亞洲首個(gè)全棱獸類(哺乳綱：全齒目)化石及其動(dòng)物地理學(xué)意義

First Asian Paleocene pantolambdid pantodont（Mammalia） and its implications to intercontinental faunal exchange

Abstract Pantodonta was one of the first groups of eutherians to evolve at the beginning ofCenozoic era， including the largest herbivores at that time. Pantodonta shows considerablediversity during the Paleocene and Eocene， with most of the species having been discoveredin Asia and North America. Here， we report on a new pantodont， represented by lower jawswith well-preserved dentition， discovered from the Middle Paleocene Nongshan Formationof the Nanxiong Basin， Guangdong Province， China. Its unique dental and mandibularcharacteristics distinguish it well from any known Asian pantodont， but are quite consistentwith North American taxa， especially Pantolambda and Titanoides. The new specimen isidentified as Nanxiongilambda yei gen. et sp. nov.， characterized by the combination of thefollowing morphological features： thick and robust mandible with a conspicuous anteroexternalflange， high positioned condyloid process， posteriorly protruding angular process， robustbut not elongated lower canines， double-rooted p1， small but distinct talonids on posteriorlower premolars， talonids nearly as wide as trigonids on lower molars， and m3 with a welldevelopedhypoconulid and a distinct entoconid. The new discovery marks the first recordof a pantolambdid pantodont outside of North America， suggesting a broader geographicaldistribution and intercontinental dispersal of this clade during the Paleocene. Consideringthe more primitive Pantolambda only found from Torrejonian to early Tiffanian NALMA（equivalent to middle-late Shanghuan to early Nongshanian ALMA）， which is slightly earlierthan Nanxiongilambda （early Nongshanian ALMA）， pantolambdids have likely undergone aparticular wave of migration from North America to Asia during the Early-Middle Paleocene.Previous researches have suggested that pantodonts had dispersed only from Asia to NorthAmerica during the Early Paleocene， but our new specimen indicates the biotic dispersal mayhave occurred in the opposite direction. The new specimen also contributes to the renewal ofthe evolutionary history of pantodonts and provides further insights into the migration anddispersal of Paleocene mammals.

Key words Nanxiong Basin， Paleocene， Nongshan Formation， Pantodonta， Pantolambdidae，Nanxiongilambda

Citation Quan S S， Wang Y Q， 2024. First Asian Paleocene pantolambdid pantodont （Mammalia）and its implications to intercontinental faunal exchange. Vertebrata PalAsiatica， 62（4）：291–312

1 Introduction

Mammals in Asia tended to be highly endemic during the Early Paleocene， with onlya few taxa closely related to the North American forms （Wang et al.， 1998）. Among the taxashared by the two continents during that time， most were above the family level; almostnone of them had clearly close phylogenetic relationships at or below the family level.Moreover， a number of mammal taxa appeared suddenly in North America without ancestralforms known from earlier horizons on the same continent， and more primitive forms of earlyPaleocene mammals were often found in Asia. This indicates a primary direction of dispersalfrom Asia to North America， namely the “East of Eden” model （Beard， 1998）. NorthAmerican mammals migrating back to “East of Eden” during the Early Paleocene， however，are rarely documented.

Pantodonta is one of the first groups of large eutherian mammals to evolve after the endof the Cretaceous （Kemp， 2005）. They thrived during the Paleocene and Eocene in East Asiaand western North America. Only a few species were found in Europe and South America.During the Paleocene， Pantodonta exhibited considerable diversity， comprising ten families（McKenna and Bell， 1997） and more than twenty genera， including some of the largest planteatingmammals of the Paleocene （Rose， 2006）.

Except for the only Holarctic distributed Coryphodontidae， no other pantodont familieshave been reported from both Asia and North America. Asian non-coryphodontid pantodontsinclude Harpyodidae， Bemalambdidae， Pantolambdodontidae， and Pastoralodontidae，while North American types include Pantolambdidae， Titanoideidae， Barylambdidae， andCyriacotheriidae. Non-coryphodontid pantodonts from North America （except for thepeculiar Cyriacotheriidae） usually have a well-separated paracone and metacone on the uppermolars， relatively wide talonids on the lower molars， moderately enlarged lower canines， andunelongated lower premolars. These characteristics， especially as a combination， can hardlyever be found in Asian types. This suggests that pantodonts had at least formed a separateclade in North America since the earliest dispersal from Asia in the Early Paleocene.

Here， we describe a relatively complete lower jaw collected from the Middle PaleoceneNongshan Formation in the Nanxiong Basin， Guangdong Province. The new specimenis clearly different from all the known Asian pantodonts， and its dental and mandibular characteristics are similar to the North American genera Pantolambda and Titanoides. Thediscovery of the new specimen in the Nanxiong Basin has not only improved our knowledge ofthe distribution of pantolambdid pantodonts， but also provided an unusual example of mammaldispersal from North America into Asia through the Beringia Route in the Early–MiddlePaleocene.

