Lucas, S. G., Hunt, A. P. & Lichtig, A. J., 2021, Fossil Record 7. New Mexico Museum of Natural History and Science Bulletin 82.
COMPLETE SPECIMENS OF THE EOCENE TESTUDINOID TURTLES ECHMATEMYS AND
HADRIANUS AND THE NORTH AMERICAN ORIGIN OF TORTOISES
ASHER J. LICHTIG1, SPENCER G. LUCAS1 and STEVEN E. JASINSKI2
1New Mexico Museum of Natural History, 1801 Mountain Road NW, Albuquerque, NM 87104, email: firstname.lastname@example.org; 2Department of
Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA 19104-6316 and Section of Paleontology and Geology, State
Museum of Pennsylvania, Harrisburg, PA 17120-0024.
Abstract—Newly described specimens of North American Eocene turtles provide valuable information
on their morphology and, more specically, variation, both intraspecic and ontogenetic. We describe
several complete and nearly complete testudinoid (Testudinoidea) specimens, including juvenile
specimens of Hadrianus corsoni, H. majusculus, Echmatemys haydeni and E. naomi. These specimens
help us determine that the oldest and most basal tortoises are from the western United States, suggesting
Testudinidae evolved in North America from one of the geoemydid-like forms in the genus Echmatemys,
which have their lowest stratigraphic occurrence in the earliest Wasatchian North American land-
mammal “age” (early Eocene, Ypresian, ~53 Ma).
The genus Echmatemys was rst named by Hay (1906)
for E. septaria from the early middle Eocene (Bridgerian
North American land-mammal “age”-NALMA) of Wyoming.
Over the next two decades this genus rapidly grew to include
all Eocene non-testudinid testudinoids in North America, with
approximately 20 named species (e.g., Hay, 1908; Roberts,
1962). Bridgeremys was separated from this group by Hutchison
(2006) and was suggested to be a stem-Rhinoclemmys or
ancestral to it.
Here, we describe three new specimens of Echmatemys
and Hadrianus, including cranial and juvenile material. We
also look at variation in a modern species (Gopherus agassizii)
to gain a better understanding of interspecic variation within
testudinids. In addition to providing further information about
these turtle taxa, we further explore the implications of these
taxa and variation for the origin of tortoises (Testudinidae).
Institutional Abbreviations: FOBU, Fossil Butte National
Monument, Kemmerer, Wyoming, USA; IGM, Geological
Institute of the Mongolian Academy of Sciences, Ulaan
Baatar, Mongolia; MSB, Museum of Southwestern Biology,
University of New Mexico, Albuquerque, New Mexico, USA;
NMMNH, New Mexico Museum of Natural History and
Science, Albuquerque, New Mexico, USA; TMP, Royal Tyrrell
Museum of Palaeontology, Drumheller, Alberta, Canada; UCB,
University of Colorado Museum of Natural History, Boulder,
Colorado, USA; USNM, United States National Museum of
Natural History, Smithsonian Institution, Washington D.C.,
USA; YPM, Yale Peabody Museum of Natural History, New
Haven, Connecticut, USA.
VARIATION IN EXTANT GOPHERUS AGASSIZII
To gain a better understanding of variation in fossil
testudinoids, it is important to understand the variation in modern
living members. The issues surrounding this shell variation are
important to understand testudinid taxonomy and phylogeny.
Vlachos and Rabi (2018) and Vlachos (2018) analyzed
testudinid relationships using a number of characters we suspect
have a much higher degree of polymorphism than is indicated
in their matrices. These were focused around the neural bones
and gular scutes of these tortoises. To provide some assessment
of the polymorphism in the characters of Testudinidae we
examined a sample of 17 Gopherus agassizii in the collections
of the Museum of Southwestern Biology to provide a baseline of
testudinid variability. Most recent phylogenies place the genus
Gopherus as sister to all extant testudinids other than Manouria,
making it a good example of a extant basal testudinid (e.g., Le
et al., 2006; Fritz and Bininda-Emonds, 2007; Mautner et al.,
2017; Takahashi et al., 2018; Vlachos and Rabi, 2018; Zhao et
al., 2020). The Gopherus agassizii specimens discussed here
come from Clark County, