Revista de Biología Tropical, ISSN electrónico: 2215-2075, Vol. 69(S1): 51-61, March 2021 (Published Mar. 30, 2021)
First record of Ubaghsicystis (Eocrinoidea - Echinodermata) from the
Cambrian (Miaolingian, Wuliuan) of Sonora, Mexico:
Biostratigraphical and paleoecological considerations
Blanca Estela Buitrón Sánchez
Francisco Javier Cuen-Romero
Matilde Sylvia Beresi
Rogelio Monreal
1. Departamento de Paleontología, Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad
Universitaria, México; blancab@unam.mx
2. Departamento de Geología, Universidad de Sonora, Blvd. Luis Encinas y Rosales, Hermosillo, Sonora, México;
francisco.cuen@ciencias.uson.mx (*Correspondence).
3. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales, Consejo Nacional de Investigaciones
Científicas y Tecnológicas, Av. A. Ruiz Leal s/n, Parque General San Martín, Mendoza, Argentina;
Received 25-V-2020. Corrected 07-IX-2020. Accepted 15-IX-2020.
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 environ-
ment. Objective: The main objective of this research work is to describe the species Ubaghsicystis cf. U. segu-
rae 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 corre-
sponding 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 asso-
ciation 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.
Key words: Eocrinoidea; Cambrian; biostratigraphy; paleoecology; Mexico.
Buitrón Sánchez, B.E., Cuen-Romero, F.J., Beresi,
M.S., & Monreal, R. (2021). First record of
Ubaghsicystis (Eocrinoidea - Echinodermata) from
the Cambrian (Miaolingian, Wuliuan) of Sonora,
Mexico: Biostratigraphical and paleoecological
considerations. Revista de Biología Tropical, 69(S1),
51-61. DOI 10.15517/rbt.v69iSuppl.1.46326
DOI 10.15517/rbt.v69iSuppl.1.46326
Revista de Biología Tropical, ISSN electrónico: 2215-2075 Vol. 69(S1): 51-61, March 2021 (Published Mar. 30, 2021)
Cambrian sedimentary rocks in Mexico
are characterized by being exposed in scarce
and isolated outcrops, but having a relatively
well-preserved biota, which has been studied
since the middle of the last century (Buitrón,
1992). The Cambrian in Sonora is exposed
in various isolated locations: Caborca, Cana-
nea, San José de Gracia, Mazatán, and Arive-
chi. These areas have been studied by several
authors (Lochman, 1948; Cooper et al., 1952;
Cooper et al., 1956; Stewart, McMenamin, &
Morales-Ramirez, 1984; González-León, 1986;
McMenamin, 1987; Almazán-Vázquez, 1989;
Vega-Granillo, 1996; Cuen-Romero et al., 2016;
Cuen-Romero et al., 2018; Cuen-Romero et al.,
2019; Beresi, Buitrón-Sánchez, Cuen-Romero,
& Palafox, 2019). The recent discovery of
Cambrian outcrops in central Sonora has been
the subject of several stratigraphic and paleon-
tological studies (Nardin, Almazán-Vázquez,
& Buitrón-Sánchez, 2009; Cuen et al., 2013;
Buitrón-Sánchez, Corona-González, Cuen-
Romero, Palafox-Reyes, & Ramírez-Guerrero,
2016; Buitrón-Sánchez, Cuen-Romero, Huerta-
Ruiz, & Montijo-González, 2017a; Buitrón-
Sánchez, Cuen-Romero, Montijo-González, &
Beresi, 2017b; Cuen-Romero et al., 2016; Cuen-
Romero et al., 2019). This research work was
carried out in a new Cambrian locality in the
central part of the state of Sonora, El Sahuaral
Hill, which has been scarcely studied but has an
abundant fossil biota. The first geologic work
of the Sahuaral Hill was carried out by Stewart,
Amaya-Martínez and Palmer (2002), who men-
tioned the presence of Cambrian rocks in the
area. Danzos-Acuña (2018) and Noriega-Ruiz
(2019), carried out a detailed biostratigraphic
study of the rocks in the region. However, the
first formal work carried out in the area is that by
Beresi et al. (2019), who documented the pres-
ence of reticulosan sponges and chancellorids.
Recently, Noriega-Ruiz et al (2019; 2020), car-
ried out a detailed stratigraphic study of the area.
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) stud-
ied to Gogia granulosa and Gogia sp. from the
same area, documenting the paleoecological
implications of these species as they are asso-
ciated with limestone, shale, and sandstone
strata, thus, showing great adaptability to dif-
ferent environments.
Regarding eocrinoids, Sprinkle (1973)
proposed the new subphylum Blastozoa
divided into four classes of extinct Paleo-
zoic “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 func-
tions, and with the presence of various respira-
tory 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.
A composite stratigraphic section of El
Gavilán Formation was measured at the south-
west flank of El Sahuaral Hill. Samples were
collected corresponding to carbonate and detri-
tal 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 pale-
ontological 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
Revista de Biología Tropical, ISSN electrónico: 2215-2075, Vol. 69(S1): 51-61, March 2021 (Published Mar. 30, 2021)
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 pres-
ence 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 hyo-
lithids, followed by limestone with oncolytic
algae, ending with dark-colored sandy lime-
stone. 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 con-
tacts 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 dolomitiza-
tion. It is 53 m thick, being exposed to the
southeast side of El Sahuaral Hill. The lower
contact is conformable with the Buelna For-
mation, 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 vary-
ing 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 conform-
able 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
Fig. 1. Location of the study area, northeast of Hermosillo, Sonora, Mexico.
Revista de Biología Tropical, ISSN electrónico: 2215-2075 Vol. 69(S1): 51-61, March 2021 (Published Mar. 30, 2021)
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).
Type species: Ubaghsicystis segurae Gil-
Cid & Domínguez-Alonso, 2002
Ubaghsicystis cf. U. segurae
Fig. 4A-4D
Occurrence: The El Gavilán Formation,
Sahuaral Hill, middle Cambrian (Miaolin-
gian, Wuliuan), Altiocculus Subzone,
Ehmaniella Zone.
Material: Two slabs with isolated plates
of the specimens USDG 354 and USDG 355.
The material is deposited in the Paleontological
Collection, Geology Department, University of
Sonora, Mexico.
Fig. 2. Stratigraphic column of the studied area. Where Sy. = System, Se. = Series, St. = Stage, Th. = Thickness, Fm. =
Formation. Modified after Beresi et al. (2019).