The samples from the Colorado, Punta del Este and Austral basins are generally rich in dinocysts, but contain variable amounts of spores, pollen, acritarchs and foraminiferal test linings. Age control is based on the correlation with the dinoflagellate range data of Williams et al . (1998b, 2004) and Brinkhuis et al . (2003). Dinocyst taxa cited in the text are listed in the Appendix 1 and are fully referenced in Williams et al . (1998a).
Cx-1 Well. Dinocysts dominate the palynological assemblages throughout the Pedro Luro Formation of the Cx-1 Well. Since all the samples are cuttings, age assignments are based on the highest occurrences of dinoflagellate species.
Danian strata extend from 1340 to 1365 m (figure 2).Ten taxa have their highest ccurrences in this interval, including Areoligera medusettiformis O. Wetzel ex Lejeune-Carpentier, Disphaerogena carposphaeropsis O. Wetzel emend . Sarjeant, Palaeoperidinium pyrophorum (Ehrenberg ex O. Wetzel) Sarjeant, and Senoniasphaera inornata (Drugg) Stover and Evitt. Based on Williams et al. (1998b), Areoligera medusettiformis O. Wetzel ex Lejeune-Carpentier, which appeared during the Late Cretaceous persisted into the Palaeogene (Lutetian). According to Williams et al . (2004) the Last Occurrence Datum (LOD) of Palaeoperidinium pyrophorum (Ehrenberg ex O. Wetzel) Sarjeant, is at about 58 Ma at mid latitudes in the Northern Hemisphere and at high latitudes in the Southern Hemisphere. Moreover, Brinkhuis et al . (2003) found the last abundant occurrence of this taxon in ODP site 1172 East Tasman Plateau (44ºS), at approximately 63 Ma (Danian). Williams et al. (2004) gave a range for Senoniasphaera inornata (Drugg) Stover and Evitt of 64.95 to 62.6 Ma within the Danian at mid latitudes. This species is thus the best stratigraphic marker, suggesting an age no younger than late Danian for this interval.
Maastrichtian or older strata occur below 1380 m, as indicated by the presence of Dinogymnium undulosum Cookson and Eisenack. Williams et al . (2004) placed the LO of the genus Dinogymnium at the Cretaceous/ Palaeogene boundary. Diconodinium lurense sp. nov. has its highest stratigraphical occurrence at 1380 m and is quite common in underlying deposits. Pedro Luro -1 Borehole. Kaasschieter (1963) defined the type section of the Pedro Luro Formation between 1360 and 1508 md (1339 to 1487 m below mean sea level) in the Pedro Luro -1 (figure 2). In our study, the highest productive palynology sample bearing dinocysts proceed from 1487- 1488 m sidewall core sample. The dinocyst assemblages from 1487-1488 m and 1492 m cutting sample are not as diverse as those from the Cx-1 Well but contain taxa such as Alisocysta circumtabulata (Drugg) Stover and Evitt, Diconodinium lurense sp. nov. and Trithyrodinium evittii Drugg. Following Williams et al . (2004), the LODs of Alisocysta circumtabulata (Drugg) Stover and Evitt is recorded in mid latitudes Northern Hemisphere at 58.5 Ma whereas the last common occurrence is placed at 57 Ma in mid-high latitudes Southern Hemisphere; Brinkhuis et al . (2003) found the LOD of Trithyrodinium evittii Drugg in the Tasman Plateau at 57.3 Ma. In this way, the age of the interval 1487-1492 m is believed to be Selandian or older. It is impossible to determine whether Diconodinium lurense sp. nov. extends above 1487 m, because of poor sample control.
Underlying sediments appear to be Campanian or early Maastrichtian in age or older. This determination is based on the presence of Odontochitina sp. At 1505 m. Helby et al . (1987, fig. 40, p. 65) showed the LOD of Odontochitina spp. in their Australasian Isabelidinium korojonense Range Zone, which is mid Campanian to early Maastrichtian. Taking into account the information published by Wilson (1984), Wood and Askin (1992) considered the ranges of Odontochitina spp. in both Australia and New Zealand to be equivalent, and placed its LOD at the end of the Campanian. Williams et al. (2004) considered that the range of Odontochitina operculata (O.Wetzel) Deflandre and Cookson extends into the early Maastrichtian (68.5 Ma) at high latitudes Southern Hemisphere and Odontochitina costata Alberti and Odontochitina operculata (O. Wetzel) Deflandre and Cookson has its LOD at 70 Ma at mid latitudes Northern Hemisphere Thus, the age of the bottom of the Pedro Luro Formation in this borehole is taken to be possibly as young as early Maastrichtian.