2 Geological backgrounds

The Nanxiong Basin is located in the northern part of Guangdong Province， South China.It is a typical intermontane basin filled by Late Cretaceous and Paleocene fluviolacustrinedeposits. The Paleocene of the Nanxiong Basin is subdivided into the Early Paleocene ShanghuFormation， the Middle Paleocene Nongshan Formation， and the Late Paleocene GuchengcunFormation （Wang et al.， 2021）. The Nongshan Formation consists of two members， the lowermember being the Zhuguikeng Member and the upper member being the Datang Member（Tong et al.， 1976; Zhang， 1984; Li and Cheng， 2007）. In the Datang Section， the ZhuguikengMember is 159.5 m thick and composed of interbedded grayish green calcareous mudstoneand purplish red mudstone， intercalated with brown sandstone; the Datang Member， 149.2 min thickness， is composed of interbedded purplish red mudstone and siltstone， intercalated withsandstone （Tong et al.， 1976）.

Based on mammalian faunal comparisons， Wang et al. （1998） considered the possibilitythat the Nongshanian Asian Land Mammal Age （ALMA） was correlated with the lateTorrejonian North American Land Mammal Age （NALMA） （To-3） through the late middleTiffanian NALMA （Ti-4）. According to paleomagnetic and isotopic results， the boundarybetween the Shanghuan and the Nongshanian ALMA is considered to be at the transitionbetween the chron C27n and C26r （Clyde et al.， 2008）. Wang et al. （2019） used this magneticpolarity reversal as the bottom boundary of the Nongshanian stage. The turnover betweenthe chron C27n and C26r also defines the boundary between the Torrejonian and TiffanianNALMA （Secord， 2008; Clyde et al.， 2010）. Ting et al. （2011） suggested that the Shanghuan-Nongshanian boundary should be correlated to the Torrejonian-Tiffanian NALMA boundaryand the Danian-Selandian Stage boundary. As such， the Nongshanian ALMA correlates withat least the first half of the Tiffanian NALMA; the lower member of the Nongshan Formation，namely the Zhuguikeng Member， approximately correlates with the early Tiffanian NALMA（Ti-1 to Ti-2）.

3 Material and methods

Our new specimen was collected in the 1980s from the Zhuguikeng Member of the Nongshan Formation in the Nanxiong Basin.

Dental nomenclature and measurements follow Muizon and Marshall （1992）;measurements are given in millimeters.

Institutional abbreviations AMNH， American Museum of Natural History， NewYork; FMNH， Field Museum of Natural History， Chicago （=CNHM， Chicago NaturalHistory Museum）; IVPP， Institute of Vertebrate Paleontology and Paleoanthropology，Chinese Academy of Sciences， Beijing; KU， University of Kansas， Lawrence; PU， PrincetonUniversity， Princeton; TMM， Texas Memorial Museum， Austin （=Texas Science and NaturalHistory Museum）; UM， Museum of Paleontology， University of Michigan， Ann Arbor; USNM，United States National Museum， Washington D. C.

4 Systematic paleontology

Order Pantodonta Cope， 1873

Family Pantolambdidae Cope， 1883

Genus Nanxiongilambda gen. nov.

Type species Nanxiongilambda yei gen. et sp. nov.

Etymology Named for both the Nanxiong Basin and Nanxiong City， Guangdong Province， China， where the holotype was found.

Diagnosis Medium to large sized pantodont. Mandible thick and robust， with aconspicuous anteroexternal flange; coronoid process of moderate height; condyloid processpositioned higher than the cheek teeth row， flat but not significantly elongated laterally inshape; the posteriormost part of the angular process approximately in the same plane as thecondyloid process. Lower canine moderately enlarged; p1–p2 double-rooted， crowns neithersectorial nor elongated anteroposteriorly; width of the talonids of the lower molars nearly equalto that of the trigonids; paraconids of the lower molars distinct; metaconids of lower molarstaller than protoconids; hypoconulid of m3 rather developed and bulging posteriorly; entoconidof m3 distinct.

Nanxiongilambda yei gen. et sp. nov.

（Figs. 1， 2）

Holotype IVPP V33263， a segment of mandible， including nearly complete right lower jaw with c1， p2–p4 and m2–m3， and partial left lower jaw with p1–m1.

Type locality and horizon Bidekeng， Youshan Town， Nanxiong City， Guangdong Province， China; Zhuguikeng Member， Nongshan Formation， Middle Paleocene.

Etymology The specific name is dedicated to Professor Ye Jie of IVPP， who kindly provided us the specimen for study.

Diagnosis As for the genus.

Description The mandible is overall stout and robust， with the maximum depth of the horizontal ramus below the m2 and notably shallow below the m3. The anterior border ofthe mandibular symphysis is nearly perpendicular. The posterior margin of the mandibularsymphysis extends to the level of the p2. The horizontal ramus is quite thick， with a prominentanteroexternal flange extending posteriorly below the lower canine to the anterior aspect ofthe lower molars， forming a distinct concavity between the anteroexternal flange and thealveolar border. Two mental foramina are visible between the anteroexternal flange and theconcavity; one is located below the p1 and the other below the p3. The coronoid process is notgreatly elongated anteroposteriorly and is of moderate height. Its posterior aspect is thinner，with a noticeable posterior projection at the apex; the anterior margin of the coronoid processthickens and curls outward， forming an angle of approximately 110° with the horizontal ramus.The medial part of the condyloid process is perpendicular to the main axis of the horizontalramus， while the lateral part is slightly oblique and extends more anteriorly. The position of thecondyloid process is relatively high and distinctly above the cheek tooth row. The massetericfossa is relatively shallow and not large. The angular process is slightly arcuate， protrudingposteriorly and inferiorly， with its posterior margin roughly as the same level as the posteriormargin of the condyloid process; its inferior margin does not reach the level of the maximumdepth of the horizontal ramus.

