First record of Ubaghsicystis (Eocrinoidea-Echinodermata) from the Cambrian (Miaolingian, Wuliuan) of Sonora, Mexico: Biostratigraphical and paleoecological considerations

Introduction: The present work constitutes the first record of Ubaghsicystis Gil-Cid & Domínguez-Alonso (Echinodermata-Eocrinoidea) in the El Gavilán Formation, exposed in the El Sahuaral Hill, a new Cambrian locality of central Sonora, Mexico. The El Gavilán Formation is a sedimentary sequence mainly made up of shale interbedded with limestone, with abundant invertebrate fossils, deposited in an open shelf marine environment. Objective: The main objective of this research work is to describe the species Ubaghsicystis cf. U. segurae for the first time in Mexico, in addition to biostratigraphic and paleoecological considerations. Methods: A composite stratigraphic section of the El Gavilán Fm. was measured in central Sonora, where samples corresponding to eocrinods were collected, the material was examined in the Microscope Leica MZ10. Results: The biotic association is composed of reticulosan sponges, chancelloriids (Chancelloria eros, Allonnia tintinopsis, Archiasterella sp.), hyolithids, brachiopods, trilobites (Quadragnostus depressa, Peronopsis sp., Tonkinella valida, and Elrathina sp.), and echinoderms (Ubaghsicystis cf. U. segurae). Regarding the age, the trilobite association establishes a chronostratigraphic position from the middle Cambrian (Miaolingian, Wuliuan), Altiocculus Subzone, Ehmaniella Zone. The material studied is constituted by isolated echinoderm plates, which probably represent various cycles of theca plates assigned to Ubaghsicystis cf. U. segurae. The genus Ubaghsicystis has been scarcely recorded in a few Cambrian regions (e.g., Burgess Shale in Canada and Los Barrios de Luna in Spain), so that the information about this taxon is limited. Conclusions: The Cambrian succession of Sonora, where Ubaghsicystis occurred, was part of the border from the Laurentia craton during this period, located near the Paleoequator during this period, where ideal conditions for increasing diversity and abundance of species were developed, including echinoderms.

The presence of echinoderms in Mexican Cambrian rocks is scarce, with few isolated elements documented for the country. Nardin et al. (2009) reported isolated plates attributed to the species Gogia granulosa, in the Cambrian section of San José de Gracia, Sonora, Mexico. Similarly, Buitrón-Sánchez et al. (2017b) studied to Gogia granulosa and Gogia sp. from the same area, documenting the paleoecological implications of these species as they are associated with limestone, shale, and sandstone strata, thus, showing great adaptability to different environments.
Regarding eocrinoids, Sprinkle (1973) proposed the new subphylum Blastozoa divided into four classes of extinct Paleozoic "pelmatozoan" echinoderms: Eocrinoidea, Paranlastoidea, Rhombifera, and Blastoidea. Echinoderms of the subphylum Blastozoa are characterized by a group often composed of stalked echinoderms, lacking free arms but with biserial brachioles with nutritional functions, and with the presence of various respiratory structures in the theca. The main objective of this research work is to describe the species Ubaghsicystis cf. U segurae for the first time for Mexico, in addition to biostratigraphic and paleoecological considerations.

MATERIAL AND METHODS
A composite stratigraphic section of El Gavilán Formation was measured at the southwest flank of El Sahuaral Hill. Samples were collected corresponding to carbonate and detrital rocks. The paleontological material was prepared in the Paleontology Laboratory at the University of Sonora. Due to the conservation of specimens, they do not show contrast when dry, so the samples were wetted. The material was photographed using a Nikon Camera D70S and manipulated with Adobe Photoshop ® .

