Acleistochelys, a new side-necked turtle (Pelomedusoides, Bothremydidae) from the Paleocene of Mali

2007·1.3 MB·English

by  Gaffney, Eugene S.

A tamerican museum
Novitates
PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024
Number 3549, 24 pp., 9 figures, 2 tables January 15, 2007
Acleistochelys, a New Side-Necked Turtle
(Pelomedusoides: Bothremydidae) from the Paleocene
of Mali
EUGENE S. GAFFNEY,1 ERIC ROBERTS,2 FAMORY SISSOKO,3
MOHAMED L. BOUARE,4 LEIF TAPANILA,5 AND MAUREEN A. O’LEARY6
ABSTRACT
The Paleocene Teberemt Formation south of the Adrar des Iforas Mountains, between
Saguirilidad and In Fargas, Mali, yielded a nearly complete skull of a new genus and species of
side-necked turtle, Acleistochelys maliensis. Acleistochelys is a member of the family
Bothremydidae Baur, 1891, because: (1) the fossa precolumellaris is absent, (2) the foramen
stapedio-temporale faces anteriorly, (3) the eustachian tube is separated from the stapes by bone,
and (4) an exoccipital-quadrate contact is present. Within the Bothremydidae, Acleistochelys
belongs to the tribe Taphrosphyini because: (1) the maxilla-quadratojugal contact is absent, (2) the
palate is dorsally arched, (3) there is only a small contribution of the palatine to the triturating
surfaces, and (4) the septum orbitotemporale is at least partially open. Acleistochelys is most
closely related to Azabbaremys because both share a narrow vomer lacking a posterior attachment
to the palatines.
The specimen was found in a marine limestone associated with crocodiles, echinoids, and
mollusks.
1 Division of Paleontology, American Museum of Natural History ([email protected]).
2 School of Geosciences, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa
([email protected]).
3 Institut des Sciences Humaines, Bamako, Mali.
4 Ecole Nationale des Ingenieurs, Bamako, Mali.
5 Department of Geosciences, Idaho State University, Pocatello, ID 83209-8072 ([email protected]).
6 Department of Anatomical Sciences, Health Sciences Center T8 (040), Stony Brook University, Stony Brook, New
York, 11794-8081, USA ([email protected]).
Copyright © American Museum of Natural History 2007 ISSN 0003-0082
2 AMERICAN MUSEUM NOVITATES NO. 3549
INTRODUCTION po postorbital
pr prootic
The Bothremydidae is an extinct group of pt pterygoid
side-necked turtles recently reviewed and qj quadratojugal
expanded in content by Gaffney et al. (2006). qu quadrate
The purpose of the present paper is to name so supraoccipital
and describe a new bothremydid, based on sq squamosal
CNRST SUNY 199 from the Paleocene of vo vomer
Mali (Tapanila et al., 2004). The skull de¬
scription follows the outline in Gaffney et al. GEOLOGY
(2006: appendix 1) for ease of comparison
Geological Setting
with other bothremydids. Based on the phy¬
logenetic analysis of Gaffney et al. (2006),
During the latest Cretaceous (Campanian-
Acleistochelys maliensis is a member of the
Maastrichtian) through middle Eocene, the
tribe Taphrosphyini as characterized in
Trans-Saharan Seaway inundated portions of
Gaffney et al. (2006). Figures, descriptions,
central West Africa (Petters, 1979). In north¬
and references to all of the bothremydid taxa
ern Mali, three transgressive-regressive cycles
referred to in this paper can be found in
are recorded by a relatively thin (<100 m)
Gaffney et al. (2006). The taxonomy also
sequence of sedimentary strata dominated by
follows that reference. The skull here named
sandstones, shales, limestones, and phos¬
Acleistochelys maliensis is included in the data
phates (e.g., Radier, 1959; Bellion et al.,
set of Gaffney et al. (2006: appendix 3) and is
1989; Moody and Sutcliffe, 1993; Tapanila et
shown in cladograms in figs. 288-314 as
al., 2004). These units were deposited in
CNRST SUNY 199. The reader should see
terrestrial, marginal marine, and open marine
this work for further discussion of this
settings within and along the margins of the
phylogenetic analysis.
Trans-Saharan Seaway, and they preserve
a rich vertebrate and invertebrate fossil re¬
Institutional Abbreviations cord.