The dental formula is 3·1·4·3， the basic pattern of placental mammals.

The specimen lacks all six lower incisors， but clear alveoli are visible. Based on the size of the alveoli， the i1 is the smallest， while the i2 and the i3 are nearly equal in size. The i1 and the i2 are directed anterodorsally， whereas the i3 is oriented more laterodorsally.

The right lower canine is nearly complete except for the broken tip. The lower canineis quite large and robust. Its length is approximately equal to its width， resulting in a roughlycircular outline in cross-section. The lower canine protrudes upwards and slightly splayslabially. There is a prominent ridge extending from the tip to the root on the lingual side ofthe lower canine. A roughly rhomboid-shaped wear facet is present at the posterior side， alsoknown as the posterior blade， which contacts the upper canine during occlusion.

The trigonids of the lower premolars resembles that of the lower molars， while thetalonids are not developed. The size of the lower premolars increases gradually from p1 to p4.The protoconids steadily increase in height， and the metaconids significantly increase in heightand width， while the paraconids remain relatively small. The talonids are undeveloped or forma small low bulge， remaining much smaller than the trigonids.

p1： The left p1 is preserved with slight damage on the cusp; the right p1 is not preserved，but its alveoli are clearly visible. The p1 has two roots. Both the paraconid and metaconid arediscernible but very small. The protoconid is noticeably taller and larger than the paraconidand metaconid. The talonid is undeveloped， and the trigonid is not elongated anteroposteriorly，resulting in a crown outline resembling an equilateral triangle. Additionally， the crown of the p1 is more inclined outwards than the other cheek teeth.

p2： Both the left and right p2 are preserved， with slight damage on the paraconid andprotoconid on the left p2 and a nearly complete right p2. The paraconid and metaconid aremore pronounced compared to those of the p1， but are still smaller than the protoconid. Theparaconid and the metaconid are the most widely separated among all the lower premolars.The talonid is not developed， appearing as a small low bulge on the lingual side posterior tothe metaconid， thus the crown outline resembles a blunt triangle or trapezoid. The wear facets are mainly on the anterior side of the paralophid and the posterior side of the protolophid. Theprecingulid， postcingulid， and ectocingulid are all clearly visible， while the lingual cingulidbelow the paraconid is better developed.

p3： The left p3 is slightly damaged on the paraconid and metaconid， and severelydamaged on the protoconid. The right p3 is relatively well-preserved， with only slight damageto the protoconid. The metaconid is larger and stouter than the paraconid， but it is still not asprominent as the protoconid. The spacing between the paraconid and metaconid is relativelysmall compared to that of the p2. The talonid remains as a small low bulge， but forms a ledgealong with the postcingulid， extending from the lingual side to the buccal side， ultimatelyconverging with the ectocingulid. Due to the presence of this ledge， the crown outline appearsmore square or trapezoidal. The wear facets and cingulids of the p3 are similar to that of thep2.

p4： The left p4 is slightly damaged on the paraconid， and severely damaged on themetaconid and protoconid. The right p4 is relatively well-preserved with all cusps almostintact， but the protolophid and the buccal side of the talonid are slightly damaged. Theparaconid is approximately equal to or slightly smaller than those of the p2 and p3. Themetaconid is almost as large as the protoconid， but is still lower than the protoconid. Theprotoconid is the tallest among all the premolars. The spacing between the paraconid andmetaconid is larger than that of the p3， but it is equal to or only slightly smaller than that of thep2. The talonid is still small， but is more developed than that of the p3. The bulge posterior tothe metaconid is longer and slightly raised compared to that of the p3; similar to p3， this bulgeforms a ledge along with the postcingulid， extending diagonally from the lingual side to thebuccal side and converging with the ectocingulid. The ledge is more developed in the p4 thanin the p3， hence the crown outline more resembles a trapezoid. The wear facets are primarilyon the anterior side of the paralophid and the posterior side of the protolophid， but the degreeof wear is moderate compared to that of the p2 and p3.

From the m1 to the m3， the size of the lower molars gradually increases， but the increasein length is more pronounced than the increase in width. Among the lower molars， the highestcusp on the trigonid is the metaconid rather than the protoconid， opposite to the situation in thelower premolars. The trigonid and the talonid of the lower molars form a double V-shape withan angle of approximately 50° in each V， except for the talonid of the m3， which is more or lessU-shaped. The talonids of the molars are slightly narrower， shorter， and much lower than thetrigonids. From the m1 to the m3， there is a tendency of gradual expansion on both the trigonidbasin and the talonid basin.

m1： The left m1 is preserved with slight damage to the paraconid and metaconid， heavydamage to the protoconid， and severe damage to the posterior part of the talonid. The right m1 is not preserved， and the alveoli are rather unclear. Based on the remaining portion of the leftm1， the paraconid is still evident and not reduced; the base of the metaconid is as robust as theprotoconid， and both are larger than the paraconid. The posterior edge of the metaconid extendsbackward to the talonid basin without the development of the metastylid. Much of the talonid is damaged， with only the cristid obliqua discernible， connecting to the posterior edge of themetaconid. The angle between the paralophid and the protolophid is about 50°. The precingulidand the ectocingulid of the m1 are complete， while the postcingulid is not discernable due toits poor preservation.

