><figcaption>Fig. 2. Fossil wood genera co-occurring with *Xenoxylon* from the Late Triassic to the Late Cretaceous in Northern Hemisphere.</figcaption></figure>
<table><caption>Table 4. Number of fossil wood genera co-occurring with *Xenoxylon* from the Late Triassic to the Late Cretaceous in Northern Hemisphere. Abbreviations: LT, Late Triassic; EMJ, Early to Middle Jurassic; LJEK, Late Jurassic to Early Cretaceous; LK, Late Cretaceous; A, Asia; E, Europe; NA, North America.</caption><thead><tr><th rowspan="2">Genus</th><th colspan="2">LT</th><th colspan="2">EMJ</th><th colspan="2">LJEK</th><th colspan="2">LK</th><th rowspan="2">Total</th></tr><tr><th>A</th><th>E</th><th>A</th><th>E</th><th>A</th><th>E</th><th>NA</th><th>A</th><th>NA</th></tr></thead><tbody><tr><td>Protocedroxylon</td><td>3</td><td>1</td><td>4</td><td>6</td><td>6</td><td>4</td><td>2</td><td>1</td><td>1</td><td>28</td></tr><tr><td>Taxodioxylon</td><td></td><td></td><td>2</td><td>1</td><td>3</td><td>1</td><td></td><td>1</td><td></td><td>8</td></tr><tr><td>Cupressinoxylon</td><td></td><td></td><td>1</td><td>2</td><td>4</td><td>3</td><td>1</td><td>1</td><td></td><td>12</td></tr><tr><td>Protocupressinoxylon</td><td></td><td></td><td>2</td><td></td><td>2</td><td></td><td></td><td></td><td></td><td>4</td></tr><tr><td>Phyllocladoxylon</td><td></td><td></td><td>1</td><td></td><td>4</td><td>1</td><td></td><td></td><td></td><td>6</td></tr><tr><td>Protophyllocladoxylon</td><td>1</td><td></td><td>2</td><td></td><td>4</td><td></td><td></td><td></td><td></td><td>7</td></tr><tr><td>Sciadopityoxylon</td><td>1</td><td></td><td>2</td><td></td><td>1</td><td></td><td></td><td></td><td></td><td>4</td></tr><tr><td>Protosciadopityoxylon</td><td></td><td></td><td>3</td><td></td><td>1</td><td></td><td></td><td></td><td></td><td>4</td></tr><tr><td>Pinoxylon</td><td></td><td></td><td>1</td><td></td><td></td><td>1</td><td></td><td></td><td></td><td>2</td></tr><tr><td>Piceoxylon</td><td></td><td></td><td></td><td></td><td>1</td><td>2</td><td></td><td></td><td></td><td>3</td></tr><tr><td>Protopiceoxylon</td><td></td><td></td><td></td><td></td><td>5</td><td>3</td><td>1</td><td></td><td></td><td>9</td></tr><tr><td>Ginkgoxylon</td><td></td><td></td><td>1</td><td></td><td>1</td><td></td><td></td><td></td><td></td><td>2</td></tr><tr><td>Protocallitrixylon</td><td></td><td></td><td>1</td><td></td><td></td><td></td><td></td><td></td><td></td><td>1</td></tr><tr><td>Sahnioxylon</td><td></td><td></td><td>1</td><td></td><td></td><td></td><td></td><td></td><td></td><td>1</td></tr><tr><td>Lioxylon</td><td></td><td></td><td>1</td><td></td><td></td><td></td><td></td><td></td><td></td><td>1</td></tr><tr><td>Podocarpoxylon</td><td></td><td></td><td>1</td><td>1</td><td>2</td><td>1</td><td></td><td></td><td></td><td>