Paleontology and Geology of Laetoli: Human Evolution in Context

Laetoli in northern Tanzania is one of the most important paleontological and paleoanthropological localities in Africa. In addition to fossil hominins, there is a diverse associated fauna. The Laetoli fauna is important because it serves as a key comparative reference for other Plio-Pleistocene sites in Africa, it samples several time periods that are generally poorly represented at other paleontological sites in East Africa, and it provides key insights into the faunal and floral diversity during the Pliocene. As a result of renewed fieldwork at Laetoli (1998–2005) more than 25,000 fossils have been collected, of which more than half are fossil mammals. Most of the fossils were recovered from the Upper Laetolil Beds (3.6–3.85 Ma), but smaller samples came from the Lower Laetolil Beds (3.85–4.4 Ma) and Upper Ndolanya Beds (2.66 Ma). These include new specimens of Australopithecus afarensis from the Upper Laetolil Beds and the first finds of fossil hominins from the Upper Ndolanya Beds, attributable to Paranthropus aethiopicus. Inferences about the paleoecology at Laetoli are important for understanding the possible range of hominin habitat preferences and ecological change in East Africa during the Pliocene. The evidence from a wide range of analyses indicates that a mosaic of closed woodland, open woodland, shrubland and grassland dominated the paleoecology of the Upper Laetolil Beds. The region would have been dry for most of the year, except for the possible occurrence of permanent springs along the margin of the Eyasi Plateau and ephemeral pools and rivers during the rainy season. The paleoecological reconstruction of the Upper Ndolanya Beds is more problematic because of conflicting lines of evidence, but it is very likely that conditions were drier than in the Upper Laetolil Beds with a greater proportion of grassland, but that closed and open woodlands were still a major part of the ecosystem.

New rodent specimens collected at Laetoli between 1998 and 2005 are described here. The material allows an updating and refinement of the previously published taxonomic lists, especially those for the Lower Laetolil Beds and the Upper Ndolanya Beds. The increased number of well-preserved cranial specimens allows the description of several new species and a better appreciation of the size and morphology of some Laetoli taxa compared to their southern and eastern African counterparts. This is especially the case for Saccostomus, for which the fossil record has recently been much improved. The new species described here include a small sciurid, two Gerbillinae, and a thryonomyid. Some species are newly recognized at certain localities, and Aethomys and Petromus are recorded for the first time at Laetoli. The distribution and stratigraphic range for Pedetes laetoliensis is extended, and it is now recorded in the Upper Ndolanya Beds. Similarly, Xerus janenschi is now identified in the Laetolil Beds. As in the previous study of the Laetoli rodents, important differences in species composition and diversity between the Upper Laetolil Beds and the Upper Ndolanya Beds are confirmed. These probably reflect differences in landscape. Compared to other Pliocene assemblages, the Laetolil Beds are characterized by a very unusual diversity of sciurids and the dominance of Saccostomus and Pedetes, but otherwise they compare well with other East African Mio-Pliocene rodent assemblages, such as those from the Omo Valley and Lemudong’o. The Laetoli assemblages are distinct from those of Lukeino, Chorora and Harasib 3, but could belong to the same faunal unit as Ibole (Manonga Valley). They also differ in some respects from those from Hadar and Pliocene South African sites. Few species are shared in common between the Laetolil Beds and Upper Ndolanya Beds, but it is uncertain whether this turnover is due to taphonomic or paleoclimatic factors. This contribution highlights the importance of Laetoli for understanding rodent evolution, as well as for its geographic position at the crossroads between East and South Africa.

