Toarcian–Aalenian (Early–Middle Jurassic) radiolarian fauna from the Los Molles Formation, Neuquén Basin, Argentina: Taxonomy and paleobiogeographic affinities

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Journal of South American Earth Sciences 31 (2011) 253e261

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Journal of South American Earth Sciences

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ToarcianeAalenian (EarlyeMiddle Jurassic) radiolarian fauna from the Los MollesFormation, Neuquén Basin, Argentina: Taxonomy and paleobiogeographicaffinities

Karlos Guilherme Diemer Kochhann a,*, Simone Baecker-Fauth a, Ignacio Pujana b,Ariane Santos da Silveira c, Gerson Fauth a,1

a Laboratório de Micropaleontologia, Universidade do Vale do Rio dos Sinos (UNISINOS), Av. UNISINOS, 950 e B. Cristo Rei/CEP: 93.022-000, São Leopoldo, RS, BrazilbDepartment of Geosciences, University of Texas at Dallas, P.O. Box 830688, Richardson, TX 75063-0688, USAc Programa de Pós-Graduação em Geologia, Universidade do Vale do Rio dos Sinos (UNISINOS), Av. UNISINOS, 950 e B. Cristo Rei/CEP: 93.022-000, São Leopoldo, RS, Brazil

a r t i c l e i n f o

Article history:Received 10 September 2010Accepted 14 January 2011

Keywords:RadiolariaPaleobiogeographyToarcianeAalenianLos Molles FormationNeuquén BasinArgentina

* Corresponding author.E-mail addresses: [email protected] (K.G.D. K

br (S. Baecker-Fauth), [email protected] (I. Pujana)(A. Santos da Silveira), [email protected] (G. Fauth1 Tel.: þ55 (51) 3037 1000x1766.

0895-9811/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.jsames.2011.01.001

a b s t r a c t

This paper reports the occurrence of a ToarcianeAalenian (EarlyeMiddle Jurassic) radiolarian fauna inthe Los Molles Formation, Neuquén Basin, Argentina, as well as comments on its paleobiogeographicaffinities. The micropaleontologic analysis was carried out in fine-grained rocks from a turbiditic sectionof the Los Molles Formation. These samples were first chemically processed using only hydrogenperoxide (H2O2), and afterward treated with acetic (CH3COOH) and hydrofluoric (HF) acids. The firstchemical procedure permitted the recovery of only few spongy spumellarians, while the second oneenabled to recover more diversified radiolarian assemblages. In general, the studied fauna presents lowdiversity and abundance, with a strong dominance of spumellarians over nassellarians. The fauna iscomposed by the genera Paronaella, Homoeoparonaella, Praeconocaryomma, Archaeocenosphaera, Orbi-culiformella, Praeparvicingula, and some unidentified spumellarians and nassellarians. According topaleobiogeographic models, the studied ToarcianeAalenian fauna presents a mid to high latitude affinity.It is possible to infer from those data a bipolar distribution of some taxa, such as Praeparvicingula andprobably Praeconocaryomma, between the Northern and Southern hemispheres since the Early Jurassic(Toarcian).

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1. Introduction

Taxonomic and biostratigraphic studies on ToarcianeAalenianradiolarian faunas were carried out in rocks of the Queen CharlotteIslands (Carter et al., 1988; Carter and Haggart, 2006), Oregon(Pessagno et al., 1989), Japan (Hori, 1997; Yao, 1997; Kojima andSaito, 2000; Suzuki and Ogane, 2004; Niwa et al., 2003; Hori andWakita, 2006), Betic Cordillera (Sandoval et al., 2008) and JulianAlps (Gori�can et al., 2003). The paleobiogeographic significance ofMesozoic radiolarian faunas have been discussed by some authorsmainly for the Middle/Upper Jurassic (Pessagno and Blome, 1986;Pessagno et al., 1993; Kiessling, 1999; Pessagno and Hull, 2002),

ochhann), sbfauth@unisinos., [email protected]).

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Middle Jurassic and Early Cretaceous (Carter and Haggart, 2006)and Cretaceous (Vishnevskaya, 2009). These paleobiogeographicstudies have contributed to the improvement of the knowledgeabout the tectonic evolution of specific areas (see Pessagno et al.,1993; Carter and Haggart, 2006).

