Revista de Biología Tropical, ISSN electrónico: 2215-2075, Vol. 69(S1): 35-50, March 2021 (Published Mar. 30, 2021)

Diversity and biostratigraphy of the late Oligocene-late Miocene sand

dollars (Echinoidea: Scutelliformes) of Argentina and Uruguay

Claudia J. del Río

Sergio Martínez

1. Museo Argentino de Ciencias Naturales-Consejo Nacional de Investigaciones Científicas y Técnicas de Argentina,

Buenos Aires, Argentina; claudiajdelrio@gmail.com

2. Facultad de Ciencias, Universidad de la República, Montevideo, Uruguay; smart@fcien.edu.uy (*Correspondence).

Received 19-V-2020. Corrected 21-VIII-2020. Accepted 07-IX-2020.

ABSTRACT

Introduction: Scutelliforms were diverse and widespread in shallow marine environments during Neogene

times in South America. Nevertheless, they have almost never been used as biostratigraphic tools. Objective:

To provide a refined stratigraphic frame useful for calibrating temporal dimensions of scutelliform diversity

from Argentina and Uruguay and its correlation with the molluscan assemblages previously proposed. Methods:

A detailed survey of their geographic and stratigraphic provenance was carried out. We revised both the bibli-

ography and collections (institutional and from our own field work). Results: The group is represented by 14

species belonging to six genera, and four assemblages were identified. Numerical dates of the Neogene marine

rocks obtained recently allowed their placement in a chronological scheme: “Iheringiella” sp. A is restricted

to the late Oligocene, the genera Camachoaster and “Eoscutella” and the species Monophoraster telfordi to

the early Miocene, Abertella gualichensis and Abertella miskellyi to the middle Miocene, and Monophoraster

duboisi, Amplaster coloniensis and Amplaster ellipticus to the late Miocene. Non-lunulate scutelliforms are not

restricted to the late Oligocene as previously supposed. The oldest occurrence of the genus Monophoraster cor-

responds to the early Miocene, and along with Iheringiella are long-living taxa that embrace the 25.3 Ma-18.1

Ma (Iheringiella patagonensis) and approximately 15 Ma-6.48 Ma (Monophoraster darwini) intervals. The

presence of Iheringiella in the early Miocene of northeastern Patagonia is corroborated, reaching there its north-

ernmost distribution. Monophoraster darwini has a temporal range from the late Miocene (where it was previ-

ously thought to be restricted) back to the middle Miocene, since this is the species yielded in the well-known

and discussed “Monophoraster and Venericor Beds”. Conclusions: The Paleogene-Neogene scutelliforms of

Argentina and Uruguay range from the late Oligocene to the late Miocene. There is a good correspondence

among the numerical ages, molluscan biozones and scutelliform assemblages.

Key words: Echinoidea; Scutelliformes; sand dollars; Paleogene; Neogene; Argentina; Uruguay.

del Río, C.J., & Martínez, S. (2021). Diversity and

biostratigraphy of the late Oligocene-late Miocene

sand dollars (Echinoidea: Scutelliformes) of

Argentina and Uruguay. Revista de Biología Tropical,

69(S1), 35-50. DOI 10.15517/rbt.v69iSuppl.1.46324

In the Southwestern Atlantic region, a

thick, shallow marine sedimentary sequence

characterized by a highly diverse molluscan

fauna is exposed, associated with sometimes

dense concentrations of scutelliforms. While

the molluscs attracted the attention of pale-

ontologists since the 19th century by means

of d’ Orbigny’s (1842) and Darwin’s (1839,

1846) publications, echinoids remained largely

ignored. It was not until the last few decades

that the taxonomic knowledge and the geo-

graphic and stratigraphic provenance of south-

ern South American scutelliforms increased and

improved, pointing to a diverse and widespread

DOI 10.15517/rbt.v69iSuppl.1.46324

Revista de Biología Tropical, ISSN electrónico: 2215-2075 Vol. 69(S1): 35-50, March 2021 (Published Mar. 30, 2021)

group that was abundant in the shallow marine

environments during Neogene times.

Echinoids, and particularly scutelliforms,

have almost never been used as biostratigraphic

tools in South America. The scarce references

dealing with this group as stratigraphic tools

are those of Ameghino (1898, 1906), Iher-

ing (1897, 1900, 1907, 1927), and Ortmann

(1902), who claimed that the “one-holed”

genus Monophoraster was restricted to the late

Miocene of Patagonia (“Entrerriense strata”)

and the “non-holed” Iheringiella to the Oligo-

cene (“Patagoniense strata”).

Ongoing numerical dating provides a

refined stratigraphic frame extremely useful in

calibrating the temporal dimensions of scutel-

line diversity and its correlation with the mol-

luscan assemblages proposed by del Río (1988,

2004). In order to attain this goal, we performed

a synthesis of the available information, bring-

ing together the highly dispersed taxonomic

bibliography with every stratigraphic and geo-

graphic distribution adequately documented.

This analysis indicates that scutelliforms are

stratigraphically constrained in this region, and

that along with the molluscan assemblages,

they are useful in the correlation of marine

units from different Patagonian basins.

Antecedents: Early collections from east-

ern Patagonia were made by Charles Darwin

during the voyage of the “HMS Beagle” along

the Patagonian coast in 1833-1834, and by Car-

los Ameghino during his 15 field trips between

1886 and 1903. In the course of the Princ-

eton Expedition (1896-1899) John B. Hatcher

made a large collection in the surroundings of

Argentino, Pueyrredón, and Posadas lakes in

western Patagonia.

