First Fossil of Tylidae (Isopoda: Oniscidea) in Kachin Amber, Myanmar, with a List of All Oniscidea Fossil Records

: A fossil of Oniscidea, Tylidae gen. et sp. indet. from Kachin amber (Cretaceous Ceno-manian), Myanmar, is described here. The convex body, the cephalon with a triangular protrusion between the antennae, and pereonites 2–6 with epimera demarcated from tergites indicate that this specimen belongs to the family Tylidae, but since it is not an adult the identification of the genus and species is uncertain. This specimen has a convex body and shows an ability to conglobate, like all Tylidae. It is the first specimen of Oniscidea with a conglobation ability found in Burmese amber. Up to now, the fossil record of terrestrial isopods has included a total of 20 families and 54 records (36 species and 18 not formally identified species), 20% of which are from the Cretaceous period. These fossil records from the Cretaceous period show that terrestrial isopods were highly diversified as early as in the Cenomanian.


Introduction
Terrestrial isopods (Oniscidea), also known as woodlice or pill bugs, are the only isopods fully adapted to live on land, occurring in all terrestrial environments from coasts to high mountains, and from forests to deserts [1][2][3][4][5].The species of Oniscidea evolved to live on land directly from the ocean instead of using freshwater [6].Like other Peracarida, terrestrial isopods show direct development, involving a marsupial and a postmarsupial phase.After exiting the marsupium the individuals become the first postmarsupial mancas, similar to adults except for the reduced pereonite 7 and seventh pereopods, and the absence of secondary sexual characters [7].
As for many other invertebrate groups, the phylogeny of terrestrial isopods is not yet completely resolved and existing research based on morphological and molecular analyses tends to consider most of the species of Oniscidea as monophyletic, maybe with the exclusion of the genus Ligia Fabricius, 1798 [8][9][10][11][12][13][14][15][16].Oniscidea consist of five principal lineages, Diplocheta, Tylida, Microcheta, Synocheta, and Crinocheta [6,17], including almost 4000 species in more than 500 genera and 38 families [2,3,[18][19][20][21][22]. Diplocheta and Tylida represent the most basal clades, mainly including semi-terrestrial species [9].Most species of Diplocheta and Tylida occur in near-shore habitats and are considered the transitional stage of Isopoda conquering the land [5].Tylida (Tylidae) comprise only two genera: Helleria Ebner, 1868 and Tylos Audouin, 1826 [16].Helleria comprises only H. brevicornis Ebner, 1868, distributed in the woods of the North Tyrrhenian sea [17,22], whereas Tylos has a worldwide distribution, occurring on coastal supralittorals [23][24][25][26].Tylidae have a conglobation ability to protect themselves from predators and to reduce water loss [27,28].Tylos juveniles can allow the swash to transport them up the beach by rolling themselves into a ball; although adults have not been observed to show this behaviour, if caught by a wave they flatten themselves against the beach and allow the water to pass over them before moving upshore [24].
Regarding fossils, the earliest Oniscidea fossil record comes from the Cretaceous Albian (105 Ma), but some indications suggest that they could have already appeared on land in the Late Palaeozoic [14,29,30].Fossil records of Oniscidea are quite scarce and mostly limited to preservation in amber [30]; until recently, they were almost entirely restricted to the Cenozoic.To our knowledge, in the Mesozoic, amber with Oniscidea inclusions is only known from three localities: France, Spain and Myanmar [30].The formation of Kachin amber is relatively old (Cretaceous, 99 Ma) [31], in the period of intense orogeny in the Mesozoic [32].Dinosaurs were still in their heyday, and fish and birds were also highly developed [33][34][35].At that time, mammals and angiosperms had just appeared [36].The biodiversity in amber from Myanmar is very high, with a lot of published documents about insects and other terrestrial arthropods [37].To date, there are only two species in two genera of Oniscidea preserved in Burmese amber [38,39].The evolutionary history of the Oniscidea is of great significance to better understand the processes of land adaptations in crustaceans.Thus, Oniscidea may also contribute to the reconstruction of the ancient environment near the Early Cenomanian amber-producing forest in northern Myanmar.
In this paper, we describe the first fossil of Tylidae from Kachin amber, Myanmar, and provide an updated catalogue of all fossil species and records of the suborder Oniscidea.This work tries to give an insight into the earliest diversity and history of Oniscidea.

