The oldest short-tailed whipscorpion (Schizomida): A new genus and species from the Upper Cretaceous amber of northern Myanmar

Cretaceous Research 106 (2019) 104227

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The oldest short-tailed whipscorpion (Schizomida): A new genus and species from the Upper Cretaceous amber of northern Myanmar € rg U. Hammel c, d, Sandro P. Müller a, Jason A. Dunlop b, *, Ulrich Kotthoff a, Jo Danilo Harms e a

University of Hamburg, Center of Natural History-Geological-Paleontological Museum and Institute for Geology, Bundesstrasse 55, D-20146, Hamburg, Germany Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, Invalidenstraße 43, D-10115, Berlin, Germany c Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Straße 1, D-21502, Geesthacht, Germany d €t Jena, Institut für Zoologie und Evolutionsforschung Mit Phyletischem Museum, Ernst-Hackel-Haus und Biologiedidaktik, Friedrich-Schiller-Universita Erbertstr 1, D-07743, Jena, Germany e University of Hamburg, Center of Natural History e Zoological Museum, Martin-Luther-King-Platz 3, D-20146, Hamburg, Germany b

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 31 August 2019

Arachnids are an ancient lineage of arthropods and orders such as scorpions, harvestmen and mites have their fossil origins in the Silurian or Devonian. Another order with potentially old origins is the shorttailed whipscorpions, or schizomids (Arachnida: Schizomida). These animals have a fragmentary fossil record with species either described or documented from Dominican amber (Miocene: Chattian), drill core sediments from the Oligocene in China, and the upper Pliocene (Zanclean) of Arizona. Here, we describe the first named example of a short-tailed whipscorpion from Upper Cretaceous amber (Myanmar: Hukawng Valley) based on male morphological features. Mesozomus groehni gen. et sp. n. cannot be assigned to any Recent genera of schizomids and preserves a unique mix of plesiomorphic (e.g. the retention of eyes) and apomorphic characters (e.g. enlarged femur IV) that merit future evaluation within a phylogenetic context. We extend the fossil record of schizomids by ca. 65 million years, from the Paleogene to the Mesozoic, and add the twelfth order of arachnids to the diverse arachnid biota documented from Burmese amber. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Arachnida Schizomida Amber Cenomanian Myanmar

1. Introduction Schizomids (Arachnida: Schizomida), sometimes referred to as short-tailed whipscorpions, are an order of small to medium-sized arachnids which are evidently closely related to the larger whipscorpions (Thelyphonida). Schizomids are one of the less diverse arachnid orders, currently with about 350 living species (Harvey, 2013; Monjaraz-Ruedas and Francke, 2016) in two families, plus a few descriptions of fairly young fossils ranging from Oligocene to Pliocene in age (see 1.1.). Extant schizomids are found throughout the world in warmer climates and are occasionally reported from European botanical gardens or similar localities (summarised by Armas and Rehfeld, 2015: Table 1) where they were presumably

* Corresponding author. Fax: þ49 30 889140 8868. E-mail addresses: [email protected] (S.P. Müller), [email protected] (J.A. Dunlop), [email protected] (U. Kotthoff), joerg. [email protected] (J.U. Hammel), [email protected] (D. Harms). https://doi.org/10.1016/j.cretres.2019.104227 0195-6671/© 2019 Elsevier Ltd. All rights reserved.

imported with tropical plants and/or soil. Schizomids are fastmoving, predatory arachnids which can be found in leaf litter, top soil or cave ecosystems. Their biology has not been investigated in detail, although useful data can be found in Kraus and Beck (1967), Rowland (1972) and Humphreys et al. (1989). The classic study of Hansen and Sørensen (1905) remains a valuable guide to schizomid morphology, while Manzanilla et al. (2016) categorised setation patterns. Adis et al. (1999) offered ecological data on Amazonian species, and Oliveira and Ferreira (2014) studied their behaviour and activity rhythms. Some schizomids have become important models in conservation research because many species have very small distribution ranges and react sensitively to habitat disturbance, or have specific ecological requirements such as a dependence on high moisture levels (Harvey et al., 2011; Harms et al., 2018). Schizomids are also increasingly used in biogeographical research because the order shows interesting patterns both at continental and more local scales (Manzanilla et al., 2016; Clouse et al., 2017). Many species are

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2. Material and methods

Table 1 Length of individual limb articles in millimetres.

