1. Introduction

Crinoids have been considered one of the most important group of invertebrates of shallow marine environments, especially during the Paleozoic. Although the fossil record of crinoids is common and diverse in Phanerozoic rocks, the abundance of disarticulated ossicles that make up their skeletons is greater than the reports of articulated specimens. In particular, assemblages of articulated crinoids are exceptional, and they are typically confined to particular bedding planes or localized lenses (^{Hess et al., 2002}). The scarcity of complete specimens is the result of the way in which the peduncle, calyx and crown that compose the body are articulated. These parts are composed of a series of plates or ossicles of calcitic-magnesian composition adjoined by means of ligamentary connective tissue, either rigid or flexible. Upon death these connective tissues quickly decay, and the great majority of fossil crinoidal remains are disarticulated by various processes (^{Hess et al., 2002}). This causes the post-mortem disjoining of crinoids to occur quickly, resulting in the common finding of isolated ossicles. This process makes it difficult to accurately identify these elements to low taxonomic levels whereby they are essentially ignored (^{Thomka et al., 2012}; ^{Donovan and Doyle, 2019}).

Considering this problem, several researchers (e.g., ^{Moore and Jeffords, 1968}; ^{Fearnhead and Donovan, 2015}; ^{Donovan and Doyle, 2019}) have proposed a classification system based on the morphology of the ossicles, working mainly on Paleozoic records. Such a scheme is based on the temporal and spatial distribution of each morph, suggesting the possibility that these fossils could be biostratigraphic or paleobiogeographic useful. In this context, all works about isolated or disarticulated ossicles are essential tasks in crinoid paleontology (^{Hess et al., 2002}), because the value of such incomplete material has been recognized as an important, but neglected, area of research (^{Thomka et al., 2012}). According to the classification of ^{Moore and Jeffords (1968)} and given that there are no previous studies on Permian crinoids from Chiapas, this paper aims to describe a series of columnar ossicles from the Grupera Formation, a unit of early Cisuralian age (Asselian‒Sakmarian). This information expands the current data on the distribution of isolated elements of Paleozoic crinoids. A second aim is to discuss the possible biostratigraphic and paleogeographic implications of the material studied in the Permian of Chiapas.

2. Geological setting

The locality, where the material studied was sampled, is about 15° 43’ 14.2” N ‒ 92° 26’ 46” W, in the Chicomuselo municipality, Chiapas, Mexico (Figure 1). The specimens were deposited in calcareous sandstones of fine grain in the Grupera Formation, considered the oldest Permian litho-stratigraphic unit from the Chicomuselo region. This formation is well- exposed in southeastern Chiapas extending from Loma Alta and La Pinta to the Guatemala frontier (^{Gutiérrez-Gil, 1956}).

Figure 1 Geological position of the locality, where crinoids occur, in the Grupera Formation. Map modified from ^{Jiménez-Hernández et al. (2005)}. 

The Paleozoic succession begins with the Santa Rosa Formation (Upper Mississippian‒Middle Pennsylvanian). This unit is mainly composed of shale, phyllite and slate (^{Gutiérrez-Gil, 1956}). Overlaying the Santa Rosa Formation is the Grupera Formation from the Asselian‒Sakmarian (Cisuralian), age assigned by ^{Thompson and Miller (1944)} by means of the foraminifera Schwagerina gruperaensis and Schwagerina chiapasensis. Its base is made up of 300 m of silicified shale intercalated with dark gray limestone; above, there are 117 m of gray shale and sandstone strata, followed by 110 m of alternations of dark gray limestone and shale (^{Gutiérrez-Gil, 1956}). The succession continues with La Vainilla Limestone, composed of limestone and argillaceous limestone.

