The Quebrada Honda Fauna
Although the fauna of Quebrada Honda is still incompletely known, it is presently the best sampled Miocene fauna from Bolivia and one of the best sampled from the middle latitudes of South America. All major groups of mammals present in South America at that time have been recorded at Quebrada Honda, with the exception of primates (Table 3). Given the general rarity of primates in the South American fossil record, it cannot be determined with confidence if they were absent from the area at the time of deposition or whether this is a taphonomic effect.
The marsupials from Quebrada Honda include several small argyrolagids and palaeothentids, a sparassocynid didelphimorphian, and a borhyaenid. The preservation of most of these taxa is exceptional: the palaeothentid Acdestis maddeni is represented by the most complete skull known for the family (Goin et al. 2003); specimens attributed to the argyrolagid Hondalagus altiplanensis include a nearly complete skull and mandibles (Sánchez-Villagra et al. 2000b); and a new borhyaenid is represented by an equally impressive skull and associated mandibles (A. Forasiepi, pers. comm. 2005). Quebrada Honda may eventually prove to be one of the richest localities for Neogene marsupial specimens.
Quebrada Honda xenarthrans include two dasypodids, several glyptodontids and probably two pilosans. Although only the glyptodontid Propalaehoplophorus andinus and the nothrothere pilosan Xyophorus villarroeli have been described in detail (Frailey 1986; Saint-André 1996), it appears that the majority of the eight xenarthran species are endemic to Bolivia (Scillato-Yané and Carlini 1999). Takai et al. (1984) provisionally identified two isolated osteoderms from Quebrada Honda as belonging to Hoplophorus, a genus of glyptodonts otherwise restricted to the Pleistocene of Patagonia (McKenna and Bell 1997). Based on the photographs of these specimens provided by Takai et al. (1984), these specimens could pertain equally well to one of the other glyptodontids recorded from Quebrada Honda. As the presence of Hoplophorus in the middle Miocene of Bolivia would represent a significant temporal range extension of the taxon, it is excluded from the faunal list pending the recovery of more diagnostic material.
None of the rodents from Quebrada Honda has been described in detail, and therefore all identifications should be considered preliminary. They are presently assigned to five families: Dasyproctidae, Eocardiidae, Octodontidae, Echimyidae and Chinchillidae. Rodents were apparently abundant at Quebrada Honda during the time the fossils were deposited, especially chinchillids (MacFadden and Wolff 1981). All chinchillids that have been collected from Quebrada Honda are lagostomines, as is the case for Patagonian Miocene localities; this contrasts sharply with the presence of only chinchilline chinchillids in the slightly older Chucal Fauna of northern Chile (Flynn et al. 2002a). Two families of rodents included in the preliminary report of MacFadden and Wolff (1981) do not appear to be present at Quebrada Honda: Caviidae and Capromyidae. As noted by MacPhee (2005) the presence of capromyids in the middle Miocene of Bolivia would be especially notable given that their distribution is otherwise restricted to the Greater Antilles. Based on the taxa now known to be present at Quebrada Honda, these apparent ‘misidentifications’ appear more likely to be attributable to changes in taxonomy; Eocardia (Eocardiidae) was once considered a caviid and Neoreomys (Dasyproctidae) was once considered a capromyid (e.g. Scott 1905).
At least seven species of endemic ungulates are present at Quebrada Honda and all are referable to distinct families. Takai et al. (1984) identified the proterotheriid litoptern Diadiaphorus based on excellent mandibular dental material, but did not refer the specimens to a particular species. The presence of a macraucheniid litoptern was noted by Hoffstetter (1977) based on an isolated lateral metapodial; no additional material has been noted in the literature. Astrapotheres were also first identified at Quebrada Honda by Hoffstetter (1977); the species appears to be a member of the Uruguaytheriinae, but its generic allocation has yet to be determined (see further discussion below).