Other boreholes. Diconodinium lurense sp. nov. Was previously recorded (as ? Lejeunia ? sp.) by Gamerro and Archangelsky (1981) in the two offshore boreholes Ranquel x-1 and Puelche x-1. In the Ranquel x- 1 Borehole, the Pedro Luro Formation extends from 2050 to 2300 m. The top of this unit is marked by the highest occurrences of several species, such us Diconodinium lurense sp. nov. as well as Manumiella ? cretacea (Cookson) Bujak and Davies (as Isabelidinium cretaceum ). The top of Pedro Luro Formation in Puelche x-1 Borehole, at 1600 m corresponds to the highest occurrences of Trithyrodinium evittii Drugg (as T. fragile ), Palaeocystodinium australinum (Cookson) Lentin and Williams and Cerodinium dartmoorium (Cookson and Eisenack) Lentin and Williams (as Deflandrea dartmooria ). Although Diconodinium lurense sp. nov. ranges into the overlying units (dated as Eocene to Oligocene), this species seems to be consistently present only below the top of Pedro Luro Formation. We therefore consider the presence of Diconodinium lurense in the Eocene to Oligocene indicates reworking.
Williams et al . (1998b) placed the LOD of Manumiella ? cretacea (Cookson) Bujak and Davies close to the end of the Danian (at about 61 Ma). A reevaluation of the stratigraphical significance of LODs of selected dinocyst species suggests that the age of the Pedro Luro Formation, and therefore the youngest deposits containing Diconodinim lurense sp. nov. are not younger than Danian. Moreover, Archangelsky et al . (1997) studied the Pejerrey x-1 Well, drilled offshore Colorado Basin, where they recorded our new species in strata assigned to Maastrichtian-Danian age.
Punta del Este Basin
According to Daners and Guerstein (2004), based on dinoflagellate cysts, the transition between Cretaceous and Palaeogene strata in the Gaviotín borehole is placed between 1717 and 1747 md (figure 2). The presence of Damassadinium californicum (Drugg) Fensome et al. between 1582 and 1658 md. suggests a Danian or earliest Selandian age since Williams et al . (2004) recorded the LOD of this species for mid latitudes in the Northern Hemisphere at 60.3 Ma. At 1717 and 1814 md the highest occurrences of Tanyosphaeridium variecalamum Davey and Williams and Alisogymnium euclaense (Cookson and Eisenack) Lentin and Vozzhennikova, confirm an age no younger than Maastrichtian for the lower part of the selected interval (Williams et al ., 1998b). Thus, Diconodinium lurense sp. nov. (as Diconodinium sp.) was recorded from Maastrichtian to lowermost Danian sediments.
The dinocyst assemblages studied in the Austral Basin are recovered from the Calafate Formation, which outcrops south of Lago Argentino, southwestern Santa Cruz Province. Diconodinium lurense sp. nov. is recorded (as Diconodinium sp.) only in the uppermost part of the section (figure 2). The presence of Manumiella druggii (Stover) Bujak and Davies and Alisocysta circumtabulata (Drugg) Stover and Evitt indicate a maximum late Maastrichtian age for the middle part of the section and upwards. Despite the ranges of the recorded dinocyst species extending into the Early Palaeogene, the absence of species first appearing in the Danian is noticeable. Sedimentological studies suggest that the Cretaceous-Palaeogene boundary deposits have been eroded in this area. Marenssi et al . (2002) recognized that southern Lago Argentino, the Eocene Man Aike Formation unconformably overlies the Late Cretaeous Calafate Formation. This is supported by the persistent presence of the megaspore Grapnelispora loncochensis Papú 1997 in the uppermost part of this section. This species, which has hitherto only been recovered from Maastrichtian deposits of Patagonia and would reinforce an age no younger than late Maastrichtian (Marenssi et al ., 2004). This data provide the evidence for considering the FO of Diconodinium lurense to be late Maastrichtian.
The distribution of Diconodinium lurense sp. nov. in the three basins confirms it as a consistent component of K/P boundary dinocyst assemblages. The new species probably appeared during the late Maastrichtian and disappeared around about the Danian/Selandian boundary. Thus, Diconodinium lurense sp. nov. seems to be a good biostratigraphical marker for deposits related to the Maastrichtian-Danian transition in the Southwest Atlantic Basins.