Radiolarian faunas were previously described from Tithonian(Late Jurassic) strata of the Neuquén Basin (VacaMuerta Formation)by Pujana (1989, 1991 and 1996). This author emphasized thepresence of the genus Parvicingula and attributed a southernTethyan affinity to the radiolarian fauna of the Vaca MuertaFormation (Pujana, 1989), inferring a possible bipolar distributionof some taxa during this time interval. Middle Jurassic radiolarianfaunas were reported for the Neuquén Basin by Pujana (1997a), andeven the presence of a highly endemic Aalenian one was brieflycommented, but not formally described, by Pujana and Tang (1992).

The purpose of this study is to report the occurrence of a Toar-cianeAalenian (EarlyeMiddle Jurassic) radiolarian fauna in the LosMolles FormationeNeuquén Basin, Argentina, as well as to infer itspaleobiogeographic affinities.

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2. Geological settings

The Neuquén Basin is located in central western Argentina andChile between the latitudes 31� and 41�S (Fig.1). Its largest area is inArgentine territory, including portions of the provinces of Neuquén,Mendoza and La Pampa. In the northern portion of the basin, it isrestricted to the Andean Belt as an elongated NeS band, while in itssouthern area it broadens eastwards into the extra-Andean domain.Limited to the west by the Andean Volcanic Arc, the basin is alsobounded by the Sierra PintadaMassif to the northeast and theNorthPatagonian Massif to the southeast. The infill of the Neuquén Basinpresentsmarine and continental (epiclastics, carbonates, evaporitesand pyroclastics) strata that range from Late Triassic to Paleocene inage (Ramos, 1998). According to Howell et al. (2005) the basin infillrecorded three main stages of tectonic evolution and correlativebase level changes (synrift, postrift and foreland phase).

Fig. 1. Site of the studied section in the southern Neuquén Basin (top) and its locationin an Ikonos satellite scene (coordinate system: South America Datum e SAD e 1969)(base). Regional map modified from Howell et al. (2005).

Fig. 2. Studied stratigraphic profile, dominated by turbiditic facies. The 78 analyzedsamples are located, as well as the radiolarian rich samples. Stratigraphic surfaces,sequences (SEQ.) and paleophysiographic element (P.E.) modified from Paim et al. (2008).For more details concerning the chronostratigraphic data, see geological settings section.

During the ToarcianeAalenian to BathonianeCallovian intervalthe Neuquén Basin underwent a thermal subsidence stage, causingthe expansion of the marine sedimentation in the basin (Franzeseet al., 2003). The marine strata of the Los Molles Formation arecomposed predominantly by gray and black mudstones, siltstonesand sandstones related to turbiditic lobes deposits (Gulisano andGutiérrez-Pleimling, 1994). Previous biostratigraphic studies,

K.G.D. Kochhann et al. / Journal of South American Earth Sciences 31 (2011) 253e261 255

based on palynology, carried out in the Los Molles Formation,described its type-section and suggested a Toarcian to possibleBajocian age to these deposits (Wolkheimer, 1973). Afterward,Quattrocchio et al. (1996) positioned the Los Molles Formation(Charahuilla reference section) into the Callialasporites turbatuspalynological subzone (Late ToarcianeEarly Bajocian), confirmingWolkheimer’s results. Despite the data exposed above, the occur-rences of characteristic ammonites, bivalves, brachiopods andcalcareous microfossils extended the base of the Los MollesFormation to the Pliensbachian (see Ricardi et al., 2000). Fora revised high resolution ammonite zonation, and its correlation tothe standard ammonite zones, see Ricardi (2008).

Paim et al. (2008), based on the analysis of sedimentary facies,assigned the Los Molles Formation to three paleophysiographicelements, which are slope rise to basin plain, slope andmid to outershelf. Those authors reported that abundant fine-grained depositsand sandy turbiditic lobe strata, as well as the dominance of radi-olarians, described in the present work, comprise the main featuresof the slope rise to basin plain paleophysiographic element.

The chronostratigraphic data presented in this paper requireparticular discussion. Despite the scarcity of biostratigraphic datafor the studied section, Ricardi (2008) recognized the ammoniteassociation zone of Phymatoceras at the Arroyo La Jardinera area,Late Toarcian in age, based on the occurrences of Phymatocerascopiapense (Moricke), P. ex. gr. lilli (Hauer), P. ex. gr. erbaense(Hauer) and P. cf. pseudoerbaense (Gab.). Based on the radiolarianoccurrences presented herein and their revised stratigraphic range(Carter et al., 2010), a Toarcian age could be attributed to the faunarecovered from the sample P-219.7 m, based on the co-occurrencesof Praeparvicingula tlellensis Carter and Homoeoparonaella reciprocaCarter. The lithostratigraphic section studied herein is a partialsection of that described by Paim et al. (2008). Those authorspresented chronostratigraphic data based on correlations ofstratigraphic surfaces to Gulisano and Gutiérrez-Pleimling, 1994stratigraphic chart for the same area, with chronostratigraphybased on ammonites, suggesting a ToarcianeAalenian age for thestudied interval. Besides all data exposed above, it is consideringherein that there is no reasonable evidence for placing the Toar-cianeAalenian boundary in the studied section, which probablyshould occur above the sample P-219.7 m (Fig. 2), being the entiresection, and consequently the studied fauna, attributed to theToarcian (below the sample 219.7 m)eAalenian interval.