As detailed previously, the notoungulates of Quebrada Honda include an hegetotheriid, a toxodontid (possibly two), an interatheriid and a mesotheriid. Among these, the hegetotheriid and interatheriid are endemic to Quebrada Honda; the toxodontid is potentially endemic to Quebrada Honda but is not represented by sufficiently diagnostic material; and the mesotheriid occurs in at least one other Bolivian locality but appears to be endemic to the region (Croft and Anaya 2004, 2006, present study). Given the age and geographical position of Quebrada Honda, the presence of many endemic ungulate species is not unexpected. What is unexpected is the abundance of the hegetotheriid Hemihegetotherium and the rarity of the other three taxa. As discussed by Croft et al. (2004) and Croft and Anaya (2004, 2006), mesotheriids are typically the most abundant notoungulates in early–middle Miocene localities of northern Chile and Bolivia, whereas hegetotheriids are much less so. In the collections from Quebrada Honda housed at the MNHN, Hemihegetotherium is represented by some three dozen specimens (Croft and Anaya 2006) but only a single specimen of the mesotheriid Plesiotypotherium has been recovered. Similarly, interatheriids and toxodontids are generally abundant in the faunas in which they occur (e.g. La Venta, Santa Cruz), but only one specimen of Miocochilius and two toxodontid teeth have been collected from Quebrada Honda. The composition of the notoungulate community at Quebrada Honda appears to be unique among middle Miocene faunas and further study may help clarify environmental or ecological factors that might have contributed to this condition.
One other noteworthy aspect of the Quebrada Honda Fauna is the nearly even distribution of species among the main groups of South American mammals (namely marsupials, xenarthrans, rodents and ungulates); each comprises 20–30 per cent of the total faunal species richness. Compared with the other Miocene faunas analysed above, this corresponds most closely (almost exactly) to the distribution of species at La Venta, excluding primates (Table 5). Given that the total species richness of La Venta is twice as great as that of Quebrada Honda, it seems unlikely that this even distribution of species is indicative of a similar habitat at the two localities. Rather, it seems more likely that this pattern results from less complete sampling of rodents and marsupials in the other faunas, especially as these groups tend to be uncommon and/or of small body size in most faunas. A testable hypothesis is that additional sampling of these Argentine faunas would disproportionately increase the number of marsupial and rodent species, creating a distribution more similar to that of Quebrada Honda and La Venta. This relatively even distribution among major groups may therefore be typical for middle–late Miocene faunas.
Table 5. General taxonomic composition of Miocene faunas examined in this study. The raw number indicates the number of species in each ordinal or supraordinal group. The percentage in parentheses indicates the proportion of the non-primate terrestrial mammal fauna represented by each group in each assemblage. The total number of species for each fauna is listed in the bottom row
| ||Collón-Curá||Quebrada Honda||La Venta||Lower Arroyo Chasicó||Upper Arroyo Chasicó|
|Marsupials||2 (5%)||6 (20%)||12 (20%)||2 (8%)||–|
|Xenarthrans||12 (32·5%)||8 (27%)||16 (27%)||5 (21%)||7 (35%)|
|Rodents||12 (32·5%)||9 (30%)||18 (31%)||9 (38%)||4 (20%)|
|Ungulates||11 (30%)||7 (23%)||13 (22%)||8 (33%)||9 (45%)|
The analysis of faunal resemblance presented above suggests that the fauna of Quebrada Honda is more similar to that of the slightly asynchronous Patagonian locality of Collón-Curá than to the synchronous low-latitude La Venta Fauna. This does not appear to result from geographical proximity, however, as Quebrada Honda is positioned roughly half-way between Collón-Curá and La Venta. Rather, it appears that this middle-latitude locality bears a greater resemblance to Patagonian localities than to low-latitude localities, despite slight age differences; one would expect that a Laventan fauna from Patagonia would demonstrate even greater faunal resemblance to Quebrada Honda than a Colloncuran one, thus lending increased support for a biogeographical tie between the middle and upper latitudes during the middle Miocene. A more robust test of this hypothesis would involve a greater number of localities of this age from all latitudes, but this must await discovery and description of additional faunas.
It would have been enlightening to include a Mayoan SALMA fauna (late middle–early late Miocene; Text-fig. 2) in the analysis of faunal resemblance to see whether Quebrada is more similar to slightly older or slightly younger Patagonian localities. Kay and Madden (1997) demonstrated that the fauna of La Venta more closely resembled the former (e.g. Santacrucian and Colloncuran faunas) than the latter (e.g. Mayoan and Chasicoan faunas) and, therefore, Quebrada Honda might be expected to show the same pattern. The lack of available faunal data for the Mayoan precludes such an analysis. The lack of inclusion of a temporally intermediate Mayoan fauna probably accounts for the separation of the five faunas into two temporal groups (i.e. middle Miocene and late Miocene) as noted above.
