Protaxodioxylon turolense Vozenin-Serra, 2011

Vozenin-Serra, Colette, Diez, José Bienvenido & Ferrer, Javier, 2011, A new species of Protaxodioxylon (Cupressaceae s. l.) from the late Albian of the Aragonian branch of the Iberian Range (Spain). Palaeoclimatic implications, Geodiversitas 33 (1), pp. 11-24 : 15-20

Protaxodioxylon turolense Vozenin-Serra , n. sp.

HOLOTYPE. — MPZ 97 /2514, Paleontological Museum , University of Zaragoza, Spain.

PARATYPES. — MPZ 97/2504 and MPZ 97/2516, Paleontological Museum, University of Zaragoza.

TYPE LOCALITY. — El Barranquillo site, municipal district of Castellote, Teruel Province, North Spain, coord. UTM 30TYL268247.

TYPE HORIZON. — Utrillas Fm, Lower Cretaceous (late Albian).

ETYMOLOGY. — The specific name refers to the Spanish denomination for the inhabitants of Teruel.

DIAGNOSIS. — Secondary homoxyleous pycnoxylic wood, without distinct growth-rings. Tracheids with polygonal lumen. Dimensions (tangential x radial): 35-66 (55) × 40-76 (70) µm. Uni- and biseriate, locally triseriate rays of 7-42 (12-22) cells in height. Density: 5-6 rays by tangential horizontal mm. Cells height: 20 to 25 µm. Rays homogeneous with smooth horizontal and end walls. Mixed radial pits mostly uniseriate in closely crowded arrangement or contiguous and separate, rarely biseriate, then opposite and surrounded by crassulae. Dimension of pits (height and width) 24- 28 × 28-32 µm. 1 or 2 taxodioid pits (occasionally 3) in cross-fields, 13-15 µm in diameter, disposed in one single horizontal line. Abundant axial parenchyma, with smooth transverse end walls.

DESCRIPTION

Transverse section

Homoxyleous pycnoxylic wood without any growth ring.The tracheids have a polygonal outline ( Fig.3A ), with radial diameter comprised between 40 and 76 (70) µm and tangential diameter of 35 to 66 (55) µm, the wall thickness varies between 3 and 5 µm.

There are from 1 to 8 files of tracheids between adjacent rays, mostly 2 to 5. The number of tracheids per mm2 varies between 224 and 256.

Tangential section`

The rays are uniseriate or completely or partially biseriate ( Fig. 3B ). Triseriate rays can occasionally be seen. They are of average height, between 7 and 26 cells, mostly between 12 and 22. However, rarely the height exceeds 30 cells and can reach as many as 42 cells. There are 5-6 rays per tangential millimetre. The parenchyma ray cells are between 20 and 25 µm high.

The axial parenchyma is abundant ( Fig. 3C ). The parenchyma cells are rectangular, between 30 and 50 µm wide and have smooth transverse end walls.

Radial section: The tracheids show radial pits of mixed type with clear araucarian tendency. Radial uniseriate pits are predominant, and mostly in closely crowded arrangement (87%, Fig. 4A ) others being contiguous or separate (6%, Fig. 4C ). Opposite biseriate pits can also be seen surrounded by bars of Sanio (7%) ( Fig. 4D ). The height of the pits varies between 24 and 28 µm, and the width between 28 and 32 µm.

The rays are homogeneous ( Fig. 4B ). Both horizontal and end walls of the ray parenchyma cells are thin and smooth; the cell height varies between 20 and 25 µm, and their length between 150 and 290 µm.

The cross-fields are rectangular in shape and show 1 or 2 taxodioid pits (occasionally 3), with a large oblique or horizontal opening included in the areola limits ( Fig. 4F, G ). The diameter of the cross-field pits varies between 13 and 15 µm, they are arranged in a single horizontal line when there are 2 or 3 pits per field.

DISCUSSION

The general aspect of the tracheids and rays, the presence of taxodioid cross-field pits and the presence of abundant axial parenchyma, indicate that the structure can be related to the taxodiaceous Cupressaceae . Recent phylogenetic analyses based on molecular and morphological data propose to consider both Cupressaceae and Taxodiaceae as a single family ( Cupressaceae s.l.), excluding Sciadopitys Siebold & Zucc. transferred to a monotypic family Sciadopityaceae Luerss. ( Gadek et al. 2000; Farjon 2005). Within this family Cupressaceae sensu lato the greatest affinities are found within extant genera that have smooth terminal walls of the parenchyma cells namely Athrotaxis , Cunninghamia , Metasequoia , Sequoia , Sequoiadendron and Taiwania . Both Cunninghamia and Taiwania show pitting of variable type in the cross-fields, while the specimens studied present only taxodioid cross-field pits. Athrotaxis is characterized by low rays ( Phillips 1948).