This unit apparently overlays the Grupera Formation; however, its actual stratigraphic position remains unclear and has been suggested that the La Vainilla Limestone could be the base of the Paso Hondo Formation (^{Gutiérrez- Gil, 1956}; ^{López-Ramos, 1979}). The Paleozoic succession ends with the Paso Hondo Formation (upper Cisuralian‒lower Guadalupian) mainly composed of 600 m of light to dark gray limestones of Artinskian‒Roadian age (^{Gutiérrez-Gil, 1956}; ^{Hernández-García, 1973}; ^{Torres-Martínez et al., 2019}) (Figure 2).

Figure 2 Paleozoic stratigraphy of the Chicomuselo region. 

3. Systematic paleontology

The material described is deposited at Museo de Paleontología of the Facultad de Ciencias, Universidad Nacional Autónoma de México. Figured and type specimens are designated in the descriptions by the prefix FCMP. The specimens are preserved in calcareous sandstone as authigenic molds. In all cases rubber casts were made.

Type species. Pentagonopternix insculptus^{Moore and Jeffords, 1968}.

Diagnosis. Heteromorphic and transversely pentastellate stem; nodals with 5 elliptical cirrus scars; internodals with 14 columnals, alternating in size; sutures indented forming pits wider than high with no development of radial pores; articular facets having a star-shaped belt of medium-length straight crenulae, their outer ends aligned in inwardly bent curve, the inner ends abutting stellate areola with a slightly concave floor; lumen moderately large and circular.

Figure 3 (A, B) Pentagonopternix? sp., samples A) FCMP 1350; B) FCMP 1349. (C, D) Cyclogrupera minor n. gen and n. sp., sample FCMP 1351. (E-N) Cyclocaudex typicus, samples E) FCMP 1354; F, G) FCMP 1357; H, I) FCMP 1352; J, K) FCMP 1355; L) FCMP 1356; M, N) FCMP 1353. (O-S) Cyclocaudex plenus, samples O, P) FCMP 1359; Q) FCMP 1360; R, S) FCMP 1358. (T, U) Cyclocaudex jucundus, sample FCMP 1361. (V) Cyclocaudex insaturatus, sample FCMP 1362. All specimens illustrated are rubber casts. Scale bars = 5 mm, except A, B, L = 2 mm; C, D = 1 mm. 

Description. Columnals with a pentastellate shape; crenularium poorly preserved; small, sub-pentagonal lumen.

Material. FCMP 1349, 1350.

Occurrence. Fine-grained calcareous sand-stones, Grupera Formation (Asselian‒Sakmarian), Chiapas, Mexico.

Remarks. The specimens are questionably assigned to the morphogenus Pentagonopternix mainly due to the stellate shape of the ossicle. The poor conservation of several features of the articular facet, as well as the lack of a noditaxis,-does not allow making a reliable assignment.

This morphogenus has been found in the Chaffin Limestone from Gzhelian (Upper Pennsylvanian) age of Texas (^{Moore and Jeffords, 1968}), in the Antiinskiy Horizon from the middle Permian (Roadian‒Wordian) of the Transbaikal region of Russia (^{Kotlyar et al., 2006}), and in the Imtachan Formation from the upper Permian (Lopingian) of Verkhoyansk, Russia (^{Biakov et al., 2016}). In Mexico, it had only been recorded in the Middle Pennsylvanian of Sierra Agua Verde, Sonora (^{Buitrón-Sánchez et al., 2015}).

Type species. Cyclogrupera minor n. sp.

Etymology. Refers to the circular shape of the articular facet, as well as to the lithostratigraphic unit where the specimens were collected.

Diagnosis. Homeomorphic stem of small diameter, gently rounded in longitudinal profile; granulose walls; articular facet circular; crenularium large, reaching the outline of perilumen; crenulae coarse, with crenellae and culmina of the same size; without areola; perilumen gently marked, delineating the lumen outline; lumen small, circular.

Etymology. Refers to the tiny size of the specimens.

Diagnosis. As the genus.

Description. With traits of the genus; besides this displays 13 crenulae, lumen 0.5 mm diameter and crenularium 0.7 mm width.

Material. FCMP 1351.