Two incomplete mandibles (plus a third tentatively referred) and an isolated P4 of Rhynchocyon pliocaenicus are reported from Localities 2, 3, and 10E, Upper Laetolil Beds, Laetoli, Tanzania, East Africa. The specimens are dated at ca. 3.7–3.6 Ma. Morphology of this newly recovered material confirms and enhances a diagnosis based previously on only the holotype and paratype. Mean jaw depth and almost all dental measurements of R. pliocaenicus are on average about 21% smaller than those of the extant species R. cirnei, R. petersi, and R. chrysopygus. Rhynchocyon pliocaenicus is diagnosed also by a p2 with a strong posterior heel with a prominent posterior basal cusp, consistent presence of an anterobuccal cingulum on p4-m2, and a posterior cingulum on p4-m1. The protoloph of P4 and M1 of R. pliocaenicus connects to the tip of the paracone. If the habitat preferences of R. pliocaenicus were similar to extant Rhynchocyon, then the presence of this fossil species suggests that closed canopy habitats were present at Laetoli ca 3.7–3.6 Ma, but probably relatively rare.

An additional specimen of a fossil galagid was recently recovered from the Upper Laetolil Beds at Laetoli in northern Tanzania. This new find represents the most complete specimen of a galagid known from Laetoli, and comprises associated partial right and left mandibular corpora. The galagid material from Laetoli can all be attributed to a single species, previously referred to as Galago sadimanensis. However, the taxon is sufficiently distinct from all extant galagids, as well as stem galagids from the Miocene of East Africa, to be placed in its own genus, Laetolia. The fossil record of galagids from the Pliocene of Africa is exceedingly poor, and Laetolia sadimanensis represents the best-known form. Laetolia can be distinguished from other galagids by its unique suite of morphological features. The stout and vertical implantation of P₂, the steeply inclined and robust symphysis, and the relatively deep corpus are all specialized features that are probably functionally linked. However, Laetolia has a less molariform P₄ than extant galagids, and it can be inferred to represents their primitive sister taxon. Based on molecular clock estimates, extant galagids shared a last common ancestor during the late Oligocene. It is interesting, therefore, to discover a sister taxon of extant galagids surviving in East Africa until at least the Pliocene, contemporary with more advanced crown members of the clade. From a paleoecological perspective, the occurrence of fossil galagids at Laetoli implies the presence of habitats with at least a sparse coverage of trees and/or thorn bush.

New finds from Laetoli have allowed a more detailed assessment of the taxonomy and paleobiology of the fossil ­cercopithecids. Most of the specimens consist of isolated teeth, jaw fragments and postcranial bones from the Upper Laetolil Beds (∼3.5–3.8 Ma), but four specimens are known from the Upper Ndolanya Beds (∼2.66 Ma) and a proximal humerus has been recovered from the Lower Laetolil Beds (∼3.8–4.3 Ma). Four species are represented: Parapapio ado, Papionini gen. et sp. indet., cf. Rhinocolobus sp., and Cercopithecoides sp. Parapapio ado is the most common species. Based on dental size and proportions and facial morphology, Pp. ado can be distinguished from all other species of Parapapio. The postcranial specimens attributed to Pp. ado indicate that it was a slender and agile semi-terrestrial monkey. A few isolated teeth represent a second species of papionin, larger in dental size than Pp. ado. Due to the paucity of the material, the taxon is left unassigned at the genus and species level. A distal humerus attributed to this taxon indicates that it was large terrestrial cercopithecid. The most common species of colobine is referred to cf. Rhinocolobus sp., based on its overall similarities to Rhinocolobus turkanaensis. The material can be distinguished from all fossil colobine species previously recognized from Africa, but without more complete cranial specimens it is not possible to diagnose a new taxon. From the postcranial material it can be inferred that it was generally adapted for arboreal quadrupedalism. The somewhat smaller species of colobine represents a previously undescribed species of Cercopithecoides. The postcranial specimens attributed to this taxon indicate that it was fully arboreal. Analysis of the distribution of the Laetoli cercopithecids provides provisional evidence of spatial patterning and temporal trends. For example, the dentition of Parapapio exhibits a trend to increase in size during the course of the Upper Laetolil Beds. As at other late Miocene and early Pliocene localities older than 3.5 Ma, the Laetoli cercopithecid community is characterized by the absence of Theropithecus and the relatively large proportion of colobines. After 3.5 Ma Theropithecus becomes the dominant cercopithecid at all East African ­localities, and the proportion of colobines declines accordingly.

