Early Cretaceous dinoflagellate cysts from the southern border of the Neuquén Basin , Estancia Santa Elena locality , Argentina

An assemblage of dinoflagellate cysts is re�orted from the Lo�er Member (Pilmatu�� Member) of the Agrio Formation (late Valanginian-Hauterivian) of the southern Neuqu��n Basin, Argentina. Thirty s�ecies are recognized and the ne� s�ecies Oligosphaeridium quattrocchioae and Muderongia sarjeantii described. The studied assemblage is com�ared �ith other Early Cretaceous assemblages from central �estern Argentina, Patagonia, the South Atlantic Malvinas Plateau, the Antarctic Peninsula and Australia. The marine dinoflagellate cyst assemblage of the Agrio Formation at the Estancia Santa Elena locality has cosmo�olitan character, �ith a �redominance of Euro�ean and Tethyan s�ecies.

The material studied is from outcro� sam�les of the Lo�er Member (Pilmatu�� Member) of the Agrio Formation, of �hich the lo�ermost 144m cro� out at Estancia Santa Elena (Figs. 1 and  2).Well-�reserved assemblages of dinoflagellate cysts are �resent only in the lo�ermost 25m of the section.Bet�een 25 and 87m the �reservation is �oor, and in the u��ermost �art of the section (from 87 to 144m) it is very �oor.The general stratigra�hic setting is �resented on Table 1 and the stratigra�hic levels bearing the dinoflagellate cyst assemblages are dis�layed in Table 2.
A synthesis of the biostratigra�hy of the Agrio Formation, based on ammonites, �alynomor�hs and calcareous nannofossils, �as given by Aguirre-Urreta et al. (1999).The ammonites from the Agrio Formation indicate a late Valanginian to early Barremian age (Table 1).The �osition of the Agrio Formation in the context of facies evolution of the Mesozoic Neuqu��n Basin �as given in Leanza et al. (2000) and Lazo (2006) studied the ta�honomy of the Pilmatu�� Member macrofauna (= Lo�er Member) of the Agrio Formation.The biofacies and sea level changes of the Pilmatu�� Member �ere analysed by Lazo (2007).His studies on the biofacies are based on corals, ser�ulids, bivalve mollusks and gastro�ods, �hich are the �rinci�al com�onents of the benthonic macrofauna of this member.Lazo et al. (2009) �resented an integrated biostratigra�hy of the Agrio Formation at its ty�e locality (Bajado del Agrio).They correlated biostratigra�hically the local section �ith the Euro�ean standard zonations.Concheyro et al. (2009) studied the micro�aleontology of the Agrio Formation in different areas of the Neuqu��n Basin.They found that in central and southern �ositions of the Neuqu��n Basin the age of calcareous nannofossil assemblages is Late Valanginian -Late Hauterivian and that the benthic microfauna indicates frequent eustatic variations (from shelves to shallo� marine) �ith conditions of normal salinity.Ottone (2009) gave a thorough revie� of the �aleoenvironmental and �aleoclimatic significance of the Cretaceous flora of the Neuqu��n Basin, �rinci�ally based on the �alynologic record, considering 191 �a�ers related to the subject.
Ottone & Aguirre-Urreta (2000), through a com�arative analysis of dinoflagellate cysts and ammonites, �ro�osed the existence of a marine link bet�een the Austral and Neuqu��n basins

