ORIGINAL PAPER

A New Species of Prolibytherium (Ruminantia, Mammalia)from Pakistan, and the Functional Implications of an AtypicalAtlanto-Occipital Morphology

Melinda Danowitz1 & Rebecca Domalski1 & Nikos Solounias1

# Springer Science+Business Media New York 2015

Abstract We describe a new species of Prolibytherium, P.fusus, sp. nov., from the lower Miocene of Pakistan, thus ex-tending the genus to Asia. Prolibytherium is otherwise knownonly from Libya. This species differs from Prolibytheriummagnieri in several basioccipital and atlanto-occipital morphol-ogies. Namely, the posterior basioccipital tuberosities are con-tinuous at the midline and lack the elevated transverse ridgeseen in P. magnieri, and the notch formed between the lateraloccipital condyles and paraoccipital process is lower. Both spe-cies of Prolibytherium have a characteristic ventrally fused oc-cipital condyle at the midline, with a notably fuller circumfer-ential articular surface. Prolibytherium magnieri also has thick-ened dorsal and ventral arches of the atlas. These specimensalso possess a longitudinal groove for the Eustachian tube ex-tending from the alisphenoid canal to the bullae, and a seconddeep grove isolating the basisphenoid bone from the temporalbone. These, plus several other atlanto-occipital morphologiesstrengthen the cervical support of the head. This is especiallyimportant for Prolibytherium, as the taxon possesses massivealiform cranial appendages. We relate the approximation of theoccipital condyles to a convergent state in two giraffids(Giraffokeryx punjabiensis and Schansitherium tafeli), each ofwhich possesses multiple pairs of ossicones, presumably neces-sitating a strengthened atlanto-occipital joint.

Keywords Prolibytherium . Atlanto-occipital joint .

Basicranium . Ruminantia . Cranial appendages

Introduction

Prolibytherium magnieri is a specialized Miocene ruminantwith massive flattened cranial appendages, and a specializedatlanto-occipital joint. Arambourg described it from GebelZelten (16 Ma) in North Africa in 1961, and Hamiltonreviewed and described it in more detail in 1973. It is knownonly from the lower Miocene of Gebel Zelten of Libya.Prolibytherium magnieri is the type species forProlibytherium, and is presently the only species describedfor the genus. We introduce a new species of Prolibytheriumfound in the Zinda Pir of Pakistan, the first introduction of thegenus in Asia. The establishment of a new and geologicallyolder (19 Ma) species is based on differences in basicranialmorphology from P. magnieri.

The phylogeny of these ruminants is currently unresolved;Prolibytherium has been placed in Sivatheriidae (Hamilton1973), Palaeomerycidae (Janis and Scott 1987; Prothero andLiter 2007; Solounias 2007), and Climacoceratidae (Pickfordet al. 2001; Morales et al. 2003; Sánchez et al. 2010). Webriefly review the systematic affiliation of Prolibytherium,and provide arguments strengthening its association withClimacoceratidae.

The unique ventral fusion of the occipital condyles inProlibytherium has been mentioned briefly by Sánchez et al.(2010) and Solounias (2007), but the implications on the sup-port and range of motion of the neck have yet to be discussed.The cranial appendages ofProlibytherium are notably atypicalfor ruminants, as they form a massive, slightly concave plate,which is oriented parallel to the dorsal aspect of the skull(Sánchez et al. 2010). Solounias (2007) suggested that thesecranial appendages encompassed two pairs of structures (an-terior and posterior), which merge and form a common web atthe base. Sánchez et al. (2010) described the appendages ashaving an asymmetric Bx^ shaped scaffold, which supports

* Melinda Danowitzmdanowit@nyit.edu

1 Department of Anatomy, New York Institute of Technology Collegeof Osteopathic Medicine, Old Westbury, NY 11568, USA

J Mammal EvolDOI 10.1007/s10914-015-9307-8

the lateral bony expansions. Hamilton (1973) described theseas large and aliform, and fused to the frontal and parietal boneswith no visible suture at the base. We relate these uniquelystructured and immense appendages to the atypical morphol-ogy of the atlanto-occipital joint.