Renewed investigations at Laetoli in northern Tanzania have led to the recovery of a number of new fossil hominins. A lower canine and a mandibular fragment from the Upper Laetolil Beds (3.63–3.85 Ma) are referred to Australopithecus afarensis, and an edentulous maxilla and a proximal tibia from the Upper Ndolanya Beds (2.66 Ma) are attributed to Paranthropus aethiopicus and Hominini gen. et sp. indet., respectively. Additional hominin specimens from earlier collections are described here for the first time, including three specimens of A. afarensis, probably from the Upper Laetolil Beds, and a possible cranial fragment of an infant from the Upper Ndolanya Beds. The chronology and provenance of the Laetoli hominins are reconsidered. The species afarensis is provisionally retained in Australopithecus to reflect its anatomical and paleobiological similarities to the other species of Australopithecus sensu lato, but a reasonable case could be made on phylogenetic grounds to transfer it to Praeanthropus. It has been argued that the Laetoli sample of A. afarensis is morphologically and temporally intermediate between A. anamensis and the Hadar sample of A. afarensis, and that A. anamensis and A. afarensis represent a single anagenetically evolving lineage. However, the new specimens from the Upper Laetolil Beds help to close the gap between the Laetoli and Hadar samples, and a critical assessment of the morphological variation in the two samples indicates that there are few consistent differences separating them. Rather than being intermediate in morphology, the Laetoli sample appears to represents an earlier population of A. afarensis, with almost the full complement of derived features that characterizes the Hadar sample, but still retaining a few primitive traits. The morphological features that distinguish A. anamensis from A. afarensis are much more extensive, and these provide adequate justification for the recognition of a species distinction. The evidence best fits an evolutionary model involving a cladogenetic event rather than a simple anagenetic transformation of a single unbranched anamensis-afarensis lineage through time. The Paranthopus aethiopicus specimen from the Upper Ndolanya Beds represents the ­oldest securely dated specimen definitively attributable to this taxon and the first definitive record outside of the Turkana Basin. The Paranthropus clade probably immigrated into eastern Africa before 2.7 Ma, and became widely distributed throughout the region soon thereafter. The timing and biogeographic patterning of the occurrence of Paranthropus and Homo suggest that their respective dispersals into eastern Africa were not coincident or synchronous. Homo appeared somewhat later than Paranthropus across most of eastern Africa, except in the Awash region of Ethiopia where Homo makes its first appearance in the absence of Paranthropus. These differences in the timing and distribution suggest that Paranthropus and Homo may have had different biogeographic histories, and that their ancestral species may have had different ecological requirements at the time of their initial influx into eastern Africa.

This paper reviews the extensive carnivoran fauna of Laetoli on the basis of collections housed in Berlin, London, Nairobi, and Dar es Salaam. Members of the Carnivora are known from both the Lower and Upper Laetolil Beds, as well as from the Upper Ndolanya Beds. Of these, the Upper Laetolil Beds are best sampled, and the material includes a minimum of 28 species of Carnivora (four Canidae, three Mustelidae, three Viverridae, six Herpestidae, five Hyaenidae, and seven Felidae). Many of the smaller Carnivora species include complete or partial skeletons and whole, undamaged crania, suggesting rapid burial and absence of trampling and other taphonomic processes that severely affected the more fragmentary larger Carnivora. The Upper Ndolanya Beds Carnivora are preserved in a similar fashion. This stratigraphic unit includes nine to ten species (one Mustelidae, two Herpestidae, one or two Hyaenidae, and five Felidae). All of these are also known from the Upper Laetolil Beds. The Lower Laetolil Beds are less well sampled, with only four species of Carnivora (one Mustelidae, one Herpestidae, and two Hyaenidae). Of these, the mustelid and one hyenid are unique to this stratigraphic unit, while one hyenid is shared with the Upper Laetolil Beds and the herpestid with both the Upper Laetolil Beds and the Upper Ndolanya Beds. Three of the Lower Laetolil Beds Carnivora (all except the herpestid) are partial skeletons, suggesting different depositional or taphonomic conditions at that time, while the presence of an otter in the Lower Laetolil Beds indicates the presence of a large, permanent body of water in the vicinity.