Trochanter Femur Patella Tibia Metatarsus Tarsus Total

Pedipalp

Leg I

Leg II

Leg III

Leg IV

0.42 0.40 0.34 0.28 e 0.20 1.64

0.24 1.02 >0.77 missing missing missing >2.03

0.15 0.60 0.29 0.41 0.34 0.27 2.06

0.16 0.44 0.26 0.32 0.38 0.28 1.84

0.21 0.95 0.38 0.58 0.47 0.40 2.99

local cave-endemics, and in some areas of the world such as Western Australia these animals have been used to investigate aspects of subterranean speciation, adaptation and biogeography (Harvey et al., 2008; Pinto-da-Rocha et al., 2016; Framenau et al., 2018). 1.1. Fossil schizomids The first fossil schizomid to be described was Calcitro fisheri Petrunkevitch, 1945 from the Pliocene (Zanclean, ca. 1.8e5.3 Ma) Onyx Marble of Arizona, USA. As well as being referred to an extinct genus, it was also placed in its own family, Calcitronidae, the validity of which is dubious (see 3.). Two more Onyx Marble fossils were described by Pierce (1950, 1951), again in extinct genera, as Onychothelyphonus bonneri Pierce, 1950 and Calcoschizomus latisternum Pierce, 1951. Of these, O. bonneri was initially treated provisionally as a whip scorpion (Thelyphonida: Thelyphonidae), while C. latisternum was placed in the schizomid family Schizomidae; now a synonym of Hubbardidae (cf. Harvey, 2003). Recently, O. bonneri was transferred to another family, Protoschizomidae, by Monjaraz-Ruedas et al. (2017) and may even fall within the extant genus Protoschizomus Rowland, 1975. As noted by Monjaraz-Ruedas et al. (2017), this raises the possibility that Onychothelyphonus Pierce, 1950 could become the senior synonym of the living genus. The next fossil to be reported was Calcitro? oplonis Lin, 1988 described in Lin et al. (1988) from the Oligocene (ca. 23e34 Ma) oil shales of the Gubei District in Shandong, China and tentatively referred to Petrunkevitch's (1945) genus. While it could represent a female schizomid with a short but unmodified flagellum, it is quite large (body length 6 mm) and we cannot exclude the possibility that it is a small whipscorpion (Thelyphonida) with a damaged flagellum. In any case, based on the published illustration of the ventral surface it is difficult to compare it meaningfully to other fossil or living species. Two unequivocal schizomids have been described from Miocene (ca. 16 Ma) Dominican Republic amber by Krüger and Dunlop (2010). They were subsequently reassigned to different genera by Armas and Teruel (2011) yielding the combinations Antillostenochrus pseudoannulatus (Krüger & Dunlop, 2010) and Rowlandius velteni (Krüger & Dunlop, 2010). Both species belong to modern Neotropical genera and do not show any character states that derivate significantly from the extant fauna in this region. The stratigraphically oldest fossil schizomids come from midCretaceous (ca. 100 Ma) Burmese amber. They were initially figured as “Schizomida indet.” by Wunderlich (2015: photo 176) and as unidentified schizomids by Xia et al. (2015: p. 175). We are also aware of several more specimens both in public (Selden and Ren, 2017: fig. 10A) and private collections, although none of this material has been named or assigned to a (sub)family. Here, we take the opportunity to formally describe the first Burmese amber schizomid which we refer to a new, extinct genus within the family Hubbardiidae e which contains about 95% of the living species e and its subfamily Hubbardinae.