Location of the study area:
The paleontological material studied comes from El Sahuaral Hill, it is located 44 km northeast of Hermosillo at geographical coordinates 29°21'5.58" N & 110°37'13.98" W, located 7 km north of the village of San José de Gracia, in the central part of the Sonora state (Fig. 1).
Stratigraphy: Although in the study area, exposed rocks vary in age from Precambrian to Holocene, the Cambrian rocks are best exposed, and consists of the Proveedora, Buelna, Cerro Prieto, and El Gavilán formations (Fig. 2).
Proveedora Formation: This formation is composed of fine-grained quartzite, with reddish spots due to oxidation, followed by brown to dark gray quartzarenite with the presence of Skolithos isp. This unit is 216 m thick and is exposed in most of the El Sahuaral Hill (Noriega-Ruiz et al., 2020).
Buelna Formation: This formation is made up of sandy limestone with fragments of hyolithids, followed by limestone with oncolytic algae, ending with dark-colored sandy limestone. In the middle part of the unit, an oolitic limestone bed is present. This unit is exposed at the southwest side of the El Sahuaral Hill with 64 m thickness. The lower and upper contacts are conformable with the Proveedora and Cerro Prieto formations, respectively (Noriega-Ruiz et al., 2020).
Cerro Prieto Formation: This formation consists of a cliff-forming dark gray oolitic limestone with evidence of some dolomitization. It is 53 m thick, being exposed to the southeast side of El Sahuaral Hill. The lower contact is conformable with the Buelna Formation, and the upper contact is conformable with the El Gavilán Formation (Noriega-Ruiz et al., 2020).
El Gavilán Formation: The lower part of this unit is composed of a red-yellow colored thin-bedded shale with reticulosan sponges, chancellorids, brachiopods, and trilobites (Fig.  3). The middle part is made up of interbedded red shale and gray limestone beds of varying thickness, with high fossiliferous content. The upper part of the unit consists of yellow sandstone. The real thickness of the unit has not been determined due to folding. Also, this unit is exposed in the valley, southwest of the El Sahuaral Hill. The lower contact is conformable with the Cerro Prieto Formation, while the upper contact is in angular unconformity with the Miocene rhyolite-andesite volcanic rocks (Noriega-Ruiz et al., 2020).

Systematic paleontology:
The nomenclature and morphological terms used to describe the specimens are those proposed by Sprinkle (1973) and Gil-Cid & Domínguez-Alonso (2002).
Phylum Echinodermata Bruguière, 1791 Subphylum Blastozoa Sprinkle, 1973 Class Eocrinoidea Jaekel, 1918 Family uncertain Genus Ubaghsicystis Gil-Cid & Domínguez-Alonso, 2002 Diagnosis: Eocrinoid with globose theca formed by approximately 40 plates, epispires present in the upper half and absent in the lower part of the theca. Anal pyramid in lateral position above the equator line of the theca. Long, homeomorphic, and holomeric peduncle with a distal fixation disc, the column is of circular section, narrow lumen, and without articular crenulations. Thin, biserial arms, with cover plates and without brachioles (Modified from Gil- Cid & Domínguez-Alonso, 2002  Description: The specimens are incomplete and poorly preserved; only plates from the lower and half part of the theca were recovered. The plates are large of variable diameter, with smooth edges (Table 1).
The specimen USDG 354 has four plates, probably corresponds to the infralateral plates of the cycle 2, as they are the largest, with a smooth external surface and sometimes a slight concavity. Over the cycle 2, the cycle of lateral plates (cycle 3) is observed, displaying an irregular and curved polygonal contour. Articulation concavity is observed in plates 2: 1-2: 2 and 2: 1-3: 2.
The specimen USDG 355 is incomplete and poorly preserved, with seven plates that correspond to 4 cycles. The plates are rounded and smooth-edged. Plate cycle 1 is subrounded. Cycle 2 (infralateral), plates are the largest and have smooth edges, possibly articulated. Cycle 3 (lateral) plates are similar in size to cycle 1 plates but differ in the hexagonal edges (3:2).
Remarks: The infralateral plates of the theca of the Mexican specimen are very similar to those of Ubaghsicystis segurae, described for the Ovielle Formation from the middle Cambrian of Spain (Gil- Cid & Domínguez-Alonso, 2002), in that they are the largest of  the theca and of smooth surface. The side plates are similar to the same specimens, and they have an irregular polygonal contour, and the 3:2 plate is presented in an almost hexagonal shape. For the rest of the plate cycles (4), it is not possible to make a comparison since this cycle in U. segurae, is of irregular contour plates without a defined pattern. However, as in the Mexican specimen, they are smaller in size, since they decrease in size as approaching the oral region (Gil- Cid & Domínguez-Alonso, 2002). A notable difference between the Mexico specimen and the example from Spain is in terms of size, as the plates of the Mexican specimen are considerably larger. Ubaghsicystis segurae collected in beds of the middle Cambrian area of the High Atlas area of Morocco (Zamora, Clausen, Álvaro, & Smith, 2010) is similar to the Mexican specimen in the polygonal and surrounded form of the plate cycles 2 and 3; however, it differs in its smaller size.
Ubaghsicystis cf. U. segurae from Mexico is very similar to ?Ubaghsicystis sp. from the middle Cambrian Burgess Shale, Canada (Sumrall & Zamora, 2015), especially in the form of the plate cycles, for which the ones from cycle 2 are also the largest and irregular in size, however in terms of size, is also smaller than the Mexican specimen.