Paleontological fieldwork between 1998–2005 by the Eyasi Plateau Expedition at Laetoli and nearby sites produced a large collection of proboscidean fossils from the early to mid Pliocene Lower and Upper Laetolil Beds and late Pliocene Upper Ndolanya Beds, and possibly older sediments at Endolele, that substantially enlarges the sample of proboscidean material recovered earlier by Louis Leakey in 1935, Kohl-Larsen in 1938–1939, and Mary Leakey in the 1970s and early 1980s. The present study of the combined proboscidean sample confirms the presence of deinotheres and loxodont elephants, and provides the first description of anancine gomphotheres from the area. In addition, the first stegodont from the Eyasi Plateau is identified. The study also suggests that gomphotheres and loxodont elephants evolved locally in the Eyasi Plateau during the early Pliocene. Inference from stratigraphic distribution of proboscidean taxa, isotopic analyses, and dental morphology corresponds with paleoecological reconstruction depicting the Eyasi Plateau during the early-mid Pliocene as covered with abundant shrub- and grassland, with more restricted gallery forest, and as drier during the late Pliocene. Age-grade mortality profiles of elephants and deinotheres from the Laetolil and Upper Ndolanya Beds indicate a chronic lack of standing water or cyclical incidences of drought in the region for a sustained interval of time.

The small sample of fossil orycteropodids (n = 27) available from the Laetolil Beds at Laetoli consists of isolated postcranials and a few cranio-dental specimens. The material can all be attributed to a single species of the genus Orycteropus. The Laetoli aardvark is generally similar in morphology to the extant O. afer, but differs in being smaller and in having a more gracile postcranial skeleton. The material is also distinguishable from O. abundulafus and O. djourabensis from the late Miocene and early Pliocene of Chad. Given its unique combination of features, the orycteropodid from Laetoli very likely represents a distinct species, but the material is not adequate to designate a new taxon. However, the evidence indicates that there was a greater diversity of aardvarks in the Pliocene of East Africa than is currently recognized.

The remains of fossil rhinoceroses from Laetoli represent at least three taxa: Ceratotherium efficax, Ceratotherium cf. simum, and Diceros sp. The great majority of the material from the Pliocene Laetolil Beds belongs to C. efficax, for which we provide a revised diagnosis. This taxon has been frequently misidentified and inaccurately referred to as C. praecox, C. germanoafricanum, or C. mauritanicum. A cranium from the Upper Ndolanya Beds shows more derived dental features, but a precise assignment to C. germanoafricanum or C. simum is presently not possible. The occurrence of true Diceros in the Laetolil Beds is demonstrated by a partial cranium with incomplete dentition, but very few other specimens can be potentially attributed to this genus. Analysis of occlusal wear patterns suggests that C. efficax was a grazer or possibly a graze-dominated mixed feeder; in either case it probably included a variable component of browse in its diet. The transition from a Diceros-like ectolophodont dentition to the full-fledged plagiolophodonty seen in extant Cera­totherium simum included a substantial period of stasis, spanning at least the interval represented by the Laetolil Beds. A shift in the dietary regime towards increased grazing had occurred by the Upper Ndolanya time, and this trend continued from the early Pleistocene to the Recent. Based on the available fossil record, the split of the two lineages leading to the extant species must have taken place in Africa during the Miocene.

Equid teeth and postcrania from the Laetolil and Upper Ndolanya Beds are described and compared with other hipparion assemblages from East and South African Plio-Pleistocene localities. The hipparion from the Laetolil Beds is morphologically similar to Eurygnathohippus hasumense from Hadar, although of slightly smaller dimensions, and is referred to Eurygnathohippus aff. hasumense. The hipparion from the Upper Ndolanya Beds closely resembles the advanced hypsodont form found at Olduvai Gorge (Bed I-IV), and is referred to Eurynathohippus aff. cornelianus.

