A Miocene pyrgodesmid millipede (Polydesmida: Pyrgodesmidae) from Mexico

A new fossil species of pyrgodesmid millipede (Polydesmida: Pyrgodesmidae) placed in the genus Myrmecodesmus Silvestri, 1910 is described. The type materials are two amber inclusions, male and female specimens that come from Miocene strata in Chiapas, Mexico. Myrmecodesmus antiquus sp. nov. has collum with 10 dorsal tubercles; without porosteles or ozopores; legs of the rings 2–9 with a short projection on the prefemur in both the female and male. Myrmecodesmus antiquus sp. nov is the first fossil record of the genus Myrmecodesmus. This is a New World taxon that belongs to the pantropical family Pyrgodesmidae. Thus, Myrmecodesmus antiquus sp. nov expands the range of the genus to the Miocene tropics in Middle America.


INTRODUCTION
The polydesmid millipedes of the family Pyrgodesmidae currently show a Pantropical distribution (Enghoff et al., 2015). However, the fossil record is limited to amber inclusions from Miocene deposits of the Dominican Republic and Mexico (Shear, 1981;Santiago-Blay & Poinar, 1992;Riquelme & Hernández-Patricio, 2018). Fossil specimens of Dominican amber have been assigned to the genera Docodesmus Cook, 1896a, Iomus Cook, 1911, Lophodesmus Pocock, 1894and Psochodesmus Cook, 1896b(Shear, 1981Santiago-Blay & Poinar, 1992), and the only fossil specimen described at the species level that is known so far is Docodesmus brodzinskyi Shear, 1981. Another fossil pyrgodesmid was found in Chiapas amber, Mexico, a female specimen identified as CPAL.117 by Riquelme & Hernández-Patricio (2018), which was initially included as a new member of the genus Myrmecodesmus within Pyrgodesmidae. In the present contribution, based on the female CPAL.117 plus another male specimen identified as CPAL.132, a new species of the genus Myrmecodesmus is now described. Below are descriptions, illustrations, and a discussion of related taxa.

GEOLOGICAL SETTING
The fossil specimens CPAL.117 and CPAL.132 come from the lignite-sandstones beds exposed in a site called Los Pocitos in the town of Simojovel, located approximately 122 km by road from the city of Tuxtla, Chiapas, southwestern Mexico. The amber-bearing beds of Simojovel are generally assigned to the Mazantic and Balumtum strata from early to mid-Miocene (Perriliat, Vega & Coutiño, 2010;Durán-Ruíz et al., 2013;Riquelme et al., 2013). Another outcrop exposed near Los Pocitos in Simojovel that contains fossil amber, is preliminarily considered the upper portion of the La Quinta strata in the late Oligocene (Graham, 1999). Here, a marine sedimentary environment is predominantly observed. The stratigraphic section and lithology of a typical amber outcrop in Chiapas are presented in Durán- Ruíz et al. (2013). It is indicated there that a portion of the marine sandstones of the La Quinta is located between the boundaries of the early Miocene and the late Oligocene. However, the complete geology of all amber deposit in Chiapas is an unresolved issue and the stratigraphic record of amber outcrops must be carefully considered (Durán-Ruíz et al., 2013;Riquelme et al., 2013). We consistently noted in the field that most of the fossil inclusions in Simojovel, Totolapa and Estrella de Belén in the Chiapas Highlands come from lignite-sandstones beds which belong to the Mazantic and Balumtum strata from early to mid-Miocene (Durán-Ruíz et al., 2013;Riquelme et al., 2013Riquelme et al., , 2014aRiquelme et al., , 2014b. Simojovel, Totolapa, and Estrella de Belén are considered the type localities of a Conservation Lagerstätte with a remarkable abundance of amber inclusions, predominantly terrestrial arthropods and plants (Riquelme et al., , 2014b. Here the sedimentary record is strongly associated with a lowland-fluvial environment close to the coastal plain (Graham, 1999;Langenheim, 2003;Perriliat, Vega & Coutiño, 2010;Riquelme et al., 2013); and paleobiota resembles those found in current humid tropics (Riquelme & Hernández-Patricio, 2018). Chiapas amber has chemical signatures that match with the extant resins of the genus Hymenaea (sensu Langenheim, 1966), which are also currently distributed in the tropics (Langenheim, 2003;Riquelme et al., 2014b)

MATERIALS AND METHODS
The fossil specimens treated in this study are listed as CPAL.117 and CPAL.132, housed at the Colección de Paleontología, Universidad Autónoma del Estado de Morelos (CPAL-UAEM), located in Cuernavaca, Morelos, Mexico. Anatomical terminology follows Hoffman (1976) and Koch (2015), and nomenclature follows Shear (2011). Preparation of material and methods used here are presented in Riquelme et al. (2013). Microphotographs were acquired using multiple image-stacking (Z ≥ 45) in a Carl Zeiss microscope, and schematic drawings were hand traced by electronic pen using a stereomicroscope and Corel Draw X7 for graphic processing. Anatomical measurements are presented in millimeters and were collected using the open-source program tpsDig V. 2.17 (Rohlf, 2013).
The electronic version of this article in Portable Document Format will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank Life Science Identifiers can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix http://zoobank.org/. The LSID for this publication is: urn:lsid:zoobank.org: pub:95759D56-205C-4FD2-A01B-0CE3F9621E0E. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central and CLOCKSS.
Diagnosis. With traits of the genus Myrmecodesmus sensu Shear (1977), plus the following combination of diagnostic characters: collum with 10 dorsal tubercles; without porosteles or ozopores; legs of the rings 2-9 with a short projection on the prefemur in both female and male.
Trunk: collum flabellate covering head in dorsal view; anterior margin divided into 10 equal rounded lobes separated and parallel to ground; dorsal surface domed with 2 transverse rows of tubercles, posterior row with 4 large tubercles, medial row with 6 medium tubercles, the rest of the collum surface with small tubercles (Figs. 3A and 3C). Midbody metazonite surface with 4 longitudinal rows of 2 paramedian (pm) and 2 dorsolateral (dl) tubercles, composed of 3 large tubercles that are basally fused; a group of 4 or 6 small tubercles intercalate between dl and pm tubercles, and 2 rows of 5 or 6 small tubercles middorsally, a variable number of medium and small tubercles between dl tubercles and paranota; collum, tergites and metazonites lacking setae. Prozonites slightly granular, some margins with tiny tubercle-like cuticular outgrowths (Figs. 4A, 4B and 5A). Paranota medium-sized, arising low on body, slightly directed anteriorly, declined, anterior margin straight, undivided, pitched slightly so anterior margin is lower than Telson: preanal sclerite with 3+3 lateral lobes and 2 small dorsal tubercles; epiproct not hidden under 18th segment, short, bluntly rounded, with 4 strong spinnerets in square array below apex; anal valves each with 2 setae near the mesal margin; subanal plate rounded-triangular with 2 setae near the apex. Sternites slightly roughened, not setose, wider than long; coxae nearly contiguous, with a transverse impression slightly deeper than longitudinal (Figs. 4C, 4D, 5B and 5C). Remarks. Gonopods are not visible in the male CPAL.132 and the epigyne and cyphopods are also not distinguishable in the female CPAL.117, as a consequence of the state of conservation of the bodies and because they are partially covered with cloudy amber. However, CPAL.132 and CPAL.117 are interpreted as male and female adults, respectably, due to the number of rings =19 (extant species of Myrmecodesmus may have either 19 or 20 rings as adults), the paranota and metazonite tubercles strongly differentiated, as well as the number of legs in ring 7 of the male (Figs. 1B, 2B and 3B-3D).