The Late Cretaceous-Paleogene Malian
AMNH American Museum of Natural
outcrops that are the focus of this study
History, New York
(fig. 1) are located in a narrow, half-graben
CNRST SUNY
Centre National de la Recherche structure, known as the Gao Trench or
Scientifique et Technologique, Bama¬ “Detroit Soudanais.” Paleontologic investiga¬
ko, Mali-Stony Brook University, tions of invertebrate faunas demonstrate that
New York the Trans-Saharan Seaway may have provided
FMNH Field Museum of Natural History, at times a dispersal route via the Gao Trench
Chicago
for marine fauna between the Tethys Sea and
South Atlantic Ocean (Kogbe, 1981; Moody
Anatomical Abbreviations and Sutcliffe, 1993). Today this vast, desolate
region is characterized by expansive low-relief
bo basioccipital
plateaus that are capped by resistant limestone
bs basisphenoid
beds. Small shifting dune fields and vast
ex exoccipital
stretches of desert pavement cover this region
fpcci foramen posterius canalis carotici
and obscure regional stratigraphic correla¬
interni
fr frontal tions.
ju jugal
mx maxilla Sedimentology of Mali 17
na nasal
op opisthotic Description
pa parietal
pal palatine The turtle specimen described herein was
pf prefrontal recovered from locality Mali 17, located south
pm premaxilla of the Adrar des Iforas Mountains, between
2007 GAFFNEY ET AL.: NEW SIDE-NECKED TURTLE FROM MALI 3
COMPOSITE
MALI 17 MEASURED SECTION
SECTION
II
10
—
UNIT 4
UNIT 3
UNIT 2
UNIT!
► 0 —I
THICKNESS
MS LS 5S CG
(METERS)
LEGEND
Lithology Fossils
- new turtle taxon
- limestone
- other reptiles
- shale - rays
Oriolampas michelini
-
- sandstone (Conical)
- sandstone w/ large - Oriolampas michelini
inclined foresets (Flat)
- interbedded m Linthia sudanensis
-
limestone & shale
Ostrea multicostata
-
,o*~ - phosphate
- nautiloid
conglomerate
- burrows
Fig. 1. Stratigraphic section of locality Mali 17, showing position of type specimen of Acleistochelys
maliensis n.gen., n.sp.
Saguirilidad and In Fargas. Mali 17 is situated chalky, micritic limestone with thin shale
within a 6-10 m thick sequence of richly partings (unit 3) overlies this unit and
fossiliferous phosphatic sandstones, shales, preserves multiple articulated and associated
and limestones. The sedimentology at Mali turtle remains, including the specimen de¬
17 is characterized by a basal 1 m thick white scribed in this report. Other well-preserved
to pale yellow phosphatic sandstone (unit 1) vertebrate and invertebrate fossils were also
with nodular chert concretions that grades recovered from this laterally extensive bed,
upward into a thin (0.5 m), dark gray sandy including the remains of crocodiles, encrusting
shale (unit 2) containing ray tooth plates and oysters (Ostrea multicostata), nautiloids
abundant bioturbation (fig. lb). A 3 m thick (Deltoidonautilus sp.), gastropods (Gistortia
4 AMERICAN MUSEUM NOVITATES NO. 3549
sp.), and echinoids (e.g., Linthia sudanensis, TRIBE TAPHROSPHYINI GAFFNEY, TONG, AND
Oriolampas michelini). The entire succession is MEYLAN, 2006
capped by a distinctive 2 m thick, cliff¬ SUBTRIBE TAPHROSPHYINA GAFFNEY, TONG,
forming, blocky, recrystallized limestone bed AND MEYLAN, 2006
(unit 4). This unit preserves rare, moldic
Acleistochelys, new genus
nautiloids and abundant trace fossils.
Type Species: Acleistochelys maliensis,
new species.
Interpretation
Distribution: Paleocene of Mali.
Mali 17 represents an upward fining, retro- Etymology: Aklystos, Greek for “shel¬
gradational marine sequence. Increasing water tered”, or “hollowed”, in allusion to the
depth is interpreted based on reduction of small, paired pits on the triturating surface;
siliciclastics and increasing purity of carbo¬ chelys, Greek for “turtle”.