This contribution discusses over 200 suid ­specimens recovered from Laetoli by the Eyasi Plateau Paleontological Expedition (EPPE) between 1998 and 2005. Earlier collections from the site included just three taxa, Notochoerus euilus and Potamochoerus porcus from the Laetolil Beds, and Kolpochoerus limnetes (= heseloni) from the Upper Ndolanya Beds. In addition, the more recent collections establish the presence of Nyanzachoerus kanamensis and Notochoerus jaegeri in the Laetolil Beds and extend the range of Kolpochoerus heseloni into this earlier geological unit. Potamochoerus material from the Upper Laetolil Beds has been assigned to P. afarensis. Metridiochoerus andrewsi is present in the Upper Ndolanya Beds. The suid fauna can now be shown to be taxonomically similar to that from other comtemporaneous sites. The autecology of the suid taxa from Laetoli has implications for the paleoenvironmental reconstruction of the site, and suggests that a range of environments were present in the region.

792 specimens attributed to the Giraffidae were recovered by the Eyasi Plateau Paleontological Expedition (EPPE) from the three Pliocene stratigraphic units at Laetoli, with Giraffa stillei the most common taxon in all three levels. Giraffids are notably well represented in the Upper Laetolil Beds, with further evidence gathered by EPPE for the three previously recognized species from this unit. In the Lower Laetolil Beds Giraffa stillei is provisionally identified, as is Sivatherium. A third, large giraffid species may also be present. Based on a specimen recovered by Kohl-Larsen’s team during the first extensive exploration of Laetoli, we now provisionally recognize Giraffa pygmaea from the Upper Ndolanya Beds, along with Giraffa stillei and Sivatherium maurusium. Evidence for Giraffa jumae in the Upper Ndolanya Beds is not as convincing, as it is based on a small number of postcranial bones. In the time between the formation of the Upper Laetolil Beds and the Upper Ndolanya Beds, it appears that Giraffa stillei increased in size, which has been documented at other contemporary East African localities. This may relate to competition from the smaller Giraffa pygmaea.

Substantial revisions are made to the account of the Bovidae of the Laetolil Beds and the Upper Ndolanya Beds given in Gentry (1987). Both beds have 17 or 18 species with a wide range of tribal affiliations. The bovids of the Laetolil Beds are unlike those in the “Plio-Pleistocene” faunas of Olduvai Gorge and the Shungura and Koobi Fora Formations. Some species are primitive, others are divergently specialized. Reduncini may well be totally absent, and a few teeth of a larger Cephalophus represent a rarely fossilized tribe. A little-advanced Hippotragus is more abundant than in later faunas. One or two links can be discerned between the bovids in the Upper Ndolanya Beds and those in the Laetolil Beds, but the differences are more striking. There are signs of a “Plio-Pleistocene” fauna being in place, and some of its species reach a large size. A new species of Aepyceros is described from the Laetolil Beds and of an alcelaphine from the Upper Ndolanya Beds.

The Pliocene of Laetoli (Tanzania) has produced a taxonomically unbalanced fauna of amphibians and squamate reptiles. Amphibians are represented by only two specimens belonging to indeterminate anurans. Similarly, lizards are comprised of two specimens that are referred to an indeterminate acrodontan and an indeterminate scincomorphan. Snakes are more numerous (more than 150 specimens) and taxonomically diverse. They include a boid (Python sebae or P. natalensis), at least three colubrids (cf. Thelotornis, cf. Rhamphiophis, one indeterminate colubrid, and another indeterminate taxon that might be a modern specimen), an elapid (Naja robusta), a possible distinct elapid, and a viperid (Bitis olduvaiensis or a new species of Bitis). The latter taxon represents the most common snake. The paleoecological implications of the fauna are not readily apparent because the most useful indicator taxa are distributed in different beds. Based on the entire fauna it may be concluded that the Laetoli area was neither a desert nor covered by forest. Bodies of water were periodically present, at least during the period of deposition of the Upper Laetolil Beds. An unusual feature of the Laetoli snake community, but similar to a few other Neogene faunas from sub-Saharan Africa, is the fact that colubrids are not the dominant taxon.