Materials and Methods
The amber specimen for our study was collected from Kachin (Hukawng Valley) in northern Myanmar before 2016, at the north end of Noije Bum, which is approximately at 26 • 15 ′ N, 96 • 34 ′ E and 18 km southwest of the town of Tanai [31].The age of Kachin amber is attributed to the Early Cenomanian (98.19 ± 0.62 Ma) [31].The specimen (NO.BXAM BA-ONI-001) is currently housed in the Institute of Zoology, Chinese Academy of Sciences (for ten years), and will eventually be deposited in the Beijing Xiachong Amber Museum, Beijing, China.
The specimen was examined and photographed under a Nikon SMZ25 microscope, coupled to a Nikon DS-Ri2 digital camera system.The line drawings were edited by Adobe Illustrator CC and Adobe Photoshop CC.
With the present paper, we also give a survey of all species of fossil Oniscidea described until today, including all valid names and taxa with uncertain or questionable taxonomic status.The families, genera and species are arranged strictly alphabetically, regardless of their systematic position.The catalogue is mainly based on Schmalfuss, (2003) [17], Schmidt, (2008) [6], and Broly, (2013) [30] Description: Maximum dimensions: 2.9 mm length × 1.7 mm width.Able to roll up into a ball; dorsal surface not distinctly granulated (Figure 1).
Cephalon (Figure 2A,B): triangular frontal process well developed; flat, semi-circular frontal shield above triangular frontal process; eyes consisting of several ommatidia.
Antennula (Figure 2A): short, composed of one or two elements.Antenna (Figures 1C and 2A-D): five peduncular articles plus flagellum of four articles.Fourth peduncular article longest; fifth peduncular article thickset.Flagellum with three subequal articles and minute terminal article.
Pereopods (Figure 2G,H): poorly discernible due to conglobating position of the animal.Dactylus and propodus with no setae; carpus longest, twice as long as propodus, covered with many setae, with three swellings on sternal margin, each with two setae; merus and ischium with few setae.
Telson (Figure 2F): sub-rectangular with widely rounded distal margin, about oneand-a-half times as wide as long; in dorsal view, fully covering uropods.
Remarks: The specimen certainly belongs to the family Tylidae, based on the following characteristics: (1) a well-developed frontal process; (2) pereonites with epimera not fused to tergites except pereonite 1; and (3) uropods in a completely ventral position, not visible in dorsal view [24,41].Even if this specimen is a manca with pereonite 7 not fully developed, it shows a clear difference from both Tylos and Helleria, i.e., the presence of a flat frontal shield above the frontal process.It differs from Helleria also in the pleon tergites not being fused and in the triangular shape of the frontal process.For the latter characteristic, this specimen seems similar to the genus Tylos.Due to the manca stage of this specimen, we refrained from establishing a new genus and species for it.More specimens with adult forms are needed to further determine its taxonomic status at genus and species level.

Catalogue of Oniscidea Fossil Records
The fossil record of Oniscidea comprises 36 species and 17 records with undefined taxon names, including the one new record described herein.Eleven fossil records from Spain, France and Myanmar come from the Mesozoic Era; thirty-nine from the Baltic, Dominican Republic, Ukraine, Kenya, India and Mexico are from the Cenozoic Era, and four have unknown ages (Table 1).

Catalogue of Oniscidea Fossil Records
The fossil record of Oniscidea comprises 36 species and 17 records with undefined taxon names, including the one new record described herein.Eleven fossil records from Spain, France and Myanmar come from the Mesozoic Era; thirty-nine from the Baltic, Dominican Republic, Ukraine, Kenya, India and Mexico are from the Cenozoic Era, and four have unknown ages (Table 1).Schmidt, 2008: 220 (comments) [6]; Broly et al., 2013: 464 (list) [30].