m2： The right m2 is relatively well-preserved， with slight displacement between thetrigonid and talonid. The paraconid is prominent and more developed than in the precedinglower cheek teeth， but it is still notably lower than the protoconid and the metaconid. Themetaconid is large and robust， and it is even taller than the protoconid. Therefore， the notchof the protolophid is in the midpoint rather than being closer to the metaconid as in thepreceding lower cheek teeth. The m2 is slightly worn on the anterior side of the protolophid，but severely worn on the posterior side of the protolophid and the talonid. The hypoconidis almost completely flattened due to wear， leaving a considerably wide worn surface. Thehypoconulid is moderately worn， next to which is an indistinct entoconid. The angle betweenthe paralophid and the protolophid is about 50°， the same as the angle between the cristidobliqua and the postcristid. The precingulid， ectocingulid， and postcingulid are complete，and the lingual cingulid below the paraconid is less developed than that of the lowerpremolars.

m3： Only the right m3 is preserved， with the posterior half of the trigonid severelydamaged. The metaconid and most of the protoconid are not preserved， while the talonid isrelatively intact. There is only a distinct paraconid left on the trigonid， closely adjacent tothe entoconid of the m2. The height of the paraconid exceeds the maximum height of theentire talonid， suggesting that the trigonid should be significantly taller than the talonid. Thetalonid of the m3 is notably inclined upward and forward， which is attributed to its raisingalveolar border. The hypoconid is heavily worn. The hypoconulid is well-developed andbulges posteriorly， but not to the extent of forming an independent third lobe. The entoconidis more distinct and notably larger than in the m2. Due to these differences， the talonidresembles a wide “U” rather than a narrow “V” in shape. The talonid basin is larger thanthat of the m2， nearly forming a rounded depression that is open on its anterolingual side.The cingulids are complete， and are most developed below the paraconid， hypoconid， andhypoconulid.

5 Comparison and discussion

5.1 Compared with Asian pantodonts

In Asia， the known pantodonts include Coryphodontidae， Harpyodidae， Bemalambdidae，Pastoralodontidae， and Pantolambdodontidae （encompassing the genera formerly classified under Archaeolambdidae） （Chow and Qi， 1978; Huang， 1995）. Coryphodontidae existedfrom the Late Paleocene to Middle Eocene， and is the only Holarctic distributed pantodont，for it is found in both North America and Euroasia. Harpyodidae existed from early tolatest Paleocene in Anhui and Jiangxi province， China （Wang， 1979; Wang et al.， 2016）.Bemalambdidae existed only in the Early Paleocene of Guangdong， Jiangxi， Anhui andHunan province， China （Wang et al.， 2019）. Pastoralodontidae existed in the Early to MiddlePaleocene of Guangdong and Anhui （Chow and Wang， 1978; Wang et al.， 1992; Wang et al.，2016） and the Late Paleocene of Nei Mongol （Chow and Qi， 1978）. Pantolambdodontidaeexisted from the Middle Paleocene to Middle Eocene， and is widely distributed; specimenshave been found in Guangdong， Jiangxi， Anhui， Ningxia， Xinjiang， Nei Mongol， andMongolia （Huang， 1995; Wang et al.， 2016）.

Coryphodontidae is a group of highly specialized pantodonts which is characterizedby the following： 1） enlarged， flattened lower incisors; 2） procumbent lower caninessignificantly protruding outward; 3） lophodont lower cheek teeth， with reduced or absentparaconids and the trigonid nearly as high as the talonid; 4） bilophodont lower molars withreduced paralophid and cristid obliqua， and a well-developed protolophid and postcristid（Chow and Wang， 1979; Lucas， 1998）. These unique characteristics clearly distinguish itfrom V33263.

Harpyodidae has only one genus， Harpyodus. Specimens of the genus include a skull（Wang， 1979） and two fragmentary maxillae （Chiu and Li， 1977; Huang and Zheng， 1997），which makes a direct comparison between Harpyodus and V33263 impossible. However，the upper canines of Harpyodus are not well-developed， suggesting that their lower caninesare likely to be small as well. Specimens of Harpyodus are also notably smaller thanV33263.

Bemalambdidae includes Bemalambda and Hypsilolambda with relatively abundantmaterial available （Chow et al.， 1973; Wang， 1975）. The size of the mandible with its welldevelopedlower canines in V33263 is more similar to that of bemalambdids than to thatof Harpyodus. However， Bemalambdidae still exhibits several differences from V33263，including： 1） a lower position of the condyloid process at the level of the lower dentition; 2） asingle-rooted p1 and more laterally compressed premolars; 3） less pronounced paraconids andrelatively higher protolophids on the lower molars， forming conspicuous shearing surfaces;4） obviously narrower talonid compared to the trigonid; 5） narrower V-shaped angle of thetrigonid than that of the talonid on the lower molars.

