Acta Palaeobotanica 59(1): 51–67, 2019 e-ISSN 2082-0259 DOI: 10.2478/acpa-2019-0006 ISSN 0001-6594 Fruit morphology, anatomy and relationships of the type species of Mastixicarpum and Eomastixia (Cornales) from the late Eocene of Hordle, southern England STEVEN R. MANCHESTER1* and MARGARET E. COLLINSON2 1 Florida Museum of Natural History, Dickinson Hall, P.O. Box 117800, Gainesville, Florida, U.S.A.; e-mail: [email protected] 2 Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey TW20 0EX, United Kingdom Received 26 October 2018; accepted for publication 29 April 2019 ABSTRACT. The Mastixiaceae (Cornales) were more widespread and diverse in the Cenozoic than they are today. The fossil record includes fruits of both extant genera, Mastixia and Diplopanax, as well as several extinct genera. Two of the fossil genera, Eomastixia and Mastixicarpum, are prominent in the palaeobotanical literature, but concepts of their delimitation have varied with different authors. These genera, both based on species described 93 years ago by Marjorie Chandler from the late Eocene (Priabonian) Totland Bay Member of the Headon Hill Formation at Hordle, England, are nomenclaturally fundamental, because they were the first of a series of fos- sil mastixioid genera published from the European Cenozoic. In order to better understand the type species of Eomastixia and Mastixicarpum, we studied type specimens and topotypic material using x-ray tomography and scanning electron microscopy to supplement traditional methods of analysis, to improve our understanding of the morphology and anatomy of these fossils. Following comparisons with other fossil and modern taxa, we retain Mas- tixicarpum crassum Chandler rather than transferring it to the similar extant genus Diplopanax, and we retain Eomastixia bilocularis Chandler [=Eomastixia rugosa (Zenker) Chandler] and corroborate earlier conclusions that this species represents an extinct genus that is more closely related to Mastixia than to Diplopanax. KEYWORDS: fruits, Mastixia, Diplopanax, fossil, Late Eocene, Tertiary INTRODUCTION Both extant genera of the cornalean family has been recognized (reviewed in Kirchheimer Mastixiaceae have distinctive endocarps that 1957, Mai 1993). are readily recognizable in the fossil record. Generic and specific nomenclature for fossil Although geographically limited to Asia mastixioid fruits is complicated because many (Diplopanax and Mastixia) and Malesia (Mas- of the fossil genera and species were named tixia) today, both genera are well documented before the fruit morphology and relationships by fossil fruits in the Cenozoic of Europe (Mai of the extant representatives were well under- 1970, Martinetto 2011, Hably & Erdei 2013) stood. The living genus Diplopanax Hand.- and North America (Tiffney & Haggard 1996, Mazz was named (Handel-Mazzetti 1933) sub- Stockey et al. 1998). In addition, a complex sequent to the designation of the extinct genera of extinct mastixioid genera with endocarps Mastixicarpum and Eomastixia by Chandler similar to those of Mastixia and Diplopanax (1926), but initially it was thought to repre- sent Araliaceae, and its affinities to Mastixia * Corresponding author were only recognized 50 years later (Zeng 1983, 52 S.R. Manchester & M.E. Collinson / Acta Palaeobotanica 59(1): 51–67, 2019 Eyde & Xiang 1990). Meanwhile, fossil fruits MATERIALS AND METHODS with similar morphology had been assigned to other genera such as Tectocarya Kirchheimer Specimens. Fruits of extant Mastixia and Diplo- (1936), Mastixiopsis Kirchheimer (1936), Mas- panax were borrowed from herbaria of the US National tixiodiocarpum Scott (Scott 1954, Manchester Museum, Washington, DC (US); Royal Botanic Gardens, 1994) and others. The recognition and distinc- Kew, Richmond, Surrey, UK (K); the Komarov Botani- tion of various fossil mastixioid genera were cal Institute, Saint Petersburg (LE), and the Missouri Botanical Garden, St. Louis (MO). Fossil specimens are informed by degrees of departure from what housed in the palaeobotany collections, Earth Depart- was then considered to be the only modern ment, Natural History Museum London (NHMUK; genus – Mastixia. Names proliferated, with up specimen numbers prefixed “V.”). Geological occurrence to 16 genera and 40 species in the literature on and locality details may be found in the original pub- lished sources (Chandler 1926, 1962, Reid & Chandler European fossil mastixioid fruits. Some were 1933), reviewed and updated by Collinson (1996) and subsequently synonomized, however, and Mai Cleal et al. (2001). The Hordle samples (Hordle bed 10) (1993) reduced the number of European fos- are from the Totland Bay Member of the Headon Hill sil mastixioid genera to seven, including five Formation of late Eocene (Priabonian) age. extinct genera plus fossil species of the two Light and scanning electron microscopy. Extant and fossil fruits were studied using natural frac- extant genera. Difficulties remain, however, ture surfaces and transverse sections cut with a paper- because some of the earliest-described taxa – thin diamond blade mounted on a Microslice II annular important because of their nomenclatural pri- saw. Naturally fractured surfaces were also studied by ority – are less well known anatomically than scanning electron microscopy (SEM) using a Hitachi some of the later-described examples. S-4000 instrument at the University of Florida. Micro-computed tomography (micro-CT). For Fossil mastixioid genera were first recog- anatomical investigation of extant material, selected nized by Chandler (1926) from the late Eocene fruits were examined by micro-CT at the Imaging and of Hordle, England. Based upon lignified fruits, Analysis Centre, NHMUK, using a Nikon Metrology she established two new genera that were sub- HMX ST 225. These included a fruit from the type sequently found to be widespread in the Euro- material of Mastixia pentandra (K704731) and another from type material of M. euonymoides (K704785), plus pean Tertiary: Eomastixia and Mastixicarpum. two other fruits of Mastixia euonymoides (G. Watt The circumscription of these genera was subse- 5899, K). To protect specimens and avoid using adhe- quently revised as new species from other sites sive, the fruits were first wrapped in cling film and were added (Kirchheimer 1957, Holý 1975, Mai then inserted carefully into Oasis TM (foam used in 1993), and by Chandler (1962) when she exam- flower arranging) for stability during the scan. Scan- ning was undertaken using a tungsten reflection target ined larger collections of topotypic material at accelerating voltage of 90 kV and current of 290 µA, from Hordle as well as material from other sites. with an exposure time of 708 ms (giving a scan time Chandler’s work, both in 1926 and 1962, was of 38 minutes). No filters were used and 3142 projec- careful and detailed, but limited by the tools of tions were taken over a 360° rotation. Voxel sizes of the time for imaging fine-structure details. Her the resulting datasets were 12 or 13 µm depending on specimen size. Three-dimensional volumes were descriptions were based on optical observations reconstructed using CT Pro (Nikon Metrology, Tring, of fruits in surface view and variously fractured UK) and TIFF stacks were exported using VG Studio to reveal internal anatomy of the fruit and seed Max (Volume Graphics GmbH, Heidelberg, Germany). features, and informed by careful comparative Datasets were visualized, and images and videos were work on available extant taxa. captured, using Avizo 8.1 (FEI Visualization Science Group, Bordeaux, France). Images were adjusted uni- For the present article we review the mor- formly overall for contrast and brightness using Adobe phology and anatomy of the extant Mastixia Photoshop® CS2 or CS6. Micro-CT scan datasets are and Diplopanax and provide detailed compara- archived at NHMUK. tive studies of the morphology and anatomy for Synchrotron radiation x-ray tomographic the type species of Eomastixa and Mastixicar- microscopy (SRXTM). For improved resolution of ana- tomical details, SRXTM was applied to selected fossil pum, based on fruits from the original locality specimens and fruits of extant Mastixia using the TOM- at Hordle, England. New anatomical details CAT beamline of the Swiss Light Source, Paul Scher- revealed by x-ray tomography and scanning rer Institut, Villigen, Switzerland, in March 2015 (fos- electron microscopy improve our understanding sils) and February 2016 (extant). For extant Mastixia, of the relationships among these modern and one fruit from type material of each of M. pentandra (K704731) and M. euonymoides (K704785) was scanned. fossil taxa, confirming that Eomastixia is close For fossil Mastixicarpum crassum specimens, the holo- to Mastixia and that Mastixicarpum is similar type V20074a [figured by Chandler 1962: pl. 19, fig. 19] to but distinct from Diplopanax. (detached valve in a separate scan from main fruit), S.R. Manchester & M.E. Collinson / Acta Palaeobotanica 59(1): 51–67, 2019 53 V68593i and V68593ii (two halves of the same speci- side of the endocarp, which becomes the ger- men, re-registered from V20076 duplicate material) mination valve. This is seen on the surface Eomas- from Hordle were scanned. Both syntypes of of the endocarp as a narrow longitudinal slit. tixia bilocularis (V20079a, V20079b, labeled E. rugosa in the collection) were scanned, as was V40768 from the This infold intrudes the locule along the plane Bournemouth Freshwater Beds, which was figured by of bisymmetry,