Occurrence. Fine-grained calcareous sand-stones, Grupera Formation (Asselian‒Sakmarian), Chiapas, Mexico.

Remarks. The Chiapas material was assigned to the family Leptocarphiidae due to the slender stem, broad crenularium and presence of a perilumen surrounding the circular lumen. Cyclocaudic-ulus differs from Cyclogrupera by the lack of areola; Nothrosterigma differs by its coarser, convex and ring-shaped perilumen surrounding the lumen, and Heterostelechus displays an elevated perilumen from the surface of the articular facet.

Type species. Cyclocaudex typicus^{Moore and Jeffords, 1968}.

Diagnosis. Stem heteromorphic; straight-sided to gently convex longitudinal profile; nodals bearing cirrus scars, not wider than internodals and slightly taller; crenularium wide with long and straight crenulae; areola small, lacking or becoming a perilumen; lumen small to medium in size, and circular, subpentagonal or quinquelobate in outline.

Description. Column with straight to slightly rounded longitudinal profile, with granulose walls; heteromorphic stem; sutures gently crenulate; articular facet subcircular; crenularium broad, with slender crenulae, many bifurcated; areola is not conspicuous, reduced in size, granulose and flat, which surrounds a large, circular lumen; inside of the lumen, a circular and minute jugula is observed.

Material. FCMP 1352-1357.

Occurrence. Fine-grained calcareous sand-stones, Grupera Formation (Asselian‒Sakmarian), Chiapas, Mexico.

Remarks. The specimens of the Grupera For-mation resemble Lomalegnum hormidium. However, C. typicus has neither the sutures strongly crenulate nor the peripheral ends of culmina typical of Lomalegnum.

The material can also be distinguished from Cyclocaudex jucundus by the lack of a broad areola with a depressed surface. Cyclocaudex typicus has been reported in the Upper Pennsylvanian of Texas in the United States (^{Moore and Jeffords, 1968}), as well as in the Carboniferous of Oaxaca (^{Villanueva-Olea et al., 2011}), Upper Pennsylva-nian of Puebla (^{Esquivel-Macías, 1996}) and mid-dle Permian of Oaxaca in Mexico (^{Vachard et al., 1997}). This is the first record of the taxon in the lower Permian of North America.

Description. Ossicles of circular shape in outline; conspicuous cirrus, which endow a greater size to the nodals; sutures slightly crenulate; ossicles of at least three different heights, which suggests a minimum noditaxis of four columnals; crenularium very wide with narrow crenulae; culmina wider than crenellae, bifurcating near margins; areola narrow and flat; lumen of reduced size, irregular to circular in outline.

Material. FCMP 1358-1360.

Occurrence. Fine-grained calcareous sand-stones, Grupera Formation (Asselian‒Sakmarian), Chiapas, Mexico.

Remarks. The material of Chiapas is distinguished from Mooreanteris waylandensis from the Graham Formation of Texas (^{Moore and Jeffords, 1968}) by its symmetric articular facet and suture strongly crenulate. Cyclocaudex plenus is dissimilar to Cyclocaudex typicus from the Thrifty Formation of the Upper Pennsylvanian of Texas (^{Moore and Jeffords, 1968}) by its poorly defined boundary between the reduced areola and the crenularium, and the crenulae bifurcated near the edge of the areola. Cyclocaudex plenus has been reported in the Upper Pennsylvanian of Texas (^{Moore and Jeffords, 1968}) and in the lower Permian of Calnali (^{Moreno and Patiño 1981}) and Pemuxco, Hidalgo (^{Arellano et al., 1998}; ^{Buitrón-Sánchez et al., 2017}).

Description. Latus of columnals slightly rounded and convex; columnals with slight differences in height; sutures crenulate; articular facet circular; crenularium of medium size, with some bifurcations at the outer half; crenulae narrow with culmina wider than crenellae; lumen large and circular; areola broad, depressed, which deepens towards the interior of the lumen.

Material. FCMP 1361.