The holotype and only known specimen (Fig. 1) originates from €hn (Glinde, Germany), specthe private collection of Carsten Gro imen no. 11212, and has now been deposited in the GeologicalPaleontological Museum of the University of Hamburg, Germany under the repository number GPIH4986. The amber piece was coated with enamel for protection and imaged immersed under Penaten baby oil to improve the refraction index. For photography a BK Plus Lab System by Dun, Inc. with an integrated Canon camera with 5x, 10x and 20x micro lenses was used, as well as a photo microscope Leica M205 A. Images were taken using the software Capture One (Dun System) and Leica Application Suite X (photo microscope). For image stacking, Zerene stacker version 1.04 was used. The specimen was also examined under a Leica M205 A stereomicroscope from which photographs of relevant structures to be illustrated were taken. These photographs were used as templates for some of the drawings, while habitus drawings were made using a stereomicroscope with a camera lucida attachment. Images were edited with Photoshop CC 2017. A micro-computer tomography scan was also available (Fig. 2), made using a Phoenix X-ray nanotome S, GE®. The resulting dataset was edited with the software Amira 6.0.1 (Thermo Fischer Scientific) to generate a 3D model (voxel size z 3.4 mm). The fossil was compared to published descriptions of schizomids (see below for details), as well as extant material in the collections of the Museum für Naturkunde Berlin. The print (Harvey, 2003) and online (Harvey, 2013) schizomid catalogues for the order provided data about the systematics and distribution of both fossil and Recent schizomids, along with the older catalogue by Reddell and Cokendolpher (1995). All measurements are in millimetres. 2.1. Burmese amber Burmese amber has emerged in recent years as one of the most important localities for fossil arachnids (reviewed by Selden and Ren, 2017) and other terrestrial arthropods. It is one of the few species-rich fossil sites for arthropods in Southeast Asia and hosts representatives of all living arachnid orders. The fossils originate from amber mines in the Hukawng Valley in northern Myanmar and are usually dated to the Late Cretaceous (earliest Cenomanian; ca. 99 Ma) following authors such as Shi et al. (2012). A recent study by Smith and Ross (2018) on bivalve borings in the amber suggested that it was buried relatively soon after deposition, such that an age estimate of ca. 100 Ma seems reasonable. General accounts of the amber and its geological setting can be found in Zherikhin and Ross (2000), Grimaldi et al. (2002) and Ross et al. (2010). For a full list of Burmese amber inclusions see Ross (2018) and for a discussion of biogeography see 4.2. 3. Systematic palaeontology Order Schizomida Petrunkevitch, 1945 Family Hubbardiidae Cook, 1899. Subfamily Hubbardiinae Cook, 1899 Remarks. Three schizomid families are currently recognised: Hubbardiidae, Protoschizomidae and the extinct Calcitronidae. A problem with Calcitronidae is that its diagnosis, to quote Reddell and Cokendolpher (1995), “…does not include characters useful in comparing this with other families.” The original description by Petrunkevitch (1945) includes a 7:5:4:4 tarsal formula (i.e. the number of tarsomeres making up the tarsi of legs IeIV) which is different from the (?:3:3:3) tarsal formula in our new amber specimen. At the same time, Petrunkevitch's diagnostic character is

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€hn collection no. 11212) of Mesozomus groehni gen. et sp. nov. (Arachnida: Schizomida: Hubbardiidae) from the mid-Cretaceous (Cenomanian) Fig. 1. Holotype (GPIH4986, ex Gro Burmese amber of Myanmar. A. Dorsal overview. B. ventral overview. CeD. Camera lucida drawings of the same. Legs numbered from IeIV. Scale bar equals 1 mm.

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not part of the expected schizomid ground pattern, in which tarsi IIeIV should invariably have only three tarsomeres. The status and validity of Calcitronidae thus merits revision, but for the present we can exclude this problematic fossil family from consideration. Harvey (2003) offered several characters which distinguish between the two remaining families. Protoschizomids are usually quite large, typically ca. 7e12 mm, except for Agastoschizomus texanus Monjaraz-Ruedas et al., 2016 which is a little over 3 mm. The new amber fossil is only 2.65 mm long (Fig. 1). The length ratio of the pedipalp claw to the metatarsus plus tarsus in protoschizomids is 0.6e1.3, whereas in the new fossil (Fig. 3EeF) the ratio is slightly below 0.5. In protoschizomids the fixed finger of the chelicerae bears only two large teeth, whereas in the fossil there are two smaller teeth between the two larger outer teeth (Fig. 3D). Less helpful is the fact that in protoschizomids the tarsal spurs are symmetrical, the cheliceral serrula are not composed of hyaline teeth and the female flagellum lacks segmentation (Harvey, 2003). In hubbardiids the tarsal spurs are (when present) asymmetrical, there is a serrula with hyaline teeth and a segmented female flagellum; however, none of these three characters can be tested in the new (male) fossil. An alternative character for excluding protoschizomids is the robust femur IV in the new amber specimen (Fig. 4AeB). In protoschizomids this limb article is usually slender (e.g. Reddell and Cokendolpher, 1995; MonjarazRuedas et al., 2017). Finally, protoschizomids are restricted today to Mexico and the southern USA where they primarily occur in cave ecosystems. The amber specimen from Myanmar is very distant from the Americas e even on a mid-Cretaceous palaeogeographic map e making a close relationship to Protoschizomidae intuitively unlikely. We should, however, note that Burmese amber also includes ricinuleid arachnids (Wunderlich, 2015) which are today only found in the Americas and West Africa. On balance, for the reasons outlined above, we assign the new fossil to Hubbardiidae. Within this family, the subfamily Megaschizominae Rowland, 1973 was erected for two species described by Lawrence (1969) from South Africa and Mozambique. As the name implies, they are large schizomids (body length ca. 8 mm). They were diagnosed by, e.g., Reddell and Cokendolpher (1995) as having a row of 8e9 setae on the base of the anterior process of the propeltidium, distinct eye-spots present, a claw/metatarsusetarsus ratio of 0.5, three teeth (one small tooth between two larger outer teeth) on the fixed finger of the chelicerae and a male flagellum with soft eversible areas and many small pores. The new fossil better fits the diagnosis of the subfamily Hubbardiinae Cook, 1899 as is smaller in body length, lacks this distinctive row of setae at the base of the anterior process (Fig. 3B), bears corneate eyes (Fig. 3AeB), has four teeth (two smaller ones between two larger outer ones) on the fixed finger of the chelicerae and no evidence for eversible areas/ pores on the male flagellum (Fig. 4CeD). Genus Mesozomus gen. nov Type species. Mesozomus groehni gen. et sp. nov., designated herein. Etymology. The generic name is derived from the Mesozoic geological era combined with Zomus: the living genus the fossil seems most closely to resemble. Diagnosis. Anterior process of propeltidium with a 1 þ 2 setal arrangement; propeltidium probably with 1:2:2:2 pattern. Corneate eyes present. Metapeltidium divided by a thin suture. Fixed finger of chelicera with two small teeth between two larger outer teeth. Opisthosomal segments XeXII not elongated; XII without posterodorsal process. Male flagellum dorsoventrally flattened, ca. twice as long as wide, oblong. Leg femur IV robust, approximately 2.1 times longer than wide; anterodorsal margin of femur IV produced at an angle of about 90 .