With the exception of the tracheid radial pitting, the specimens studied show the characteristic of the genera Sequoia , Sequoiadendron and Metasequoia . These genera, mainly differentiated by their leaves and reproductive organs, show very similar wood characteristics. Some differences proposed to differentiate these genera on the wood anatomy are the size of the radial pitting in earlywood, smaller in Metasequoia (10-13 µm) than in Sequoia and Sequoiadendron (14-24 µm) ( Gromyko 1982), the maximum height of the rays (higher in Sequoia ) the number of cross-fields pits (from 2-8, and sometimes 10 in Sequoia , and less than 6 in Metasequoia and Sequoiadendron ). The specimens found at El Barranquillo combine the characteristics of these three genera. The fossil woods related to the subfamily Sequoioideae have predominant abietoid radial pitting on the tracheids as does the extant Sequoia . However, the structure studied here has a clearly mixed structure with chief araucarian characteristics (87%). Many coniferous fossil genera, especially Jurassic and Cretaceous, show all the characteristics of the modern groups to which they are related, with the exception of their mixed radial pitting

AFFINITIES

The presence of mixed radial pits in the tracheids is characteristic of some Mesozoic conifers. These morphogenera are usually named by adding the prefix proto- to the name of the modern genus with which they have the most similar characteristics. Kräusel (1917) proposed the artificial group of the Protopinaceae to accommodate these morphogenera, as they were anatomically well defined and localized in time. This point of view has been (and still is) a subject of controversy amongst researchers. Hollick & Jeffrey (1909) claimed that the tracheid radial pitting had a predominant taxonomic value, and considered that the mixed structures were the ancestral forms of modern Araucariaceae . Gothan (1907) on the other hand, rejected the preponderance of this characteristic in comparison to more important ones such as the morphology of the rays and cross-fields and considered that the Mesozoic mixed structures are the ancestral forms of abietineous conifers.

Bailey (1933) and Bailey & Faull (1934) were opposed to the use of the group proposed by Kräusel, arguing that mixed structures with clear araucarian tendency are observed in the roots and axes of the cones of some modern abietineous conifers ( Cedrus ), which means they are within the variation limit of this group. Grambast (1960) affirmed that pitting of a mixed type, far from being characteristic of a particular group of conifers of the Mesozoic era, can in fact be seen not only amongst certain Tertiary and modern conifers, but also amongst many Permo-Carboniferous gymnosperms in southern regions.

Without wishing to enter into these debates about the usage of the Protopinaceae artificial group, the affinities of the wood structure from the Upper Albian of El Barranquillo with fossil and modern species of the Sequoioideae subfamily indicate that, in this particular case of araucarioid pitting clearly predominant, the mixed type possibly corresponds to an ancestral feature of modern taxodiaceous Cupressaceae .

Fossil woods with mixed radial pitting and closely related to taxodiaceous Cupressaceae were firstly named Prototaxodioxylon by Vogellehner (1968). This genus was not based on proper observations by Vogellehner but on an erroneous interpretation of cross-fields pits in the wood Protocupressinoxylon chouberti Attims (1965) , from Morocco. Moreover this last wood deposited in the “Laboratoire de Paléobotanique-Paris” was reexamined by Nadjafi (1982) who discovered the presence of true spiral thickenings like in the family Taxaceae . So, Nadjafi attributed it to Prototaxoxylon Kräusel & Dolianiti (1958) . Consequently it was impossible to consider the Prototaxodioxylon genus as valid. Nadjafi proposed the name Metataxodioxylon for such woods related to the former Taxodiaceae and without spiral thickenings. Unfortunately this last name has never been published, so it is invalid. Following the determination key to morphogenera of Mesozoic conifer-like woods by Philippe & Bamford (2008), we can attribute our Spanish wood to the genus Protaxodioxylon Bamford & Philippe (2001) characterized by mixed radial tracheid pitting and taxodioid cross-field pits. To our knowledge this genus comprises one single species Protaxodioxylon romanensis (Philippe) Bamford & Philippe (2001) , known from the Lower Toarcian of Doubs, France and Lower Pliensbachian of Bas-Rhin, France. Our specimens are close to P. romanensis but the absence of growth rings and the ray height ( P. romanensis has lower rays) led us to create a new species P. turolense . It is interesting to notice that during the Jurassic, can be found structures with mixed radial pitting and true abietinean radial pitting, so Taxodioxylon lemoignei Philippe (1994) from the Oxfordian of Bourgogne, France, has a typical abietoid radial pitting, and is very close to Taxodioxylon gypsaceum (Göppert) Kräusel, 1949 . Later on, some wood specimens were attributed to the genus Protaxodioxylon ( Philippe et al. 2006, 2010, Philippe pers. comm.) (cf. list hereafter):

– Hungary: Pécsbányatelep: Sinemurian or Hettangian given as Taxodioxylon sp. by Greguss & Kedves ( Philippe & Barbacka 1997);

Vasas and Pecs: Hettangian ( Philippe et al. 2006). – Poland: Gnaszyn: Bathonian ( Philippe et al. 2006).