Darwin collected sand dollars from the stra-

ta later known as San Julián and Puerto Madryn

Formations, mentioned as Scutella in his “Jour-

nal…” (Darwin, 1839) and “Geological obser-

vations...” (Darwin, 1846). Based on Darwin´s

specimens, Desor (1847) described three nomi-

nal species in a brief account, naming them as

Monophora darwini, Echinarachnius juliensis,

and Scutella patagonensis. Subsequently, Cot-

teau (1884) added Monophora duboisi from

the Paraná Formation, a unit exposed in north-

eastern Argentina. Lahille (1898) synonymized

Desor´s E. juliensis and S. patagonensis under

a new and monotypic genus: Iheringia (type

species: Iheringia patagonensis). Homonymy

problems led Berg (1898) and Lambert and

Thiéry (1921) to change Iheringia to Iheringi-

ella and Monophora to Monophoraster.

Thereafter, and for many years, scutel-

liforms from Argentina and Uruguay were

believed to be represented by only three spe-

cies: the one-holed Monophoraster darwini

and M. duboisi, and the non-holed Iheringiella

patagonensis (e.g. Loriol, 1901; Loriol,1902;

Ortmann, 1902; Mortensen, 1948; Bernasconi,

1959; Durham, 1966). The genus Amplaster

was added to these by Martínez (1984) to hold

the late Miocene Uruguayan species Amplas-

ter coloniensis. Later, the middle Miocene

Amplaster alatus Rossi de Garcia and Levy,

1989 and the late Miocene Amplaster ellip-

ticus Mooi, Martínez and Parma, 2000 were

also included in this genus. Abertella, a genus

that had been thought to be restricted to North

America, was discovered in the Argentinean

middle Miocene by Martínez, Reichler and

Mooi (2005) and later, another new species

of this age was described by Kroh, Mooi, del

Río and Neumann (2013). Mooi, Martínez

and del Río (2016) increased the specific rich-

ness of Monophoraster through the inclusion

of the new species, Monophoraster telfordi

(early Miocene), and Mooi, Martínez, del Río

and Ramos (2018) described another non-

holed genus from the Neogene of Argentina:

Camachoaster.

There remain some controversial species:

“Eoscutella” mirandae, which as discussed

by Mooi et al. (2018) is not an Eoscutellidae

but probably an Abertellidae, and two unde-

scribed taxa that further increase the rich-

ness of the group (“Iheringiella” sp. A and

“Iheringiella” sp. B).

In total, since the three species were rec-

ognized in the early 20th century, the group

now includes 14 species distributed in six gen-

era and four families that are endemic to the

American continent.

Revista de Biología Tropical, ISSN electrónico: 2215-2075, Vol. 69(S1): 35-50, March 2021 (Published Mar. 30, 2021)

MATERIALS AND METHODS

We carried out a bibliographic analysis that

retained only those references with adequate

descriptions or illustrations concerning South

American scutelliforms, and re-examined more

than 300 specimens housed in different institu-

tions, including their types when available.

To the previously known classic fossilifer-

ous sites mentioned in the geological literature,

we add other new ones (Fig. 1).

The repositories and their acronyms are as

follows:

CASG: California Academy of Sciences, Geo-

logy collection, San Francisco, USA.

CIRGEO-PI: Centro de Investigaciones en

Recursos Geológicos, currently housed at

MACN-Pi, Ciudad Autónoma de Buenos

Aires, Argentina.

CPBA: Cátedra de Paleontología de la Univer-

sidad de Buenos Aires, Ciudad Autónoma

de Buenos Aires, Argentina.

FCDP: Departamento de Paleontología, Facul-

tad de Ciencias, Universidad de la Repú-

blica, Montevideo, Uruguay.

MACN-Pi: Colección Paleoinvertebrados,

Museo Argentino de Ciencias Naturales

Bernardino Rivadavia, Ciudad Autónoma

de Buenos Aires, Argentina.

MAS-Pi: Museo de Ciencias Naturales y Antro-

pológicas “Pro. Antonio Serrano”, Paraná,

Argentina.

MB: Museum für Naturkunde, Berlin, Germany.

MCZ: Museum of Comparative Zoology, Cam-

bridge, Massachusetts, USA.

MNA-CPO: Museo Nacional de Antropolo-

gía, Colección Paleontológica Oliveras,

currently housed at the Museo Nacional

de Historia Natural, Montevideo, Uruguay.

PRI: Paleontological Research Institution, Itha-

ca, USA.

SEGEMAR: Servicio Geológico Minero

Argentino, San Martín, Buenos Aires,

Argentina.

RESULTS

Stratigraphic setting of scutelliform

beds: The shallow marine late Oligocene-late

Miocene sedimentary sequence exposed in

Argentina and Uruguay is represented by the

San Julián Formation (late Oligocene in the

Austral Basin, early Miocene in the Mazarredo

Sub-basin), El Chacay, Estancia 25 de Mayo

and Monte León Formations (early Miocene,

Austral Basin), Chenque Formation (early

Miocene-middle Miocene, San Jorge Basin),

Gaiman, Bajo del Gualicho, Vaca Mahuida

Formations, and the “Isla Escondida Beds”

(middle Miocene) and the Puerto Madryn,

Río Negro, Paraná and Camacho Formations

(late Miocene).