Occurrence. Fine-grained calcareous sand-stones, Grupera Formation (Asselian‒Sakmarian), Chiapas, Mexico.

Remarks. Cyclocaudex jucundus occurs in the Upper Pennsylvanian of Texas (^{Moore and Jeffords, 1968}), in addition to the Pennsylvanian of Calnali, Hidalgo (^{Buitrón et al., 1987}), the Upper Pennsylvanian (^{Velasco de León and Buitrón, 1992}) and Permian (^{Esquivel et al., 2000}) of San Salvador Patlanoaya, Puebla, the lower Permian of Pemuxco, Hidalgo (^{Arellano et al., 1998}), Middle- Upper Pennsylvanian of Sonora (^{Buitrón-Sánchez et al., 2012}; ^{Villanueva-Olea et al., 2016}) and in the middle Permian of Sonora (^{Buitrón et al., 2007a}).

Description. Articular facet circular; crenularium very wide, reaching the rim of the lumen; crenulae very narrow, with culmina wider than crenellae, some bifurcating in the medium part; lumen of medium size and circular.

Material. FCMP 1362.

Occurrence. Fine-grained calcareous sand-stones, Grupera Formation (Asselian‒Sakmarian), Chiapas, Mexico.

Remarks. The specimen of Chiapas differs from Cyclocaudex plenus by its larger lumen and crenulae reaching the rim of the lumen. Cyclocaudex insat-uratus is dissimilar to Cyclocaudex aptus from the Lower Mississippian of Kentucky (^{Moore and Jeffords, 1968}) by its coarser and less abundant crenulae and a larger lumen. The species differs from Cyclocaudex congregalis of the Lower Mississip-pian of Kentucky (^{Moore and Jeffords, 1968}) by its flat crenularium, without inner rounded edges. Cyclocaudex insaturatus occurs in the Middle Penn-sylvanian of Kansas (^{Moore and Jeffords, 1968}), as well as in the Pennsylvanian of Calnali, Hidalgo (^{Buitrón-Sánchez et al., 1987}) and the Middle Mississippian and Lower-Middle Pennsylvanian of Santiago Ixtaltepec, Oaxaca (^{Villanueva-Olea et al., 2011}). This is the first record of C. insaturatus from the Permian.

Type species. Floricyclocion heteromorpha n. sp.

Etymology. Refers to the floriform lumen.

Diagnosis. Floricyclidae characterized by a straight and heteromorphic stem columnals with median length crenulae, flat areola and medium sized stellate lumen.

Figure 4 (A, B) Floricyclocion heteromorpha n. gen and n. sp., sample FCMP 1363. (C) Lamprosterigma erathense, sample FCMP 1364. (D-H) Preptopremnum cf. laeve, samples D, E) FCMP 1365; F) FCMP 1368; G, H) FCMP 1366. (I-N) Heterostelechus jeffordsi, samples I) FCMP 1369; J) FCMP 1370; K) FCMP 1373; L, M) FCMP 1370; N) FCMP 1372. (O, P) Heterostelechus cf. keithi, samples O) FCMP 1371; P) FCMP 1375. (Q-V) Cyclocaudiculus regularis, samples Q) FCMP 1377; R, S) FCMP 1376; T) FCMP 1378; U, V) FCMP 1379. Scale bars = 2 mm, except C = 5 mm; G, H = 1 mm. 

Etymology. Refers to the heteromorph stem.

Diagnosis. As the genus.

Description. Column straight in lateral view, heteromorphic, with columnals of different length; articular facet circular; crenularium of medium size, with straight and coarse crenulae; culmina slightly coarser than crenellae; areola flat and granulose; lumen medium sized and stellate.

Material. FCMP 1363.

Occurrence. Fine-grained calcareous sand-stones, Grupera Formation (Asselian‒Sakmarian), Chiapas, Mexico.

