Gaps In The Fossil Record

1. Introduction

Hylobatid origins are shrouded in mystery. Despite being the most speciose group of living apes with a historically big distribution over Due east and Southeast Asia (figure 1) [i–5], the fossil record of hylobatids (=gibbons and siamangs or 'lesser apes') is woefully incomplete, with just a handful of teeth widely recognized every bit stalk hylobatids before the Middle Pleistocene [6,7]. The paucity of fossil lesser apes is particularly vexing given that molecular data consistently estimate their divergence from other primates by at least xx Ma [eight–10], and their sister group, the great apes, are represented past a large and diverse fossil record in Asia past at to the lowest degree approximately 12.vii Ma [11]. Therefore, fossil hylobatids should be nowadays in the African and/or Asian record well before the get-go widely recognized fossil taxon, Yuanmoupithecus, in the Late Miocene (approx. 7–nine Ma) of Yunnan, China [7,12]. Hither, we report a new small-scale-bodied ape specimen from the late Middle Miocene site of Ramnagar (figure 1), a classic locality in the Indian Lower Siwaliks correlating to the middle or lower half of the Chinji Germination on the Potwar Plateau, Pakistan [13–17]. Specimen VPL/RSP2 is a right lower third molar (Chiliad_iii) with strong morphological affinities to extant hylobatids, even stronger than Yuanmoupithecus, thereby extending the known fourth dimension range of fossil hylobatids past approximately 5 Myr and providing an updated minimum historic period for their development and dispersal into Asia coeval to that of slap-up apes. Equally this specimen is distinct from all other known fossil apes, we draw it as a new genus and species below and discuss other Asian Miocene specimens previously mentioned in the context of catarrhine evolution and hylobatid origins.

Figure 1. — Figure one. Elevation: map illustrating the location of *Kapi* (black star) relative to modern (dark dark-green) and historical (lite greenish) populations of hylobatids and the approximate distribution of stem hominoid sites in East Africa (blue triangles). Green triangles mark the location of the hylobatid fossil taxa *Bunopithecus* and *Yuanmoupithecus*; xanthous rectangles marker the location of the fossil catarrhine taxon *Dionysopithecus* sp. from Centre Miocene sites in Pakistan (see text). Bottom left: full general geological map of the Siwalik Group surrounding Ramnagar with satellite imagery (GeoEye-1) of the Ramnagar region respective to the dashed insert of the geological map; bottom right: simplified stratigraphic section and photos of sequence at Sunetar 2 highlighting the *ex situ* discovery levels of primate specimens VPL/RSP1 (*Ramadapis*) and VPL/RSP2 (*Kapi*). Map by Free Vector Maps: http://freevectormaps.com.

Note that this published work and the nomenclatural acts it contains have been registered in ZooBank. The Life Scientific discipline Identifiers (LSIDs) for this publication are: urn:lsid:zoobank.org:human activity:2D7C942A-EC42-46FF-AF8F-A2341B5C87E5 and urn:lsid:zoobank.org:act:436CC8FF-118D-4F39-92D8-5C92B18005C4.

2. Systematic palaeontology

Order Primates Linnaeus, 1758

Suborder Anthropoidea Mivart, 1864

Infraorder Catarrhini Geoffroy St. Hilaire, 1812

Superfamily Hominoidea Gray, 1825

Family Hylobatidae Gray, 1870

Kapi ramnagarensis gen. et sp. november.

(a) Etymology

Genus proper name from the Hindi word for a mutual anthropoid ape or monkey (kapi). Species name in reference to Ramnagar (Jammu and Kashmir), India, where the type specimen was constitute.