An important framework for the analysis

is made up of the late Paleogene-Neogene

molluscan assemblages and their relative ages

proposed by del Río (1988, 2004). Late Mio-

cene molluscs were gathered in the Aequi-

pecten paranensis Zone (del Río, 1988), and

the late Oligocene-middle Miocene ones were

separated by del Río (2004) into five informal

assemblages known as: 1) Panopea sierra-

na-Parinomya patagonensis Assemblage (PP)

(late Oligocene); three assemblages of early

Miocene age: 2) Jorgechlamys centralis-Retic-

ulochlamys borjasensis Assemblage (JR), 3)

Reticulochlamys zinsmeisteri-Struthiolarella

patagoniensis-Pleuromeris cruzensis Assem-

blage (RSP), and 4) Pseudoportlandia glabra-

Antimelatoma quemadensis Assemblage (PA);

and the middle Miocene: 5) Nodipecten sp.-

Venericor abasolensis-Glycymerita camarone-

sia Assemblage (NVG).

These assemblages were subjected to

numerical dating, and the results support their

relative ages. As shown in Fig. 2, the PP, PA

and RSP Assemblages are restricted to the

Eastern sector of the Austral Basin and have

not yet been identified in Western Patagonia.

The PP Assemblage is recognized in the San

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Fig. 1. Late Oligocene-late Miocene localities with scutelliforms in Argentina and Uruguay. 1. Yacimiento La Rinconada;

2. Puesto Astorga; 3. Barranca Final; 4. Paraná-Diamante area; 5. Barranca de los Loros; 6. Cerro Bautista; 7. El Manzano;

8. Arroyo de las Limetas; 9. San Pedro; 10. El Doradillo; 11. Cerro Prismático; 12. Eje Tentativo; 13. Punta Piaggio-Punta

Pirámides area; 14. Lote 39; 15. Salina Grande; 16. Estancia San Lorenzo; 17. Cerro Avanzado; 18. Bahía Craker; 19.

Trelew; 20. Isla Escondida; 21. surroundings of Camarones; 22. Pico Salamanca; 23. Estancia Busnadiego; 24. Punta

Maqueda-Punta Delgada; 25. Punta Casamayor-Cañadón El Lobo; 26. Punta Nava; 27. Cerro Blanco; 28. Estancia

Floradora; 29. Playa La Mina; 30. Playa Drake/Punta Cuevas; 31. Oven Point; 32. Cerro Pan de Azúcar; 33. Meseta Chica;

34. Darwin Section; 35. Yegua Quemada-Jack Harvey creeks; 36. Río Zeballos; 37. Río Tarde-Veranada Cárcamo-Laguna

Oriental; 38. Shell-Gap; 39. upper Chalía river; 40. Estancia La Laurita; 41. Estancia Quién Sabe.

Revista de Biología Tropical, ISSN electrónico: 2215-2075, Vol. 69(S1): 35-50, March 2021 (Published Mar. 30, 2021)

Julián Formation (25.32 Ma-22.7 Ma), and the

PA and RSP ones are restricted to the Monte

León Formation (19.36 Ma-17.91 Ma).

The JR Assemblage embraces the early

Miocene deposits of the San Julián Formation

in the Mazarredo Sub-basin and the basal Para-

sequences 1 and 2 of the Chenque Formation

in the San Jorge Basin (19.69 Ma-15.68 Ma).

On the basis of its paleontological content, the

JR Assemblage was correlated with the PA and

RSP Assemblages by del Río (2004), a correla-

tion later supported by numerical ages obtained

by Parras, Dix and Griffin (2012) and Cuitiño,

Scasso, Ventura-Santos and Mancini (2015a).

The NVG Assemblage was recognized

by del Río (2004) in Parasequence 5 of the

Chenque Formation, and in the Gaiman and

Camarones Formations, and in the lower and

middle section of the Bajo del Gualicho For-

mation. There are no numerical ages for these

units, but the NVG is considered as of middle

Miocene age.

Finally, the Aequipecten paranensis Zone

is recognized in the Puerto Madryn, Bar-

ranca Final, Río Negro (Facies Balneario La

Lobería), Paraná and Camacho Formations that

embrace the interval 11.4-6.60 Ma (del Río,

Martínez, McArthur, Thirwall, & Pérez, 2018).

Fig. 1 shows the fossiliferous localities

with sand dollars, Figure 2 the lithostratigraphic

units mentioned above, Fig. 3 the geographical

and temporal distribution of the studied taxa,

Fig. 2. Chronostratigraphic chart of late Oligocene-late Miocene marine units based on

Sr/

Sr data calculated by: 1. Parras

et al. (2012); 2. Cuitiño et al. (2015b); 3. Cuitiño et al. (2012); 4. Cuitiño et al. (2015a); 5. del Río et al. (2018); 6. Palazzesi

et al. (2014); (*) no isotopic data available for these units. Abbreviations: PP = Panopea sierrana-Parinomya patagonensis

Assemblage; RSP = Reticulochlamys zinsmeisteri-Struthiolarella patagoniensis-Pleuromeris cruzensis Assemblage;

JR = Jorgechlamys centralis-Reticulochlamys borjasensis Assemblage; NVG = Nodipecten sp.-Venericor abasolensis-

Glycymerita camaronesia Assemblage (modified from del Río et al., 2018).

Revista de Biología Tropical, ISSN electrónico: 2215-2075 Vol. 69(S1): 35-50, March 2021 (Published Mar. 30, 2021)

Fig. 3. Temporal and geographic distribution of late Oligocene-late Miocene scutelliforms of Argentina and Uruguay

(fossiliferous sites in Fig. 1).

Revista de Biología Tropical, ISSN electrónico: 2215-2075, Vol. 69(S1): 35-50, March 2021 (Published Mar. 30, 2021)

Fig. 4 the correspondence between molluscan

assemblages and scutelliforms. The appendix

1 includes the positively identified materials

with their temporal, lithostratigraphic, and geo-

graphic locations.

