Anthrasimias gujaratensis Sp. Nov.
Etymology.
After Gujarat state of western India, the provenance of this species.
Holotype.
IITR/ SB/VLM 1137, a left M1 (Fig. 2 A and C).
Fig. 2.
The dentition of Anthrasimias gujaratensis sp. nov. (A) Occlusal stereopair of IITR/SB/VLM 1137, a left upper first molar. (B) Occlusal stereopair of IITR/SB/VLM 1100, a left upper second molar. (C) Lingual view of IITR/SB/VLM 1137. (D) Lingual view of IITR/SB/VLM 1100. (E) Occlusal stereopair of IITR/SB/VLM 1017, a right lower third molar. (F) occlusal and occluso-lingual view of IITR/SB/VLM 1201, a right dP4. (Scale bars, 1 mm.)
Hypodigm.
IITR/SB/VLM 1100, a left M2 (Fig. 2 B–D), IITR/ SB/VLM 1017, a right M3 (Fig. 2E), IITR/SB/VLM 1201, a dP4 (Fig. 2E).
Horizon and locality.
Early Eocene Cambay Shale, Vastan Lignite Mine, Surat District, Gujarat, western India (2). The Anthrasimias stratigraphic level contains a diverse early Eocene terrestrial mammalian fauna (1⇓–3, 17, 18). Age-diagnostic dinoflagellate cysts indicate a basal Eocene (Sparnacian, ca. 54-55 Ma) age for the mammal horizon (19). This estimate is a revision of the earlier age assessment of basal Cuisian, ca. 53 Ma, from shallow benthic foraminifera (4, 6, 20).
Diagnosis.
Equivalent in size to Altiatlasius (of Africa) and
the smallest Asian eosimiids with described dental remains. Differs
from eosimiids in having a more triangular occlusal outline (i.e., less
transverse buccolingually) and in having a cuspate hypocone (vs. absent
to cristiform). Differs from other eosimiid primates (except Phileosimias kamali) and from Altiatlasius in having less well developed buccal and lingual cingulae. Conules slightly larger than in Eosimias, Phenacopithecus, and Bahinia, but smaller than in Phileosimias.
Comparisons.
Anthrasimias shares with other eosimiids a suite of dental features noted by Beard and Wang (3)
to be diagnostic of eosimiids but not found together in omomyoids,
including strong development of pre- and postprotocristae, absence of a Nannopithex fold, and reduced conules. The distolingual expansion of the talon, present in Anthrasimias, is common among eosimiids (21). The lingual cingulum of Anthrasimias is incomplete, unlike that of Eosimias, Phenacopithecus, Bahinia, and Phileosimias brahuiorum, but is similar to P. kamali.
The steep incline of the buccal wall of the paracone and metacone in Anthrasimias is common in eosimiids, particularly Eosimias and Phenacopithecus. Like other eosimiids, and especially like Eosimias
(and unlike most omomyoids), the parastyle is a large distinct cusp,
and the metastyle is present as a swelling along the postmetacrista. The
protocone is canted mesially, such that it is closer to the mesial edge
of the tooth, as in Eosimias, Bahinia, and Phenacopithecus but not Phileosimias. There is a distinct molar waisting, especially in the area of the metaconule, as in Eosimias, Phenacopithecus, and Bahinia, but less markedly in Phileosimias.
Eosimiids generally lack metaconule cristae and a
postparaconule crista. Instead, the postprotocrista leads to the base of
the metacone or to a small metaconule that connects in turn with a
hypometacrista. Anthrasimias has an intermediate morphology:
the postprotocrista is straight, not distally bowed, and connects with
the metaconule, which sends a strong but buccally directed premetaconule
crista up the lingual aspect of the metacone. We interpret this
arrangement of the premetaconule crista as a precursor to the
hypometacrista.
Several notable features of the M3 are
eosimiid-like: the trigonid is open lingually and supports a small
centrally placed paraconid, the protocristid is transverse, and the
hypoconulid is small and does not project posteriorly as a distinct
distal lobe.
