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Palaeoworld 17 (2008) 41–46

Research paper

The first described Arsinoitherium from the upper Eocene Aydim Formation of Oman: Biogeographic implications

Abdul Razak Al-Sayigh a, Sobhi Nasir a,∗, Anne S. Schulp b, Nancy J. Stevens c

a Department of Earth Sciences, P.O. Box 36, 123-Al-Khod, Sultan Qaboos University, Oman b Natuurhistorisch Museum Maastricht, De Bosquetplein 6-7, NL6211KJ Maastricht, The Netherlands

c Department of Biomedical Sciences, College of Osteopathic Medicine, 228 Irvine Hall, Ohio University, USA

Received 4 April 2006; received in revised form 8 March 2007; accepted 18 July 2007 Available online 3 August 2007

bstract

A new fossiliferous locality is discovered from the upper Eocene Aydim Formation, in Dhofar, Southern Sultanate of Oman. A left ulna of rsinoitherium is described, and cranial and postcranial specimens found in close proximity are referred to the same taxon. The locality is

romising for the recovery of additional fossil specimens. Moreover, the presence of Arsinoitherium in Oman is of biogeographic significance; as he Red Sea did not exist during the late Eocene, these large-bodied animals were able to freely travel between what is now the Arabian Peninsula nd continental Africa. ublished by Elsevier Ltd on behalf of Nanjing Institute of Geology and Palaeontology, CAS.

eywords: Arsinoitherium; Oman; Biogeography; Afro-Arabia; Aydim Formation

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. Introduction

Cenozoic fossil vertebrates from the Arabian Peninsula onvey an important contribution to palaeontological under- tanding, representing vertebrates that occupied a myriad of cological niches, large and small in body size, aquatic and ter- estrial in habitat. Yet despite the important geographic position f Arabia with respect to other landmasses, relatively little is nown about its early Paleogene large-bodied terrestrial mam- alian fauna. Arabian fossils are meaningful for unraveling

he Tertiary migrational patterns of different vertebrate clades mong the landmasses of Africa, Europe and Asia. During he Paleocene and Eocene, paleobiogeographic reconstructions redict broad similarities between the terrestrial vertebrates of ontinental Africa and the Arabian Peninsula. Subsequently,

arge-scale interchange of terrestrial faunas between Arabia and urasia took place beginning in the early Miocene via the colli- ion of Arabia and Asia (e.g., Whybrow and Clements, 1999).

∗ Corresponding author. E-mail address: [email protected] (S. Nasir).

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871-174X/$ – see front matter. Published by Elsevier Ltd on behalf of Nanjing Insti oi:10.1016/j.palwor.2007.07.005

Until recently, regional biogeographic inferences were based argely on Arabian marine sequences, and from terrestrial faunas ocumented in the better-known palaeontological successions n neighboring landmasses. As evidence of Mesozoic verte- rates has grown rapidly in recent years from continental Africa, adagascar, and India (e.g., Krause et al., 1997, 1999, 2006; ay et al., 1999; Whybrow and Hill, 1999; Sanders et al., 004; Sereno et al., 2004; Durand, 2005; O’Connor et al., 006), vertebrate fossils (dinosaur, turtle and crocodile) have lso been recovered from the Late Campanian–Maastrichtian l-Khod Conglomerate of the Sultanate of Oman (Schulp et al., 000; Nolan et al., 1990; Buscalioni et al., 2004). Yet whereas ligocene mammals and other vertebrates have been described

rom the Dhofar region of Oman (e.g., Thomas et al., 1999), the ajority of Tertiary vertebrate research on the Arabian Peninsula

as focused upon Miocene and later deposits in Saudi Arabia Anon, 1975; Whybrow and Hill, 1999) and the United Arab mirates (Whybrow and Hill, 1999).

The tropical shelf deposits of Oman offer a promising rospect for documenting intermediate stages of mammalian volution during the Eocene. In this paper we describe an rsinoitherium specimen collected from within the Eocene

tute of Geology and Palaeontology, CAS.