Remarks. In assessing the generic affinities of the amber fossil we are faced with two problems. Some extant genera, such as the ubiquitous Schizomus Cook, 1899, are not very clearly delimited even among the extant material (see e.g. comments in Reddell and Cokendolpher, 1995) and older descriptions may not mention the full suite of characters used in modern taxonomy. Second, several characters routinely used to describe living species, such as the internal female genitalia or details of setation in the mouthparts, are either unavailable for the fossil or difficult to resolve unequivocally, even with the application of modern methods such as computed tomography (Fig. 2). Bearing these problems in mind, our initial approach was to compare the Burmese amber material with genera known primarily from the IndoePacific region, with a particular focus on genera whose living members include species with corneate eyes, a large femur IV, and the peculiar arrangement of setae on the propeltidium. A caveat here is that outgroup comparison (e.g. whipscorpions or whipspiders) suggests that retention of lateral eyes is plesiomorphic for schizomids, thus we must be cautious about using the presence of corneate eyes as the primary diagnostic character. Corneate eyes have so far only been recorded from four extant genera/species: Javazomus oculatus (Cokendolpher & Sites, 1988) from Indonesia, Neozomus tikaderi Cokendolpher et al., 1988 from India, Oculozomus biocellatus Sissom, 1980, from Indonesia and Zomus bagnallii (Jackson, 1908) from various localities in Southeeast Asia through to Fiji, with several introductions into Europe. The Burmese amber fossil differs from J. oculatus which is larger, has a 2 þ 1 setation on the anterior process of the propeltidium (1 þ 2 in the fossil, Fig. 3B), an entire metapeltidium (divided in the fossil, Fig. 1C), opisthosomal segments XeXII elongated (not elongate in the fossil, Fig. 1), four small teeth on the fixed finger of the chelicerae (two in the fossil, Fig. 3C) and a narrower femur IV (broad in the fossil, Fig. 4A). The amber inclusion differs from N. tikaderi which again has a 2 þ 1 setation on the anterior process, an entire metapeltidium, opisthosomal segments XeXII elongated, and in this species five small teeth on the fixed finger of the chelicerae and a bulbous, strongly convex male flagellum. However N. tikaderi does share a broad femur IV with the amber specimen. The fossil differs from O. biocellatus which has a larger body size, a 2 þ 1 setation on the anterior process, and a strongly convex flagellum with two dorsolateral elevations; other comparative characters are equivocal based on the original description. Of the living eyed schizomids, the new fossil appears closest to Zomus bagnallii, a widespread species in Southeast Asia that has also been introduced to Europe (Harvey, 2013), and was described and illustrated by Sissom (1980), Reddell and Cokendolpher (1995) who also recognised and named the monotypic genus, Harvey (2001), and Manzanilla (2010). The fossil shares with this species not only the corneate eyes, but also the fixed finger of the chelicerae bearing two small teeth between the larger teeth and a broad leg IV femur which is about twice as long as wide. However, there are still differences compared to Z. bagnallii such as a 2 þ 1 setation pattern on the anterior process and four or five pairs of dorsal setae on the propeltidium; in the fossil there is a medial seta followed by a series of paired setae. In standard notation this would be (2 þ 1:2:2:2:2) for Z. bagnallii versus (1 þ 2:1:2:2:2) in the amber fossil. It is also worth remarking that our fossil is male and that only females of Z. bagnallii have been discovered (Manzanilla, 2010), leading the suspicion that the living species could be parthenogenetic (Harvey, 2001). There are also surprising differences in the sizes published for the species, ranging from 2.8 mm (Harvey, 2001) to 11.6 mm (Manzanilla, 2010). As noted above, retention of eyes may be plesiomorphic, thus we expanded our comparisons to eyeless genera which share at least some of the fossil's characters. Setation of the anterior