5</td></tr><tr><td>Protopodocarpoxylon</td><td></td><td></td><td>3</td><td>4</td><td>3</td><td></td><td></td><td></td><td></td><td>10</td></tr><tr><td>Haplomyeloxylon</td><td></td><td></td><td>1</td><td></td><td></td><td></td><td></td><td></td><td></td><td>1</td></tr><tr><td>Protojuniperoxylon</td><td></td><td></td><td></td><td></td><td>1</td><td></td><td></td><td></td><td></td><td>1</td></tr><tr><td>Perisemoxylon</td><td></td><td></td><td>1</td><td></td><td></td><td></td><td></td><td></td><td></td><td>1</td></tr><tr><td>Protaxodioxylon</td><td></td><td></td><td>1</td><td>1</td><td>2</td><td></td><td></td><td></td><td></td><td>4</td></tr><tr><td>Circoporoxylon</td><td></td><td></td><td>1</td><td></td><td>1</td><td></td><td></td><td></td><td></td><td>2</td></tr><tr><td>Protocircoporoxylon</td><td></td><td></td><td></td><td>1</td><td>1</td><td></td><td></td><td></td><td></td><td>2</td></tr><tr><td>Protelicoxylon</td><td></td><td></td><td>1</td><td></td><td></td><td></td><td></td><td></td><td></td><td>1</td></tr><tr><td>Phoroxylon</td><td></td><td></td><td></td><td></td><td>1</td><td></td><td></td><td></td><td></td><td>1</td></tr><tr><td>Glyptostroboxylon</td><td></td><td></td><td></td><td></td><td>1</td><td></td><td></td><td></td><td></td><td>1</td></tr><tr><td>Prototaxodioxylon</td><td></td><td></td><td></td><td>1</td><td></td><td></td><td></td><td></td><td></td><td>1</td></tr><tr><td>Simplicioxylon</td><td></td><td></td><td></td><td>1</td><td></td><td></td><td></td><td></td><td></td><td>1</td></tr><tr><td>Paraphyllocladoxylon</td><td></td><td></td><td></td><td>1</td><td></td><td></td><td></td><td></td><td></td><td>1</td></tr><tr><td>Protobrachyoxylon</td><td></td><td></td><td></td><td>1</td><td></td><td></td><td></td><td></td><td></td><td>1</td></tr><tr><td>Anomaloxylon</td><td></td><td></td><td></td><td></td><td></td><td>1</td><td></td><td></td><td></td><td>1</td></tr><tr><td>Keteleerioxylon</td><td></td><td></td><td></td><td></td><td></td><td>1</td><td></td><td></td><td></td><td>1</td></tr><tr><td>Palaeopiceoxylon</td><td></td><td></td><td></td><td></td><td></td><td>1</td><td></td><td></td><td></td><td>1</td></tr><tr><td>Laricioxylon</td><td></td><td></td><td></td><td></td><td></td><td>1</td><td></td><td></td><td></td><td>1</td></tr><tr><td>Juniperoxylon</td><td></td><td></td><td></td><td></td><td></td><td>1</td><td></td><td></td><td></td><td>1</td></tr><tr><td>Taxaceoxylon</td><td></td><td></td><td></td><td></td><td></td><td>1</td><td></td><td></td><td></td><td>1</td></tr><tr><td>Brachyoxylon</td><td></td><td></td><td></td><td>3</td><td></td><td>1</td><td></td><td></td><td></td><td>4</td></tr><tr><td>Agathoxylon</td><td></td><td>2</td><td>9</td><td>1</td><td></td><td></td><td></td><td>1</td><td></td><td>13</td></tr></tbody></table>