Materials and Methods

Prolibytherium magnieri

Fairly complete specimen (braincase with cranial append-ages): NHM UK PVM21901

Fragmentary specimens, with fairly complete braincases:NHM UK PVM99897, NHM UK PVM99896a, NHM UKPVM26679

Atlas: NHM UK PVM99896b

Prolibytherium fusus, sp. nov.

Braincase with cranial appendages broken off: PMNH Z 162The original specimens were examined for morphological

characteristics and anatomical descriptions.Institutional abbreviations—AMNH,AmericanMuseum

of Natural History, New York, New York, USA; PMNH, ThePakistan Natural History Museum, Islamabad, Pakistan; NHMUK, The Natural History Museum, London, U.K; HPM,Hezheng Paleozoological Museum, Hezheng, China.

Systematic Paleontology

Order Artiodactyla Owen, 1848Family cf. Climacoceratidae Hamilton, 1978Genus Prolibytherium Arambourg, 1961

Type species Prolibytherium magnieri Arambourg, 1961

Generic Diagnosis—Medium-sized (similar in size toRangifer tarandus) with flat, aliform cranial appendages ori-ented horizontal to the braincase, with the anterior and poste-rior pairs merged at a webbed base (Fig. 1a). The anterior pairis smaller than the posterior pair. The cranial appendages growfrom the frontal bone, and are situated posterior to the supra-orbital foramina. Their surface is smooth with shallow, thinvascular impressions. Internally, the cranial appendages ap-pear to have a medulla of cancellous bone surrounded by athick cortex (Fig. 1b). The maximum thickness of the cranialappendages is ~300 mm. There are two lacrimal foramina atthe edge of the orbit. Ventrally, the occipital condyles arefused at the median plane (Fig. 2). The posterior basioccipitaltuberosities are separated from the condyles by a deep, narrowgroove. The Eustachian tube left a long impression along the

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Fig. 1 Prolibytherium magnieri a, massive aliform cranial appendages(NHM UK PVM21901) and their b, internal structure (NHM UKPVM99896a)

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Fig. 2 Prolibytherium magnieri braincase (NHM UK PVM99897) inventral view. (1)—the longitudinal groove extending from the level ofthe alisphenoid to the bullae for the Eustachian tube. (2)—the medial deepgroove extending from the level of the alisphenoid to the posteriorbasioccipital tuberosities that separates the basioccipital from thetemporal bone. (3)—the posteriorly-directed posterior basioccipital tuber-osities. (4)—the ventrally fused occipital condyles characteristic ofProlibytherium. (5)—the narrow groove separating the approximatedbasioccipital tuberosities

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basisphenoid bone, extending from the level of the alisphenoidcanal to the bullae. In addition, there is a second, more medialgroove between the Eustachian groove and the lateral edge ofthe basioccipital-basisphenoid. This groove on thebasisphenoid extends from the alisphenoid to the posteriorbasioccipital tuberosities, and separates the basisphenoid fromthe temporal bone. The bullae are large and spherical. Thedorsal and ventral arches of the atlas are thickened (Fig. 3).The premolars and molars are brachydont, and the upper P2is slightly longer than the P3. The masseteric insertion on themandible forms a distinct notch from the body of the mandible.This notch is anterior to the fossa for the masseter profundus,and the greatest depth of the notch is in the same plane as thesuperior margin of the foramen magnum. The mastoid processis situated in the same plane as the occipital. Hamilton (1973)included a more complete description of the genus.

Prolibytherium magnieri Arambourg, 1961

Type Locality—Gebel Zelten, Libya (middle Miocene)Holotype—A cranium with badly shattered cranial ap-

pendages described by Arambourg (1961).Referred Specimens—Hamilton (1973) listed 34 speci-

mens from Natural History Museum-London, MuséumNational d’Histoire Naturelle- Paris, and Museum ofPaleontology University of California Berkley. This includedan almost complete skull, cranial fragments, several dentitionsand mandibles, and numerous postcranial specimens.

Diagnosis of type species—As for the genus.Comments—Two studies have focused on P. magnieri.

Hamilton (1973) described in detail the material, andSánchez et al. (2010) reviewed Prolibytherium and proposedthat sexual dimorphism existed, with females possessing cra-nial appendages that retain the same pattern but are thin andcylindrical and without the webbed aliform connection. Barryet al. (2005) briefly compared Prolibytherium to Progiraffa,and they referred a Zinda Pir braincase (PMNH Z 162), alongwith a large sample of fragmentary specimens, to Progiraffa.We are currently reassigning this braincase to Prolibytherium.