Two species of tortoises are known from Pliocene sites on the Eyasi Plateau, including Laetoli. The most common species is a medium-sized tortoise, Stigmochelys brachygularis, which is well represented in the Laetolil Beds (∼3.6–4.4 Ma) and Upper Ndolanya Beds (∼2.66 Ma). The giant tortoise, “Geochelone” laetoliensis, is known only from the Laetolil Beds, and is much less common than S. brachygularis. Stigmochelys brachygularis is represented by a number of relatively complete and partial shells, as well as numerous isolated and associated shell fragments, some postcranial remains and eggs. It is generally similar in size and overall morphology to the extant leopard tortoise, S. pardalis, but a number of features serve to distinguish the two species. Stigmochelys brachygularis and S. pardalis are inferred to be closely related, and are most likely sister taxa that represent time-successive species of a single lineage. The age structure of the fossil sample, in conjunction with evidence of carnivore damage on the shells, indicates that S. brachygularis was subject to relatively high levels of predation in comparison with modern-day S. pardalis. If S. brachygularis is presumed to have been ecologically similar to modern S. pardalis, it would have been capable of living in a wide range of habitats from semi-desert and savanna to open woodland. The material attributed to “Geochelone” laetoliensis is more fragmentary, and as a consequence it is not possible to determine its precise phylogenetic or taxonomic relationships. However, it likely represents a distinct genus, possibly with affinities to Astrochelys from Madagascar. A more thorough assessment of the relationships of “G.” laetoliensis with Miocene and Pliocene giant tortoises from Africa and with extant genera will have to await the recovery of more complete material from Laetoli. Giant tortoises, such as “G.” laetoliensis, became extinct on mainland Africa during the late Pliocene, possibly associated with the appearance of early Homo and stone tool using behaviors at 2.6 Ma.

The study of the entire fossil bird collection from Laetoli, including the specimens collected by Mary Leakey (1974–1981) and the recent ones collected by the Eyasi Plateau Paleontological Expedition (1998–2005), is presented here. The 247 bird fossils allow the identification of 21 different taxa in 11 families (and eight orders). One fossil is from the Lower Laetolil Beds, 229 from the Upper Laetolil Beds, and 17 from the Upper Ndolanya Beds. The large majority of the fossils belong to francolins (Phasianidae) and guineafowl (Numididae), followed in decreasing order of abundance by Columbidae, Accipitridae, Strigidae, Passeriformes, Coliidae, and – one fossil each – Ardeidae, Falconidae, Scolopacidae and Tytonidae. Some genera or species constitute the earliest known representatives of modern lineages, among which are the falcon Falco cf. eleonorae, the Vulturine Guineafowl Acryllium vulturinum, a dove Streptopelia sp. and the eagle-owl Bubo cf. lacteus. Acryllium vulturinum, from the Upper Laetolil Beds, indicates semi-open or open country habitats in the paleoenvironment. A heron, Ardea sp. (Upper Ndolanya Beds) and a calidrine wader (Upper Laetolil Beds) imply aquatic settings. Most other birds, including most Galliformes, the Columbidae and the Coliidae (mousebirds) indicate a minimal cover of at least scattered trees or bushes. The fossil birds yield additional information regarding biogeo-graphy, differences between stratigraphic units, environment, new perspectives on the study of bird prints and eggs from diverse localities at Laetoli, and finally some clues about the probable status of some birds (i.e., breeding, passage or wintering migrant).