Kay and Madden (1997, fig. 30.1) reconstructed La Venta as sitting on a peninsula or island during the middle Miocene and suggested that this palaeogeographical isolation might have partially contributed to that fauna's high level of endemism. This was supported by faunal resemblance indices for La Venta ranging from 13 to 19 among comparisons with older faunas, and 2 to 10 among comparisons with younger faunas (Kay and Madden 1997, table 30.9). These values differ only slightly from those obtained in the present study (Table 4). The fact that the La Venta Fauna shares approximately the same proportion of taxa with Quebrada Honda as with the Patagonian Collón-Curá Fauna, despite the much closer geographical position of Quebrada Honda, lends support to the idea of a geographical or environmental barrier isolating the northern portion of South American during the middle Miocene.
South American Land Mammal ‘Ages’
A fundamental question in South American palaeomammalogy is whether the informal system of Land Mammal ‘Ages’ is applicable on a continent-wide scale (Flynn and Swisher 1995; Madden et al. 1997; Flynn and Wyss 1998). In contrast to the northern continents, this is an issue in South America for several reasons: South America spans a large range of latitude (approximately 65°); its system of land mammal ages is based on localities in a very limited geographical area (Patagonia) relative to the size of the continent; and the SALMA sequence lacks directly superimposed faunas (and therefore has gaps) in many sections (Flynn and Swisher 1995; Madden et al. 1997). Madden et al. (1997) examined the question of the continent-wide applicability of SALMAs by comparing the fauna of La Venta with other Miocene faunas and noted (p. 505) that ‘As presently defined and characterized, the Miocene SALMAs do not appear to be very powerful tools for refined temporal correlation across the latitudinal extremes of the continent.’
One acknowledged shortcoming of the aforementioned analysis, however, was the lack of contemporaneous pairs of faunas spanning the latitudinal range of South America; with La Venta being the only well-represented low-latitude fauna, it could only be compared with slightly older and slightly younger faunas from Patagonia (Madden et al. 1997). New middle-latitude faunas from localities such as Chucal (c. 18° S; Flynn et al. 2002a; Croft et al. 2004) and Quebrada Honda (c. 22° S) are approximately contemporaneous with high-latitude Patagonian faunas (Santa Cruz and Pampa Castillo in the case of Chucal; Flynn et al. 2002b; Croft et al. 2004) or low-latitude tropical faunas (La Venta in the case of Quebrada Honda) and, therefore, should provide more robust tests of the continent-wide utility of SALMAs. Can a fauna such as Chucal or Quebrada Honda confidently be referred to a particular SALMA in the absence of radioisotopic dates? If not, then such designations may be superfluous for discussions of faunas spanning more than a regional scale, at least in certain areas and during certain intervals of time.
Comparisons of the ungulates of Chucal with those of more southerly Santacrucian faunas have not only highlighted significant late early Miocene provinciality in South America but also permitted confident referral of the Chucal Fauna to the Santacrucian SALMA (Croft et al. 2004). Thus, SALMAs appear to be useful for intracontinental correlation on a limited scale (i.e. not necessarily for the entire length of the continent), at least during the early Miocene. More detailed comparisons between Chucal and other faunas must await description of the non-ungulate components of the fauna.
In the case of Quebrada Honda, the applicability of SALMAs is less clear. Although Madden et al. (1997, p. 519) were ‘equivocal about the proposition of a formal “Laventan LMA”’, they did define a Laventan Stage and corresponding Laventan Age. The boundaries of the Laventan Stage (and Age) were based on the coincident stratigraphic first and last occurrences of a variety of characteristic La Venta taxa, collectively referred to as the ‘Miocochilius Assemblage Zone’ (Madden et al. 1997; see Appendix). This assemblage zone was named after the interatheriid notoungulate Miocochilius, the most abundant and characteristic taxon of La Venta. A single-taxon range zone based on the stratigraphic occurrence of this genus was proposed informally, but the authors preferred the assemblage zone definition in the ‘belief that it will eventually prove to be more stable and more useful for biocorrelation’ (Madden et al. 1997, p. 509). The Laventan SALMA, though never formally proposed, subsequently has been considered part of the standard SALMA sequence (e.g. Flynn and Wyss 1998).
Of the mammals present in the Miocochilius Assemblage Zone, only a single taxon definitively is present at Quebrada Honda: Miocochilius (Table 3; Appendix). The presence of this genus at both localities despite the great distance between them supports its informal designation as the ‘indicator taxon’ for the Laventan SALMA. It should be taken in account, however, that M. federicoi does not itself occur at La Venta and that the precise generic designation of the species (presently referred to Miocochilius) is not obvious (see ‘Systematic palaeontology’, above). If subsequent phylogenetic analyses of the Interatheriinae were to indicate that M. federicoi and M. anomopodus do not share a common ancestor exclusive of other interatheriines (e.g. Protypotherium spp.), then Miocochilius would no longer appear to be useful for biostratigraphic correlation. For the present, the genus appears to be the most useful biochronologic indicator of this time interval.