nates upward through the sequence. The Diagnosis: A bothremydid pleurodire
fauna recovered from Mali 17 are consistent with these unique characters among the tribe
with relatively low-energy, normal marine Taphrosphyini: small pit formed by jugal,
conditions. maxilla, and palatine on triturating surface;
Strata of Mali 17 likely belong to the middle jugal exposed on triturating surface; accessory
to upper portion of the Teberemt Formation ridge present on anterior triturating surface;
of Moody and Sutcliffe (1991, 1993), which wide palatine-basisphenoid contact separating
they interpreted as Paleocene. Paleogene pterygoids on midline; supraoccipital-quad-
echinoderms, such as Oriolampas michelini rate contact present; basioccipital narrowly
and Linthia sudanensis, recovered from the enters condylus occipitalis; palatine-jugal con¬
turtle-bearing unit 3 preclude a Cretaceous age tact in small septum orbitotemporale. Other
assignment, while putative Eocene (Ypresian) distinguishing characters are: skull relatively
vertebrates documented from stratigraphically long and narrow; fossa pterygoideus deep and
higher phosphate conglomerates (fig. 1A; narrow as in Nigeremys but in contrast to
—68 m level) at nearby Tamaguilelt (Pascal Taphrosphys', foramen posterius canalis caro-
and Traore, 1989; Patterson and Longbottom, tici interni formed by pterygoid, basisphenoid,
1989; Moody and Sutcliffe, 1993; O’Leary et and quadrate in contrast to Phosphatochelys',
al., 2006) provide an upper age limit. Moody small remnant of septum orbitotemporale
and Sutcliffe (1993) also correlated the present consisting of ventral parietal process
Teberemt Formation with a similar strati¬ as in Phosphatochelys but in contrast to
graphic sequence on the west side of the Taphrosphys.
Tilemsi valley, near Tichet, which Bellion et al. Discussion: See table 1 for a comparison
(1989) dated as late Paleocene using benthic of genera in the tribe Taphrosphyini.
foraminifera and ostracodes. Based on faunal
analysis and detailed outcrop correlation with
strata exposed below and above Mali 17, we Acleistochelys maliensis, new species
tentatively agree with the Paleocene age
Type Specimen: CNRST SUNY 199
assignment of Moody and Sutcliffe (1991,
(figs. 2-5), partial skull including fragments
1993) for these deposits, although an early
of the lower jaw, lacking some of the left
Eocene age cannot necessarily be precluded.
temporal region, and both posterior parts of
the skull roof. Skull measurements are in
table 2. Associated shell fragments include the
SYSTEMATICS
anterior margin of the nuchal (fig. 6), neurals
one and four (fig. 7), and peripheral two
(fig. 7). A partial cervical vertebra (figs. 8, 9)
ORDER TESTUDINES LINNEAUS 1758 OR
BATSCH 1788 and some pelvic fragments are present.
Type Locality: Mali 17, located south of
INFRAORDER PLEURODIRA COPE, 1864
the Adrar des Iforas Mountains, between
FAMILY BOTHREMYDIDAE BAUR, 1891 Saguirilidad and In Fargas, Mali.
2007 GAFFNEY ET AL.: NEW SIDE-NECKED TURTLE FROM MALI 5
Horizon: Middle to upper portion of the Structures: The frontal in Acleistochelys
Teberemt Formation of Moody and Sutcliffe enters the orbital margin, similar to that seen
(1991, 1993). in Azabbaremys and Nigeremys.
Depositional Environment: Near shore
marine.
Parietal
Diagnosis: As for genus.
Etymology: Named for the country of Preservation: Both parietals are present
discovery. but damaged. The dorsal plate on the left side
Referred Material: None. retains only its anteromedial portion. The
Previous Work: CNRST SUNY 199 is right dorsal plate is more complete laterally,
included in the phylogenetic analysis of but it also lacks all of the temporal margin.
Gaffney et al. (2006). The processus inferior parietalis, however, is
nearly complete on both sides, the left one
having some breakage ventrally.
DESCRIPTION Contacts of dorsal plate: The dorsal plate
in CNRST SUNY 199 preserves the midline
Prefrontal
parietal contact and the medial part of the
Preservation: Both prefrontals are present: frontal contact anteriorly. The right parietal
the right one is nearly complete, the left is has the anterolateral contact with the post¬
missing some of its lateral edge. orbital preserved.