dated as Campanian to Maastrichtian, from the Central North Slope, Alaska (Spicer and Parrish 1990).

The first association of *Xenoxylon* with *Taxodioxylon* is mentioned from the Liassic (Lower Jurassic) of the Toyama Prefecture, Japan (Suzuki et al. 1982). This time frame is also the First Appearance Date (FAD) for the latter genus. Group B (*Taxodioxylon* group) is found associated with *Xenoxylon* during the entire Jurassic–Cretaceous interval. This group is one of the few groups that remained associated with *Xenoxylon* during the Late Cretaceous (Fig. 3).

*Xenoxylon* is also associated at several stages during the Late Triassic–Early Cretaceous interval with the *Phyllocladoxylon* group (group C) (Fig. 3). It must be emphasised, however, that some of the wood specimens included in the group C appear to be poorly preserved and/or taphonomically biased *Xenoxylon* specimens.

In contrast, *Xenoxylon* is rarely associated with the *Agathoxylon* or *Brachyoxylon* groups (groups E and F) (Fig. 3). Such associations are commonly reported only from the Middle Jurassic, mostly in Europe and sometimes in the Middle East and East Asia (e.g., Gothan 1906; Holden 1913; Kim et al. 2005; Poole and Ataabadi 2005; Marynowski et al. 2008). However, the Middle Jurassic fossilised wood record in East Asia, is rather limited (Oh et al. 2011). Neither of the two genus groups (E and F) co-occurred with *Xenoxylon* during the Late Triassic. After the Middle Jurassic, the genera *Agathoxylon* and *Brachyoxylon* became almost exclusively disassociated from *Xenoxylon* in Far-East Asia and in Europe as well (Fig. 3). During the Late Cretaceous, *Agathoxylon* was only found associated with *Xenoxylon* once, in Sakhalin (Nishida and Nishida 1986).

Several other genera of wood specimens are only occasionally associated with *Xenoxylon*. The small number of data, as well as the broad spectrum of possible biological affinities for these fossilised wood genera (specifically, *Podocarpoxylon* Gothan, 1906) make interpretations difficult and risky.

**Evolution of the diversity of *Xenoxylon*-taphocoenoses.**—
For the Late Triassic, the fossilised wood assemblages containing *Xenoxylon* have a low diversity globally (3 genera) (Fig. 4). After the Triassic, the global generic diversity of *Xenoxylon*-taphocoenoses increased to 27 genera for the Early to Middle Jurassic interval, as shown in Fig. 4, with a well distributed share for the different genera, although the *Protocedroxylon* group (A), the *Podocarpoxylon* group (D), and *Agathoxylon* (E) are co-dominant (Fig. 3). The diversity remained high (28 genera) during the Late Jurassic to Early Cretaceous interval, but the contributions from groups D, E, and F decreased, in favour of those from groups B (*Taxodioxylon* group) and C (*Phyllocladoxylon* group) (Figs. 3, 4). For the Late Cretaceous, the number of associations reported in the database is low (Fig. 4), and their generic diversity is

<table><caption>Table 5. Number of data for the seven taxonomical groups and for four intervals. Abbreviations: LT, Late Triassic; EMJ, Early to Middle Jurassic; LJEK, Late Jurassic to Early Cretaceous; LK, Late Cretaceous (for the composition of each taxonomical group see Table 2).</caption><thead><tr><th>Group</th><th>LT</th><th>EMJ</th><th>LJEK</th><th>LK</th><th>Total number</th></tr></thead><tbody><tr><td>A</td><td>4</td><td>10</td><td>21</td><td>2</td><td>37</td></tr><tr><td>B</td><td></td><td>5</td><td>11</td><td>1</td><td>17</td></tr><tr><td>C</td><td>2</td><td>5</td><td>9</td><td></td><td>16</td></tr><tr><td>D</td><td></td><td>11</td><td>7</td><td></td><td>18</td></tr><tr><td>E</td><td></td><td>11</td><td>1</td><td>1</td><td>13</td></tr><tr><td>F</td><td></td><td>6</td><td>3</td><td></td><td>9</td></tr><tr><td>G</td><td></td><td>8</td><td>5</td><td></td><td>13</td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>Fig. 3. Number of data for each taxonomical wood group (for the composition of each taxonomical group see Table 2); LT, Late Triassic; EMJ, Early to Middle Jurassic; LJEK, Late Jurassic to Early Cretaceous; LK, Late Cretaceous.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4. Generic diversity of *Xenoxylon* wood-assemblages during the Mesozoic times—global curve and curve for specific geographic areas. Note that the real diversity was probably much lower, because of taxonomical bias. Abbreviations: LT, Late Triassic; EMJ, Early to Middle Jurassic; LJEK, Late Jurassic to Early Cretaceous; LK, Late Cretaceous.</figcaption></figure>