Atlas (Fig. 3) The dorsal and ventral arches are thickened.The cranial articular facets, which articulate with the occip-ital condyles, are deep with a strongly bent lateral bound-ary. The facets are joined with a thickened bony connectionon the dorsal and ventral surfaces. There is a widened notchformed on the lateral edge between the dorsal and ventralaspects of the cranial articular facet. This notch articulateswith the junction between the posterior and ventral surfacesof the occipital condyles. The ventral tubercle is strong,and forms a large and thickened protrusion on the cau-dal aspect of the ventral arch. The ventral tubercle ex-tends from the posterior edge of the vertebra and thecaudal articular facets as a triangular bony expansion.

The anterior edge of the dorsal and ventral arches formsa U-shape with a small notch at the midline on theventral surface. The dorsal arch has a median longitudi-nal keel, with a double bony protrusion on the posteriorend. The caudal articular facets are oval shaped, widestmedially, and are concave laterally. The caudal articularfacet extends laterally onto the postero-lateral process ofthe alar wing.

Prolibytherium fusus, sp. nov.(Fig. 4)

Etymology—From the Latin fusus Bthe fused one,^ in ref-erence to the characteristic fused occipital condyles

Holotype—PMNH Z 162, caudoventral portion ofbraincase

Locality and Geological Age—Locality Z 124 in theVihowa Formation of Pakistan (early Miocene). Locality Z124 is located at the top of a normal polarity zone that hasbeen correlated with the 6n polarity interval and would beslightly older than 18.7 Ma (Lindsay et al. 2005).

Differential Diagnosis—Prolibytherium fusus differs fromP. magnieri in the following characters: the anteriorbasioccipital tuberosities are less distinct, and their surfacecontains longitudinal ridges instead of small bumps seen onP. magnieri. The elongated fossa between the posterior andanterior basioccipital tuberosities is one unified surface in

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Fig. 3 Prolibytherium magnieri atlas (NHM UK PVM99896b) in a,dorsal and b, caudal views

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P. magnieri, and is separated into two plates by a midlinelongitudinal groove in P. fusus. In P. fusus, the posteriorbasioccipital tuberosities are more approximated with the ar-ticular surface of the occipital condyles, and are thicker andshorter. The posterior basioccipital tuberosities are also ap-proximated across the midline in P. fusus, separated only bya narrow, deep groove, and the caudal surface contains severalbony growths concentrated medially. In P. magnieri, the pos-terior basioccipital tuberosities are separated at the midline,also by a deep groove, and the surface contains a distinctelevated transverse ridge, and no bony growths. The U-shaped ventral margin on the occipital condyles is more shal-low in P. fusus, and the condyles are oriented laterally, where-as the condyles are oriented dorso-laterally and have a deeperU-shaped ventral margin in P. magnieri. The notch between

the paraoccipital process and the lateral occipital condyles isthicker and lower in P. fusus (Fig. 4: character 5).

Description—The Zinda Pir specimen PMNH Z 162 is apartial braincase. The cranial appendages are broken off. Amore complete description is in Barry et al. (2005).

Description of the Prolibytherium magnieriand P. fusus Basicranium and Occipital Condyles

Anterior Basioccipital Tuberosities These are slight ovoidbony swellings, which are separated at the midline.

Fossa Between Anterior and Posterior Basioccipital Tu-berosities, and Shape of Basioccipital Bone The fossa isconcave and contains longitudinal striations. There is a deeplongitudinal groove on the basisphenoid extending from thealisphenoid to the posterior basioccipital tuberosities that sep-arates the basioccipital from the temporal bone (Figs. 2 and 4:character 2). Lateral to this, there is also a distinct groove onthe basisphenoid from the alisphenoid to the bullae for theEustachian tube (Figs. 2 and 4: character 1).

Posterior Basioccipital Tuberosities The posteriorbasioccipital tuberosities are positioned rostral to the occipitalcondyles, and are separated from the condyles by a distinct deepgroove. The articular surface of the condyles is not continuouswith the tuberosities. The posterior basioccipital tuberosities areprominent, thick bony protrusions, which are approximatednear the midline. The tuberosities are angled slightly posteriorly(Figs. 2 and 4: character 3), and follow the orientation of theoccipital condyles. On two of the four P. magnieri specimens,the medial surface of the tuberosities expands rostrally, forminga triangular extension onto the basioccipital.