Nine fossil beetles and seven fossil brood balls made by dung beetles are described from Laetoli (Pliocene). Seven beetles are Tenebrionidae, tribes Tentyriini and Molurini, one is a June beetle of the tribe Schizonychini (Scarabaeidae: Melolonthinae) and one a rhinoceros beetle (Scarabaeidae: Dynastinae) described as Calcitoryctes magnificus sp.n. Seven fossil dung beetle brood balls are described as Coprinisphaera laetoliensis ichnosp. n. and C. ndolanyanus ichnosp. n., the first formally described scarab ichnofossils from Africa. Two specimens of C. laetoliensis show the largest known traces of kleptoparasites described as Lazaichnus amplus ichnosp. n. The fossil beetles and brood balls of Laetoli weakly indicate a grassland, rather than a dense woodland habitat.

The Laetolil Beds of Pliocene volcanic ashes contain numerous trace fossils. Many of them resemble the hives and foraging passages of extant termites, having been preserved and more or less altered by deposition of calcite and other minerals from ground water. The most abundant nest-like structures at Locs. 9 and 10 resemble hives of the termite Macrotermes herus that still occurs at the site. At Loc. 10W the commonest nest-like structures are flattened ovoids, which seem to have been small discrete hives filled with thin carton shelves. No exact modern counterpart is known. Several rarer types of hives may have been built by other Macrotermes species, and some by Apicotermitinae. At Kakesio a series of small, distinctive fossil hives with thick carton shelves and walls seem to have been built by another termite, of which again no modern counterpart is known. The extant termite fauna at Laetoli is briefly described. The general lack of information about the structures built by modern termites is discussed.

This paper reports on a study of 892 fossil gastropod specimens from the Laetolil and Upper Ndolanya Beds at Laetoli and other sites on the Eyasi Plateau, Tanzania, collected between 1998 and 2005. The material examined represents a sample of a larger collection of over 5,000 specimens, including a large number of urocyclid slug shells. Sixteen gastropod species have been identified in the material and all can be assigned, with varying degrees of confidence, to recent genera of East African Mollusca. Ten species previously described from the site by B. Verdcourt in 1987 have been identified in the material, and a further six species have been newly recognized. The latter includes two new species that are described formally as Euonyma harrisoni sp. nov. and Halolimnohelix rowsoni sp. nov., and a further four species that can be assigned to generic level only – Cerastus sp., Subuliniscus sp., Streptostele sp. and Gulella sp. Overall the material is dominated by medium to very large shelled species, and, unlike many recent East African land-snail faunas, it contains very few micromollusks. It is considered unlikely that this reflects a methodological bias against small specimens during sampling, so it is probable that small species were absent from the Pliocene fauna or that they have not been preserved. The gastropod data and knowledge about the ecological affinities of the taxa concerned have been used to make inferences about the environmental conditions that prevailed during the Pliocene. Within the Upper Laetolil Beds, specimens have been assigned to one of a series of four stratigraphic subunits, thus enabling finer examination of the variation in environmental conditions over this period. The gastropod fauna from the Lower Laetolil Beds is highly distinctive, being dominated by large, robust achatinids and lacking urocyclid slugs. This suggests that conditions were relatively dry at that time, although not to the extent that they could be classified as semi-arid. Savanna habitats may have been prevalent over this period and large trees were probably absent or scarce. Several taxa vary in frequency across the subunits of the Upper Laetolil Beds, thus suggesting changes in ­environmental conditions over this period. The three lower subunits (up to Tuff 7) are characterized by high frequencies of Euonyma and Subulona, which would indicate the presence of woodland. Based on the gastropod data, woodland in the Upper Laetolil may have peaked between Tuffs 3–5. Above Tuff 7 the gastropod fauna changes to one dominated by Edouardia and Trochonanina, suggesting the return of more xeric conditions, perhaps comprising a savanna or bushland ecosystem associated with scattered trees or patches of woodland. The gastropod fauna of the Upper Ndolanya Beds is characterized by Euonyma and Subuliniscus and a high frequency of urocyclid slugs. This suggests the occurrence of woodland or possibly forest conditions, which appear to have been relatively widespread in the area since the same fauna appears to be present at all of the Upper Ndolanya Bed localities sampled. This conclusion differs from other reconstructions of the Upper Ndolanya environment using alternative lines of evidence, which suggests that the area was dominated by open woodland-bushland and grassland.