Remarks. The morphogenus Cyclocion is distin-guished from Floricyclocion by its very short crenulae and larger lumen; sutures strongly crenulate and nodals strongly keeled. On the other hand, the genus Lamprosterigma displays a depression in the areola, in addition to very slender and numerous crenulae. Likewise, the Floricyclus differs from the new morphogenus by its very marked and gran-ulose perilumen, with division of some crenulae. Floricyclocion is assigned to the family Floricycli-dae by its floriform lumen and the presence of a straight longitudinal profile of the stem.

Type species. Lamprosterigma mirificum^{Moore and Jeffords, 1968}.

Diagnosis. Homeomorphic stem; low-height columnals; longitudinal profile straight or a little convex; sutures noncrenulate, not indented; articular facet with moderately broad crenularium; crenulae straight, set off from the areola by a sharp edge; areola bowl-shaped with smooth surface, extending to the edge of the pentalobate broad lumen, or separated from it by a narrow perilumen.

Description. Articular facet circular; crenularium wide with slender crenulae, some bifurcating close to the periphery; culmina wider than crenellae; areola granulose, set off from the crenularium by a depression and grades into a discrete perilumen; lumen wide, floriform, with poorly developed petals.

Material. FCMP 1364.

Occurrence. Fine-grained calcareous sand-stones, Grupera Formation (Asselian‒Sakmarian), Chiapas, Mexico.

Remarks. The specimen was compared with Lamprosterigma mirificum (Middle Pennsylvanian) of Kansas (^{Moore and Jeffords, 1968}) however L. mirificum does not display a developed perilumen. Plummeranteris from the Pennsylvanian of Texas (^{Moore and Jeffords, 1968}) is dissimilar to the morphogenus Lamprosterigma by its coarse and straight crenulae, and strongly floriform perilumen, with well-developed petals. Lamprosterigma erathense is reported from the Middle Pennsylvanian of Texas (^{Moore and Jeffords, 1968}) and the Middle Pennsylvanian (Atokan) of Sonora (^{Buitrón-Sánchez et al., 2007b}). This is the first occurrence of L. erathense in Permian rocks.

Type species. Preptopremnum rugosum^{Moore and Jeffords, 1968}

Diagnosis. Medium-sized column; articular facets slightly narrower than the largest column width; rounded longitudinal profile; crenularium narrow; medium to coarse crenulae set off from a broad and flat areola, slightly concave or convex, with marks of vermiculate or granulose irregularities; large and circular lumen, bordered by a narrow perilumen or an accentuated edge of the areola; lumen with claustra at mid-height of columnal plates, jugulum and jugular ramparts in the center of claustra.

Description. Column with a slightly rounded longitudinal profile; articular facet circular with a very slender crenularium and coarse crenulae; culmina wider than crenellae; areola wide and granulose in the area surrounding lumen, slightly elevated, resembling a broad perilumen; lumen circular.

Material. FCMP 1365-1368.

Occurrence. Fine-grained calcareous sand-stones, Grupera Formation (Asselian‒Sakmarian), Chiapas, Mexico.

Remarks. Preptopremnum rugosum is distinguished from P. laeve by the longer crenulae, a granulose and rounded medium-sized areola, and a circular large lumen. Nevertheless, the Chiapas specimens are slightly different to those described by ^{Moore and Jeffords (1968)} because of the occurrence of great quantity of granules on the latus of the columnals. Preptopremnum laeve occurs in the Middle Pennsylvanian of Texas in the United States (Moore and Jeffords, 1968), as well as in the Pennsylvanian of Sonora (^{Buitrón-Sánchez et al., 2005}, ^2007b, ²⁰¹²; ²⁰¹⁵) and in the Middle Pennsylvanian-lower Permian of Sierra Las Pintas, Baja California, Mexico (^{Navas-Parejo et al., 2018}).

Diagnosis. Stem heteromorphic; columnals rounded in longitudinal profile; indented and crenulate sutures; one or more cirrus scars in nodals, which are taller than internodals, but not much wider; coarse straight crenulae of medium length; areola concave, elevated tract around circular lumen, also classifiable as a perilumen.