Altiatlasius (late Paleocene, Africa) exhibits some but not all of the above-mentioned symplesiomorphies with Anthrasimias
and other eosimiids. Like eosimiids, the postprotocristae leads to the
base of the metacone and the preprotocrista to the paracone. In both Altiatlasius and eosimiids, a Nannopithex fold is absent, and like most eosimiids (but not Anthrasimias), there is a complete lingual cingulum. The steep incline of the buccal wall of the paracone and metacone in Altiatlasius also is common in eosimiids. Furthermore, like Anthrasimias and other eosimiids, especially Eosimias
(and unlike most omomyoids), the preparacrista and postmetacrista are
angled buccally and supported by a large parastyle and somewhat smaller
metastyle, respectively. However, unlike Anthrasimias and other eosimiids, the talon of Altiatlasius
is not noticeably expanded distolingually, the protocone is not canted
mesially, and the molar waisting is indistinct. Further, Altiatlasius lacks a hypometacrista.
Phylogenetic analysis.
A parsimony analysis was undertaken by using PAUP parsimony software (22) to determine the phylogenetic position of Anthrasimias and other Indian early Eocene primates (Fig. 3).
A notable feature of all maximum parsimony trees is that crown
Haplorhini is sister to all Omomyoidea, not nested within it as often
argued (8, 11, 22).
Fig. 3.
The 50% majority consensus of 11
equally parsimonious trees. Tree length, 148,887; consistency index
(CI), 0.230; retention index (RI), 0.554; rescaled CI (RCI), 0.127. Red,
Adapoidea; green; Omomyoidea; blue, crown Haplorhini. Branching
sequences are supported in 100% of the trees unless indicated by a
percentage. Suratius and Asiadapis are not included on the tree. Circled letter A indicates branch placement of Asiadapis when it is run without Suratius included. Circled letter B indicates branch placement of Suratius when Asiadapis is not included. When Suratius and Asiadapis
are run together, they are placed together at branch B. The list of
characters and their states and character-taxon matrix is provided in supporting information (SI) Text, Figs. S1–S3, and SI Appendices 1 and 2.
All trees place Marcgodinotius near the base of Adapoidea. The latter is a sister taxon to crown Strepsirrhini. Marcgodinotius is similar to the European early Eocene adapoid Donrussellia in many primitive features (5). Vastanomys is placed near the base of the Omomyoidea. Vastanomys is primitive for omomyoids in retaining a large canine and a large, although single-rooted, P2 (5). It appears to be more primitive than North American Steinius, argued by some to be the most primitive omomyoid (24).
The 50% majority consensus tree places Anthrasimias
at the base of the eosimiids. A plausible alternative places this taxon
at the base of the tarsiid clade or in an unresolved trichotomy with
tarsiids and eosimiids. The eosimiid placement is consistent with
morphological characters, mentioned in the diagnosis above, considered
most critical to reconstructing eosimiid evolution (3, 25). All trees also support placement of Altiatlasius with the Eosimiidae (10, 26, 27).
The phylogenetic position of late Eocene amphipithecids of
Asia is a subject of considerable debate. Mandibular and dental
similarities and the structure of an isolated talus suggest an
anthropoid association (28, 29).
In this analysis, we accept the view that some other isolated bones
allocated to this taxon are not primate or belong to a large
strepsirrhine (30, 31). Our analysis using dental, gnathic, and talar characters supports placement of amphipithecids within Anthropoidea.
The analysis is equivocal concerning placement of Asiadapis and Suratius.
Phylogenetic analysis of all taxa in our dataset links the two and
places them at the base of the noneosimiid Anthropoidea. In separate
analyses, however, Asiadapis, considered alone without Suratius, falls with adapoids Aframonius and Mahgarita, whereas Suratius, run alone without Asiadapis, is linked with eosimiids.
Adaptations.
Our findings of very small body size in basal members of
all radiations indicate that insects rather than plants were the primary
source of protein for early primates (Table 1) (32).
There is no support for the hypothesis that basal Anthropoidea were
large, despite the relatively large size of most Oligocene-Recent
species (15). Anthrasimias, at 75 g, was smaller than all living primates with the exception of some species of Galagoides (the dwarf galago) and Microcebus (the mouse lemur) (Table 1). No size trends are evident among eosimiids. Anthrasimias (and African Altiatlasius)
were slightly smaller than middle to late Eocene Asian eosimiids known
from dental material (85–150 g), but some tarsal bones suggest that some
middle Eocene eosimiids may have been shrew-sized (33). Likewise, Vastan omomyoids and adapoids were very small animals: Vastanomys and Marcgodinotius ranged up to 130 g. Suratius and Asiadapis
were slightly larger, up to 270 g. Thus, early Eocene members of the
three radiations of crown primates, omomyoids, stem strepsirrhines, and
crown haplorhines also weighed <300 g.