3. Material and methods

The micropaleontologic analysis was carried out in 78 samplesof fine-grained sedimentary rocks, dominantly mudstones withhigh levels of dispersed silica, which compose a turbiditic section ofthe Los Molles Formation (Fig. 2). These samples were collectedduring the description of a lithostratigraphic profile (located ata road cut along the Ruta 46, near the bridge over the La JardineraCreek) by Paim et al. (2008).

The samples were first chemically processed using a solution of29% of hydrogen peroxide (H2O2) by 24 h and washed and frac-tionated in four meshes (45, 63, 180 and 250 mm). This chemicalprocedure allowed the recovery of a few spongy spumellarians. Inface of the high levels of dispersed silica, the samples were laterchemically treated in accordance to Gori�can et al. (2003) with acetic(CH3COOH, solution at 15e20%) and Hydrofluoric (HF, solution at5%) acids, for the disaggregation of occasional carbonatic and sili-ceous cements, respectively. This second chemical method enabledthe recovery of more diversified radiolarian assemblages. Speci-mens were handpicked and representative radiolarians imaged inScanning Electron Microscope (SEM) at Centro de Pesquisas daPETROBRAS-CENPES.

4. Taxonomy

The specimens figured in the present paper are held in thecollections of Museo de La Plata, Colección de Micropaleontología,Argentina, under the curatorial numbers MLP-Mi 1835 to MLP-Mi1852. The reference collection is held at Museu de Paleontologia,Universidade do Vale do Rio dos Sinos (UNISINOS), Brazil, under thecuratorial numbers ULVG-8029 to ULVG-8113. The supragenericnomenclature is in accordance to the scheme proposed by DeWever et al. (2001).

Order SPUMELLARIA Ehrenberg, 1875Family Xiphostylidae Haeckel, 1881, sensu Pessagno and Yang,1989 in Pessagno et al., 1989, emend. De Wever et al., 2001

Genus Archaeocenosphaera Pessagno and Yang in Pessagnoet al., 1989

Archaeocenosphaera sp. cf. A. ruesti Pessagno and Yang inPessagno et al., 1989

Fig. 3; 1cf. 1989 Archaeocenosphaera ruesti Pessagno and Yang n. sp.;Pessagno et al., p. 203, pl. 1, Fig. 9 and pl. 9, Fig. 23.

Figured specimen: MLP-Mi 1835.Remarks: The recorded specimens are a little larger than the

holotype (see Pessagno et al., 1989). The poor preservation does notenable a more precise taxonomic characterization.

Material: Seven specimens in sample P-219.7 m (ULVG-8029 toULVG-8034).

Occurrence: Toarcian of Arroyo la Jardinera section, Los MollesFormation, Neuquén Basin, Argentina.

Family Conocaryommidae Lipman, 1969, emend. DeWever et al.,2001

Genus Praeconocaryomma Pessagno, 1976Praeconocaryomma sp. cf. P. whiteavesi Carter in Carter et al.,

1988Fig. 3; 2

cf. 1988 Praeconocaryomma whiteavesi Carter n. sp.; Carter et al.,p. 31, pl. 1, Fig. 3, 6.cf. 2006 Praeconocaryomma whiteavesi Carter; Gori�can et al., p.330, pl. PRY04.

Figured specimen: MLP-Mi 1836.Remarks: This species presents some similarities with P. white-

avesi Carter, but its smaller size and the poor preservation of allrecovered specimens difficult a more precise statement.

Material: 34 specimens in sample P-219.7 m (ULVG-8037 toULVG-8069).


Praeconocaryomma sp.Fig. 3; 3

Figured specimen: MLP-Mi 1837.Description: Cortical shell with well developed mammae. Distal

surfaces of the mammae seem to be imperforate, somewhat flat-tened, pentagonal or hexagonal in outline; primary spines/nodesproject from the center of the mammary area. Mammae areapparently interconnected by bars which generate relatively largepores in the intermammary area. Internal features could not beobserved.