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Fig. 2. Volume rendering of m-CT data of the specimen shown in Fig. 1. A. Dorsal view. B. Ventral view. Scale bar equals 1 mm.

process of the propeltidium is an important feature (Reddell and Cokendolpher, 1995) which broadly separates New World and African schizomids (usually with a 1 þ 1 pattern) from IndoePacific schizomids (usually with a 2 þ 1) pattern; a distinction consistent with the latest molecular results (see 4.2). The inferred 1 þ 2 pattern in the amber fossil is thus unusual and otherwise only seen in Artacarus liberiensis Cook, 1899 from Liberia, two species of the African genus Lawrencezomus Armas, 2014 as well as Gravelyzomus chalakudicus (Bastawade, 2002) from India and e the species geographically closest to the source of the fossil e Burmezomus cavernicola (Graverly, 1912) from modern Myanmar. Of these, Artacarus Cook, 1899 lacks a detailed modern description for comparison, while Lawrencezomus differs in having a dorsal process on opisthosomal segment XII (absent in the fossil, Fig. 1). G. chalakudicus also has the broad femur IV and two small cheliceral teeth, but differs in a larger body size, absence of eyes or even eyespots, no mesal spur on the pedipalp trochanter (present in the fossil, Fig. 3EeF), a large palpal claw and opisthosomal segments XeXII elongate (unmodified in the fossil, Fig. 1) (e.g. Bastawade, 2002; Kulkarni, 2012). B. cavernicola differs from the new fossil again by a larger body size, five small

cheliceral teeth, an entire metapeltidium (divided in the fossil, Fig. 1), and opisthosomal segments XeXII elongate. Two small cheliceral teeth are seen in several genera, e.g. Apozomus Harvey, 1992 from Australia, Gravelyzomus Kulkarni, 2012 from India, Burmezomus Bastawade, 2004 from Myanmar, Lawrencezomus from Africa, Caribezomus Armas, 2011 from the Caribbean, and Adisomus Cokendolpher and Reddell, 2000, Calima z and Villarreal Manzanilla, 2012 and Naderiore Moreno-Gonzale Pinto-da-Rocha et al., 2016 all from South America. A broad femur IV is seen in several genera such as Adisomus and Calima (South America), Caribezomus (Central America), Orientzomus Cokendolpher and Tsurasaki, 1994 (Micronesia), and Mahezomus Harvey, 2001 and Secozomus Hansen in Hansen and Sørensen, 1905 (Seychelles). Femur size/shape may, in isolation, be of limited systematic value. In general, somatic characters in the genera listed above do not suggest particularly close affinities to the present fossil. Perhaps the closest match would be Mahezomus apicoporus Harvey, 2001, which is small (3.45 mm), has 12 þ 2 setae on the anterior sternum, distinct eye-spots (but not cornetae eyes) and lacks tarsal spurs (Harvey, 2001), which are either absent or equivocal in the amber fossil. However, M. apicoporus has four small

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Fig. 3. Details of the anterior prosomal region. A. Propeltidium largely in dorsal view, including the anterior projection with a 1 þ 2 setal arrangement and corneate lateral eyes (arrowed). B. Interpretative drawing of the same. C. Chelicerae largely in lateral view, showing two larger teeth on the fixed finger interspersed by two smaller teeth. D. Interpretative drawing of the same. E. Pedipalp, including the mesal spur on the trochanter (arrowed). F. Interpretative drawing of the same. Scale bars equal 0.5 mm.

cheliceral teeth and a claw/metatarsusetarsus ratio of 0.65 (shorter in the fossil). The male is still unknown. In conclusion, we were unable to find an exact match between the characters preserved in the amber fossil and any living hubbardiid genus. Zomus Reddell and Cokendolpher, 1995 perhaps

comes closest based on the corneate eyes, the fixed finger of the chelicerae bearing two small teeth and a broad leg IV femur, but differences in the taxonomically significant setation pattern on anterior process of the fossil suggest that our species belongs to a new genus that has no obvious modern relatives.