Late Oligocene: Fig. 5. This interval is

represented by the San Julián Formation where

sand dollars are very frequent found, contained

in rocks exposed in two areas near to each

other: the Bajo de San Julián (lower Gran Bajo

Member), and the surroundings of the San

Julián Port (upper Meseta Chica Member).

The Gran Bajo Member (Meseta Chica, Pan

de Azúcar, Manatial Salado) contains one of

the densest concentrations of scutelliforms in

Patagonia, represented by Iheringiella patago-

nensis and by two as yet undescribed new

species (“Iheringiella” sp. A and “Iheringiella”

sp. B). Dated at 24.40 Ma and 25.32 Ma (late

Oligocene) (Parras et al., 2012), these beds

record the oldest proven occurrence of scu-

telliforms in Patagonia, and are constituted

by matrix- and bioclast-supported calcareous

bioclastic sandstones (facies Gmm of Parras &

Casadío, 2005) deposited below fair-weather

wave-base, in lower shoreface to offshore envi-

ronments. In the Meseta Chica section, sand

dollars are exceptionally abundant and found

in multi-event deposits where monospecific

beds with well-preserved imbricated or chaoti-

cally oriented entire tests are intercalated with

monospecific, thin horizons of brachiopods,

Zygochlamys geminata, or oysters in life posi-

tion. A similar dense concentration is present in

the surroundings of the San Julián Port (Playa

La Mina, Oven Point, Playa Drake/Punta Cue-

vas). In this area, the Meseta Chica Member

was dated by Sr- isotope chronostratigraphy

at 22.7 Ma to 23.53 Ma (Parras et al., 2012)

and is characterized by multi-event, massive,

medium, ochreous sandstones that contain in

Fig. 4. Correspondence between the temporal range of the studied scutelliforms and that of the molluscan assemblages.

Abbreviations in Results.

Revista de Biología Tropical, ISSN electrónico: 2215-2075 Vol. 69(S1): 35-50, March 2021 (Published Mar. 30, 2021)

situ accumulations of I. patagonensis and the

undescribed species, “Iheringiella” sp. B.

Early Miocene: Fig. 6. Marine deposits of

early Miocene age are widely exposed in the

eastern (Monte León Formation) and western

(Estancia 25 de Mayo and El Chacay Forma-

tions) sectors of the Austral Basin, and in the

San Jorge Basin (lower Chenque Formation

and San Julián Formation).

In contrast to the San Julián Formation,

scutelliforms are not frequent in the Monte

León Formation, where they are represent-

ed only by I. patagonensis recovered from

the finely laminated siltstones and sandstones

exposed at the Darwin Section, suggesting a

subtidal- or even intertidal environment (Par-

ras & Griffin, 2009), deposited between 19.36

Ma-19.27 Ma ago (Parras et al., 2012). Ihering-

iella patagonensis is also recorded from the

uppermost beds of the unit exposed at Yegua

Quemada and Jack Harvey, both localities

being correlated with the Las Cuevas section,

dated at 17.91 Ma (Parras et al., 2012).

Largely correlated with the San Julián

Formation in its type area (southern Austral

Basin), the age of the marine rocks exposed

in the Mazarredo Sub-basin of Bellosi (1995)

(San Jorge Basin) are today recognized as

early Miocene (del Río et al., in prep.), and

correlated with the Monte León Formation at

its type locality (southeastern Austral Basin).

Exposures in Punta Casamayor-Cañadón El

Lobo-Punta Nava area consist of 40 m thick

yellowish fine to medium sandstones and silt-

stones capped by calcareous shell-beds, where

echinoids are represented by I. patagonensis,

Hypechinus patagonensis, Schizaster sp., and

Isechinus praecursor, associated with abun-

dant, articulated specimens of the pectinids

Zygochlamys jorgensis and Jorgechlamys cen-

tralis. The same deposits are exposed at Cerro

Blanco where the base of the San Julián For-

mation is represented by massive yellowish

medium sandstones containing “Eoscutella”

mirandae followed by highly fossiliferous

sandstones containing I. patagonensis associ-

ated with Neoinoceramus ameghinoi, J. cen-

tralis, J. juliania, and brachiopods (Parma,

1985). The I. patagonensis reference from

Cerro Blanco was not included in our database

because the material was not illustrated and

there are no specimens in the collections. This

record is considered unconfirmed herein.

Fig. 5. Representative scutelliforms from the late Oligocene of Argentina, aboral and oral surfaces shown in each pair of

figures for a given species. A. Iheringiella patagonensis, PRI 66829, Oven Point. B. Iheringiella patagonensis MACN-Pi

5144, Pan de Azúcar. C. “Iheringiella” sp. A, MACN-Pi w/n, Meseta Chica. D. “Iheringiella” sp. B, MACN-Pi w/n, Meseta

Chica. E. “Iheringiella” sp. B, CPBA 20181, Playa La Mina. Scale bar = 10 mm.

Revista de Biología Tropical, ISSN electrónico: 2215-2075, Vol. 69(S1): 35-50, March 2021 (Published Mar. 30, 2021)