Remarks: Praeconocaryomma sp. differs from Praeconocaryommasp. cf. P. whiteavesi Carter by having prominent mammae, withapparently imperforate distal surfaces.


Fig. 3. 1: Archaeocenosphaera sp. cf. A. ruesti Pessagno and Yang, sample P-219.7 m (MLP-Mi 1835); 2: Praeconocaryomma sp. cf. P. whiteavesi Carter, sample P-219.7 m (MLP-Mi1836); 3: Praeconocaryomma sp., sample P-219.7 m (MLP-Mi 1837); 4: Paronaella sp. 1, sample P-219.7 m (MLP-Mi 1838); 5: Paronaella sp. 2, sample P-219.7 m (MLP-Mi 1839); 6:Homoeoparonaella reciproca Carter, sample P-219.7 m (MLP-Mi 1840); 7: Orbiculiformella sp., sample P-219.7 m (MLP-Mi 1841); 8: Orbiculiformella sp. cf. O. incognita (Blome), sampleP-219.7 m (MLP-Mi 1842); 9, 10: Spumellaria indet. 1, samples P-415.5 m and P-450.5 m, respectively (MLP-Mi 1843); 11, 12: Spumellaria indet. 2, sample P-408 m (MLP-Mi 1844);13: Spumellaria indet. 3, sample P-219.7 m (MLP-Mi 1845); 14: Praeparvicingula tlellensis Carter, sample P-219.7 m (MLP-Mi 1846); 15: Praeparvicingula elementaria (Carter), sampleP-219.7 m (MLP-Mi 1847); 16: Praeparvicingula sp. 1, sample P-219.7 m (MLP-Mi 1848); 17: Praeparvicingula sp. 2, sample P-219.7 m (MLP-Mi 1849); 18: Nassellaria indet. 1, sampleP-290.15 m (MLP-Mi 1850); 19: Nassellaria indet. 2, sample P-219.7 m (MLP-Mi 1851); 20: Nassellaria indet. 3, sample P-219.7 m (MLP-Mi 1852).




Family Angulobracchiidae Baumgartner, 1980Genus Paronaella Pessagno, 1971Paronaella sp. 1Fig. 3; 4

Figured specimen: MLP-Mi 1838.Description: Test lacks rays with bracchiopyle. Rays nearly equal

in length, with wider areas at distal extremities. Meshwork irreg-ular, comprised of irregular polygonal pore frames (predominantlypentagonal and hexagonal). Pore frames composed by bars con-nected to weakly developed nodes.

Remarks: The specific classification is not possible due to theapparently dissolved aspect of the specimens.

Material: Three specimens in sample P-219.7 m (ULVG-8035 andULVG-8036).


Paronaella sp. 2Fig. 3; 5

Figured specimen: MLP-Mi 1839.Description: Test with three rays (two of them broken). The

preserved ray is long, subcircular in section and terminates ina large rhombohedral tip, with a broken spine, probably originatingfrom the central portion of the tip. Meshwork of the rays and tipspoorly preserved.

Remarks: This specimen presents some resemblances to Paro-naella snowshoensis (Yeh) (see Gori�can et al., 2006), but its poorpreservation does not enable specific characterization.

Material: One specimen in sample P-219.7 m.Occurrence: Toarcian of Arroyo la Jardinera section, Los Molles

Formation, Neuquén Basin, Argentina.

Famiy Patulibracchiidae Pessagno, 1971, emend. De Weveret al., 2001Genus Homoeoparonaella Baumgartner, 1980Homoeoparonaella reciproca Carter in Carter et al., 1988Fig. 3; 6

1988 Homoeoparonaella reciproca n. sp.; Carter et al., p. 28, pl. 7,Figs. 2, 3.2006 Homoeoparonaella reciproca Carter; Gori�can et al., p. 198,pl. HOM02.Figured specimen: MLP-Mi 1840.Remarks: The characteristic pattern of alternated rows of square

pore frames and double triangular pore frames is not present in thefigured specimen, however, Carter et al. (1988) reported that thisfeature is observable only in well preserved material. The dimen-sions of the studied specimen exclude the possibility of taxonomicidentification as H. argolidensis Baumgartner and H. hydensis Yeh(see Gori�can et al., 2006).

Material: One specimen in sample P-219.7 m.Occurrence: Toarcian of Whiteaves and Phantom Creek forma-

tions, Queen Charlotte Islands, British Columbia (Carter et al., 1988);Toarcian of Arroyo la Jardinera section, Los Molles Formation,Neuquén Basin, Argentina.