SYSTEMATICS
A com�lete taxonomic list is given as an a�-�endix at the end of the text.In this section, the descri�tion of ne� s�ecies of the genera Oligos-�haeridium and Muderongia and taxonomic considerations on a s�ecies of the genus Muderongia are �resented.
Supplementary observations.The second sulcal �rocess is exce�tionally slender and may readily esca�e observation.It is not constantly �resent; it is visible on the s�ecimen illustrated in figure 6.2, but lacking from the s�ecimen sketched (fig.6.1).No indication of an anterior sulcal �rocess �as seen in any s�ecimen.The �hragma surface is laevigate to finely granulate in all s�ecimens seen.The a�ical �rocesses are more slender than those on the loisthocyst, but have com�arably com�lex terminations.Their �ro�ortionate length could not be measured.
The degree of com�lexity of the �rocess ti�s is highly variable.In vertical vie� they may a��ear like a ragged-edged �latform �ith an irregular scatter of �erforation of variable size; in lateral vie�, they are seen to be secate to cauliflorate.T�o �rocesses on the loisthocyst, l'" (Iu) and l� (X), are markedly smaller than the others; the anta�ical �rocess is of a size com�arable to those of the �re-and �ostcingular �ara�lates.
For com�arison �ith other skolochorate genera of dinoflagellate cysts see figure 3 4) has short �rocesses, but these are �ro�ortionately much thicker than in O. quattrocchioae, giving a totally different, heavier a��earance to the subs�ecies mentioned.
Nevertheless, the size of �rocesses is not reliable for differentiating chorate s�ecies, as �rocess size is not only determined by genetics and therefore might not be a good characteristic to differentiate s�ecies in one genus.As an exam�le, size of �rocesses in Lingulodinium polyhedrum changes �hen salinity of sea �ater does.In normal salinity sea �ater this s�ecies forms normal cysts �ith normally long �rocesses.But, �hen the cysts develo� in less than normal salinity, the �rocesses are shorter (Helenes, verbal communication).
Suborder CERATIINEAE Fensome et al., 1993 Family CERATIACEAE Lindemann, 1928 Genus Muderongia Cookson & Eisenack, 1958emend. Monteil, 1991b Type Species.Muderongia mcwhaei Cookson & Eisenack, 1958, �l. 6, fig. 2 Muderongia siciliana Torricelli, 1997 (Figs. 4, A-C) Remarks.The Argentinian s�ecimens corres�ond in all essential features �ith those from Sicily.Ho�ever, it should be noted that the o�erculum is by no means al�ays free, often Description.Large cysts, ceratioid, cornucavate, �ith three �rominent horns, a�ical, anta�ical and right lateral.The horns are broadbased, ta�ering gently to a blunt and irregular o�en ti�.The a�ical horn is �olar in �osition, its length less than that of the central body.The anta�ical horn may be �olar in �osition or may be �ositioned slightly or markedly to the left of the anta�ical �ole; its length is less than that of the central body.The right lateral horn is �ostcingular in �osition; it may be shorter than the other horns or may a��roach them in length.It may be straight or recurved and may exhibit a cingular notch.
Endoblast s�heroidal to elli�soidal in ambitus or bulging into the base of the right lateral horn.It may exhibit some dorsoventral flattening or may be rotund.Periblast may exhibit fe� lo� ridges bounding the cingulum, �hich may be shallo�ly concave.Alternatively, the cingulum may not be directly indicated.Sulcus and �aratabulation marked by faint lines or not at all.Endo�hragm and �eri�hragm laevigate or finely granulate or �unctate.
Derivation of name.In honour of Dr. William Antony S�ithin Sarjeant, distinguished �alynologist and great author of science fiction.

REGIONAL COMPARISON
The assemblage of the Lo�er Member of the Agrio Formation at Santa Elena is com�ared �ith other Early Cretaceous assemblages of dinoflagellate cysts from the southern hemis�here and related gond�anic areas.In order to avoid unnecessary extension of the list of References, for cita-   A, B, F, G, x 1000, figuras C,D, E, H-M, x 550.tions of �a�ers older than 2004, on dinoflagellate cyst taxonomy, �lease see the list of references in Fensome & Williams (2004).

Patagonia
The late Valanginian to Hauterivian and Barremian S�ringhill Formation of southern Patagonia contains, in addition to an abundant (and �revailing) terrestrial microflora, an assemblage of dinoflagellate cysts sur�risingly similar to that of the Agrio Formation of the Neuqu��n Basin.Based on this similarity, Ottone & Aguirre-Urreta (2000) �ostulated the �resence of a marine corridor bet�een the Austral Basin (Patagonia) and the Neuqu��n Basin, at least during

Antarctic Peninsula: Livingston and Snow Islands
The Byers Grou� (Late Jurassic-Early Cretaceous) of Livingston and Sno� islands, Antarctic Peninsula �ere de�osited in a fore-arc setting and is com�rised by the Late Jurassic Anchorage Formation, the Berriasian President Beaches Formation, the Valanginian Chester Cone Formation and overlying, non-differentiated non-marine strata.Duane (1996) em�hasizes the strong affiliation of the Byers Grou� �alynoflora to those from the Mesozoic of Australia and southern South America and the marked �rovincialism �ithin the Valanginian marine micro�lankton content.