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Fig. 4. Details of the fourth leg in lateral view with its large and robust femur (ca. twice as long as wide) and the flagellum at the posterior end of the opisthosoma which is generally flattened, but has a proximal elevation. A. Leg IV. B. Interpretative drawing of the same. C. Flagellum. D. Interpretative drawing of the same showing proposed setal notation sensu Manzanilla et al. (2016). Scale bar equals 0.5 mm (A) and 0.25 mm (C).

Mesozomus groehni gen. et sp. nov €hn collection no. 11212. Holotype. Adult male, GPIH4986, ex Gro Type-locality. Burmese amber, Hukawng Valley, Myanmar. Upper Cretaceous (lowermost Cenomanian). €hn, who Etymology. The species name acknowledges Carsten Gro kindly made the holotype available for study. Diagnosis. As for the genus Description. Almost complete adult male (Fig. 1). Colour in amber yellow to brownish red, except propeltidium and abdominal tergites IVeVIII with silvery shimmer. Total length from anterior margin of propeltidium to base of flagellum 2.65. Prosoma length 1.12; length

of propeltidium 0.85, width 0.42. Remarkably long (0.09) anterodorsal process of propeltidium, slightly bent downwards bearing two visible setae. Posterior seta not directly in a line with anterior one, thus a 1 þ 2 setation pattern (with the right seta of the posterior pair having become lost) can be inferred (Fig. 3B). Further setation of propeltidium: 1:1:1:1. Except first of these setae, which is positioned medially, setae not in any particular pattern and (similar to setae above) may originally have been paired too (Fig. 3B) and thus 1:2:2:2 in life. Corneate eyes present (Fig. 3AeB), left one elevated above surface of propeltidium. Mesopeltidia form two sclerites, left one 0.17 wide, right one not visible. Metapeltidium divided by thin suture, each plate 0.23 wide (Fig. 1C). Ventrally, anterior sternum slightly

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longer than its greatest width (Fig. 1D), with 11 þ 2 sternapophysal setae. Metasternum bears six setae. Fixed finger of chelicera with two small teeth between two larger outer teeth (Fig. 3D). Movable finger without accessory tooth, serrula present; guard tooth equivocal (not visible if originally present.) Setation on mesal side of chelicerae largely equivocal, but G6-1 setae present (Fig. 3D) sensu Manzanilla et al. (2016). Total length of pedipalp (without claw): 1.64, ca. 0.62 times body length; lengths of individual articles in Table 1. Pedipalps (Fig. 3EeF) with trochanter produced anteriorly and mesal spur produced at an angle of ca. 90 . Setation consists of seven strong, long ventral setae (last of which lies on mesal spur), and only one epical seta; although some setae probably missing. Femur shorter than trochanter. Setation consists of row of nine dorsal setae: 6th, 8th and 9th lost, 4th, 5th and 7th truncated, at least two long setae on mesal surface, three ventroectal. Patella shorter than femur. Setation consists of five dorsomesalemesal, two long dorsal, three ectal and two ventroectal. Tibia shorter than patella. Setation consist of six dorsal, three dorsoectal, threeefour ectal and three ventroectal; mesal setation not visible. Metatarsusetarsus approximately half length of femur. Setation consist of five ventroectal, four ectal and 4e6 dorsoectal. Two slightly larger, curved setae at the position of tarsal spurs (Fig. 3F); tarsal spurs themselves equivocal. Claw (pretarsus) slightly shorter than half the length of metatarsusetarsus. Leg formula from longest to shortest [1?]423; lengths of individual articles in Table 1. First pair of legs slender, but truncated at the patella (left) or femur (right). Leg II coxae with long spur, length 0.13. Leg IV slightly (1.17 times) longer than the body. Femur IV robust (Fig. 4AeB), ca. 2.1 times longer than its greatest depth. Tarsi of legs IIeIV all with three tarsomeres. Opisthosoma length 1.53, with noticeable (taphonomic) lateral compression in this region (Fig. 1C) where the tergal cuticle still reaches the opisthosomal edge. Width measured at 0.46, but probably closer to 0.66 in life. Opisthosomal segments XeXII not elongated (Fig. 1), XII without a posterodorsal process. Setation patterns of tergites IeIII equivocal. Tergite IV with pair of dorsal setae (right one probably missing). Tergite V with pair of dorsal setae. Tergite VI with pair of dorsal setae (right one missing). Tergite VII with pair of dorsal setae and pair of dorsolateral setae (right one lost). Tergite VIII with pair of dorsolateral setae (right one lost) and pair of lateral setae. Tergite IX with pair of long ventrolateral setae, and pair of dorsal or dorsolateral setae. Last three segments (XeXII) ring-like, forming postabdomen. Segment X with pair of short lateral setae and pair of long ventrolateral setae. Segment XI with pair of small dorsolateral setae and pair of long ventrolateral setae. Segment XII with pair of short lateral, pair of long lateral, pair of small dorsolateral, and pair of large, strong dorsal setae positioned in line with the stalk of the flagellum (right one missing). Ventral setation equivocal. Flagellum modified (Fig. 4CeD), as is typical for mature males, length 0.31, width 0.16, dorsoventrally flattened with one proximal elevation, oblong with rounded tip. Stalk noticeably long (0.13), making up nearly half the total flagellum length. Setation pattern (Fig. 4D) sensu Manzanilla et al. (2016): Dm1, Dm4, Vm1, Vm2 paired, Vm4, Vm5, Dl3 paired, the right one truncated, Vl1, the right one lost, Vl2 paired, two areas can be identified as having probably originally borne microsetae. 4. Discussion Most arachnid orders have a fossil record extending into the Palaeozoic (Dunlop, 2010) with, for example, scorpions going back as