The Chenque Formation is composed of

a 500 m thick marine sedimentary sequence

exposed in the surroundings of Punta Maqueda

(Parasequence 2 of Bellosi, 1995) northwards

to the Comodoro Rivadavia region (Parase-

quences 1-5). Its lowermost strata (Parase-

quences 1 and 2) were dated by Sr-isotope

chronostratigraphy by Cuitiño et al. (2015a) to

between 19.69 Ma-15.68 Ma (early Miocene-

basal middle Miocene). The Chenque Forma-

tion exposed at Punta Maqueda is represented

by a 12 m thick intercalation of grayish fine

sandstones with ochreous and greenish, mod-

erately fossiliferous sandstones, deposited in

a distal lower shoreface environment at the

base, to a middle shoreface environment at

the top of the section. One thin bed, up to 15

cm thick and composed of very fine-grained

sandstones, contains one of the most diverse

assemblages of echinoderms in the Neogene of

Argentina and Uruguay. These are represented

by patches of asteroids (Astropecten sp.), the

ophiuroid Ophiocrossota kollembergorium, the

spatangoid Brisaster iheringi, the non-lunulate

scutelliform Camachoaster maquedensis, and

the lunulate Monophoraster telfordi. These

taxa are recorded in thin, autochthonous assem-

blages in life position, deposited in distal-

middle shoreface environments, dominated by

a low hydraulic energy regime and affected by

sporadic, weak storms. Laterally associated

is a less diverse but abundant fauna, mainly

made up of census assemblages of the bivalves

Atrina magellanica and Crassostrea hatcheri

in life position, and thin, chaotically oriented

accumulations of the pectinids Swiftopecten

iheringi, Z. jorgensis, Pixiechlamys quemaden-

sis, and the gastropod Trophon santacruzensis.

Fig. 6. Representative scutelliforms from the early Miocene of Argentina, aboral and oral surfaces shown in each pair of

figures for a given species. A-G. Iheringiella patagonensis. A. CPBA 16492, Punta Lobo. B. PRI 4530, Punta Nava. C.

MACN 4547, Yegua Quemada. D. PRI 66838, Pueyrredón Lake. E. PRI 66836, Shell-Gap. F. PRI 66834, Río Chalía. G.

PRI 83920, Estancia La Laurita. H. “Iheringiella” sp. B, PRI 83635, Estancia La Laurita. I. “Iheringiella” sp. B, MACN-Pi

w/n, Estancia Floradora. J. Camachoaster maquedensis MACN-PI 5809 (Holotype), Punta Maqueda (modified from Mooi

et al., 2018). K. Monophoraster telfordi, MACN-PI 5807 (Holotype), Punta Maqueda (modified from Mooi et al., 2016). L.

“Eoscutella” mirandae, CPBA 12901 (Holotype), Cerro Blanco. Scale bar = 10 mm.

Revista de Biología Tropical, ISSN electrónico: 2215-2075 Vol. 69(S1): 35-50, March 2021 (Published Mar. 30, 2021)

The Estancia 25 de Mayo and El Chacay

Formations are exposed in the Western Austral

Basin. The former unit crops out in the Lago

Argentino area and was dated by Cuitiño et al.

(2012), who proposed an

Sr/

Sr age ranging

between 20 to 19 Ma. Sand dollars are rare and

recorded in the lowermost beds of the Quien

Sabe Member (Estancia Quien Sabe, Estancia

La Laurita), which is composed of shell beds

and highly bioturbated sandstones deposited in

shallow, moderately low energy environments

(Cuitiño & Scasso, 2010). The recognized

scutelliforms are I. patagonensis and “Iheringi-

ella” sp. B, associated with a poorly preserved

and rich molluscan fauna mainly represented

by C. hatcheri, Panopea nucleus, Panopea

panis, undetermined cardids, Austrocallista

australis, Ameghinomya argentina, Miomelon

petersoni, Valdesia cuevensis, Valdesia dalli,

and Perissodonta ameghinoi.

The El Chacay Formation, exposed

between the Lago Buenos Aires to the north

and the Lago Posadas to the south, was dated

by Cuitiño,Ventura-Santos, Muruaga and Scas-

so (2015b) between 20.3 Ma to18.1 Ma, and

scutelliforms are slightly more common and

diverse than in the Estancia 25 de Mayo For-

mation. The assemblages mostly come from

the highly bioturbated sandy beds intercalated

with bioclastic horizons placed at the lower

middle part of the unit (Facies B, lithofacies

Smb of Cuitiño et al., 2015b), deposited in

a lower shoreface environment (Cuitiño et

al., 2015b). Main fossiliferous exposures are:

Río Zeballos, Río Tarde, Veranada Cárcamo,

Laguna Oriental and Shell-Gap. Iheringiella

patagonensis is associated with the echinoids

Platipygus posthumus, Isechinus praecursor

and Brisaster sp. (Chiesa, Parma & Camacho,

1995), and to a rich molluscan fauna repre-

sented by C. hatcheri, Z. jorgensis, A. argen-

tina, P. nucleus, Valdesia collaris, Perissodonta

ameghinoi, Cirsotrema rugulosa, M. petersoni,

and Miomelon orbignyana.

Middle Miocene: Fig. 7. Marine beds of

this age correspond to the uppermost strata of

the Chenque Formation and to other lithostrati-

graphic units that have not yet been studied

in detail: Vaca Mahuida, Bajo del Gualicho,

Gaiman, and Camarones Formations, and the

Isla Escondida Beds. Except for the Chenque

Formation, there are no numerical dates for

these units, and in some cases there are still

discrepancies about their relative ages. The

presence of the NVG molluscan assemblage

makes them contemporaneous with the middle

Miocene upper beds of the Chenque Forma-

tion. Echinoids are not frequent elements but

constitute a moderately high diversity group.

The uppermost strata of the Chenque

Formation exposed at Estancia Busnadiego

Fig. 7. Representative scutelliforms from the middle Miocene of Argentina, aboral and oral surfaces shown in each pair of

figures for a given species except in B and F. A-C. Monophoraster darwini. A. MACN-PI 4580, Estancia Busnadiego. B.

CPBA 8740, Pico Salamanca. C. MACN-Pi w/n, Bajo del Gualicho. D. Abertella gualichensis, MACN-PI 4714 (Holotype),

Bajo del Gualicho. E. Abertella miskellyi, MB E.7463 (Holotype), near Camarones City (modified from Kroh et al. 2013).