Discussion
The specimen described above is the second fossil record of the family Tylidae, the first of this family for Kachin amber (Late Cretaceous) [31].As all the other taxa in this family, this specimen is able to roll up into a ball [17,41].Conglobation ability is common, in mammals, such as in hedgehogs, armadillos, and pangolins; and in arthropods, such as pill millipedes (Glomerida) [89], beetles (Ceratocanthinae) [90], cockroaches (Blaberidae) [91], and pill bugs (Oniscidea).In terrestrial isopods, there are many groups (e.g., Armadillidae; Armadillidiidae) with the conglobation ability.The conglobation ability has appeared several times independently within the Oniscidea [40].
The fossil records of Oniscidea are rare.While the earliest fossil record of Oniscidea only dates back to the Early Cretaceous, based on phylogenetic studies and the palaeobiogeographic context of fossil specimens an origin of Oniscidea in the Late Paleozoic has been proposed [14,30].To date, there are 54 terrestrial isopod fossil records within 20 families [Table 1].The classification position of some groups is not clear due to the lack of detailed morphological characteristics [see list].Among them, the earliest fossil records are from Spanish amber (Cretaceous Albian, 105 Ma) [29,92].
Forty-one records of terrestrial isopods have been reported from the Cenozoic deposits of the Baltic, Dominican Republic, Ukraine, Kenya, India, and Mexico.All the major modern families of woodlice were already present and widely distributed geographically at that time.Twelve records of terrestrial isopods come from the Mesozoic (Cretaceous), mainly from Spain, France, and Myanmar [Table 1].It seems unlikely to find inclusions of oniscideans in amber older than Cretaceous because the amber-producing plants, although recorded from Carboniferous, did not produce enough sap to trap macroscopic invertebrates until the Late Jurassic [57].The fossils belong to four out of five major groups of Oniscidea: Diplocheta and Tylida, which are the earliest diverged clades, and Synocheta and Crinocheta, at the higher phylogenetic level [93].Among crustaceans, three major groups have invaded terrestrial environments: the peracarid orders Amphipoda.Isopoda, and Decapoda [94].In Isopoda, except for Oniscidea, species in other suborders either live in the ocean or in fresh water.Terrestrial isopods (Oniscidea) are the only isopods fully adapted to live on land (except a few species that have become secondarily adapted and returned to aquatic habitats [5]).It is widely believed that the invasion of terrestrial environments from the sea by isopods has involved a transitional step within the supralittoral, instead of using freshwater [95].
The two earliest diverged families, Ligiidae and Tylidae, successfully colonised the supralittoral [5,26,96].The evolution of terrestrial isopods from ancestral marine isopods was proposed to have proceeded from a Ligia-like ancestor [95].This statement has been recently questioned with molecular evidence [13,16] that showed Ligia to be related to marine isopods and not to the rest of Oniscidea.All the extant species of Tylos live in intertidal and supralittoral areas, and a large number of marine taxa are found in Burmese amber [37].These data confirm the paleogeographic reconstruction of the area with the Burmese amber indicating the influence of a marine environment [96].This could mean that the Tylidae also lived near the sea during the Cretaceous.

Conclusions
In this paper, we report the first fossil record of a terrestrial isopods with conglobation ability from amber of Kachin State, which is the second fossil record of Tylidae.To date, a total of 54 records of isopod fossils from 20 families have been reported.The fossils of Oniscidea are largely biased toward preservation in amber and, until recently, most terrestrial isopod fossils dated from the Cenozoic period.Due to the manca stage of the specimen described, its precise taxonomic position at genus and species level cannot be established.However, the morphological characteristics of the specimen show that it is undoubtedly a member of Tylidae, morphologically close to the genus Tylos.