Pastoralodontidae and Pantolambdodontidae share several common characters（Chow and Qi， 1978; Huang， 1995） that differ from V33263： 1） absence of a prominentanteroexternal flange; 2） small lower canines， either incisorform or premolarized （except inNanlingilambda and Celaenolambda）; 3） p1–p2 elongated anteroposteriorly and compressed laterally; 4） a wider “U”-shaped outline of the trigonid instead of a narrower “V” shape onthe lower molars; 5） significantly narrower talonid compared to the trigonid on the lowermolars; 6） a narrow elliptical talonid on the m3， with the hypoconulid invisible or vestigial.Apart from these characteristics， Pastoralodontidae differs from V33263 in the presence ofa metastylid on its lower molars， as well as in having a notably shorter and lower premolarseries compared to the lower molar series. Pantolambdodontidae differs from V33263 in itsstraight horizontal ramus without a distinct constriction below the m3. Among the generaof Pantolambdodontidae， a Middle Paleocene genus Nanlingilambda in Guangdong andJiangxi province （Tong， 1979， 1982; Huang and Zheng， 2003） and an Early Eocene genusCelaenolambda in Shandong province （Tong and Wang， 2006） are special in their relativelywell-developed lower canines that have a circular cross-section and are not incisorform orpremolarized. However， the lower canines of Nanlingilambda and Celaenolambda are stillsignificantly smaller than those of V33263 in both relative and absolute size; the two generafurther differ from V33263 in their greater length/width ratio of p1–p2， more buccallylocated cristid obliqua on p3–p4， a narrower talonid relative to the trigonid on the m3， and aless pronounced hypoconulid on the m3.

In summary， V33263 clearly differs from all known Asian pantodonts. In Europe，the only known pantodonts belong to Coryphodontidae （Rose， 2006）， which is evidentlydifferent from V33263. Till now， only one species of pantodont has been discovered inSouth America， namely Alcidedorbignya inopinata （Muizon and Marshall， 1987）， whichis characterized by a hook-like angular process， simple and single-rooted p1–p2， relativelysmall lower canines， and smaller individual size compared to V33263 （Muizon and Marshall，1992）.

5.2 Compared with North American pantodonts

Non-coryphodontid pantodonts in North America include Barylambdidae，Cyriacotheriidae， Pantolambdidae， and Titanoideidae （Rose， 2006）. Barylambdidaeexisted from the late Tiffanian to Clarkforkian NALMA （Simons， 1960; Secord， 2008）;Cyriacotheriidae existed from the middle Torrejonian to early middle Tiffanian NALMA （Scott，2010）; Pantolambdidae first appeared in the middle Torrejonian NALMA and extended into themiddle Tiffanian NALMA （Lofgren et al.， 2004; Secord， 2008; Halverson and Eberle， 2018）;Titanoideidae existed from the middle Tiffanian to Clarkforkian NALMA （Simons， 1960;Rose， 2006）.

Barylambdidae is a group of heavily built pantodonts and differs from V33263 by thefollowing characters： 1） a relatively larger size; 2） a straight horizontal ramus with a consistentheight; 3） a gradually sloping rather than steep anterior wall of the mandibular symphysis;4） lack of an anteroexternal flange; 5） a notably taller coronoid process; 6） relatively smaller lower canines， either incisorform or premolarized to some degree; 7） a single-rooted p1 andelongated p1–p2.

Cyriacotheriidae is a quite special clade amongst the North American pantodonts. Itdiffers from other pantodonts in its W-shaped rather than V-shaped ectolophs on P3–P4，extremely labial situated mesostyles on upper molars， and relatively more molariform lowerpremolars （Rose and Krause， 1982; Lucas， 1998; Scott， 2010）. Hence， whether it should beincluded in Pantodonta is still debatable （Lucas， 1982， 1998; Muizon and Marshall， 1992）.Cyriacotheriidae is clearly different from V33263 in its more molariform p3–p4 bearing welldevelopedtalonids， which are as wide as the trigonids.

Pantolambdidae is characterized by the relatively well-developed lower canines witha subcircular cross-section， the broad talonids of the lower molars， the well-developed andposteriorly bulged hypoconulid of the m3， the fused mandibular symphysis， a mandiblewith an anteroexternal flange， the constricted horizontal ramus posterior to the m3， andthe posteriorly protruding angular process. These characteristics are similar to those of V33263.

Pantolambdidae is comprised of Pantolambda and Caenolambda. Pantolambda isusually considered to be the oldest and most primitive pantodont in North America （Simons，1960; Rose， 2006）. Currently there are three species in the genus Pantolambda， namely P.bathmodon Cope， 1882 （the type species）， P. cavirictum （erected from “cavirictus” by Rose，1981 for agreement in gender with Pantolambda） Cope， 1883， and P. intermedium （erectedfrom “intermedius” by Rose， 1981 for agreement in gender with Pantolambda） Simpson，1935. The holotype of P. intermedium （USNM 8384） is too fragmentary， and due to the lack ofa refered specimen with complete lower jaws， a detailed comparison between P. intermediumand V33263 is difficult to conduct.