F. Amplaster alatus, SEGEMAR 15526 (Paratype), Estancia Isla Escondida. Scale bar = 10 mm.

Revista de Biología Tropical, ISSN electrónico: 2215-2075, Vol. 69(S1): 35-50, March 2021 (Published Mar. 30, 2021)

and Pico Salamanca (Parasequence 5 of Bel-

losi, 1995) have been named as “Estratos

con Monophoraster y Venericor” by Cama-

cho (1974) and although they have not been

numerically dated, are surely younger than

the underlying Parasequence 3 (15.37 Ma),

and older than the beginning of the deposition

of the overlying continental sediments of the

Santa Cruz Formation dated to 15 Ma (Cuitiño

et al., 2015b), embracing in this way the middle

Miocene. These beds are characterized by

Monophoraster darwini, associated with the

bivalves Neovenericor austroplata, Glycymer-

ita camaronesia, A. argentina, Retotapes

striatolamellata, and the gastropods Pachy-

cymbiola ameghinoi, Penion subrecta, Perisso-

donta iheringii, and Perissodonta ameghinoi,

among the commonest species.

Exposures of the Vaca Mahuida Formation

have been recorded by Uliana and Camacho

(1975) and Concheyro (1988) south of the

Colorado River in a narrow strip that stretches

from the boundary between the Río Negro,

Neuquén and La Pampa provinces eastwards to

Lomita Colorada. These beds consist of a 50 m

thick layer of sandstones, mudstones, and mas-

sive or cross-stratified limestones, deposited

in intertidal and subtidal environments. Uliana

and Camacho (1975) found a poorly diversified

and scarce marine assemblage at Yacimiento

La Rinconda, represented by the molluscs N.

austroplata and Torcula hautali, and identified

a scutelliform as I. patagonensis. Unfortunate-

ly, that material is lost, and illustrations pro-

vided are not conclusive. Scutelliforms from

the Vaca Mahuida Formation housed at the

Cátedra de Paleontología (Universidad de Bue-

nos Aires, Argentina) (CPBA 15842-15849)

are the only ones available for study, but these

specimens were severely encrusted, hamper-

ing a proper taxonomic assignment. For these

reasons, this material has not been included in

the appendix 1.

The Bajo del Gualicho Formation is

exposed at Salinas del Gualicho, and its lower

and middle sections were correlated by del

Río (2004) with the upper part of the Chenque

Formation on the basis of the NVG Molluscan

Assemblage. At Puesto Astorga, scutelliforms

are found in thin, intercalated monospecific

accumulations of M. darwini or Abertella gua-

lichensis, floating in massive medium sand-

stones. Upwards in the section, fragments or

entire tests of both species are found together,

associated with N. austroplata, Nodipecten

salis and Pachycymbiola arriolensis.

In the surroundings of Gaiman and Trelew

cities, the Gaiman Formation is composed of a

50 m thick succession of thin coquinas, whitish

to yellowish tuffs, tuffaceous sandstones, and

tuffaceous mudstones, deposited in a shallow,

storm-dominated marine environment. Badly

preserved tests of M. darwini are recorded

in the uppermost shell-beds associated with

N. austroplata.

Kroh et al. (2013) described Abertella

miskellyi from an old and still unknown fos-

siliferous locality in the surroundings of Cama-

rones city, where the Camarones Formation is

exposed containing the typical NVG Molluscan

Assemblage represented by N. austroplata, G.

camaronesia and T. hautali.

There is little agreement among authors in

identifying the marine “Isla Escondida beds”.

The sedimentary succession exposed in this

area consists of a 150 m thick intercalation of

whithish, ochreous, and yellowish sediments

represented by tuffs, tuffaceous sandstones,

tuffaceous mudstones, sandy tuffs, and poorly

fossiliferous beds. Rossi de García and Levy

(1989) described Amplaster alatus from the

upper section, species later discussed by Mooi,

Martínez and Parma (2000). Amplaster alatus

is associated with N. austroplata, G. cama-

ronesia, and T. hautali, placing the unit in the

middle Miocene.

Late Miocene: Fig. 8. During the late

Miocene, a new transgression of the sea, a

product of various events spread over a large

area from Cabo Buentiempo (southernmost

Argentina) northwards to Uruguay. Its deposits

have been intensely studied and are recognized

as Puerto Madryn, Barranca Final, Río Negro

(Facies Balneario La Lobería), Paraná, and

Camacho Formations.

Revista de Biología Tropical, ISSN electrónico: 2215-2075 Vol. 69(S1): 35-50, March 2021 (Published Mar. 30, 2021)

The Puerto Madryn Formation records

one of the most abundant and dense accumula-

tions of scutelliform tests in the Neogene of

Patagonia, similar to those observed in the late

Oligocene San Julián Formation at Bajo San

Julián. The molluscan assemblage belongs to

the Aequipecten paranensis Zone (del Río,

1988), recently dated to between 11.4-9.25 Ma

(del Río et al., 2018). The only known scutel-

liform species is M. darwini, which is evenly

distributed from the base to the top of the

unit. Composition and taphonomy of the fos-

siliferous concentrations have been discussed

by del Río, Martínez and Scasso (2001), who

described two types of taphonomic accumula-

tions of scutelliforms: the MO Assemblage (M.

darwini and oysters) and the OMA Assemblage

(oysters, M. darwini, Aequipecten paranensis).

The first type is constituted by up to 4 cm

thick census assemblages of entire tests in life

position in medium to fine sandstones or in

heterolithic beds, deposited in a shallow and

very high-energy environment, with strong

wave action, in upper shoreface settings situ-

ated just seawards of the breaking wave zone.