Contacts: The prefrontal in Acleistochelys Structures of dorsal plate: The degree of
has the same contacts as in Azabbaremys, temporal emargination in Acleistochelys, al¬
Nigeremys, and Arenila: prefrontal on midline, though not determinable exactly, was proba¬
maxilla anteroventrally, and frontal poste¬ bly not as extensive as in Nigeremys, because
riorly. There is no palatine or parietal contact. the remaining broken margin is relatively
Structures: The dorsal plate of the pre¬ thick and extends about as far as the natural
frontal in Acleistochelys is similar to that in margin in Nigeremys (and probably Arenila,
Azabbaremys, but not strongly convex dorsal- although that skull also lacks most of the
ly as in Azabbaremys. The dorsal margin of parietal dorsal plate).
the apertura narium externa in Acleistochelys The septum orbitotemporale (see Gaffney et
is well posterior to the ventral margin in al., 2006: fig. 78) in the tribe Taphrosphyini,
contrast to Azabbaremys in which the dorsal may be reduced or absent (Gaffney et al.,
margin is almost directly above the ventral 2006: character 28). This is best seen in
margin. Phosphatochelys (Gaffney et al., 2006: fig.
The size of the fossa nasalis and sulcus 202). However, the group that lacks the
olfactorius are similar in both Acleistochelys septum (that is, Taphrosphys, Labrostochelys,
and Azabbaremys. The ventral process of the Rhothonemys, Ummulisani) also lacks the
prefrontal is narrow in Acleistochelys. ventral parietal process seen in Phospha¬
tochelys (Gaffney et al., 2006: fig. 202). This
process is a remnant of the lateral wall of the
Frontal
septum orbitotemporale and reaches the pal¬
Preservation: The right frontal in CNRST atine above the base of the processus tro-
SUNY 199 is nearly complete; it is missing chlearis pterygoidei lateral to the sulcus
only its posterolateral corner. The left one is palatinopterygoideus. This process also occurs
missing most of its lateral half. in Acleistochelys, although there are differ¬
Contacts: The frontal in Acleistochelys ences from Phosphatochelys: In Acleistochelys
contacts the prefrontal anteriorly, the other it is thinner and longer, and contacts the
frontal medially, the postorbital posterolater- palatine only, whereas in Phosphatochelys it is
ally, and the parietal posteriorly, all as in wider and shorter and contacts the palatine
Azabbaremys, Nigeremys, and Arenila, and in anteriorly and the pterygoid posteriorly. The
contrast to Phosphatochelys and Rhothonemys, entire ventral portion of the septum orbito¬
which lack a postorbital contact. temporale remnant differs in Acleistochelys
AMERICAN MUSEUM NOYITATES NO. 3549
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AMERICAN MUSEUM NOVITATES NO. 3549
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Fig. 2. Acleistochelys maliensis n.gen., n.sp. Partially restored skull based on CNRST SUNY 199. A,
dorsal; B, ventral; C, right lateral.
and Phosphatochelys in the position of the damaged, the processus inferior parietalis in
jugal and palatine (see Jugal, Palatine). Acleistochelys seems to be relatively thin, as in
Whether this structure should be considered Azabbaremys, but still seems to contact the
homologous in Acleistochelys and Phospha¬ palatine, a condition that is unclear in
tochelys is a fascinating question. Azabbaremys.
Contacts of processus inferior parietalis: Structures of processus inferior parieta¬
In Acleistochelys the vertical wall of the lis: The foramen interorbitale is relatively
parietal contacts the pterygoid ventrally, large in Acleistochelys, as in Azabbaremys
the prootic posteroventrally, and the sup- and Phosphatochelys, and the processus in¬
raoccipital posteriorly. Although somewhat ferior parietalis is narrow, also as in
2007 GAFFNEY ET AL.: NEW SIDE-NECKED TURTLE FROM MALI 9
Fig. 3. Acleistochelys maliensis n.gen., n.sp. Partially restored ventral view of CNRST SUNY 199.
Azabbaremys. The foramen nervi trigemini, and the right preserves the anterodorsal
preserved on both sides, is formed by the usual margin. The medial process on both sides is
bones: parietal anterodorsally, prootic pos- present.
terodorsally, and pterygoid ventrally. Contacts of lateral plate: The jugal in
Acleistochelys has a long anteroventral contact
with the maxilla as in Azabbaremys and at
Jugal
least an anterodorsal contact with the post¬
Preservation: Neither jugal is complete, orbital, but the length of the bone is in¬
both are missing their posterodorsal margins. determinate. The jugal contacts the quadrate
The left jugal preserves the ventral margin, posteroventrally in a suture that is more
10 AMERICAN MUSEUM NOVITATES NO. 3549
Fig. 4. Acleistochelys maliensis n.gen., n.sp. Photographs of CNRST SUNY 199. A. dorsal; B, ventral;
C, anterior; D, right lateral; E, posterior; F, left lateral.