– France: Lixhausen, Bas-Rhin: Bathonian ( Süss & Philippe 1993);

La Grandville, Ardennes: Hettangian ( Thévenard et al. 1995);

Romains, Doubs: Toarcian ( Philippe 1995); Larzac, Aveyron: Middle Bathonian ( Garcia et al. 1998);

Essonne: Middle Callovian ( Garcia et al. 1998).

– Portugal: Guimarota: Kimmeridgian ( Mohr & Schultka 2000; Philippe et al. 2010).

– England: Cleveland Basin, North Yorkshire given as Taxodioxylon spp.: late Pliensbachian Cleveland Ironstone Formation to late Bathonian Scalby Formation ( Morgans 1999).

The anatomy of these woods could be precised in order to know if they belong to different species. All of them are Jurassic in age.

Otherwise Iamandei & Iamandei (2004) created a new species attributed to the genus Prototaxodioxylon : P. marisii without any mention of the former genus Protaxodioxylon creation by Bamford & Philippe (2001). Prototaxodioxylon marisii comes from the late Cretaceous-early Tertiary of Romania and there is an ambiguity between the description and the illustrations: textfig. 3 of Iamandei & Iamandei (2004) shows abietoid and araucarioid radial pitting with cupressoid cross-field pits and not taxodioid, while radial pitting in pl. I is clearly araucarioid and crossfields pits indistinct. Further precision would be necessary.

Taxodiaceous woods have been mainly identified from Upper Cretaceous to Cenozoic sediments. In addition to the morphogenus Protaxodioxylon , some species referred to Taxodioxylon were also recorded from the Jurassic and Lower Cretaceous:

– Taxodioxylon lemoignei Philippe, 1994 (= Dadoxylon sp. of Lemoigne & Thierry 1968) from the Jurassic (Oxfordian) of France with uniseriate or biseriate abietoid radial pitting, and 1-3 (4) taxodioid cross-field pits in the earlywood cross-fields.

– Taxodioxylon sp. from the Wealden facies of the Mons Basin, Belgium ( Gerards et al. 2007).

– Taxodioxylon szei Yang & Zheng, 2003 , Lower Cretaceous of the Jixi Basin, China shows 1-3 – seriate abietoid pitting, abundant wood parenchyma, rays (1-50 cells high) with some idioblasts and 1-6 taxodioid pits in cross-fields.

– Taxodioxylon albertense (Pen.) Shimakura 1937 , known from Canada, USA, Japan (Barremian to Santonian) is characterized by abietoid pitting surrounded by bars of Sanio and rays very high (up to 70 cells).

– Taxodioxylon sp. (Falcon-Lang et al. 2007) discovered in the Lower Cretaceous Chaswood Formation ( Canada), preserved as charcoal, is characterized by opposite radial pitting, taxodioid cross-fields and absence of axial parenchyma.

In the high latitudes (78°N), Harland et al. (2007) collected permineralized woods from the South Spitzberg (Aptian/Albian). Taxodioxylon was the dominant morphogenus amongst the conifers (25% of the sampled set).

In the southern hemisphere, podocarps and araucarian conifer forests associated with some Taxodiaceae are known in the late Albian of SE Alexander Island and Antarctic Peninsula, palaeolatitude 75°S (Falcon-Lang & Cantrill 2000; Falcon-Lang et al. 2001). The Alexander Island taxon ( Taxodioxylon ) is fairly similar to T. lemoignei described in France.

Undeniably, the anatomical characteristics of the “El Barranquillo” specimens fit well into the morphogenus Protaxodioxylon . Our wood indicates that Protaxodioxylon , known till now from Jurassic only, might have extended at least during the Lower Cretaceous.

Figures

FIG. 3. — Protaxodioxylon turolense Vozenin-Serra,n. sp. (holotype MPZ 97/2514):A, transverse section without growth rings; B, tangential section showing uni- or sometimes partly biseriate rays and abundant axial parenchyma; C, tangential view of smooth transverse end wall in axial parenchyma. Scale bars: 100 μm.

FIG. 4. — Protaxodioxylon turolense n. sp.,radial sections:A, B, E, G, holotype MPZ 97/2514;C, D, F, paratype MPZ 97/2504);A, araucarioid uniseriate radial pitting; B, smooth and thin tangential walls in ray cells and araucarioid uniseriate radial pitting; C, mixed radial uniseriate pits with araucarian affinity; D, abietoid bordered pits on the radial walls of the tracheids and crassulae;E, smooth horizontal walls of ray cells and taxodioid cross-field pits; F, G, distribution of taxodioid cross-field pits. Scale bars: 100 μm.