low as well (4 genera). The *Protocedroxylon* group remained dominant (Fig. 3). The evolution of the generic diversity in the *Xenoxylon* wood assemblages from the various geographic areas is relatively similar, while Asia hosts more diversity for every stage. North America only had a *Xenoxylon* wood flora in its northernmost regions (Canada) and only during a short period (late Early Cretaceous) (Gordon 1932; Arnold 1953; Parrish and Spicer 1988; Selmeier and Grosser 2011).

**The palaeobiogeographic distribution of *Xenoxylon*-taxa**
**coenoses.**—From the Late Triassic, only four fossilised wood assemblages containing *Xenoxylon* are documented, all from eastern Asia and the Arctic region (Fig. 5A). The *Protocedroxylon* group and the *Phyllocladoxylon* group are associated with *Xenoxylon* only in the three northernmost super-localities. The southernmost one only has wood specimens from the *Phyllocladoxylon* group associated with *Xenoxylon*. According to our data, in the Early to Middle Jurassic, the global distribution of *Xenoxylon*-containing fossilised wood floras

expanded widely, while their diversity also increased sharply (Fig. 5B). It is worth mentioning, however, that the southernmost *Xenoxylon* flora in Vietnam was isolated from the main range of the genus as this region belongs to the Indochina block, which most likely shifted significantly southwards during the Cretaceous (Carter and Bristow 2003).

During the Late Jurassic and Early Cretaceous, the global distribution of *Xenoxylon*-containing fossilised wood floras was somewhat reduced globally and shifted slightly northward (Fig. 6A). This pattern is particularly clear in Europe, but not so marked in Asia, while, curiously, *Xenoxylon* extended its range in North America. For the Late Cretaceous (Fig. 6B), there are, unfortunately, few assemblages containing *Xenoxylon* reported in the Arctic or in Asia, all at relatively high palaeolatitudes (Nishida and Nishida 1986; Spicer and Parrish 1990). The Late Triassic and Late Cretaceous figures are strikingly similar for both the distribution and the composition of fossilised wood assemblages containing *Xenoxylon*.

## Discussion

**Limits to the interpretation of the results.**—In most cases the fossil wood taxa do not represent true biological entities, what introduces some degree of uncertainty in integrating the Mesozoic fossilised wood taxa in the plant systematics. Distribution data (Figs. 5A, B, 6A, B) show that *Xenoxylon* had a narrower latitudinal distribution than most coeval fossilised wood genera and therefore most likely had a more restricted ecology. Limiting the area being studied to the area of *Xenoxylon* distribution (i.e., taking into account, for the purpose of this discussion, only the floras containing *Xenoxylon*) limits the systematic bias somewhat by focusing on an ecologically more coherent unit. By the Late Triassic, all significant conifer clades were present (Renner 2009), although wood from the B, C, D, and F groups are only documented with certainty from the Early Jurassic onwards. All wood groups studied are represented from the Early Jurassic to the Late Cretaceous. Other limits encountered are the geological megabiases (Benson et al. 2009), in particular the fact that the various geological units do not outcrop on equivalent surfaces, and have not been subjected to the same investigation pressures. A global approach decreases this type of bias.