far as the Silurian, harvestmen and pseudoscorpions in the Devonian and spiders in the Carboniferous. Exceptions to this Palaeozoic record are parasitiform mites, palpigrades and schizomids. Among the parasitiforms, ticks have been known for some time from Cretaceous ambers while both palpigrades (Engel et al., 2016) and schizomids (Wunderlich, 2015; Xia et al., 2015; this study) have now been found in Cretaceous Burmese amber. This extends the known fossil record of schizomids by more than 65 million years, although there are still significant gaps in the fossil record that remain to be filled. Our data offers a minimum age constraint of ca. 100 Ma for the origins of Schizomida, although the lineage is almost certainly much older (see 4.1.) with the relatively small size and weakly sclerotized bodies of schizomids reducing their potential for preservation as fossils. In this context there is a Carboniferous (ca. 315 Ma) whip scorpion, Proschizomus petrunkevitchi Dunlop & Horrocks,1996, from the British Middle Coal Measures which appears to show some trends towards the schizomid condition such as loss of median eyes (Dunlop and Horrocks, 1996; Tetlie and Dunlop, 2008) and may thus represent the sistergroup of crown-group Schizomida (Clouse et al., 2017). If schizomids are essentially miniature whip scorpions, these late Carboniferous fossils could represent a lineage from which the modern schizomids developed and be attributed to the Schizomida stem-lineage. In retaining eyes, Mesozomus groehni gen. et sp. nov. expresses a potentially plesiomorphic character. It should also be stressed that, as the oldest formally described schizomid, it shows typical schizomid features such as the tripartite carapace and subraptorial pedipalps as well as the most convincing autapomorphy of Schizomida, namely a modified male flagellum with a setation pattern (Fig. 4D) directly comparable to that of the Recent fauna (Manzanilla et al., 2016). In this sense or fossil is another example of evolutionary stasis, with a quite specific morphology conserved over deep time. 4.1. Molecular dating An alternative approach to assessing schizomid origins is to look at molecular data. Clouse et al. (2017) estimated that schizomids split from whip scorpions at ca. 333.0 ± 17.9 Ma, which is consistent with presence of late Carboniferous whip scorpions and the hypothesis that P. petrunkevitchi could be a stem-schizomid. In the Clouse et al. (2017, fig. 4) hypothesis crown-group schizomids, i.e. the Protoschizomidae/Hubbardiidae split, date to the Permian (ca. 270.0 ± 31.2 Ma). Of particular interest is the fact that they also recovered a large IndoePacific clade, which was estimated to have originated in the early to mid-Cretaceous (ca. 110 ± 10 Ma). In other words, Burmese amber fossils fall within the same time frame, and potentially the same geographical area (see 4.2.), as this inferred radiation in the Asian-Pacific region. These observations would be compatible with the Clouse et al. (2017) hypothesis that schizomids evolved in the Americas and later spread across the globe, perhaps during the mid-Cretaceous. The new amber fossil also offers a constraint/calibration for the family Hubbardiidae and its subfamily Hubbardiinae. It is, however, less useful for constraining any of the modern genera. As discussed above, Mesozomus groehni gen. et sp. nov. does not fit comfortably into any of the known living genera and is referred here to a new, extinct taxon. Although it shows similarities with animals like the modern IndoePacific genus Zomus, there is no comprehensive morphological phylogeny into which we can integrate the new fossil in order to resolve its living sister-group. We should also reiterate that there are several more unpublished Burmese amber schizomids, including fossils without corneate eyes and with a different shape to the leg IV femur and the flagellum (pers. obs.) and that the taxonomy for the extant genera is rather chaotic and requires revision. We suspect that further genera/species will be