Pantolambda bathmodon is the smallest species in this genus. The p1–p2 of P.bathmodon are single-rooted and show no trace of distinct accessary cuspids; the p3–p4are double-rooted and have more distinct paraconids and metaconids. The widths of thetalonids of the m1 and m2 are equal to or slightly greater than that of the trigonids （Matthew，1937）. On the m3， the hypoconulid is strong， equal in size to the hypoconid， and recurvedforward to a marked degree， but does not form a distinct third lobe; the entoconid is notdistinct. P. cavirictum is significantly larger than P. bathmodon in size， with a relativelylarger subcircular lower canine. The p1 of P. cavirictum is single-rooted and is followedby a long diastema; the p2–p4 are double-rooted and appear to be approximately square inshape. The hypoconulid on the m3 bulges posteriorly， but it is not as developed as it is in P.bathmodon; the entoconid on the m3 is more distinct than that of P. bathmodon. Apart fromthe dental characteristics， the mandible of P. cavirictum shows greater development of theanteroexternal flange on the horizontal ramus; the angular process exhibits less posterior extension than that of P. bathmodon， but still reaches the plane of the condyloid process;the mandibular symphysis is heavier and the depth of horizontal ramus is relatively greaterthan that of P. bathmodon. These characters of P. cavirictum are quite consistent with that ofV33263， except for the p1.

Another pantolambdid genus， Caenolambda， is more derived than Pantolambda. Thegenus includes two species， C. pattersoni Gazin， 1956 and C. jepseni Simons， 1960. Theholotype of C. pattersoni is a skull lacking zygomatic arches and the mandible （USNM21036）， and due to the lack of a definitive referred specimen， it cannot be directly comparedwith V33263. Caenolambda jepseni is represented by a lower jaw with complete dentition，several upper teeth， and a partial post-cranial skeleton （PU 14863）. It has a prominentanteroexternal flange， relatively developed lower canines， double-rooted p2–p4， pronouncedledges on the p3–p4， and a pronounced hypoconulid on the m3， which are characteristicsconsistent with V 33263. However， its significantly elongated and laterally compressed（described as “sectorial” by Simons， 1960） p1 indicates its specialization and distinguishes itfrom V33263.

Titanoideidae is characterized by its well-developed lower canines， which exceede thoseof any other known pantodonts in proportion （Simons， 1960; Rose， 2006）. Titanoideidaecurrently comprises only one genus， Titanoides， which exhibits several distinctive features（Gingerich， 1996）： p1–p2 with double roots that are triangular in shape rather than elongatedor laterally compressed; distinct paraconids on the p1–m3; the talonid of m3 relatively broadwith a well-developed hypoconid and a distinct entoconid; and well-developed cingulids onthe lower cheek teeth. These characteristics are notably similar to those of V33263. However，the lower canines of Titanoides are significantly anteroposteriorly elongated rather than beingsubcircular in cross-section， the hypoconulid of the m3 is less pronounced and does not bulgeposteriorly， the coronoid process is low， and the angular process does not extend posteriorlyto the plane of the condyloid process （except for in a refered specimen of T. zeuxis， TMM41217-1） （Schiebout， 1974）， and the mandibular symphysis is not fused. These characteristicshighlight differences between Titanoides and V33263. The only character that is shared byV33263 and Titanoides but not found in Pantolambdidae is the double-rooted p1. However，the shape of the p1 in Pantolambdidae varies considerably： single-rooted and simple in P.bathmodon; single-rooted and followed by a long diastema in P. cavirictum; elongated andlaterally compressed in C. jepseni. The double-rooted and triangular p1 in Nanxiongilambdayei may contribute to this diversity rather than represent a synapomorphy shared withTitanoides.

After analyzing a comparison of these features， V33263 shows considerable similaritywith Pantolambda， Caenolambda， and Titanoides. Its dental and mandibular characteristicsseem to show a mosaic pattern： its lower canine is subcircular in cross-section， has a posteriorly bulged hypoconulid on the m3， displays a fused mandibular symphysis， and itsuncontracted angular process more resembles that of Pantolambda and Caenolambda; whileits double-rooted p1 more resembles that of Titanoides. The elongation of its lower canine，the development of the ledges on the p3–p4， and the width ratio between the trigonid andtalonid of the lower molars are intermediate between Pantolambdidae and Titanoideidae.Therefore， V33263 probably shows an intermediate evolutionary stage between the twoclades. We finally assign V33263 to a new genus and species within Pantolambdidae，namely Nanxiongilambda yei gen. et sp. nov. due to the following key characters： 1） thedevelopment of the hypoconulid on the m3 of V33263 exceeds any known species ofTitanoides， but it is similar to that of P. cavirictum and C. jepseni; 2） the angular processof V33263 reaches the plane of the condyloid process as in Pantolambdidae， rather thanbeing contracted as in Titanoides; 3） the mandibular symphysis is fused in V33263 andPantolambdidae， but unfused in Titanoides; 4） the length/width ratio of the lower canine ofV33263 is evidently smaller than that of any known species of Titanoides， but more similarto that of Pantolambda and Caenolambda.

5.3 Paleobiogeographic implications

Through detailed comparison， we recognize that V33263 differs from any knownAsian pantodonts， but it is rather similar to the North American pantolambdid pantodonts.Pantolambda were distributed from the Torrejonian to the early Tiffanian NALMA， whereasmore derived Caenolambda are believed to have lived during the early to middle TiffanianNALMA， either Ti-2 to Ti-3 （Lofgren et al.， 2004） or Ti-3 to Ti-4a （Secord， 2008）; Titanoidesmainly thrived throughout the Tiffanian NALMA （Lofgren et al.， 2004）.