We evaluated the quality of the data set by calculating a diversity index (DI) for each time interval (LT, EMJ, LJEK, and LK). For this index, we calculated $DI = GD / N$, where GD is the number of genera found associated with *Xenoxylon* for a given interval, and N is the number of data for this interval (see Table 5). The results are given in Fig. 7. For the Late Triassic, the diversity index is low. This finding, however, is congruent with the fact that fossilised wood flora are poorly diversified globally at this time, including those that do not feature *Xenoxylon*. With the DI decreasing slightly from the LT to the EMJ, while the GD is nine times larger, it can legitimately be hypothesised that the sampling
<figure><img src="image_7.png" /><figcaption>Fig. 5. Distribution and diversity of the fossil wood groups associated with *Xenoxylon*. A. Late Triassic (modified from the DINODATA; www.dinodata.org; accessed Oct. 11. 2011) (1, Japan; 2, Korea; 3, Southern China-Guangdong; 4, Arctic. B. Early to Middle Jurassic (1, Japan; 2, Korea; 3, Northeast China-Beijing, Liaoning, Jilin, Heilongjiang, and Inner Mongolia; 4, Central China-Henan; 5, Vietnam; 6, Uzbekistan; 7, Iran; 8, Lithuania; 9, Poland; 10, Georgia; 11, Germany; 12, UK; 13, Greenland). Abbreviations: A, *Protocedroxylon*-group; B, *Taxodioxylon*-group; C, *Phyllocladoxylon*-group; D, *Podocarpoxylon*-group; E, *Agathoxylon*; F, *Brachyoxylon*-group; G, others (for the composition of each taxonomical group see Table 2).</figcaption></figure>

was good for this time period. Increasing the number of data would most likely not reveal new associated genera. The figure is similar for the LJEK. For the Late Cretaceous, the DI doubles. This result might be due to either a decrease in the number of data or to an increase in the GD. Because even in well-documented Late Cretaceous fossilised wood flora containing *Xenoxylon* the generic diversity is always low, we interpret the increase in DI as reflecting a strong reduction in *Xenoxylon* distribution and thus in the amount of data. Once this was considered, we assumed that the data set in Table 4 was not strongly biased.

**Was *Xenoxylon* part of a diversified vegetation type?—**
From our data, *Xenoxylon* is typically associated with other genera in the outcrops. Our data originate from taphoconoses, which represent the post-mortem gathering of fossilised wood pieces. There is a little evidence from in situ fossilised wood floras that include *Xenoxylon* (Shimakura

1936; Ogura et al. 1951; Wang et al. 2000), with *Protopi-ceoxylon* (a type of wood from the *Protocedroxylon* group) being reported in Wang et al. (2000). Nevertheless, the fact that, among fossilised wood assemblages, *Protocedroxylon* co-occurs with *Xenoxylon* more than eight times more than with any other fossilised wood genus from the Late Triassic to the Late Cretaceous demonstrates that *Protocedroxylon* was a component of the *Xenoxylon*-phytocoenoses. The results for the other genera are not as conclusive. From our data, it is impossible to assert that *Xenoxylon* was growing locally in exclusive stands. It seems, however, that when sufficient numbers of wood specimens are analysed from a locality, *Xenoxylon* is usually associated with other taxa.

As far as we know, *Xenoxylon* has never been reported as co-occurring with angiosperm wood. There have been reports of angiosperm wood specimens from Cretaceous deposits, mostly from the Upper Cretaceous, in North America,
<figure><img src="image_8.png" /><figcaption>Fig. 6. Distribution and diversity of the fossil wood groups associated with *Xenoxylon*. A. Late Jurassic to Early Cretaceous (1, Japan; 2, Northeast China-Beijing, Liaoning, Jilin, Heilongjiang, and Inner Mongolia; 3, Southeast Mongolia; 4, Western China-Xinjiang; 5, Primorye; 6, Arctic; 7, Greenland; 8, Canada-Alberta; 9, Canadian Arctic Archipelago; 10, Alaska-Central North Slope). B. Late Cretaceous (1, Sakhalin; 2, Alaska-Central North Slope). Abbreviations: A, *Protocedroxylon*-group; B, *Taxodioxylon*-group; C, *Phyllocladoxylon*-group; D, *Podocarpoxylon*-group; E, *Agathoxylon*; F, *Brachyoxylon*-group; G, others (for the composition of each taxonomical group see Table 2).</figcaption></figure>

Japan, Europe, and Antarctica (e.g., Wheeler et al. 1987, 1994; Meijer 2000; Poole and Francis 2000; Takahashi and Suzuki 2003). These are, however, mostly from low- to mid-latitudes, while during this time span, the *Xenoxylon* range shifted towards high latitudes.