(b) Generic diagnosis

Kapi differs from Oligocene and Miocene catarrhine taxa such as propliopithecids, pliopithecids, and dendropithecids in the combination of the following lower molar features: transverse orientation of the mesial cusps with the metaconid even with or slightly mesial to the protoconid, reduced buccal cingulum, peripheral placement of the cusps creating reduced basal crown flare, small entoconid–hypoconulid pair, and a broad, open occlusal basin. Further differs from most pliopithecids in the mesiodistal orientation of the cristid obliqua (also constitute in some pliopithecines), the transverse orientation of the hypoconid and entoconid, the more central placement of the hypoconulid on M_iii, and the lack of any crests between the protoconid and hypoconid associated with the pliopithecine triangle. Differs from Oligocene–Miocene proconsulids in the combination of the reduced entoconid–hypoconulid pair, transversely aligned mesial and distal cusps, the more than peripheral placement of the cusps on the tooth crown (leading to reduced crown flare), and the reduction of the buccal cingulum (although a reduced cingulum is also present in some proconsulids). Differs from most hominids and hylobatids in the retention of a reduced merely moderately adult buccal cingulum and a relatively long, broad mesial fovea. Further differs from hominids in its relatively modest size. Further differs from known hylobatid genera in its overall more ovoid and relatively narrower shape (except Symphalangus), distal tapering, and less inflated cusps (a more than detailed diagnosis of Kapi can be found in the electronic supplementary material).

(c) Specific diagnosis

Every bit for genus.

(d) Holotype

VPL/RSP2 (Vertebrate Paleontology Laboratory, Panjab University Department of Geology/Ramnagar Sunetar Primate 2); a consummate and slightly worn right Yard_iii crown (figure 2).

Figure 2. VPL/RSP2 in various views. Clockwise from top left: Oc, Occlusal; Li, Lingual; Di, Distal; Me, Mesial; Bu, Buccal; Ob, Oblique. A 3-dimensional surface rendering derived from µCT scans of the specimen is available at MorphoSource.org (media # M53248-96377; www.morphosource.org/Detail/MediaDetail/Prove/media_id/53248).

(e) Hypodigm

The holotype is the only known specimen.

(f) Horizon

Lower Siwalik deposits; approximately 12.v–13.8 Ma (see electronic supplementary material, Geological groundwork section).

(g) Localities/sites

Sunetar two; approximately 4.v km S/SE of Ramnagar, Jammu and Kashmir, India (effigy one).

(h) Clarification

VPL/RSP2 corresponds to a low-crowned, bunodont M_three from a catarrhine slightly smaller than Hoolock in molar size (figure two; mesiodistal (MD) = 7.8 mm; buccolingual (BL) = 6.3 mm). It is mesiodistally longer than wide (breadth–length alphabetize of 0.79 calculated from photos; see electronic supplementary fabric for extended description), indicating proportions about similar to those of typical proconsulids, but considerably broader, on boilerplate, than those of pliopithecids, and slightly broader than those of mod Symphalangus, propliopithecids, as well every bit dendropithecids, although much overlap exists between individual specimens. It is relatively narrow compared to many mod gibbons, and slightly narrower than Yuanmoupithecus (0.81) and Bunopithecus (0.82).

The crown of VPL/RSP2 is ovoid in occlusal outline, tapering distally such that the distal moiety is narrower than the mesial moiety. There are v well-developed cusps, low and conical in shape, arranged around the periphery of the crown. The buccal wall of the crown displays a reduced, semi-continuous cingulum. The metaconid is the well-nigh voluminous and highest cusp, followed past the hypoconid and protoconid, which are subequal in elevation. The entoconid is similar in elevation to the hypoconid and protoconid, but relatively smaller in basal area. Every bit is typical for apes, the hypoconulid is the smallest of the five cusps and located slightly towards the buccal side of the crown (figure 2).

The protoconid has a short but well-developed preprotocristid and postprotocristid. The metaconid is slightly mesial to the protoconid and has a brusk and rounded premetacristid. The metaconid and entoconid are widely spaced by a long postmetacristid. The hypoconid has a brusk prehypocristid (cristid obliqua) that is parallel to the long axis of the crown. Both the postentocristid and the posthypoconulid cristid are low and ill-divers. The mesial fovea is broad and rectangular, delimited distally past a well-differentiated mesial transverse crest (hypometacristid and hypoprotocristid). The mesial marginal ridge is relatively abrupt and well developed. The distal fovea is intermediate in size, but poorly divers.