IVPP V33263 was discovered in the Zhuguikeng Member of the Nongshan Formation inthe Nanxiong Basin， Guangdong Province， China. The Nongshanian ALMA correlates with thefirst half of the Tiffanian NALMA; the lower member of the Nongshan Formation， namely theZhuguikeng Member， approximately correlates with the early Tiffanian NALMA （Ti-1 to Ti-2）. Therefore， the age of Nanxiongilambda yei is confined to the early Tiffanian. Since the newspecies shares a series of common characteristics with North American taxa rather than Asiantaxa， and considering that the more basal North American clade Pantolambda lived throughTo-2 to Ti-1， it is reasonable to interpret Nanxiongilambda as an immigrant of pantolambdidsfrom North American.

The earliest and most primitive pantodonts discovered in Asia are Bemalambdidae，represented by Bemalambda and Hypsilolambda. They are exclusively documented fromthe Early Paleocene Shanghuan ALMA， which corresponds to the Puercan and TorrejonianNALMA. The earliest confirmed pantodont with relatively complete material in North Americais Pantolambda， which appeared slightly later than Bemalambdidae. In South America， theonly known pantodont is Alcidedorbignya inopinata， which lived during the TiupampanSouth American Land Mammal Age. Recently， Muizon and Ladevèze （2020） and Muizonand Billet （2022） discussed the age of Tiupampan SALMA and proposed a correlation ofthe Tiupampa beds to chron 28r （corresponding to Pu-3 to To-1）. If this result is accurate，Alcidedorbignya predates Pantolambda （To-2） and shares a similar temporal range withBemalambda. However， according to a recent phylogenetic analysis （Muizon et al.， 2015），Alcidedorbignya is very closely related to North American pantodonts （represented byPantolambda） but more distant from Asian pantodonts （represented by Bemalambda andHarpyodus）. Additionally， Clemens （2017） reported a single upper molar of a pantodontCrustulus fontanus dated to the latest Puercan （Pu-3） in North America. Crustulus is moremorphologically similar to Alcidedorbignya than Asian pantodonts （such as Bemalambdaand Harpyodus） and more derived North American pantodonts （such as Pantolambda），supporting the hypothesis that Alcidedorbignya derived from North American taxa （Muizonet al.， 2015）.

When compared with pantodonts from America， Bemalambda and Harpyodus showsome key primitive characteristics， such as closely appressed or connate paracones andmetacones. They also differ from American species in some other characteristics， such asa wide stylar shelf and a deeply incised ectoflexus （Lucas， 1998; Rose， 2006）， indicatingan early divergence between Asian and American pantodonts. Furthermore， some featuresof Bemalambda are clearly specialized， such as particularly wide and anteroposteriorlycompressed upper cheek teeth， well-developed canines， and much narrower talonids thantrigonids on the lower molars， suggesting that Bemalambda may not be the ancestral formof pantodonts but rather an early specialized clade. From the earliest common ancestor ofpantodonts （which should be older than Bemalambda in age）， two main lineages emerged;one remained in Asia and gave rise to forms such as Archaeolambda and Pantolambdodon，while the other likely migrated to North America during the Puercan NALMA and thenquickly migrated to South America as represented by Alcidedorbignya. The main lineageremaining in North America evolved into forms such as Pantolambda during the TorrejonianNALMA， and Titanoides and Barylambda during the Tiffanian NALMA. The migrationof pantodonts from Asia to North America before the middle Torrejonian （To-2） is widelyaccepted （Beard， 1998; Lucas， 1998; Rose， 2006）.

Nanxiongilambda yei evidently shows more similarity with North Americanpantodonts （especially Pantolambda and Titanoides） than with Asian pantodonts. Therefore，Nanxiongilambda is likely to have evolved from North American Pantolambda or its closerelatives no later than the earliest Tiffanian.The appearance of Nanxiongilambda fromNanxiong Basin represents the reversed wave of pantodont migration from North America toAsia.

Lucas and Williamson （1995） proposed that the mammalian fossil assemblage of theShanghu Formation correlates with the North American Puercan， indicating a migrationof primitive Carnivora， Mesonychia， Tillodontia， and Pantodonta from Asia to NorthAmerica around the Puercan-Torrejonian boundary. Wang et al. （1998） conducted a moredetailed comparison of various mammalian taxa， suggesting that fossils of Mesonychidae，Carnivora， and Arctocyonidae indicate a correlation of the Shanghuan with some part ofboth the Puercan and Torrejonian NALMA. The similarities and comparability of somecertain mammal taxa at the family level between the Shanghuan fauna and Puercan andTorrejonian faunas suggest some degree of biotic exchange between Asia and NorthAmerica， but this kind of exchange appears to be rather limited since no particular species ofmammal is known to have inhabited both continents during this period （Wang et al.， 1998）.This is indicated by the strong endemism of Shanghuan mammals in Asia， representedby the prevalence of anagalids， simplicidentates， duplicidentates， and didymoconids，none of which occurred in North America during this age （Ting et al.， 2011）. Evidence of mammalian exchange during the Nongshanian is more abundant， with taxa such asPalaeoryctidae， Nyctitheriidae， Carpolestidae， Viverravidae， Alagomyidae， Arctostylopidae，Coryphodontidae， and Prodinoceratidae found in both Asia and North America （Tongand Wang， 2006）. The discovery of Nanxiongilambda provides new evidence for bioticexchanges between Asia and North America at the family level in the early NongshanianALMA and early Tiffanian NALMA.