The global diversity of *Xenoxylon* floras, as judged from wood assemblages, was low during the Late Triassic but increased sharply at the Triassic–Jurassic boundary. This is in accordance with a global southward expansion of *Xenoxylon* during the Early Jurassic (Philippe and Thévenard 1996), as well as with a simultaneous strong diversification of wood types (Philippe and Harland 2007). During the Late Jurassic to Early Cretaceous interval, the diversity of the *Xenoxylon* flora is similar to that during the Early to Middle Jurassic, although it decreased slightly in the southern part of the range. By the Late Cretaceous, the diversity of *Xenoxylon*-floras decreased and their distribution was reduced.

<figure><img src="image_9.png" /><figcaption>Fig. 7. Diversity index (DI = GD/N, where GD is the number of genera associated with *Xenoxylon*, and N the number of occurrences) for each time period. Abbreviations: LT, Late Triassic; EMJ, Early to Middle Jurassic; LJEK, Late Jurassic to Early Cretaceous; LK, Late Cretaceous.</figcaption></figure>
**Xenoxylon-phytocoenoses and palaeoclimates.**—The Pangaean supercontinent became fragmented during the Triassic, while the climate remained warm and dry globally. It is noteworthy that *Xenoxylon*-phytocoenoses appeared worldwide during the Carnian, with *Protocedroxylon* as an associated element, during a time that might correspond to a cooling and pluvial event (Simms and Ruffell 1989). Continents became increasingly separated during the Early Jurassic, while concomitantly, the world climate became more equable and more humid than during the Triassic (Chandler et al. 1992; Golonka and Ford 2000; Ruckwied and Götz 2009). Such a globally more humid climatic environment was most likely the primary cause of the spread and the increase in the diversity of the *Xenoxylon*-phytocoenoses.

This diversity increased slightly during the Late Jurassic to Early Cretaceous interval, except in northern East Asia. The Cretaceous is regarded as having been globally warmer than the Jurassic (Spicer et al. 1993; Ziegler et al. 1993), although significant cooling events most likely occurred during the Early Cretaceous (Pucéat et al. 2003; Maurer et al. 2012). In Europe, specifically after the Middle Oxfordian, the climate became more arid (Dromart et al. 2003), while the range of *Xenoxylon* decreased northward (Philippe and Thévenard 1996). However, in Far-East Asia, this distribution restriction did not occur for *Xenoxylon*, and the associated phytocoenoses remained rich and diverse, especially in northeast China (Liaoning, Jilin, Heilongjiang, and Inner Mongolia provinces). Recent geochemical results (Amiot et al. 2011) evidenced that cool climates prevailed in northern Far-East Asia, at least during the Early Cretaceous (Barremian–early Albian). The prevalence of a cool environment during the Early Cretaceous most likely determined the persistence of diverse *Xenoxylon*-phytocoenoses in northern Far-East Asia.

The globally warm Late Cretaceous saw a concomitant severe decrease in the distribution of *Xenoxylon* and a sharp decrease in the diversity of *Xenoxylon*-phytocoenoses, confirming a relationship, for this genus, between its distribution and associated diversity. The most recent data for *Xenoxylon*-phytocoenoses, and for the genus, are from northern Far-East Asia and Alaska, supporting the idea that this area remained cool and/or humid throughout the Cretaceous, although Maastrichtian data from fossilised leaves indicate a significant local warming slightly to the south (Hermann and Spicer 1996).