The talonid basin is expansive and has a simple Y-shaped groove blueprint with no secondary wrinkling. A well-adult postcristid and hypoentocristid link the hypoconulid and entoconid, forming the mesial-most boundary of the distal fovea, separating it from the talonid basin. The metaconid is damaged, but there may be traces of a small mesostylid or tubercle on the postmetacristid. A minor tubercle is besides present on the preprotocristid. There is no evidence of a pliopithecine triangle and no retentiveness of the paraconid.

3. Morphometric and phylogenetic analyses

Two-dimensional morphometric analyses of M₃ shape equally well as a cladistic analysis of 272 craniodental and postcranial features in extant and fossil catarrhine taxa back up Kapi as a stalk hylobatid. We quantified M_three crown shape and cusp position as characterized by fourteen homologous landmarks (post-obit [18]; see electronic supplementary material, figure S1 and table S1) and conducted a phylogenetic analysis using parsimony inference on a modified version of a recent matrix (electronic supplementary textile, datasets S1–S2) [19]. Our comparative morphometric sample includes 166 M₃ specimens: five crown hylobatid genera (due north = 79), three crown hominid genera (due north = 56), 2 propliopithecid genera (north = 6), vi pliopithecid genera (n = 9), 4 dendropithecid genera (n = 7), 5 proconsulid genera (due north = 7), the stem hylobatid Yuanmoupithecus (northward = i), and Kapi (n = i) (electronic supplementary material, table S2, effigy S2, dataset S3). Landmark information were imported into MorphoJ [20] and Morphologika2 [21] and and so subjected to a generalized least-square Procrustes superimposition to focus on size-adapted shape variables. A Chief Components Assay (PCA) was performed using Procrustes coordinates and wireframe models were created to visualize the extreme landmark configurations. Using Discriminant Function Analysis pairwise tests implemented in MorphoJ [twenty], we too created wireframes and deformation grids to notice shape deformations from the mean shape configuration (reference configuration) of each of our major taxonomic groups to the shape of VPL/RSP2 (M₃) (target configuration; electronic supplementary material, figure S3). Post-obit [18], hierarchical phenetic trees were obtained from Procrustes distances using a neighbour-joining (NJ) cluster analysis with propliopithecids assigned equally the outgroup.

Although catarrhine Thou₃s are variable in morphology and have been previously discounted in taxonomic identification [22], our multivariate results demonstrate that extant hylobatid Thou₃s are singled-out from stem catarrhines and stem hominoids, at least at the broad taxonomic levels analysed hither (effigy 3; electronic supplementary material, tables S3–S4; see as well [3]; and [23–26] for other hominoids). These results are in line with recent research suggesting that, while morphologically variable within a taxon, anthropoid M₃s evolve more quickly and are more distinctive between taxa, thereby making them more taxonomically informative than Chiliad₁south and Chiliad₂s in many cases [27]. VPL/RSP2 falls exclusively within crown hominoid infinite in the PCA plot, well within the crown hylobatid minimum convex polygon and closest to a number of crown hylobatid specimens on PC1 and PC2. While crown hylobatids practise overlap in PC space with crown hominids (great apes), all crown hylobatids are easily differentiated from hominids on the basis of size, which is excluded from the shape assay presented hither (figure 3; see as well electronic supplementary material, figure S2). Yuanmoupithecus plots within the small surface area of overlap between crown hylobatid, crown hominid, and stem catarrhine/hominoid taxa, merely closest to crown hominoid specimens on PC1 and PC2. The Pleistocene gibbon Bunopithecus falls exclusively within crown hylobatid morphospace.

Figure 3. PCA resulting from two-dimensional morphometric analysis of overall M₃ crown shape characterized by 14 homologous landmarks (see wireframes; cusps = black circles). *Kapi* plots comfortably within hylobatid space (=greenish polygon), and completely exterior the sampled distribution of stem catarrhine and stem hominoid taxa. By contrast, *Yuanmoupithecus* plots within the small area of overlap between stem and crown catarrhine/hominoid taxa, including crown hylobatids. (Online version in colour.)