Occipital Condyles (Ventral View) The two condyles arethickened and fused at the midline, with no visible fusion line,forming one unified articular surface. This forms a U-shapedventral margin to the foramen magnum. The surface of thefused condyles is full, rounded, and bulging. The junctionbetween the posterior and ventral articular surfaces is roundedand not distinct. There is a notch on the antero-dorsal aspect ofthe lateral surface of the condyle. The fusion of the occipitalcondyles makes these taxa unique among artiodactyls.

Occipital Aspect of the Skull The posterodorsal articularsurface of the occipital condyles is expanded. The notchformed between the paroccipital process and condyle isdisplaced ventrally, so that the notch is more ventral than thedorsal edge of condyle. The occipital surface is on the sameplane as the mastoid bone and paroccipital process. The at-tachment for the nuchal ligament, the external occipital

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Fig. 4 Prolibytherium fusus, sp. nov. braincase (PMNH Z 162) in a,ventral, b, left posterolateral, and c, posterior views. (1)—thelongitudinal groove extending from the level of the alisphenoid to thebullae for the Eustachian tube. (2)—the medial deep groove extendingfrom the level of the alisphenoid to the posterior basioccipital tuberositiesthat separates the basioccipital from the temporal bone. (3)—theposteriorly-directed posterior basioccipital tuberosities. (4)—theventrally fused occipital condyles characteristic of Prolibytherium.(5)—the narrow groove separating the approximated basioccipitaltuberosities. (6)—the low-positioned notch between the paraoccipitalprocess and the occipital condyle. (7)—the alisphenoid canal

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protuberance, has a central elevated ridge on the midline.There are two deep pits positioned lateral to the ridge.

Discussion

Assignment of the Zinda Pir Braincase to Prolibytherium

Barry et al. (2005) referred the braincase PMNH Z 162 fromZinda Pir to Progiraffa exigua, as well as numerous teeth, amaxilla, postcranial elements and an isolated cranialappendage. Pilgrim (1908) established Progiraffa exiguabased on a left second and third molar from the Upper Naribeds (now the Chitarwata Formation [Antoine et al. 2013]) ofDako Nala in Bugti Hills of Baluchistan (Pakistan). The spec-imen was figured in Pilgrim (1911: pl. I, fig. 1) together with asecond lower molar (pl. I, fig. 2) identified as Progiraffa sp.from the Blower Siwaliks^ of Sind. A third lower molar (pl. I,fig. 3) and two upper molars were identified as Progiraffasivalensis from the lower Siwaliks of the Potwar (Pilgrim1908). These teeth are all primitive-looking giraffid teeth;the protoconid, metaconid, and the other cuspids form wallsthat are buccally and lingually inclined and slightly convex,rather than straight and vertical.

The basis of the referral by Barry et al. (2005) was size,geographic proximity, and the age of the fossils. While it iscommon in the Miocene to find three or more giraffid speciesper locality, and thus there may have been multiple species oflarge ruminants in the lower Miocene of southern Asia, werefer the Zinda Pir braincase to Prolibytherium. At this time,we do not exclude the possibility of the presence of Progiraffain the Zinda Pir collection. We simply exclude the braincasefrom the remainder of the Progiraffa sample, because it bettermatches Prolibytherium. The new identification extends therange of Prolibytherium into Asia and strengthens faunal sim-ilarities between southern Asia and northern Africa.

Barry et al. (2005) noted the distinct thickness of the dorsalaspect of the Z 162 braincase, suggesting the presence of largecranial appendages as are seen in Prolibytherium. The brokenedge of the cranial appendages is posterior and covers thewidth of the calvaria confirming the presence of structuressimilar to those of P. magnieri. In addition, we note threeadditional characters that place the braincase closer toProlibytherium thanProgiraffa. First, in the specimen in ques-tion and Prolibytherium, the mastoid process is in the sameplane with the occipital. In all Giraffidae and presumably inProgiraffa, these processes are more rostral than the occipital,which protrudes and forms an hourglass shape. Second,Prolibytherium and the braincase in question both possess adeep groove separating the basisphenoid from the temporalbone, extending from the level of the alisphenoid canal tothe posterior basioccipital tuberosities. This groove is absentin Cervidae, Bovidae, and Giraffidae. Third, the impression of

the Eustachian tube is elongated and horizontal, and embed-ded lateral to this groove. This Eustachian tube morphologyseen both in Prolibytherium and the Z 162 braincase is unlikethat of other ruminants.