The OMA Assemblage is not as common as the

MO Assemblage and is represented by highly

fragmented tests that may be part of autochtho-

nous single-event or of allochthonous multi-

event, time-averaged accumulations that infill

large erosive tidal channels of high-energy

shoreface environments.

The Barranca Final Formation and the

Facies Balneario La Lobería (Río Negro For-

mation) crop out in the cliffs situated along the

northern littoral of San Matías Gulf. The base

of the section at Balneario La Lobería consists

of 2 m of barren, ochreous, cross-stratified,

medium sandstones, overlain by 7 m of yel-

lowish, massive, fine sandstones, and yields

well preserved M. darwini in life position

associated with A. paranensis, Ostrea sp., and

Pododesmus camachoi. These beds have been

recently dated to 7.08-6.55 Ma by del Río et

al. (2018) using Sr-isotope chronostratigraphy.

The Barranca Final Formation, located 117

km west from Balneario La Lobería, consists

of an intercalation of mudstones, very fine

sandstones, and thin fossiliferous strata that

yield M. darwini, A. paranensis, Moirechlamys

actinodes, and oysters. According to Palazzesi

et al. (2014), the sediments were deposited in a

low-energy shallow marine/estuarine environ-

ment, dated in 9.61 Ma (lower section) to 6.48

Ma (middle part of the section).

Numerical values recently obtained by

del Río et al. (2018) place the Paraná Forma-

tion between 7.55-6.67 Ma. Scutelliforms are

scarce, being represented by Monophoraster

duboisi, A. alatus (mentioned herein for the

Fig. 8. Representative scutelliforms from the late Miocene of Argentina and Uruguay, aboral and oral surfaces shown

in each pair of figures for a given species except in E and F. A. Monophoraster darwini, FCDP 1789, Eje Tentativo. B.

Monophoraster darwini (col. Cuitiño thesis w/n), Barranca Final. C. Monophoraster duboisi, MNHN B33423, Paraná.

D. Abertella sp., MAS-Pi 511, Paraná. E. Amplaster coloniensis, MNA-CPO 3426 (Holotype), Barranca de los loros. F.

Amplaster ellipticus, MNA-CPO 3425 (Paratype), San Pedro. Scale bar = 10 mm.

Revista de Biología Tropical, ISSN electrónico: 2215-2075, Vol. 69(S1): 35-50, March 2021 (Published Mar. 30, 2021)

first time from this unit), and by an undescribed

new species of Abertella (Pérez et al., 2011)

(Fig. 8D) from Punta Gorda Sur (Diamante),

where a 2.5 m thick intercalation of sandstones

and mudstones are capped by a 30 cm thick

amalgamated shell bed that contains Anadara

bonplandeana, Crassostrea cf. rhizophorae, A.

paranensis, Leopecten oblongus, and Chionop-

sis munsterii (del Río et al., 2018).

The Camacho Formation (7.20 Ma-6.92

Ma) yields a richer assemblage than the Paraná

Formation, represented by M. duboisi, A. ala-

tus, Amplaster ellipticus, and Amplaster col-

oniensis, contained in shallow, subtidal pelitic

and sandy rocks, usually deposited by storms

(Martínez, 1994).

DISCUSSION

Fig. 4 shows the temporal distribution of

scutelliforms and its correlation with the mol-

luscan assemblages. No Paleogene-Neogene

scutelliforms older than the late Oligocene or

younger than the late Miocene have been found

or mentioned in the literature for the studied

area. The oldest occurrences belong to the

abundantly represented Iheringiella patago-

nensis in the Southern Austral Basin (San

Julián Formation, ca 25 Ma-23 Ma), associated

with the less common “Iheringiella” sp. A and

“Iheringiella” sp. B, approximately 25-23 Ma.

Iheringiella patagonensis survived until 18 Ma

(Monte León, El Chacay and Estancia 25 de

Mayo Formations). This species is also record-

ed in northern Patagonia (San Jorge Basin,

Mazarredo Sub-basin), in the San Julián For-

mation dated to 21 Ma (del Río et al., in prep.),

which represents the northernmost occurrence

of this species.

Around 18 Ma, scutelliforms were widely

distributed in Patagonia and represented by

the surviving Iheringiella patagonensis and

“Iheringiella” sp. B (Austral Basin), along with

Camachoaster maquedensis and Monopho-

raster telfordi (San Jorge Basin), this being

the oldest and southernmost known occur-

rence for the genus Monophoraster. At this

point it is important to emphasize that in

contrast to previous claims (e.g. Ameghino,

1898; Ameghino,1906; Ihering, 1897; Ihering,

1900; Ihering, 1907; Ihering, 1927; Ortmann,

1902; Teisseire, 1928; Bernasconi, 1959),

the non-holed genus Iheringiella was coeval

with the one-holed Monophoraster during the

early Miocene.

Iheringiella was extinct by the end of the

early Miocene, but Monophoraster, Amplaster,

and Abertella diversified to make up the char-

acteristic middle Miocene genera of northern

Patagonia, surviving into the late Miocene.

Monophoraster darwini and I. patagonensis

constituted long-living taxa, existing approxi-

mately from 15 Ma to 6.48 Ma and 25.3 Ma

to18.1 Ma, respectively. This temporal range is

longer than that estimated by molecular studies

in Recent related scutellines such as Mellita

and Encope (Coppard et al., 2013; Coppard,

2016; Coppard and Lessios, 2017), leaving

open a question concerning possible existence

of cryptic species among the fossil taxa.