PC1 is most clearly driven past differences in the position of the hypoconulid relative to the protoconid and hypoconid (in a straight line buccally in pliopithecids, more than central/slightly buccal in hylobatids), the position of the cusps/width of the occlusal basin relative to the outline of the crown (pliopithecids = internally placed cusps, narrow occlusal basins, increased flare, big cingulum; hylobatids = peripherally placed cusps, wide occlusal basins, reduced flare, reduced cingulum), and the alignment of buccal and lingual cusps (pliopithecids = buccal cusps more mesial than lingual cusps, hylobatids = buccal and lingual cusps aligned transversely). VPL/RSP2 exhibits a negative value on PC1 due to its transversely aligned and peripherally placed mesial and distal cusps, broad occlusal basin, reduced cingulum, low degree of flare and more centrally positioned hypoconulid. PC2 does not split up well-nigh taxa (except propliopithecids on the positive end), just appears related to crown elongation and distal tapering (negative values = more elongated and tapered, positive values = less elongated and tapered), along with like features as seen on PC1 including the position of the cusps relative to the crown outline, the position of the hypoconulid, and the alignment of buccal and lingual cusps. VPL/RSP2 exhibits slightly negative values, consistent with its slight distal tapering. The NJ cluster analysis based on the morphometric data places Yuanmoupithecus and Kapi in a cluster with crown hominoids, with Yuanmoupithecus at the base of operations of the cluster and Kapi as the sister to hylobatids. Dendropithecids, proconsulids, and pliopithecids are placed in a split cluster as the sister grouping to Yuanmoupithecus + crown hominoids (figure four).

Figure 4. Top: neighbour-joining cluster assay derived from Procrustes distances of the same 14 landmark morphometric dataset as in figure 3. Numbers higher up branches indicate bootstrap values based on x 000 replicates. Bottom: strict consensus of 18 about parsimonious trees (MPTs) resulting from phylogenetic assay of stem catarrhine taxa including 272 characters (craniodental + postcranial), with outgroups constrained successively. Numbers beneath branches indicate whatever bootstrap values over l%. *Kapi* is reconstructed as a stem hylobatid in both the cluster analysis and all MPTs. Tree length = 1455 steps. (Online version in color.)

The resulting trees from our cladistic analysis are consequent with the morphometric analyses and recover both Kapi and Yuanmoupithecus every bit crown hominoids, and both fossil taxa are most parsimoniously reconstructed every bit stem hylobatids (figure 4; electronic supplementary textile, datasets S1–S2 for graphic symbol list and matrix). In all 18 almost parsimonious trees (MPTs), Yuanmoupithecus is the sis taxon to a crown hylobatids + Kapi clade. Aside from the inclusion of Yuanmoupithecus and Kapi, the relationships among other catarrhines are broadly the aforementioned as those previously presented [19].

4. Discussion

Based on the available bear witness of lower molar anatomy, Kapi ramnagarensis represents the commencement new hominoid species discovered at Ramnagar in nearly 100 years. While caution is necessary given that only a single tooth is documented, the analyses presented here demonstrate that Kapi is more similar to extant hylobatids in its known morphology than the widely accepted stalk hylobatid Yuanmoupithecus. Thus, if one considers Yuanmoupithecus a stem hylobatid, Kapi is every bit if non more likely to exist one also, making information technology the earliest known hylobatid in the fossil tape (figures 3 and 4). The phylogenetic placement of these two taxa within hominoids, however, is admittedly hard to appraise in the absenteeism of additional cloth. Based on shared similarities with extant hylobatids in the premolars and anterior dentition, Yuanmoupithecus peradventure represents a slightly unlike combination of dental morphologies in early hylobatid development compared to Kapi. Farther specimens of both taxa are necessary to confidently resolve the polarity of stem hylobatid dental features.

The discovery of Kapi in approximately 12.5–13.8 Ma Lower Siwalik deposits helps to fill temporal, morphological, and biogeographic gaps in hominoid development. While much of hylobatid evolution remains unknown, information technology is now probable that they dispersed to Asia from Africa by the end of the Heart Miocene, possibly at the same time as great apes such as Sivapithecus just after the Centre Miocene Climatic Optimum [28,29]. Judging by the affinities of both Kapi and Yuanmoupithecus in our analyses, it seems virtually likely that hylobatids evolved from an African taxon dentally similar to dendropithecids or proconsulids, the ii advanced catarrhine groups outside of crown hominoids with specimens closely approaching hylobatids in the multivariate and phylogenetic analyses. Therefore, it is entirely possible that early stem hylobatids are currently represented by some of the fossil material in the extensive Eastward African Early Miocene record, but cannot all the same exist distinguished based on the lack of clear hylobatid dental synapomorphies among these fragmentary taxa.