Systematic Position of Prolibytherium

The familial affiliation of this Miocene ruminant is currentlyunresolved. It is unlikely, however, that Prolibytherium be-longs to the Giraffidae (Sivatheriinae or Sivatheriidae;Hamilton 1973). There are general similarities in the shape ofits cranial appendages to the ossicones of Sivatherium; bothtaxa possess massive plate-like cranial appendages that arefused to the parietal bone. The cranial appendages ofProlibytherium, however, are not likely to be ossicones, be-cause no sutures are visible separating the cranial appendagesfrom the skull. In addition, considering differences in the den-tition and geologic age, these similarities are more likely due toconvergence. The dentition of Prolibytherium is different fromthat of Giraffidae in that the teeth are slender and slightly morehypsodont, and the enamel is not as crenulated (Hamilton1973). Sivatherium (~2 Ma) is also significantly younger thanProlibytherium (~19–16 Ma) (McKenna and Bell 1997).

Prolibytherium has been previously assigned toPalaeomerycidae based on the cranial appendages (Janis andScott 1987; Prothero and Liter 2007; Solounias 2007).Palaeomerycids were geographically widespread in NorthAmerica and Eurasia, and had simple supraorbital appendageswith a median single occipital appendage (Ginsburg and Heintz1966; Qiu et al. 1985; Prothero and Liter 2007). None of thepalaeomerycids have a forward projection-extension of the cra-nial appendages, as is seen in Prolibytherium. In addition, thecranial appendages of palaeomerycids have been classified asfrontal ossicones, due to the presence of visible suture lines withthe skull (Astibia et al. 1998; Sánchez et al. 2010). TheProlibytherium cranial appendages, however, are fused to theparietal bone with no visible suture. The internal structure oftrue giraffid ossicones is composed of cartilaginous material aswell as dense connective tissue (Spinage 1968; Ganey et al.1990). The Prolibytherium cranial appendages, however, ap-pear to consist of cancellous bone surrounded by a thick bonycortex (Fig. 1b), further negating their identification asossicones. Lastly, the Prolibytherium dentition lacks theBPalaeomeryx fold,^ shared by many Palaeomerycidae(Prothero and Liter 2007).

Current observations suggest a closer relationship toClimacoceratidae. Morales et al. (2008) described specializa-tions of the cubonavicular that are shared only betweenClimacoceratidae and Prolibytherium. In addition, theProlibytherium magnieri cranial appendages are oriented paral-lel to the dorsal aspect of the skull. This orientation matches thatof the anterior beam of the cranial appendage of Climacocerasgentryi (Hamilton 1978: Fig. 21). The curvature between the

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anterior and posterior beams of Climacoceras gentryi alsomatches the dish shape of the Prolibytherium plate-like append-ages. The cranial appendages of Climacoceras are similar tocervids in shape but have no basal burr, negating a cervid typedetachment. Pickford et al. (2001), Morales et al. (2003), andSánchez et al. (2010) placed Prolibytherium near Climacoceras.If the presumed cervid-like-shaped cranial appendage fromGebel Zelten (specimen NHM M 26690) is the female ofProlibytherium, as Sánchez et al. (2010) have proposed, thenthe affinity of Prolibytherium to Climacoceratidae is strength-ened. However, evidence strongly suggests that early Miocenefemale giraffoids would have been hornless (Solounias 2007).We feel that the specimen described as a female Prolibytherium(NHM M 26690) is more likely a male representing a newtaxon. We agree, however, with the similarities proposed be-tween theClimacoceras andProlibytherium cranial appendages.