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ydim rock Formation in Dhofar region of Southern Oman Fig. 1). Oman’s location between Egypt and India provides n important data point, facilitating more robust comparisons ith Arsinoitherium and other Tethyan vertebrates known from

ontinental Africa. This discovery has implications for under- tanding past environments and biotas, filling a gap in the Eocene fro-Arabian fossil record.

. Stratigraphy and geological setting of the Aydim ocality

In the eastern part of the Gulf of Aden, rifting of the Afro- rabian plate began during the Oligocene and continued until

arly Miocene, followed by oceanization (Lepvrier et al., 2002). fter the final regression of the Cretaceous sea, a new major

ransgressive–regressive series was deposited during the Ter-

iary in southern Oman (Platel et al., 1992). This depositional eries involves three sedimentary groups (Platel et al., 1992; latel and Roger, 1989; Lepvrier et al., 2002) corresponding to re-rift, syn-rift and post-rift stages of deposition (Fig. 2): the

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Fig. 1. Location of t

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adramawt, Dhofar and Fars groups, respectively. Specimens etailed herein were discovered within the Hadramawt Group, brief description of which follows.

The Hadramawt Group, Paleocene (Thanetian) to late Eocene Priabonian) in age, rests unconformably upon Cretaceous strata. n its type area, the Hadramawt Group constitutes an exten- ive sequence of typical carbonate shelf deposits comprising our formations. The Umm Er Radhuma Formation (Beydoun, 964), up to 600 m thick, is a late Thanetian–middle Iler- ian limestone with shale intercalations and chert nodules. The atest early Lutetian–Bartonian Dammam Formation, and the artonian–Priabonian Aydim Formation each reach more than 00 m in thickness. The thin (60 m) dolomitic and evaporitic idal-flat deposits of the Rus Formation (late Ilerdian to Cuisian) eveloped in between. The Aydim Formation was defined in hofar in order to distinguish the upper Eocene limestone that

verlies the Dammam Formation (Platel et al., 1987; Roger et al., 989, 1992; Lepvrier et al., 2002). The marine limestone of the ydim Formation, 100–120 m thick, has very few equivalents in

he Arabian Peninsula, as much of the region was emerged dur-

he study area.

A.R. Al-Sayigh et al. / Palaeoworld 17 (2008) 41–46 43

relate

i c t t t o T g g t b s

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Fig. 2. Mesozoic and Cenozoic stratigraphic units and rift-

ng the late Eocene (Platel et al., 1987). In Somalia these deposits orrespond to the Karkar Formation (Merla et al., 1979). The ype-section for the Aydim Formation has been established in he Aydim area (Platel et al., 1987; Roger et al., 1992) where he formation consists of four members of contrasting lithol- gy: the Heiron, Moosak, Tagut and Haluf members (Fig. 3). he basal very thin (1–5 m) Heiron Member comprises a soft reen marl, occasionally transitioning into beds of very fine,

reen mudstone and pink sandy siltstone (2 m). Above this is he Moosak Member, a 20–30 m thick unit of thin-bedded beige, ioclastic limestone, with an interval of cross-bedded yellowish iltstone and calcarenite, rich in Nummulites. The macrofauna

Fig. 3. Lithostratigraphic log of the Aydim Formation.

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d sequences in Southern Dhofar (after Roger et al., 1989).

f these lagoonal facies comprises branched and solitary corals, astropods, bivalves, and echinoids. The calcarenitic base of the agut Member, some 6 m thick, is easily recognizable and con-

ains numerous foraminifera, particularly Nummulites. Above his is a thick sequence (50 m) of thin-bedded, whitish, bioclas- ic or micritic, generally strongly recrystallized limestone, rich n echinoids, corals and foraminifera. The overlying Haluf Mem- er, about 25 m thick, consists of alternating soft, white to yellow halky marl, and fairly indurated beige to brown biomicritic imestone. In this area, the upper part of the Aydim Formation has characteristic stepped morphology. Brown algal laminations

re abundant at the top of the formation.

