Anatomical characteristics of fossil wood collected from the Manchar Formation (Miocene), Thano Bula Khan, Sindh, Pakistan

The characterization of petrified wood provides valuable information about paleoclimatology and geological history and helps to reconstruct the past forest flora of different parts of the earth. The present study was undertaken to evaluate the anatomical characteristics of fossil wood of the Miocene age collected from the Manchar Formation exposed at Thano Bula Khan, Sindh, Pakistan. In order to carry out a detailed anatomical investigation, three-dimensional sections were prepared using a petrotome. The microscopic analysis allowed us to study vessel size and arrangement, wood parenchyma, fibers, and xylem rays. Based on the comparison between recent and fossil wood, we concluded that the investigated characters are comparable with those of the genus Atalantia Corrêa of the Rutaceae family. Therefore, it was named as Atalantioxylon thanobolensis sp. nov. with reference to the location of Thano Bula Khan from which the fossil wood was collected.


introduction
The anatomical study of fossil wood has long been proven as an effective instrument for determining the flora of the paleo-forest. The anatomical study of fossil wood provides useful features for the taxonomy of fossil plant and represents an important tool in determining the flora of paleo-forests. Moreover, xylotomical data can also be useful for paleo-ecological reconstruction. As suggested by Visscher and Jagels (2003), the identification of fossil wood gives valuable information on paleo-ecosystems and paleo-environments in the absence of reproductive or vegetative plant organs. Recently, Acarca et al. (2018) evaluated and identified the silicified wood belonging to Miocene forests. Based on paleobotanical studies, a variety of dicot wood flora was described by Akkemik et al. ( 2018) from the Miocene age in Ankara Turkey, confirming the paleoclimatic conditions (a xeric-low mountainous forest prevailing under a semi-dry climate). Anatomical studies of dicotyledonous fossil wood species from Sindh region of Pakistan were reported by various authors, such as Khan and Rehmatullah (1968), Rehmatullah (1971), Khan and Rajput (1976), Bhutto et al. (1993), Ahmed et al. (2007, Shar et al. (2010), Soomro et al. (2016) and Mangi et al. (2020). Rajput and Khan (1984) identified gymnosperm and monocot wood from the Sindh province. De Franceschi et al. (2008) also found some dicotyledonous fossil wood in the lower portions of the Chitarwal Formation, Sulaiman Range, eastern Baluchistan, while fossil woods from the province of Punjab (Pakistan) have also been documented by Soomro et al. (2016a, b;2017). The current work aims to characterize the fossil wood obtained from the Manchar Formation of the Miocene Age at Thano Bula Khan, Sindh (Pakistan).

Material and methods
The fossil wood sample of the Manchar Formation (TB35) was taken from Thano Bula Khan (25°24'35"N,67°46'27"E,district of Jamshoro,Sindh,Pakistan). The size of the sample was 20 cm in length and 4.5 cm in width. The colour of the fossil wood was noted to determine the depositional material. Using the ground thin-section method described by Opała-Owczarek et al. (2020), nine thin sections were prepared (cross, tangential and radial planes). All the samples were carefully observed under the microscope and all their anatomical features were noted. Photography was carried out with a digital microscope available at the Paleobotany Laboratory, Institute of Plant Sciences, University of Sindh, Jamshoro.
Holotype. The specimen was given the name "TB 35" (holotype shown in Fig. 2). It consists of silicified wood collected 10 km south-west of Thano Bula Khan, by the first author.
Horizon. Manchar Formation. Age: Pliocene to Upper Miocene. Morphological description. The present fossil was anatomically identified from a well-preserved secondary wood sample measuring 20 cm in length and 4.5 cm in width. The color of the fossilized wood is light brown with shine indicating deposition of silicates.
Anatomical analysis. Cross section. Wood diffuse porous, growth ring present, demarcated by a line of terminal parenchyma vessels small to medium in size, solitary and mostly in radial multiples of 2-5, mostly evenly distributed but in some places showing crowding at the beginning of the growth ring, circular to oval when  solitary, sometimes elliptical due to pressure during fossilization. Tylosis present, parenchyma paratracheal, terminal, and apotracheal; the latter diffuse, while paratracheal parenchyma is sparse, present as few cells around some of the vessels; terminal parenchyma forms 2-3 seriate continuous lines demarcating the growth rings; diffuse parenchyma very sparse, difficult to locate in cross section, fibers thick-walled and non-septate (Fig. 2).

Comparison with modern wood
The principal anatomical characters of the petrified wood samples are: small to medium sized vessels, thin bands of terminal parenchyma along with scanty and diffuse paratracheal parenchyma; 1-3 (mostly 2) seriate, homogeneous xylem rays and moderately thick-wall, non-septate fibers strongly indicate the affinity of this fossil wood with the Rutaceae (Metcalfe and Chalk 1950;Stoel and Borman 2008). A detailed anatomical study of various genera of this family revealed a close resemblance of the studied fossil wood with the modern woods of Atalantia Corrêa. A comparison was made with the wood of Atalantia monophylla DC., Atalantia. missionis Oliv., and Limonia acidissima L. The fossil wood under consideration resembles very closely the woods of both these species in all the anatomical characters, such as distribution of vessels, vessel shape and size, parenchyma arrangement, 1-3 seriate, homogeneous xylem rays, and non-septate fibers. The only difference observed between the fossil wood and the modern wood of the fore-mentioned species is the presence of crystalliferous apotracheal parenchyma in the modern wood while it is absent in the fossil wood. Given the resemblance of the fossil wood with the wood of both Atalantia and Limonia, we proposed a new genus Atalantioxylon.
The genus Atalantia is limited to the Indian subcontinent where it is present with four species and two varieties. The infra-generic classification and the species limits of the genus are, however, not well established due to the presence of intermediate forms. Two species, A. monophylla (L.) DC. and A. racemosa Wight & Arn. are extensively distributed, while the third species, A. wightii Yu.Tanaka is endemic to Pakistan (Rameshkumar et al. 2020).

Comparison with fossil wood
Chitaley and Shallon (1962) described a fossil wood from the Deccan near Nagpur; they placed their fossil wood in the family Rutaceae but from its photographs and text figures it does not appear to belong to this family. It also differs markedly from the fos- sil wood under investigation in the absence of terminal parenchyma and in having two types of xylem rays, short and long, made up of both heterogeneous procumbent cells and erect cell. The fossil wood of Atalantioxylon indicum from Madhya Pradesh in India was the first authentic record of a member of the Rutaceae in fossil state (Lakhanpal et al. 1978). The differences observed between the fossil wood under investigation and the previously reported fossil wood from India regard the size of vessels and slightly dissimilar parenchyma cells (Table 1). Hence, the studied fossil wood is assigned a new species name, viz. Atalantioxylon thanobolensis sp. nov.

Conclusion
A new species, Atalantioxylon thanobolensis is described from Sindh, Pakistan. The presence of other Rutaceae fossil species of Atalantioxylon in the subcontinent and their resemblance with the actual genus Atalantia suggest that a tropical climate existed in the past in the sub-continent.