PALEOBIOGEOGRAPHIC AND PALEOGEO-GRAPHIC IMPLICATIONS OF THE ASSEM-BLAGE OF DINOFLAGELLATE CYSTS OF THE AGRIO FORMATION AT SANTA ELENA LOCALITY
Of a total number of 27 mor�hoty�es studied at the s�ecies level, 13 are from NW and W Euro�e, six from the Tethyan Realm, four from Australia and t�o from Canada.Table 4. Stratigra�hic range of dinoflagellate cyst s�ecies �resent in the lo�ermost section of the Agrio Formation at Estancia Santa Elena locality./ Distribución estratigráfica de las especies de quistes de dinoflagelados presentes en la parte inferior de la Formación Agrio en la localidad Estancia Santa Elena.
sho�ing ventral attachment.The age of the Sicilian s�ecimens (early Hauterivian) is very close to that of the Argentinian ones (latest Valanginian to earliest Hauterivian).Muderongia sarjeantii Volkheimer, s�.nov.(Figs. 4, E-H) Diagnosis: A s�ecies of Muderongia in �hich all three horns are of com�arable dimensions, very broad-based, ta�ering gently to blunt, rather ragged-looking ti�s and having a length bet�een t�o-fifths and three-fifths of the length of the central body.Cingulum marked by faint lines and a slight central de�ression.Paratabulation not oth-er�ise directly indicated.Endoblast almost s�heroidal, but bulging into the base of the right lateral horn.Endo�hragm finely to coarsely granular; �erifragm levigate or very finely granulate.
The cosmo�olitan character of the assemblage of marine dinoflagellate cysts of the Agrio Formation at Estancia Santa Elena locality indicates for the late Valanginian and early Hauterivian an o�en marine connection bet�een the Neuqu��n Basin, the Tethyan Realm and �estern/north�estern Euro�e.It also indicates a lo� gradient of �aleo-tem�eratures bet�een �aleoequator and �oles.Based on ammonite faunas, Aguirre-Urreta et al.(1999)established a correlation of the Agrio Formation �ith the column of the �estern Tethys, com�lemented by studies on �alynomor�hs and calcareous nannofossils.Their results sho� that an o�en marine connection, as above mentioned, continued through the Hauterivian and Barremian.In a later contribution, Aguirre-Urreta et al.(2008)sho� that early in the Cretaceous the ammonites are re�resented in the Neuqu��n Basin by six families (belonging to 20 genera/subgenera) of tethyan affinities and that �aleotem�eratures in the Neuqu��n Basin �ere similar to those in the Tethys.

on Table 4 .
The latter are Cyclonephelium vannophorum Davey extending to the Cenomanian, �ystrichodinium pulchrum Deflandre and Circulodinium distinctum (Deflandre and Cookson) Jansonius, both �ith holoty�es of Senonian age.The kno�n distributions of the t�o endemic forms Oligosphaeridium quattrocchioae nov.s�. and Muderongia sarjeantii nov.s�. are limited to the late Valanginian and early Hauterivian strata of the study area.CONCLUSIONS 1.In an assemblage of dinoflagellate cysts from the Pilmatu�� Member of the Early Cretaceous Agrio Formation (Neuqu��n Basin) 30 marine s�ecies are recognized.2. The ne� s�ecies Oligosphaeridium quattrocchioae and Muderongia sarjeantii are endemic.3. Com�arison of the dinoflagellate cyst assemblage of the Pilmatu�� Member of the Agrio Formation at Santa Elena locality �ith other Early Cretaceous assemblages from the southern hemis�here, the Tethyan realm and north�estern Euro�e, sho�s the cosmo�olitan character of most com�onents of the assemblage here studied and indicates a lo� gradient of tem�eratures bet�een the �aleoequator and �oles during Early Cretaceous times.