Biostratigraphical considerations:
The trilobite fauna from the El Gavilán Formation of the El Sahuaral section is represented by Quadragnostus depressa, Peronopsis sp., Tonkinella valida, and Elrathina antiqua. Altogether, the trilobite fauna indicates a middle Cambrian (Miaolingian, Wuliuan) chronostratigraphic position. The biotic content corresponds to the Altiocculus Subzone, the upper part of the Ehmaniella Zone, defined by Sundberg (1994), as an assemblage zone that occurs above the Glossopleura Zone and below the Bolaspidella Zone, dividing it into four subzones: Proehmaniella, Elrathiella, Ehmaniella, and Altiocculus.
For Gondwana, Ubaghsicystis is known from the Cambrian of Spain and Morocco (Gil- Cid & Domínguez-Alonso, 2002;Zamora et al., 2010;Sumrall & Zamora, 2015). In Laurentia, it had only been recorded in the Cambrian of Burgess Shale (Sumrall & Zamora, 2015), and we are reporting herein the first occurrence of the genus in Mexico. In Spain, Ubaghsicystis segurae has been documented from the Ovielle Formation (Miaolingian, Wuliuan) of Barrios de Luna, Province of León, Northern Spain (Gil- Cid & Domínguez-Alonso, 2002). In Morocco, it has been studied from the High Atlas, where complete specimens of Ubaghsicystis segurae were recovered, also from the middle Cambrian (Miaolingian, Wuliuan) (Zamora et al., 2010). In North America ?Ubaghsicystis has been documented from the Burgess Shale, Canada, where poorly preserved specimens were recovered, which are included in the Ehmaniella Zone (syn. Bathyuriscus-Elrathina Rasetti, 1951) of middle Cambrian (Miaolingian-Wuliuan) age. As previously mentioned in Mexico only isolated and disarticulated plates of Ubaghsicystis have been collected from the El Gavilán Formation in the El Sahuaral section, central Sonora, where it is included, in the upper part of the Ehmaniella Zone, middle Cambrian (Miaolingian-Wuliuan). The ability of pelmatozoa to adapt to a variety of marine environments was cited by Zamora et al. (2010). Different morphological innovations allowed them to adapt to soft, clay offshore substrates, and sometimes to be able to attach on shell fragments.

Paleoecological and paleogeographic
As mentioned above, the benthic community of the middle Cambrian of the El Sahuaral was developed in an open shelf marine environment with a soft substrate. This is indicated by the abundance of shale, related to the deposition of clay as fine material, along with the presence of agnostid trilobites (Quadragnostus depressa and Peronopsis sp.), which have only been recorded in open shelf marine environments (Robison, 1976;Liñán, 1996;Cuen-Romero et al., 2019).
The fossiliferous assemblage from the middle Cambrian of the El Sahuaral sections consists of complete chancelloriid sclerites and fragments of scleritomes (bodies) partially articulated of Chancelloria eros, together with isolated sclerites belonging to Allonnia tintinopsis, and Archiasterella sp. (Beresi et al., 2019).
In the present report, chancellorids, hyolithids, linguloid brachiopods, and trilobites were also documented. This faunal content is  practically identical to the assemblage cited from the type section of San José de Gracia, Sonora, located 10 km to the south of the study area ( Fig. 1) (Cuen et al. 2013;Cuen-Romero et al., 2016).
The El Gavilán Formation has yielded a diverse faunal assemblage including small shelly fossils as Chancelloria, Archiasterella, and Allonnia. Numerous isolated complete chancelloriid sclerites with central rays and 6-7 marginal rays belonging to Chancelloria eros and diverse fragmentary shells of brachiopods dominate most samples. Additionally, eocrinoid remains, few hyolithids, and trilobites are represented in the assemblage. Few conical hyolithids are badly preserved on bedding surfaces. Moreover, some brachiopods as Lingulella sp. Acrothele sp., Prototreta sp., Dictyonina sp., and Linnarsonia sp. were found at the type section of San Jose de Gracia of the El Gavilán Formation , of which some are also present in the El Sahuaral section.
Results demonstrate a wide distribution and palaeobiological importance of the middle Cambrian fauna in Sonora, Mexico as part of the Laurentian shelf, characterized by carbonates deposited in shallow and deeper shelf areas under tropical conditions. Ethical statement: authors declare that they all agree with this publication and made significant contributions; that there is no conflict of interest of any kind; and that we followed all pertinent ethical and legal procedures and requirements. All financial sources are fully and clearly stated in the acknowledgements section. A signed document has been filed in the journal archives.

ACKNOWLEDGMENTS
The authors are gratefully to Frederick A. Sundberg (Museum of Northern Arizona, USA), who collected the material. The authors thanks the financial support provided by the project PRODEP UNISON-PTC-301: "Paleoecología de los ecosistemas marinos del Cámbrico de Sonora, México: Bioestratigrafía, Paleobiogeografía y su relación con el cratón de Norteamérica". We also thank the Department of Geology at the University of Sonora, as well as the Instituto de Geología, UNAM, for the support given to carry out this research.