A second endemic ungulate from the Miocochilius Assemblage Zone of La Venta might be present at Quebrada Honda: a ‘large, advanced astrapothere … represented by a highly fragmented skull that consists of the cranium and pieces of the palate and upper teeth’ (Frailey 1987, p. 5). This specimen was referred by Frailey (1987) to ?Xenastrapotherium, a taxon otherwise known from Venezuela and Colombia. Hoffstetter (1977) had earlier referred isolated dental specimens from Quebrada Honda to ‘le groupe Uruguaytherium-Xenastrapotherium’ and Johnson and Madden (1997) chose to refer these and the specimen described by Frailey (1987) to Uruguaytheriinae gen. et sp. incertae sedis, pending more detailed analyses of the Quebrada Honda astrapothere material. Johnson and Madden (1997, p. 377) did observe that ‘Among the Astrapotheriidae for which this portion of the cranium is known, the general shape and configuration of the base of the neurocranium in UF 26679 [the Quebrada Honda specimen described by Frailey 1987] is most similar to Granastrapotherium snorki’, a species known only from La Venta. If the Quebrada Honda material does, indeed, pertain to Granastrapotherium (presently a monospecific taxon), the genus could represent a useful biochronologic indicator for the Laventan SALMA. Similarly, referral of the Quebrada Honda taxon to Laventan Xenastrapotherium kraglievichi (another uruguaytheriine astrapothere) would suggest that this species (but not the genus) is a useful biochronologic indicator; the genus as a whole spans a broader range of ages, based on occurrences in Brazil, Colombia, Ecuador and Venezuela (Johnson and Madden 1997). An undescribed juvenile astrapothere skull from Quebrada Honda (MNHN 3672) will undoubtedly provide important new data for determining the precise affinities of the taxon and should clarify the biochronologic utility of middle Miocene uruguaytheriine astrapotheres.
Of the non-ungulate taxa from Quebrada Honda, only a single characteristic taxon of the Laventan is potentially present: Neoreomys sp. (Table 3; Appendix). Neoreomys huilensis from La Venta was originally described by Fields (1957), but later work by Walton (1990, 1997) recommended that the species be referred to a distinct (as yet unnamed) genus. Preliminary identification of the species from Quebrada Honda suggests that it is more closely allied to Patagonian Neoreomys than to ‘Neoreomys’huilensis from La Venta, but this must be confirmed by more detailed study and collection of additional material. The glyptodontid Asterostemma has been recorded at both La Venta and Quebrada Honda, but this is not a characteristic taxon of the Laventan, and the species from La Venta may be generically distinct from Patagonian Asterostemma (Carlini et al. 1997).
Overall, the Quebrada Honda Fauna provides weak support, at best, for the continent-wide applicability of SALMAs. Of the 15 defining and characteristic non-primate mammals of the Miocochilius Assemblage Zone (Madden et al. 1997, fig. 29.6), only three are potentially present at Quebrada Honda, and one of these is doubtful. Three genera are potentially shared by the two faunas (two of them characteristic of the assemblage zone), representing 11·5 per cent of currently recorded Quebrada Honda genera; this is approximately half the proportion shared by Quebrada Honda and Collón-Curá and only slightly more than the proportion shared between Quebrada Honda and the Lower Arroyo Chasicó (Table 4).
By contrast, 100 per cent of the genera recorded from the Santacrucian Pampa Castillo locality of southern Chile also occur at the type locality for the Santacrucian SALMA (Flynn et al. 2002b); similarly, 57 per cent of the ungulates from the Santacrucian Chucal locality of northern Chile are also known from Santa Cruz (Croft et al. 2004) despite the significant difference in latitude. Combined with the data presented here, these early and middle Miocene faunas suggest that SALMAs are likely to be useful for biocorrelation among high- and mid-latitude faunas, but that low-latitude faunas should rely on a distinct system of LMAs for temporal correlation. Although the proportion of taxa shared among these faunas will certainly be modified with better sampling and further taxonomic studies, it appears unlikely that the overall pattern of faunal similarity will change significantly enough to alter these broad patterns.