Family Spongodiscidae? Haeckel, 1862Genus Orbiculiformella Kozur and Mostler, 1978Orbiculiformella sp.Fig. 3; 7

Figured specimen: MLP-Mi 1841.Description: Discoidal spongy skeleton. Margins slightly inflated,

with larger pores than the central area. Central area is slightlydepressed.

Remarks: The figured specimen does not present the charac-teristic spines of the genus, but this could be attributed to its poorpreservation.



Orbiculiformella sp. cf. O. incognita (Blome, 1984)Fig. 3; 8

cf. 1988 Spongotrochus (Stylospongidium) sp. aff. S. (S.) echino-discus Clark and Campbell; Carter et al., p. 46, pl. 10, Fig. 7, 10.cf. 2003 Orbiculiforma? incognita Blome; Gori�can et al., p. 296, pl.3, Figs. 5e7.cf. 2006 Orbiculiformella incognita Blome; Gori�can et al., p. 262,pl. ORB06.

Figured specimen: MLP-Mi 1842.Remarks: This species, similarly to Orbiculiformella sp., does not

present the characteristic spines of the genus, possibly as a conse-quence of the poor preservation, which also prevents a moreprecise taxonomic statement. The groove running along the testmargin described by Gori�can et al. (2006) is present in the studiedspecimens.

Material: Four specimens in sample P-219.7 m (ULVG-8082 toULVG-8084).


SPUMELLARIA INCERTAE SEDISSpumellaria indet. 1Fig. 3; 9 and 10

Figured specimens: MLP-Mi 1843, from samples P-415.5 m and P-450.5 m.

Description: Spherical cortical shell, lacking spines. Polygonalpore frames (triangular, quadrangular and pentagonal) composedby small plates radially aligned. Radially aligned plates form thethick cortical shell.

Remarks: This species cannot be assigned to the familyXiphostylidae by its irregular pore frames arrangement. However,due to its general aspect, this species could be closely related to thegenus Archaeocenosphaera Pessagno and Yang.

Material: One specimen in samples P-239.8 m (ULVG-8097), P-408 m (ULVG-8098) and P-415.5 m; five specimens in sample P-450.5 m (ULVG-8099 to ULVG-8102).

Occurrence: ToarcianeAalenian of Arroyo la Jardinera section,Los Molles Formation, Neuquén Basin, Argentina.

Spumellaria indet. 2Fig. 3; 11 and 12

Figured specimens: MLP-Mi 1844, from sample P-408 m.Description: Spongy spumellarian, with cortical shell bearing

small and irregular pores.Remarks: This species presents some resemblance to Spumel-

laria indet. 1, but the frames of the pores are more irregular and arenot composed by the radially aligned plates.

Material: Three specimens in sample P-190.4 m (ULVG-8103to ULVG-8105); four specimens in sample P-408 m (ULVG-8106and ULVG-8107); one specimen in sample P-415.5 m (ULVG-8108); five specimens in sample P-450.5 m (ULVG-8109 to ULVG-8113).

Occurrence: ToarcianeAalenian of Arroyo la Jardinera section,Los Molles Formation, Neuquén Basin, Argentina.

Spumellaria indet. 3Fig. 3; 13

Table 1Samples with radiolarian occurrences. Numerical data demonstrate the low abundance and diversity of the fauna. For location of the samples in the lithostratigraphic section,see Fig. 2.

Spum

ella

ria

inde

t. 1

Spum

ella

ria

inde

t. 2

Nas

sella

ria

inde

t. 1

Par

onae

lla

sp. 1

Par

onae

lla

sp. 2

Hom

oeop

aron

aell

a re

cipr

oca

Pra

econ

ocar

yom

ma

sp. c

f. P

. whi

teav

esi

Pra

econ

ocar

yom

ma

sp.

Arc

haeo

ceno

spha

era

sp. c

f. A

. rue

sti

Orb

icul

ifor

mel

la s

p.

Orb

icul

ifor

mel

la s

p. c

f. O

. inc

ogni

ta

Spum

ella

ria

inde

t. 3

Pra

epar

vici

ngul

a tl

elle

nsis

Pra

epar

vici

ngul

a el

emen

tari

a

Pra

epar

vici

ngul

a sp

. 1

Pra

epar

vici

ngul

a sp

. 2

Nas

sella

ria

inde

t. 2

Nas

sella

ria

inde

t. 3

P-190.4 m 3

P-219.7 m 3 1 1 34 7 7 7 4 1 3 1 4 3 6 1

P-239.8 m 1

P-290.15 m 1

P-408 m 1 4

P-415 m 1 1

P-450.5 m 5 5


Figured specimen: MLP-Mi 1845.Description: Ellipsoidal to subspherical cortical shell. Cortical

shell possesses a spine, which presents a pattern of alternatinggrooves and ridges.