Monophoraster darwini and Monopho-

raster duboisi are the youngest representatives

of the Monophoraster lineage, and along with

Amplaster coloniensis, Amplaster ellipticus

and Amplaster alatus characterized the latest

Miocene beds represented by the Paraná and

Camacho Formations exposed in northeastern

Argentina and Uruguay.

As can be concluded from the data sum-

marized above, the scutelliform species rich-

ness increased from three (two undescribed)

species in the late Oligocene to five in the

early Miocene and to six in the late Miocene.

Fig. 4 shows that Monophoraster embraces the

entire Miocene, discarding its importance as a

precise biostratigraphic tool. However, occur-

rences of Iheringiella (late Oligocene-early

Miocene), “Eoscutella” (early Miocene), and

Abertella and Amplaster (middle Miocene-late

Miocene), point to a more restricted interval,

and prove to be slightly better biostratigraphi-

cally constrained. On the other hand, at the

species level, the group as a whole turns

out to be a useful temporal indicator, since

there are taxa restricted to the late Oligocene

(“Iheringiella” sp. A), to the early Miocene

Revista de Biología Tropical, ISSN electrónico: 2215-2075 Vol. 69(S1): 35-50, March 2021 (Published Mar. 30, 2021)

(Camachoaster maquedensis, “Eoscutella”

mirandae, Monophoraster telfordi), to the mid-

dle Miocene (Abertella gualichensis, Abertella

miskellyi) and to the late Miocene (M. duboisi,

A. ellipticus, A. coloniensis).

When compared with the molluscan zones,

the middle and late Miocene scutelliform gen-

era Amplaster and Abertella correlate with the

NVG Assemblage (middle Miocene) and the

Aequipecten paranensis Zone (late Miocene).

Camachoaster and “Eoscutella” are restricted

to the early Miocene, with the early Miocene

JR Assemblage (San Jorge Basin and Mazar-

redo Sub-basin) and the PA and RSP Assem-

blages (Austral Basin). Only one taxon seems

to be restricted to the late Oligocene PP Assem-

blage (“Iheringiella” sp. A).

Ethical statement: authors declare that

they all agree with this publication and made

significant contributions; that there is no con-

flict of interest of any kind; and that we fol-

lowed all pertinent ethical and legal procedures

and requirements. All financial sources are

fully and clearly stated in the acknowledge-

ments section. A signed document has been

filed in the journal archives.

ACKNOWLEDGEMENTS

M. Tanuz, Beatriz Aguirre-Urreta from the

Universidad de Buenos Aires, M. Longobucco

from the Museo Argentino de Ciencias Natura-

les and Alejandra Rojas (Facultad de Ciencias,

UdelaR) assisted with material housed in the

mentioned institutions, J. Cuitiño and L. Pérez

kindly provided specimens for study, A. Kroh

and R. Mooi gave us permission to include

their photos in the present paper, and A. Oleinik

assisted us with the bibliography. The sug-

gestions of the reviewers Andreas Kroh, Rich

Mooi and James Nebelsick, were very useful

for improving the manuscript.

RESUMEN

Diversidad y bioestratigrafía de las galletas de mar

(Echinoidea: Scutelloida: Scutellifomes) del Oligoceno

tardío-Mioceno tardío de Argentina y Uruguay

Introducción: Realizamos una síntesis actualizada

de la composición taxonómica de las asociaciones de escu-

télidos del Oligoceno tardío-Mioceno tardío de Argentina

y Uruguay. Objetivo: Ubicar los escutélidos en un marco

estratigráfico refinado, a los efectos de observar su diversi-

dad a lo largo del tiempo y la correlación de sus ensambles

con los ya conocidos de moluscos. Métodos: Revisamos

en detalle la procedencia geográfica y estratigráfica de los

ejemplares y su asignación taxonómica, basándonos tanto

en la bibliografía como en colecciones institucionales y

producto de nuestras campañas de colecta. Resultados:

El grupo está representado por 14 especies agrupadas en

seis géneros, siendo identificadas en cuatro ensambles. Las

edades numéricas recientemente obtenidas permitieron su

ubicación en un esquema cronológico: “Iheringiella” sp.

A se restringe al Oligoceno tardío, Camachoaster y “Eos-

cutella”, y Monophoraster telfordi al Mioceno temprano,

Abertella gualichensis y Abertella miskellyi al Mioceno

medio, y Monophoraster duboisi, Amplaster coloniensis y

Amplaster ellipticus al Mioceno tardío. Los escutélidos sin

lúnula no están restringidos al Oligoceno tardío como se

suponía. Monophoraster aparece en el Mioceno temprano

y junto con Iheringiella abarcan largos períodos de tiempo,

encontrándoselos entre los 25.3 Ma-18.1 Ma (Iheringiella

patagonensis) y aproximadamente los 15 Ma-6.48 Ma

(Monophoraster darwini). Se corrobora la presencia de

Iheringiella en el Mioceno temprano del noreste de Pata-

gonia, donde alcanza su punto más septentrional. Mono-

phoraster darwini se encuentra desde el Mioceno medio

hasta el Mioceno tardío (lapso para el cual previamente se

consideraba restringido), ya que se trata de la especie pre-

sente en los bien conocidos “estratos con Monophoraster y

Venericor”. Conclusiones: Los escutélidos del Paleógeno-

Neógeno de Argentina y Uruguay se encuentran desde el

Oligoceno tardío hasta el Mioceno tardío. Hay una buena

correspondencia entre las edades numéricas, las biozonas

de moluscos y los ensambles de escutélidos.

Palabras clave: Echinoidea; Scutelliformes; galletas de

mar; Paleógeno; Neógeno; Argentina; Uruguay.

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