Table 1.
Body mass estimates for Eocene and
early Oligocene South Asian primates (Thailand, Myanmar, Pakistan,
India) and representative early taxa of early Eocene Omomyoidea and
Adapoidea
Previous studies on the diet of fossil anthropoids have
relied on comparative evidence from living taxa and the morphology of
the lower teeth of anthropoid taxa from the Fayum of Egypt dating back
to the late Eocene (34).
Those late Eocene anthropoids show a diet that was predominantly
frugivorous, but their >750 g body size suggests leaves, not insects,
as an important source of dietary protein. However, the 20-million-year
separation of Fayum anthropoids from basal members of the anthropoid
clade makes them poor candidates from which to infer possible adaptive
shifts at the base of the group.
Body mass alone may tell us something about the likely
source of dietary protein, but tooth structure gives further details
about the relative importance of fruit vs. animal prey. Among
small-bodied extant prosimians, a strong relationship exists between the
summed lengths of shearing crests of the lower molar teeth and the
amount of animal prey in the diet (32). A similar phenomenon occurs with the upper molars (Fig. 4, Table 2). From dental anatomy (combined with small size), we infer that Anthrasimias had a mixed diet of fruit and some insects similar to that of the mouse lemur Microcebus. The development of shearing crests on the upper and/or lower teeth of Asiadapis, Vastanomys, and Marcgodinotius
likewise suggests a mixed frugivorous/insectivorous diet. Unlike
proposed reconstructions of body mass >1 kg, our body mass and
dietary reconstructions of taxa basal to Eocene primate clades provide
broadly based evidence that the earliest primates relied, at least in
part, on insects or other animal prey. Our conclusion is consistent with
the visual predation hypothesis but does not rule out coevolution with
angiosperms. There is no evidence to indicate that changes in body mass
or diet accompanied the cladogenic splitting of haplorhines from
strepsirrhines or anthropoids from omomyoids.
Fig. 4.
Measurements of shearing crest development on the molar teeth of Vastan primates. (A) Ratio of second lower molar length to summed lengths of six principal M2 shearing crests. (B)
Ratio of first upper molar length to summed lengths of four principal
buccal shearing crests. Color-coded bars (blue, insects; red, fruit;
yellow, gums) indicate principal dietary item (34). Asiadapis, Vastanomys, and Marcgodinotius fall within the range of extant prosimian fruit and gum eaters such as the extant mouse lemur Microcebus, which also eats a substantial amount of insects (Table 2).
Table 2.
Diet and shearing crest lengths of extant and extinct primates used in the text
Temporal and biogeographic implications.
Hitherto, the oldest undisputed eosimiids were recovered from the Chinese middle Eocene (≈45 Ma) (3, 35). Anthrasimias is the first eosimiid from the Indian subcontinent and extends the Asian fossil record of anthropoids by 9–10 million years. Anthrasimias may also be the oldest anthropoid in the world. However, our analysis supports the hypothesis that Altiatlasius from the late Paleocene of Africa is possibly an eosimiid anthropoid (10, 26, 27). Nevertheless, others consider it to be an omomyoid (36), a plesiadapoid (37), or of indeterminate subordinal affinities (1).
In any event, the cooccurrence of an anthropoid taxon alongside adapoid
and omomyoid primates in the early Eocene of Asia gives further
evidence that the cladogenesis of crown haplorhines and strepsirrhines
was ancient, in the Paleocene or even Cretaceous, as molecular evidence
suggests (14, 38).
The Vastan Indian fauna shows strong links with Laurasian early Eocene faunas (5, 6, 17, 18). The presence of an anthropoid in India before 54 million years ago and possibly even earlier in Africa (if Altiatlasius is an anthropoid) fleshes out the picture of early Cenozoic interchange between Laurasia and Africa (26, 39) and between the Indian and Asian plates, the latter in the context of their tectonic collision (17).
The Vastan anthropoid testifies to the early importance of India as an
important center for the differentiation of all of the major groups of
primates.