Remarks: Cortical shell possibly with bipolar primary spines, butin the two specimens recovered in the present work only part ofone spine is preserved.

Material: One specimens in sample P-219.7 m.Occurrence: Toarcian of Arroyo la Jardinera section, Los Molles


Order NASSELLARIA Ehrenberg, 1875Family Parvicingulidae Pessagno, 1977Genus Praeparvicingula Pessagno, Blome and Hull in

Pessagno et al., 1993Praeparvicingula tlellensis Carter in Gori�can et al. (2006)Fig. 3; 14

1988 Parvicingula sp. E; Carter et al., p. 56, pl. 5, Fig. 13.2006 Praeparvicingula tlellensis Carter; Gori�can et al., p. 340, pl.PCA02.

Figured specimen: MLP-Mi 1846.Material: Three specimens in sample P-219.7 m (ULVG-8085 and

ULVG-8086).Occurrence: Toarcian of Whiteaves Formation, Queen Charlotte

Islands, British Columbia (Carter et al., 1988); Toarcian of Arroyo laJardinera section, Los Molles Formation, Neuquén Basin, Argentina.

Praeparvicingula elementaria (Carter) in Gori�can et al., 2006Fig. 3; 15

1988 Eucyrtidium elementarius Carter n. sp.; Carter et al., p. 60,pl. 17, Fig. 13.1988 Parvicingula sp. B; Carter et al., p. 56, pl. 18, Figs. 3, 4.2006 Praeparvicingula elementaria (Carter); Gori�can et al., p. 334,pl. PVG02.

Figured specimen: MLP-Mi 1847.Material: One specimen in sample P-219.7 m.

Occurrence: Toarcian of Whiteaves Formation, Aalenian ofPhantom Creek Formation and Bajocian of Graham Island Forma-tion, Queen Charlotte Islands, British Columbia (Carter et al., 1988);Toarcian of Arroyo la Jardinera section, Los Molles Formation,Neuquén Basin, Argentina.

Praeparvicingula sp. 1Fig. 3; 16

Figured specimen: MLP-Mi 1848.Description: Test conical. Cephalis is hemispherical, possibly

imperforate; thorax is slightly trapezoidal and sparsely perfo-rate. Abdomen and post-abdominal segments are slightly trap-ezoidal, continuously widening distally as added. Abdominaland post-abdominal segments possess three rows of hexagonalpore frames. Outer rows of pore frames staggered with respectto the central row. Weakly developed circumferential ridgesseparating segments on distal chambers, being absent in prox-imal ones.

Remarks: Praeparvicingula sp. 1 differs from Praeparvicingulatlellensis Carter by having weakly developed circumferential ridges,present only between distal segments, and by the continuouswidening of the post-abdominal segments.

Material: Four specimens in sample P-219.7 m (ULVG-8087 toULVG-8089).


Praeparvicingula sp. 2Fig. 3; 17

Figured specimen: MLP-Mi 1849.Description: Test conical, almost rounded apically. Hemi-

spherical cephalis and trapezoidal thorax sparsely perforated.Abdomen and post-abdominal segments slightly trapezoidal,continuously widening distally, giving a conical outline to the test.Abdominal and post-abdominal segments with three rows ofhexagonal pore frames, being the outer rows of pore frames


staggered with respect to central row; circumferential ridgesseparating chambers are poorly developed.

Remarks: Praeparvicingula sp. 2 differs from Praeparvicingula sp.1 by having poorly developed circumferential ridges, and by itssmaller size.

Material: Three specimens in sample P-219.7 m (ULVG-8090 andULVG-8091).


NASSELLARIA INCERTAE SEDISNassellaria indet. 1Fig. 3; 18

Figured specimen: MLP-Mi 1850.Remarks: Nassellarian internal mold. Probably, the mold of the

cephalis is lacking.Material: One specimen in sample P-290.15 m.Occurrence: ToarcianeAalenian of Arroyo la Jardinera section,

Los Molles Formation, Neuquén Basin, Argentina.

Nassellaria indet. 2Fig. 3; 19

Figured specimen: MLP-Mi 1851.Remarks: Specimens highly dissolved, with conical outline.Material: Six specimens in sample P-219.7 m (ULVG-8092 to

ULVG-8096).Occurrence: Toarcian of Arroyo la Jardinera section, Los Molles


Nassellaria indet. 3Fig. 3; 20

Figured specimen: MLP-Mi 1852.Description: Dicyrtid nassellarian with a solid bladed apical

horn. Cephalis is small and hemispherical and the large thorax isopened distally. Cephalis and thorax possess pentagonal andhexagonal pore frames, which are smaller in the cephalis than inthe thorax. Well developed nodes protrude from the vertices ofpore frames; nodes are bigger on the cephalis than on the thorax.