. Arsinoitherium: an intriguing herbivore

Arsinoitherium is an extinct mammalian taxon known pri- arily from the late Eocene and early Oligocene of Egypt and thiopia (e.g., Andrews, 1906; Sanders et al., 2004). The genus

s a member of the enigmatic group, the Embrythopoda, of which he phylogenetic relationships with other early Cenozoic mam-

alian groups remain poorly understood (e.g., Tabuce et al., 007). Once allied with hyraxes (Andrews, 1906), more recent tudies group the embrythopods most closely with either the pro- oscideans (Court, 1993) or the tethytheres (Gheerbrant et al., 005). The genus Arsinoitherium was first described by Beadnell 1902) from the Jebel Qatrani Formation, Fayum Depression f Egypt, a geologic unit comprised of ∼350 m of fluvial andstones, mudstones, siltstones and conglomerates, and esti- ated at 31 Ma of age (Seiffert, 2006). Subsequent discoveries

f Arsinoitherium indicate that this animal lived throughout frica browsing in warm, humid, highly vegetated environments

Moseley, 1988; Kappelman et al., 2003). This taxon has been

econstructed as a massive, graviportal herbivore with forelimbs dapted for strong forelimb retraction rather than adduction, fea- ures consistent with occupying a semi-aquatic habitat in which orelimbs provide strong forward propulsion during locomotion Court, 1993).

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. Systematic palaeontology

lass Mammalia Linnaeus, 1758 uperorder Paenungulata Simpson, 1945 rder Embrithopoda Andrews, 1906 amily Arsinoitheriidae Andrews, 1904 enus Arsinoitherium Beadnell, 1902

Arsinoitherium sp.

.1. Material

Well-preserved partial left ulna (SQU-287). Fragmentary cra- ial elements (SQU-289) and pedal phalanges (SQU-288) found n close proximity (∼distance; e.g., 0.5 m) to the ulna are here onservatively referred to the same taxon. Specimens are housed n the collections of the Sultan Qaboos University (SQU).

.2. Description

SQU-287 is a partial left ulna, robust in form, and mea- uring 38 cm in length (Fig. 4). The proximal portion of the pecimen exhibits a wide semilunar notch. A large rugosity is resent on the posterior aspect of the olecranon process, asso- iated with strong muscle attachments in life, although what emains of the proximal aspect of the process is badly abraded. ortions of the facets supporting both the medial and lateral umeral condyles are present, creating a saddle-shaped sur- ace separated by a V-shaped notch for the articulation with

he radius. As in other Arsinoitherium specimens (e.g., Court, 993), the proximal radius does not appear to have been fused ith the ulna. The ulnar diaphysis is flat anteriorly, but triangular

n cross-section owing to the prominent ridge running distally

t e O e

Fig. 4. Left ulna of Arsinoitherium (A) medial view; (B) an

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rom the olecranon process. The midshaft narrows, then expands istally; however, epiphyseal articulations for the carpus are ot preserved. SQU-287 differs from the similarly sized Paleo- astodon in having a more robust ulnar shaft that dramatically

xpands distally. Moreover, SQU-287 also lacks the deep radial roove running nearly the length of the diaphysis as observed in aleomastodon. Although lacking in specific autapomorphies,

he specimen is consistent in size and morphology with other pecimens attributed to the genus Arsinoitherium (Andrews, 906).

.3. Remarks

Currently there are two recognized species of Arsinoitherium, . zitteli from the Fayum of Egypt, and the larger A. gigan-

eum recovered from the Chilga region of northwestern Ethiopia. n additional species (“A. andrewsi”) has also been proposed, ased on molar size differences within the Fayum sample (e.g., ankester, 1903; Andrews, 1906). However, Sanders et al.