In many ways, Kapi represents a logical intermediate or mosaic dental morphology between Early on Miocene dendropithecids/proconsulids and extant hylobatids; information technology clearly displays the bunodonty, peripherally placed cusps, expanded basin and transversely aligned cusps as seen in living hylobatids, only also retains primitive features such every bit a (reduced) buccal cingulum, a relatively long mesial fovea, and perchance a vestigial mesostylid non typically observed in living gibbons and siamangs. It likewise does non display the expanded cusp areas typical of living hylobatids. Features of extant hylobatid molars, particularly the bunodonty and expansion of the occlusal basin relative to stem catarrhine taxa (effigy 3; electronic supplementary cloth, figures S2–S3), indicate an adaptive shift to a dedicated diet of frugivory. The presence of these characters in Kapi suggest that this shift had begun by the end of the Eye Miocene, consequent with the silencing of the uricase gene (another proposed adaptation to frugivory) in hylobatids during this fourth dimension period as well [thirty].

While other Eurasian fossils accept been advanced every bit possible hylobatids in the past, none have held up to closer scrutiny and an improved understanding of the catarrhine fossil record. Pliopithecids (i.due east. pliopithecoids), a well-represented group of catarrhines establish in the Early to Tardily Miocene of Asia (east.g. Dionysopithecus, Platodontopithecus, Pliopithecus, and Laccopithecus), resemble hylobatids in sure cranial features, including a relatively brusque face, projecting inferior orbital rims, and a broad interorbital altitude. However, they also lack a key synapomorphy institute in all crown catarrhines, namely a completely ossified tubular ectotympanic (ear tube). In addition, they generally possess a unique combination of archaic features (east.m. very broad upper molars and an entepicondylar foramen in the distal humerus) along with autapomorphic lower molar anatomy (including the pliopithecine triangle), leading most experts to conclude that they are, in fact, late-occurring stem catarrhine taxa (figure 4) [6,7,19,31–33].

A worn M³ from the Middle Siwalik locality of Haritalyangar, Republic of india, was initially referred to as a possible hylobatid ancestor and ultimately placed in its own genus, Krishnapithecus [34,35]. However, Krishnapithecus has recently been demonstrated to be a tardily-occurring pliopithecid, with lower molars displaying a distinctive pliopithecine triangle among other pliopithecid features [36]. Notably, our cladistic analysis reconstructs the recently described and debated Pliobates from the Middle/Late Miocene boundary of Spain every bit a pliopithecid taxon besides (see also [19]).

One other taxon from South asia, Dionysopithecus sp., represented by a handful of isolated teeth from the Lower Siwalik Kamlial Formation and Manchar Germination in Pakistan (approx. 16–17 Ma), has been discussed every bit a possible dendropithecid, proconsulid, pliopithecid, and fifty-fifty stalk hylobatid [7,37–39]. Thus, the affinities of H-GSP 8114/609, the sole lower molar (Thousand₁) assigned to Dionysopithecus sp., were re-examined given its proximity in time and space to the G₃ from Ramnagar as well as its possible status every bit a stem hylobatid or advanced stem catarrhine/hominoid (dendropithecid or proconsulid) in Asia. Although roughly like in size, GSP 8114/609 is morphologically distinct from Kapi in its much college crown, ameliorate developed cingulum, stronger occlusal crests, more restricted occlusal basin, and more than centrally located cusps, clearly representing a dissimilar taxon. We conducted a split morphometric analysis on GSP 8114/609 and a large sample of catarrhine M₁southward (electronic supplementary textile, figures S4-S5, tables S4–S6, dataset S4). There is more overlap among all groups on the first two components of the M₁ PCA, and GSP 8114/609 falls completely outside hylobatid multivariate space. Instead, it falls within an area of overlap betwixt dendropithecids, pliopithecids, and crown hominids. Thus, while the taxonomic placement of the Manchar/Kamlial specimens is still unclear, they seem unlikely to vest to a fossil hylobatid.

Finally, a proximal humerus (GSP 28062) from site Y499 in the Chinji Formation (approx. 12.05 Ma) [40–42] is the only other minor-bodied catarrhine specimen currently known from the Siwaliks close to the likely fourth dimension range represented at Ramnagar. Interestingly, this specimen displays none of the specializations for extreme mobility present in living hylobatids and bully apes, and instead retains a primitive catarrhine morphotype similar to pliopithecids and dendropithecids [42]. While an association with an every bit yet undiscovered Chinji-level pliopithecid or dendropithecid is perhaps about probable, if this proximal humerus is attributable to a stem hylobatid (e.g. Kapi), information technology would advise that the suspensory features exhibited by extant hylobatids evolved independently from groovy apes and inside the last around 12.five–13.8 Myr from a more primitive catarrhine morphotype. Such an association would too advise that the common ancestor of crown apes was not a highly suspensory fauna, a hypothesis that nosotros consider probable (encounter also [43–45]), but that stands in dissimilarity to the consensus view of ape development for much of the past century [46–49]. Additional early catarrhine and stem hylobatid fossils such every bit Kapi, especially with associated postcrania, are necessary to resolve these competing views and gain a clearer insight into the first approximately 6–8 Myr of hylobatid evolution.

Data accessibility

Original raw µCT image browse data and derived three-dimensional surface rendering of VPL/RSP2 are available on MorphoSource.org (world wide web.morphosource.org/Detail/MediaDetail/Bear witness/media_id/53248; https://doi.org/x.17602/M2/M96377). All data used in the morphometric analyses are provided equally MorphoJ input files and the matrix used in the cladistic analysis is provided every bit a text file in the electronic supplementary fabric associated with this commodity.

Authors' contributions

C.C.G., A.O., and K.D.P. designed the study. C.C.M., C.J.C., B.A.P., North.P.S., and R.P. did field research. C.C.G., A.O., K.D.P., B.A.P., J.G.F., N.P.S., and R.P. collected and analysed comparative data on primate dental morphology. B.A.P. performed µCT imaging analyses. C.J.C. and R.P. studied the geological context and provided the geological background. C.C.G., K.D.P., B.A.P., N.P.South., and R.P. identified fauna at Ramnagar. A.O., C.C.G., and K.D.P. performed morphometric analyses. Yard.D.P. and C.C.G. performed the phylogenetic analyses. All authors wrote the paper.

Competing interests

We declare nosotros have no competing interests.

Funding

This piece of work was supported past the Leakey Foundation, the PSC-CUNY kinesthesia honor plan, Hunter College, the AAPA professional evolution programme, the University of Southern California, the Establish of Man Origins (ASU), and the National Science Foundation (BCS Award nos. 1945736, 1945618). In addition, R.P. and Northward.P.S. were supported by MoES/P.O. (Geosci)/46/2015 and SERB-HRR/2018/000063.

Acknowledgements

Terry Harrison provided access to Yuanmoupithecus and comments that greatly improved the manuscript. Kai He, Hira Naseer, and Nastassia Chittumuri assisted with photography and wireframe representation of the tooth specimens. Two anonymous reviewers and an acquaintance editor provided constructive comments that improved the manuscript. We thank Judy Galkin (AMNH), Eric Delson (AMNH), Terry Harrison (NYU), Hannah Taboada (NYU), Eileen Westwig (AMNH), Eleanor Hoeger (AMNH), Neb Kimbel (ASU), Julie Lawrence (ASU), Frieder Mayer (ZMB), Darrin Lunde (NMNH), Mark Omura (Harvard), Jessica Cundiff (Harvard), Larry Flynn (Harvard), and David Pilbeam (Harvard) for access to original specimens and casts in their intendance. We also give thanks Morgan Hill and the Microscopy and Imaging Facility at the AMNH for admission and assistance with µCT scanning VPL/RSP2. Luci Betti-Nash prepared the map in effigy 1.

Footnotes

Electronic supplementary material is bachelor online at https://doi.org/10.6084/m9.figshare.c.5098945.

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