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9

However, since India first came into contact with the rest of Asia in the Eocene, and could have transported African faunal elements with it, we should be cautious about inferring any relationships between fossils from Myanmar and modern Indian species. 5. Concluding remarks

Fig. 5. Palaeomap reconstruction for the Turonian (ca. 90 Ma) showing the likely position of the Burmese amber forest which, in at least some scenarios, should already have been connected to Eurasia by the Cretaceous. After Blakey [Blakey palaeomaps now available at: https://deeptimemaps.com/] and Google Earth Pro (2018).

Mesozomus groehni gen. et sp. nov. is the first record of Schizomida from Burmese amber to be formally described and named, and represents at the same time the oldest known schizomid species. It confirms that this arachnid order was present in the IndoePacific by the mid-Cretaceous and is consistent with molecular data predicting a radiation of this group in this region at about this time. The preserved character combination is notable for including the retention of corneate eyes, but does not precisely match diagnoses of modern genera. This suggests that at least one extinct schizomid lineage was present in the amber forests of what is now modern Myanmar. Acknowledgements

identified in due course, and these should contribute to a fuller picture of the faunal composition and diversity of schizomids in the Burmese amber forest.

€hn for access to his collections, Paul SelWe thank Carsten Gro den for useful discussions and two anonymous reviewers for helpful comments on the typescript.

4.2. Biogeography

References

Selden and Ren (2017, and references therein) summarised two hypotheses about the geographical position of the West Burma terrane at the time of amber deposition. In the first hypothesis, this terrane rafted off from Australia during the Jurassic, colliding with Eurasia at ca. 80 Ma. In other words, in this scenario the amber forest would have been on an island in the mid-Cretaceous and could have transported Gondwanan faunal elements with it which were originally present in Australia. A second, perhaps now better supported, hypothesis has the West Burma terrane separating in the Devonian, i.e. before the estimated origination dates of Schizomida (see 4.1.), and colliding with Eurasia (Fig. 5) during the Jurassic. In this scenario the Cretaceous amber forest could have been colonised by Eurasian faunal elements. Clouse et al. (2017) concluded that while the IndoeMalaya region the Pacific Islands and Australia host considerable numbers of schizomids today, ancestral area reconstruction suggests that these are not the areas where this arachnid order originated but, instead, were colonised from what is now the Americas. This would tend to contradict the first biogeographical scenario, in which Burmese amber was on an island which split off from Australia in the Jurassic. Bearing in mind the Coal Measures whip scorpions, Clouse et al. (2017: fig. 6) suggested that the origins of whip scorpions and schizomids should be sought in the tropics of Pangea during the Carboniferous. These regions now correspond to North America and Europe, the latter without native schizomids. The first branches of the schizomid molecular tree, including Protoschizomidae and some Hubbardiidae, are invariably Neotropical (Clouse et al., 2017: fig. 4) and the next node recovered in their phylogeny was a fairly fundamental (Cretaceous?) split of the remaining hubbardiidids into an AmericaneAfrican and an IndoePacific clade. Precisely how and when the ancestors of the IndoePacific clade reached Southeast Asia is a topic for future research, but we suspect that Mesozomus groehni gen. et sp. nov. was part of this IndoePacific radiation e and potentially offers an explicit calibration point for the clade. It is also interesting to note that the unusual 2 þ 1 setation pattern on the anterior process of the fossil is seen in most, but not all (see Neozomus tikaderi above), of the Indian schizomids today.

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