Description. Column with a straight profile; noditaxis of three different sizes in the height of columnals (nodals and internodals 1IN and 2IN); sutures crenulate; articular facet circular; medium-sized crenularium with coarse crenulae; culmina and crenella of the same size; areola elevated and granulose; medium-sized subpentagonal lumen.

Material. FCMP 1369-1373.

Occurrence. Fine-grained calcareous sandstones, Grupera Formation (Asselian‒Sakmarian), Chiapas, Mexico.

Remarks. The material of Chiapas was assigned to the morphospecies by the articular facet features. Heterostelechus jeffordsi is distinguished from the rest of the morphospecies mainly by its column profile slightly rounded. This taxon has been recorded in the Late Pennsylvanian of Texas (^{Moore and Jeffords, 1968}) and Guerrero, Mexico (^{Flores de Dios and Buitrón-Sánchez, 1982}; ^{Esquivel-Macías et al., 2004}). This is the first report of the morphospecies in the early Permian.

Description. Plate with a smooth and straight latus; articular facet circular; crenularium of medium size; coarse and straight crenulae, with crenellae and culmina of the same width; areola slightly concave, forming a pentagonal outline in the inner edge of the crenularium; lumen small and circular.

Material. FCMP 1374, 1375.

Occurrence. Fine-grained calcareous sand-stones, Grupera Formation (Asselian‒Sakmarian), Chiapas, Mexico.

Remarks. The material resembles Heterostelechus keithi but its straight longitudinal profile and smaller lumen does not allow making a reliable assignment. Heterostelechus keithi has been recorded in the Pennsylvanian of Texas (^{Moore and Jeffords, 1968}), in addition to Hidalgo (^{Buitrón-Sánchez et al., 1987}), Puebla (^{Velasco de León and Buitrón-Sánchez, 1992}) and Sonora in Mexico (^{Buitrón-Sánchez et al., 2007b}, ²⁰⁰⁸, ²⁰¹⁵). This is the first report in the Permian of North America.

Type species. Cyclocaudiculus regularis^{Moore and Jeffords, 1968}.

Diagnosis. Stem with a straight longitudinal profile; nodals bearing one single scar; crenularium with coarse and straight crenulae; lumen circular, small or medium.

Description. Stem heteromorphic, straight in longitudinal profile; articular facet circular which occupies the width of the columnal; crenularium with coarse crenulae; culmina wider than crenellae; areola flat with perilumen; crenularium small and subcircular.

Material. FCMP 1376-1379.

Occurrence. Fine-grained calcareous sand-stones, Grupera Formation (Asselian‒Sakmarian), Chiapas, Mexico.

Remarks. Cyclocaudiculus regularis is reported in the Gzhelian (Upper Pennsylvanian) of Texas in the United States (^{Moore and Jeffords, 1968}) and in the Pennsylvanian of Sierra Las Mesteñas, Sonora, Mexico (^{Villanueva-Olea et al., 2016}). This is the first record of the taxon in the lower Permian.

4. Discussion

5. Conclusions

The report of eleven morphospecies of crinoid ossicles has allowed recognizing part of the marine invertebrates that inhabited the Chicomuselo region during the early Permian (Asselian- Sakmarian). The record of these taxa is the first made in Chiapas, Mexico, highlighting the occurrence of two new morphogenera: Floricyclocion and Cyclogrupera. These crinoids were deposited in a restricted marine environment of the inner ramp, characterized by well-oxygenated conditions, high productivity and good illumination. Previously, the taxa Cyclocaudiculus regularis, Heterostelechus keithi, Lamprosterigma erathense and Cyclocaudex insaturatus had only been recorded in different localities from the Pennsylvanian of North America but their occurrence in rocks from the Grupera Formation allows extending their stratigraphical range at least until the lower Cisuralian. The crinoid ossicles were deposited in a geographic region associated with the biotic Grandian Province of North America which groups different Permian localities of Mexico, Venezuela and the United States.