The millipedes and centipedes of Chiapas amber

An inventory of fossil millipedes (class Diplopoda) and centipedes (class Chilopoda) from Miocene Chiapas amber, Mexico, is presented, with the inclusion of new records. For Diplopoda, 34 members are enumerated, for which 31 are described as new fossil records of the orders Siphonophorida Newport, 1844, Spirobolida Bollman, 1893, Polydesmida Leach, 1895, Stemmiulida Pocock, 1894, and the superorder Juliformia Attems, 1926. For Chilopoda 8 fossils are listed, for which 3 are new records of the order Geophilomorpha Pocock, 1895 and 2 are of the order Scolopendromorpha Pocock, 1895.

Excluding the probable millipede specimen ?Xylobius mexicanus Müllerried 1942 from a upper Jurassic/mid-Cretaceous horizon in Puebla, Central Mexico, whose taxonomic identity is doubtful, the fossil material of both millipedes and centipedes in Mexico generally comes from the Miocene amber localities in the Chiapas Highlands in southwestern Mexico.These sites are part of the Chiapas amber Lagerstätte with remarkable fossil preservation of terrestrial arthropods, including myriapods (Riquelme et al. 2013(Riquelme et al. , 2014a, b), b).
Here we address the current knowledge of fossil millipedes and centipedes in Miocene Chiapas amber.An inventory of fossil specimens is given, which mostly includes new fossil records (Table 1).Other records, as presented in published papers, are also mentioned.A major part of this work is the result of a short period of fieldwork and represents only an introductory account due to the difficulties surrounding the collection of fossil specimens.We estimate that in 2016 about 60 amber specimens of both millipedes and centipedes, but mostly millipedes, were found in the Chiapas amber area.But nearly 50 fossil specimens were lost in the same period due to commercial trading and smuggling.In the context of this loss of material, the present inventory of fossil specimens contributes significantly to the aim of estimating taxonomic diversity of both Diplopoda and Chilopoda in Mexico.It highlights new records of the order Siphonophorida, Spirobolida and Polydesmida in the Diplopoda and new records of the order Geophilomorpha and Scolopendromorpha in the Chilopoda.

Methods
A set of fossil specimens for this study was collected during 3 short field trips in 2015, 2016 and 2017 in the amber areas near the towns of Simojovel and Totolapa, Chiapas, Mexico.A second set of specimens was examined from the amber inclusion collection that belongs to the Museo del Ambar de Chiapas (MACH) and a third set of specimens was examined from the collection of the Museo del Ambar Lilia Mijangos (MALM), both located in San Cristobal de las Casas, Chiapas.The amber inclusion collections of these 2 museums are formally certified by the Instituto Nacional de Antropología e Historia (INAH), a federal agency that protects the paleontological heritage in Mexico.Exporting fossils from Mexico without INAH registration certificate is illegal under federal laws.Furthermore, 3 millipede specimens, currently lost by commercial trade, are also listed in this paper.These fossils were photographed and identified in the field using the informal code AM.CH., which in Spanish means Ámbar de Chiapas, followed by an identification number (Id).
Taxonomic treatment of each fossil specimen includes the extended version of the traditional Linnaean classification ranks.There is also morphological annotations and additional information of the current geographic distribution of closely related extant taxa in Mexico.Data from the 4 previously described fossil species are compiled from literature; however, the catalogue presented here is predominantly taxonomic, and not bibliographic.Terminology follows Enghoff et al. (2015) and Koch (2015) for the Diplopoda, and Bonato et al. (2011) for the Chilopoda.The microphotographs of the specimens were obtained by using multiple images stacking for 3-dimensional focus expansion in a Carl Zeiss microscope (Riquelme et al. 2014a, b).
Identification.Head partially lost, trunk smooth, body cylindrical and elongated that matches Juliformia, but the specimen is strongly damage by the fossilization process, so no judgment is made as to its specific taxonomic position.
Identification.Head partially damaged or not fully visible in CPAL.104, body cylindrical, 2 apodous segments before telson, tergites with a longitudinal groove, ozopore present, legs long, telson short, epiproct minute, a pair of stout paraprocts with setae along the anal opening is also present.All fossil specimens match Stemmiulida, but for more specific taxonomic position they need further revision.
Remarks.Extant species of Stemmiulida are recorded almost exclusively at the southern region of Mexico, including the states of Veracruz, Campeche and Quintana Roo (Bueno-Villegas et al. 2004).

Millipedes preliminary identified but not determined
Class Diplopoda de Blainville in Gervais, 1844 Diplopoda sp.indet.
Identification.Two specimens surrounded by other millipede specimens within the same disc-shaped piece of amber (Figure 2A-B), which does not allow a more accurate description of their anatomy at the present.
Identification.The fossils specimens were observed in the field, body and general appearance matches juliform millipedes.These fossils were sold days after their discovery, so no judgment is made as to their taxonomic position.
Identification.This fossil specimen was observed in the field, it seems closely related with the extant representatives of the Family Chelodesmidae, particularly with the fossil species M. paachtun as described in Riquelme et al. (2014a).Locality.Mexico, Chiapas?Simojovel area?The origin of this material is unproven; there are no data about the exact locality or strata provided by Ross et al. (2016).

Centipedes
Remarks.Specimen undetermined; micrograph published in Ross et al. (2016).Accordingly, the specimens resemble Geophilomorpha.Ross et al. (2016)  Locality.Mexico, Chiapas?Simojovel area?The origin of this material is unproven; there is no data about the exact locality or strata provided by Ross et al. (2016).

Discussion
It is not possible to check the identity of ?Xylobius mexicanus because the fossil specimen is currently missing (Müllerried 1942).Its geological age is also uncertain.
In the absence of the specimen and additional evidence (no images were found), it is reasonable to consider it as a highly doubtful record.Currently the fossil record of Mexico confidently assignable to Diplopoda and Chilopoda is limited to the mid-Cenozoic of Chiapas (Table 1); for Diplopoda this includes 4 orders, 6 families, 5 genera and 3 species; whereas for Chilopoda this comprises 3 orders, 3 families, 3 genera and 1 species.
(Bueno-Villegas et al. 2004iphonophorida in Mexico are currently found in the northern states of Sonora, Baja California Sur, Nuevo León, and Tamaulipas.They also found in the southern states of Veracruz and Yucatan(Bueno-Villegas et al. 2004).
granulated dorsal sculpture in the metategite.Sex is indeterminable in MALM.21 by the position of the body.Both specimens represent the first fossil record of the family Siphonophoridae in Chiapas amber.Remarks.Spirobolida sp.indet.
Locality.MACH.20:Mexico,Chiapas, Simojovel, La Pimienta site, 17°08ʹ29ʺ N, 092°45ʹ46ʺ W.Identification. MACH.20 without head or terminal segments, body with more than 40 pairs of legs; fossil specimen match Geophilomorpha but this shows poor preservation that makes a more detailed description difficult.