Krause and Mass （1990） suggested the existence of the Beringia Route during thePaleocene， but noted its high paleolatitude may have posed some barriers to migration.Beard （1998） suggested that the Beringia Route could have functioned as a significant filterto dispersal， and the migration across the Beringia may have been influenced by sea levelchanges， regional tectonics， and climate change. This filtering effect concept is supportedby a number of authors （Brikiakis， 2014）. Based on a comprehensive comparison betweenmammalian faunas of Asia and North America during the Paleocene and Early Eocene，Tong and Wang （2006） suggested that the biotic exchange between the two sides of theBeringia should have been relatively unimpeded. Some conservative taxa （such as Anagalidaand Bemalambdidae） were likely to remain in situ while more widely adaptive taxa weredistributed across both continents. Given the relatively warm climate during the Paleocene，it seems possible for Nanxiongilambda to have migrated across the Beringia between NorthAmerica and Asia， as noted for Dissacus （Brikiakis， 2014）， Tillodontia （except Deltatherium），Rodentia， and Coryphodon （Lofgren et al.， 2004）.

6 Conclusion

Nanxiongilambda yei is a distinctive Asian pantodont that shows more similaritiesto North American clades than it does to Asian clades. It is obviously different from anyknown Asian pantodont in its dental and mandibular characteristics. The discovery ofNanxiongilambda yei has not only enriched the Nongshanian fauna in South China but has alsoadded to the diversity of Asian pantodonts.

However， Nanxiongilambda possesses mosaic characteristics of both Pantolambdaand Titanoides from North Amercia， such as the posteriorly protruding angular process， thesubcircular lower canine in cross-section， the double-rooted p1–p2， and the well-developedhypoconulid and distinct entoconid on the m3. The appearance of Nanxiongilambda suggests amigration of derived pantolambdids from North America to Asia by passing through BeringiaRoute during the early-middle Paleocene. The presence of pantolambdid pantodonts in Asiaalso suggests that migration of Paleocene mammals back to “East of Eden” may have beenmore common than currently recognized.

Acknowledgements We appreciate Prof. Ye Jie for providing the specimen and ZhangYuping for contributing to the field work. We also thank Prof. Tong Yongsheng for his helpfuldiscussions， Mr. Li Shijie for preparing the specimen， and Mr. Gao Wei for photographing it.

摘要：全齒目是新生代早期首批出現(xiàn)的真獸類哺乳動(dòng)物之一，其中不乏當(dāng)時(shí)體型最大的一些植食性動(dòng)物。全齒目在古新世和始新世期間相當(dāng)多樣化，在亞洲和北美洲具有較豐富的化石記錄。報(bào)道并描述了一件采自廣東南雄盆地中古新統(tǒng)濃山組竹桂坑段的全齒類下頜骨化石材料。其獨(dú)特的下頜和牙齒特征明顯區(qū)別于任何已知的亞洲全齒類，而與北美的全齒類，尤其是全棱獸屬（Pantolambda）和巨脊齒獸屬（Titanoides）較為相似。新標(biāo)本被命名為葉氏南雄脊齒獸（Nanxiongilambda yei gen. et sp. nov.）， 其鑒定特征包括：下頜骨較粗壯且具前外凸緣，髁突位置顯著高于頰齒齒列，角突最后端向后延伸至略超過(guò)髁突后緣的位置，下犬齒粗壯但并不前后向延長(zhǎng)，下前臼齒具雙齒根，下臼齒下跟座發(fā)育，m3下次小尖發(fā)育且向后伸展，m3下內(nèi)尖獨(dú)立且清晰可辨。形態(tài)特征的綜合對(duì)比顯示，南雄脊齒獸與北美洲的全棱獸類有著明確的親緣關(guān)系。這是全棱獸類在北美以外的首次發(fā)現(xiàn)，表明古新世期間全棱獸類具有比此前認(rèn)為更加廣泛的地理分布。鑒于更加原始的早期全棱獸類僅存在于北美洲（Torrejonian–early Tiffanian， 相當(dāng)于上湖期中晚期-濃山期早期）， 略早于南雄脊齒獸（濃山期早期）， 全棱獸類很可能在早古新世晚期-中古新世早期經(jīng)歷了一次從北美到東亞的遷徙。以前認(rèn)為全齒類的擴(kuò)散僅發(fā)生在早古新世從亞洲至北美的方向上，新材料的發(fā)現(xiàn)表明在早-中古新世期間，全齒類仍有可能從北美擴(kuò)散到亞洲。南雄脊齒獸的發(fā)現(xiàn)不僅為全齒類的演化歷史提供了新的信息，也為古新世期間哺乳動(dòng)物的洲際遷徙與擴(kuò)散提供了新的證據(jù)。

關(guān)鍵詞：南雄盆地，古新世，濃山組，全齒目，全棱獸科，南雄脊齒獸

中圖法分類號(hào)：Q915.873 文獻(xiàn)標(biāo)識(shí)碼：A 文章編號(hào)：2096–9899（2024）04–0291–22

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