Remarks: Although the preservation is regular, taxonomicalaffinities are unknown.

Material: One specimen in sample P-219.7 m.Occurrence: Toarcian of Arroyo la Jardinera section, Los Molles


5. Results

Seven out the 78 analyzed samples from the studied Toar-cianeAalenian strata presented radiolarians (Table 1). Generalinformation indexes were determined according to Kiessling (1996)in the radiolarian richest sample (P-219.7 m). The Nassellarians/Spumellarians ratio (N/S) was 0.227, the abundance was of164.4 rads/g and the preservation index ¼ 5e6.

The fauna presents low diversity and abundance, with domi-nance of spumellarians over nassellarians. It is composed by thegenera Paronaella, Homoeoparonaella, Praeconocaryomma, Archae-ocenosphaera, Orbiculiformella, Praeparvicingula, and some uniden-tified spumellarians and nassellarians (Table 1). Some spongyspumellarians with unknown taxonomic affinities are recorded(which may be endemic, i.e. Spumellaria indet. 1 and Spumellariaindet. 2); Praconocaryomma is the most abundant and Praeparvi-cingula is the richest genus. In general, the recovered fauna presentsa dominance of forms with sturdy skeletons (with heavily silicifiedwalls) overweak ones, which could be related to preservational bias(taphonomic processes and/or preferential dissolution during

chemical treatments). Similarities at generic level are observedbetween the fauna described herein and Japanese ToarcianeAale-nian ones (Yao, 1997; Kojima and Saito, 2000; Suzuki and Ogane,2004; Niwa et al., 2003; Hori and Wakita, 2006).

Despite the poor preservation of the specimens, the faunal studyachieved some remarkable results. The species Praeparvicingula tlel-lensis Carter and Homoeoparonaella reciproca Carter, which co-occurin sample P-219.7 m, were reported for the Toarcian by Carter et al.(1988) in Queen Charlotte Islands, British Columbia, and consideredbiostratigraphic markers by Carter et al. (2010), allowing to suggesta Toarcian age for the sample P-219.7 m in the present study. Prae-parvicingula elementaria (Carter), also recorded herein in sampleP-219.7m, was reported for the ToarcianeAalenian interval by Carteret al. (1988), for the Queen Charlotte Islands, British Columbia.

6. Discussions

6.1. Paleobiogeographic affinities

Pessagno and Blome (1986) proposed a radiolarian paleo-biogeographic model for the Middle/Upper Jurassic based on theoccurrences of Ristola/Pantanelliids and Parvicingula. According tothose authors the Tethyan Realm is characterized by the increasingabundance and diversity of Pantanelliids and/or Ristola, while theBoreal Realm (mid to high latitudes) is characterized by thedominance of Parvicingula (later Praeparvicingula was included asa typical Boreal taxa e see Pessagno et al., 1993). There is a transi-tional zone between these realms, where both taxa could co-occur.Afterward, studies carried out by Pessagno et al. (1993), Kiessling(1999) and Pessagno and Hull (2002) subdivided the realms inprovinces, updating the model and expanding it to the SouthernHemisphere. Although this model has been extensively used,Hagstrum and Murchey (1996) pointed out a number of defi-ciencies as, for instance, the influence of preservational bias and thescarcity of paleomagnetic control data. Recent data suggested thatthis model is not strictly applicable to the Southern Hemisphere(see the description of a well preserved Tithonian radiolarian faunafrom Antarctic Peninsula, by Kiessling (1999), with Pantanelliids).

Carter and Haggart (2006) reported useful ecological andmorphological characteristics to assess the mid to high latitudeaffinities of radiolarian assemblages, based on the original model ofPessagno and Blome (1986) and subsequent research. The criteriaused are the absence of low latitude and the presence of highlatitude taxa, the occurrence of taxa with unknown affinities anda latitudinal diversity gradient, which increases towards low lati-tudes. Besides these faunal aspects there are morphological trends,such as the larger body size, presence of either heavier, thick-walledsmooth shells or shells with heavily silicified walls, which tend tobe more conspicuous at high latitudes. Those authors tested thismodel in Early to Middle Jurassic and Early Cretaceous radiolarianassemblages from Queen Charlotte Islands, British Columbia.