Two genera (*Brachyoxylon* and *Agathoxylon*), found episodically associated with *Xenoxylon*, have special palaeoclimatic significance. *Brachyoxylon* was found in southwestern Europe, during the Middle to Late Jurassic interval, to be bound mostly to the tropophilous climates of tropical lowlands and perireefal systems (Garcia et al. 1998), while during the Early Jurassic, it had a more diverse palaeoecology (Philippe 1995). *Brachyoxylon* is only rarely associated with *Xenoxylon* after the Early Jurassic, except in Greenland, an exception that might be due to a taxonomic bias. Indeed, *Brachyoxylon* is related to diverse types of plants, and the Greenland associations might be caused by the local persistence of the more hygrophilous Early Jurassic *Brachy-

oxylon* plants. *Agathoxylon*, most likely a “bin” taxon for several botanical groups (pteridosperms, caytoniales, possibly also some cycads and Bennettitales) does not present a clear palaeoecological pattern. In Eurasia, it was, however, distributed mostly in the south throughout the Middle Jurassic–Late Cretaceous interval. After the Middle Jurassic, in Asia, *Agathoxylon* and *Xenoxylon* appear to be exclusive (Oh et al. 2011), whereas this is not the case in Europe. However, *Agathoxylon* might include taxa there that were not represented in Asia and that inhabit a different ecology.

The results and discussions confirm that *Xenoxylon* was bound to “northern climates”, i.e., cooler and/or humid ones, and that it was associated with a more diverse flora in the southernmost (i.e., warmest) parts of its distribution. This result suggests that the occurrence of *Xenoxylon* at lower latitudes is bound to more humid climatic episodes, which were not necessarily cooler.

## Conclusion

From its appearance in the Late Triassic to its disappearance at the K–T boundary, *Xenoxylon* most likely lived in association with other genera. During the globally most arid/warmest intervals (Late Triassic and Late Cretaceous), *Xenoxylon*-phytocoenoses had a limited diversity, with *Protocedroxylon* being the primary associated genus, and a northern distribution. During the globally more humid/cooler times, and especially during the Early Jurassic, *Xenoxylon*-phytocoenoses are encountered in significantly more southern regions and are more diverse, often including *Taxodioxylon*, particularly during the Early Cretaceous. The distribution patterns for *Xenoxylon*-phytocoenoses in Europe, North America and Asia differ sharply. Our results suggest that within *Xenoxylon*'s range, corresponding phytocoenoses were differentiated on a latitudinal gradient according to the climate change patterns during the Mesozoic, and the occurrence of diverse *Xenoxylon*-phytocoenoses at mid- to low palaeolatitudes characterise more humid episodes.

## Acknowledgements

Thanks are owed to Marion Bamford (University of the Witwatersrand, Johannesburg, South Africa), Fabrice Malartre (Ecole Nationale Supérieure de Géologie, Nancy, France), Kazuo Terada (Fukui Prefectural Dinosaur Museum, Fukui, Japan), Wu Zhang (Shenyang Institute of Geology and Mineral Resources, Ministry of Land and Resources, Shenyang, PR China), Shaolin Zheng (Shenyang Institute of Geology and Mineral Resources, Ministry of Land and Resources, Shenyang, PR China), and an anonymous reviewer for constructive discussions. Our thanks also go to Maria Doludenko (Geology Institute, Moscow, Russia), Leszek Marynowski (University of Silesia, Sosnowiec, Poland), Frank Wittler (Karlsruhe University, Germany), and Michał Zatoń (University of Silesia, Sosnowiec, Poland), who donated specimens; and to Peta Hayes (Natural History Museum, London, UK), David Cantrill (Royal Botanic Gardens Melbourne, Melbourne, Australia), Gilles Cuny (Natural History Museum of Denmark, Copenhagen,
Denmark), Dario de Franceschi (Muséum national d'Histoire naturelle, Paris, France), Paul Kenrick (Natural History Museum, London, UK), Harufumi Nishida (Chuo University, Tokyo, Japan), and Mitsuo Suzuki (Tohoku University, Sendai, Japan) for their help while reviewing collections. Kazuo Terada is thanked for kind permission to use his in situ *Xenoxylon* photo.

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