Functional Implications of the Fused Condyles

The atypical morphology of the occipital condyles inP. magnieri and P. fusus has functional implications. We hy-pothesize that the fusion and increase in thickness on theventral margin of the occipital condyles allows for morecentralized range of motion and support of the head. Theconvergence of the condyles is so complete that there is novisible fusion line, indicating that the merging happens earlyin ontogeny. This thickened increase in central bony materialsuggests a median plane focus of forces during combat. Thefusion also provides a greater articular surface area betweenthe occipital condyles and the atlas during flexion of the head.Hamilton (1973) also described large anterior swellings on thebasioccipital region of P. magnieri, which according to Mead(1906), provided strength and support to the atlanto-occipitaljoint during combat. In addition, the deep groove extendingfrom the level of the alisphenoid canal to the posteriorbasioccipital tuberosities, which is absent in other ruminants,appears to isolate the basioccipital bone from the temporalbone laterally. Presumably, this is congruent with a more cen-tralized modality of fighting. Lastly, the thickened dorsal andventral arches of the atlas would provide further support to thestrengthened atlanto-occipital joint.

Presumably, midline ventral fusion of the occipital condyleswould increase the range of motion for rotation of the head.Conversely, dissections of the atlanto-occipital bone of severalsheep specimens revealed that rotation is limited by a V-shapednotch on the anterior articular facets of the atlas, which articu-lates with the junction between the posterior and ventral artic-ular occipital condylar surfaces. This barrier to rotation preventsthe articular surface of the atlas from reaching the midline of theoccipital condyles, regardless of midline fusion. TheProlibytherium occipital condyles, however, have a roundedjunction between the posterior and ventral articular surfaces,

and a widened notch on the atlas, suggesting that their rotationrange of motion was in fact likely increased.

Relationship of Cranial Appendages to the SpecializedAtlanto-Occipital Joint

The approximation of the occipital condyles provides greatercentral support of the head, which is especially important forspecies with large and heavy cranial appendages.Prolibytherium magnieri possesses massive Bbutterfly wing^shaped appendages encompassing the dorsal surface of thefrontal, parietal, and occipital aspects of the skull (Sánchezet al. 2010). These large cranial appendages are orientedsomewhat parallel to the calvaria, and the surface extendsanterior to the orbit, lateral to the margins of the skull, andsignificantly posterior to the occipital. This plate-like orienta-tion of the cranial appendages is a unique feature ofProlibytherium and is not seen in any extant or extinct rumi-nant. The exceptionally large and flattened cranial appendageswere likely utilized in a specialized mode of combat requiringstrong reinforcement of the atlanto-occipital joint.

While the ventral fusion of the occipital condyles is a fea-ture unique to Prolibytherium , two specimens ofSchansitherium tafeli (AMNH 32505, HPM 6217) and onespecimen of Giraffokeryx punjabiensis (AMNH 19475) (be-longing to Giraffidae) exhibit approximated condyles, with anarrow intercondylar groove. Both of these giraffids possesstwo pairs of ossicones, potentially necessitating stronger sup-port of the atlanto-occipital joint. The Giraffokeryx specimenhas a pair of ossicones on the anterior frontal bone, and alarger pair of ossicones above the orbit. The posteriorossicones sit on an expanded boss and extend postero-laterally from the frontal bone (Solounias 2007). BothSchansitherium specimens have a small pair of ossicones ex-tending in front of the orbit, oriented anteriorly, and large,straight posterior ossicones situated above the orbit. The oc-cipital condyles of these three specimens are approximated onthe rostral ventral margin, separated only by a narrow groove,which is especially shallow in one Schansitherium specimen(HPM 6217) (Solounias 2007). The massive appendages ofProlibytherium, and the multiple pairs of ossicones seen inGiraffokeryx and Schansitherium likely require enhancedcranio-cervical support, contributing to the approximation ofthe occipital condyles.

Acknowledgments We acknowledge the NYIT-COM AcademicScholars program. We thank Pip Brewer and Jerry Hooker at the NHM.We thank Eileen Westwig, and the Departments of Mammalogy andPaleontology of the AMNH for access to specimens. We also thank TaoDeng, Zhanxiang Qiu, BanyueWang, and Sukuan Hou at IVPP.We thankJohn Barry for discussions on the systematics and geologic information.We also thank Larry Flynn and Michelle Morgan and the Peabody Mu-seum of Harvard. We also thank Everett Lindsay. Funds were covered byNS.

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