The species richness of Praeparvicingula and the absence ofPantanelliids are determinant aspects according to the paleo-biogeographicmodel presented by Pessagno andHull (2002). Thosecharacteristics could include the studied fauna in the Austral Realm,however, interpretations should be cautious due preservational bias(Praeparvicingula possesses sturdier skeletons than Pantanelliids(Hagstrum and Murchey, 1996)). Pujana (1997b) reports Pan-tanelliids in some stratigraphic levels of the Los Molles Formation,and, although absent in the studied samples, Pantanelliids couldhave been present in the original biocenosis.

Applying the paleobiogeographic criteria proposed by Carterand Haggart (2006), the fauna recovered is characterized by theabsence of low latitude taxa, presence of high latitude ones (i.ePraeparvicingula), low diversity, presence of taxa with unknown


taxonomic affinities and the occurrence of forms with heavy shellsand strongly silicified walls. Although these criteria includea considerable number of variables, preferential preservation couldalso bias the results, both in presence and absence of taxa, as well asin the general morphological pattern (e.g. see Kiessling’s, 1996preservation index).

According to Kiessling (1999), the genus Praeconocaryommaincreases in abundance through the Austral Realm during theTithonian and spongy spumellarians were highly abundant inthe Upper Jurassic of Antarctic region (Austral Realm). Theseobservations fit well with some faunal trends presented in thestudied material (e.g. high abundance of the genus Praeconocar-yomma, the presence of some spongy-endemic spumellarians andthe low N/S ratio in sample P-219.7 m, determined in accordanceto Kiessling (1996)).

In accordance to the paleobiogeographic model of Pessagno andBlome (1986), updated by Pessagno and Hull (2002), and theecological and morphological criteria proposed by Carter andHaggart (2006), it is possible to infer a mid to high latitudeaffinity for the ToarcianeAalenian fauna reported in the presentstudy. Although this interpretation needs to be done carefully, dueto the preservational bias, the studied fauna presents taxonomicaffinities with Aalenian ones, previously attributed to mid to highlatitude areas (see Hori and Wakita, 2006; Carter and Haggart,2006). These data allow to propose that the bipolar distribution ofsome taxa, as Praeparvicingula and probably Praeconocaryomma,began in the Early Jurassic (Toarcian) times. This pattern of radio-larian bipolarity was previously observed for the Upper Jurassic(Pujana, 1989) and Cenozoic (e.g. Lazarus et al., 2008).

6.2. Main conclusions

The application of the paleobiogeographic model revised byPessagno and Hull (2002) and the criteria proposed by Carter andHaggart (2006) suggests a mid to high latitude affinity for thestudied fauna. These data suggest a bipolar distribution, latitudi-nally controlled, of some taxa (Praeparvicingula and probably Prae-conocaryomma) since the Early Jurassic (Toarcian). Preferentialpreservation and/or dissolution can play an important role in theapplicability of those models; consequently, further studies areneeded to improve the discussions presented herein.

Considering the poor preservation (preservation index ¼ 5-6according to Kiessling, 1996), probably due to turbiditic deposi-tional processes and/or the possible differential dissolution duringthe HF treatment (Bartolini et al., 1999), paleoecological interpre-tations based on both abundance and diversity patterns have to bedone carefully.

The description of a ToarcianeAalenian radiolarian fauna in theSouthernHemisphere, presentedherein, cancontribute to thebetterunderstanding of the paleobiogeographic distribution of some taxaand the improvement of forthcoming paleoceanographic studies.

Acknowledgments

The authors are indebted to the staff of the Laboratório deMicropaleontologia at Universidade do Vale do Rio dos Sinos(UNISINOS) by all support and helpful discussions, as well as toPaim P.S.G., Lavina E.L. and Faccini U.F. for providing the samplesstudied herein. The authors are grateful to Martins R. (CENPES/PETROBRAS) for the SEM images, to Bergue C.T. by the critical andgrammatical revision and to the two anonymous reviewers whogreatly improved the first version of the manuscript.

This researchwas carried out within the terms of the Agreementfor Cooperation and Technical Assistance signed by the Uni-versidade do Vale do Rio dos Sinos (UNISINOS e Brazil) and the

Secretaría de Estado de Energía y Minería de la Provincia delNeuquén (Argentina) at 2 February 2008, with funding provided byPetróleo Brasileiro S.A. (PETROBRAS).

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Documents

Toarcian–Aalenian (Early–Middle Jurassic) radiolarian fauna from the Los Molles Formation, Neuquén Basin, Argentina: Taxonomy and paleobiogeographic affinities