2004) argued that the range of variation in Fayum arsinoithere ooth size could comfortably be encompassed in a single, sexu- lly dimorphic extant herbivore species, and relegated all Fayum pecimens to A. zitteli. Due to the incomplete nature of SQU- 87, no attempt is made to assign it beyond the level of the genus t this time. Approximately 30 fragmentary specimens referable o Arsinoitherium have been recovered from the Aydim locality o date, only three of which are identifiable to element. Impor-

antly, more complete bones remain in situ and await continued xcavation. Hence more precise taxonomic placement of the mani arsinoithere material awaits the recovery of additional

lements.

terior view; and (C) lateral view. Scale bar = 50 mm.

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.4. Occurrence

The specimens described herein were discovered in the upper ocene calcarenite beds of the Moosak Member of the Aydim ormation (Fig. 3), west of Salalah; approximately 46 km from

he Yemen border close to Aydim (Fig. 1).

. Discussion and biogeographic implications

The location of Oman between Africa and Asia undoubt- dly made it an important portal for the global dispersal of early enozoic mammals. During the Cretaceous, the vertebrate com- osition of Oman included taxa reflecting clades that arose prior o the breakup of Gondwanan landmasses (e.g., crocodyliforms: uscalioni et al., 2004). Still attached to Africa during the Creta- eous and Paleogene, the Arabian Peninsula was mainly a broad, hallow, carbonate-dominated tropical shelf, similar to the South hina Sea today. Much of the region was covered by water ntil the late Eocene (Alsharhan and Nairn, 1997; Abed, 2005). eparation from what is now continental Africa began during

he Oligocene and continued until the early Miocene (Beydoun, 964, 1966, 1970, 1982; Abbate et al., 1988; Bosellini, 1992; antozzi, 1996; Fantozzi and Sgavetti, 1998; Lepvrier et al., 002). Southern Yemen, Oman, and northern Somalia together omprise the site of an Oligo-Miocene extension, related to rift- ng in the Gulf of Aden (Beydoun, 1964; Lepvrier et al., 2002). quivalent rift-related deposits are exposed along the conjugate argins of these regions today. As such, the period of rifting and

ctive faulting in the central part of Gulf of Aden lasted about 5 Ma, starting approximately 35 Ma in the late Eocene-early ligocene, climaxing around 5 Ma later and reaching comple-

ion with the deposition of the post-rift sediments beginning round 18 Ma (Lepvrier et al., 2002).

The recovery of Arsinoitherium from the Aydim Formation of outhern Oman supports the notion that during the late Eocene, he Red Sea did not yet provide a significant barrier to prevent nimals from freely crossing between the Arabian Peninsula and frica. The subsiding area between Dhofar and Somalia in the

ate Eocene provided a first stage in the opening of the Gulf of den due to rifting processes. This scenario explains the persis-

ence of marine deposits in this area, despite the general marine egression associated with a global fall in sea level beginning at he close of the middle Eocene (Roger et al., 1992, 1993). Until his time, the mammalian fauna is essentially African in char- cter (Whybrow and Hill, 1999), with subsequent increases in sian components, such as rodents (Whybrow and Hill, 1999).

. Conclusions

Based on a fairly complete left ulna and several associ- ted elements, the genus Arsinoitherium is reported from the aleogene Aydim Formation of Oman. This newly discovered ertebrate assemblage offers promise for comparisons among

aleogene Afro-Arabian faunas. Moreover, exposures in Wadi ydim vary from bioclastic limestone to calcareous sandstone, udstone and siltstones similar to those of the Fayum Depres-

ion of Egypt. These discoveries suggest that the Aydim locality

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n Dhofar preserves an Arabian snapshot of a littoral Fayum-like nvironment. At almost 36 Ma, the Omani mammal is perhaps ne of the older arsinoitheres in the world.

cknowledgments

We wish to thank Samir Hanna for his assistance with this ork. Thanks are also due to William Sanders for his help with

dentification of the specimen. We thank Marcelo Reguero, Eric oberts, Chen Siwei and an anonymous reviewer for construc-

ive comments.

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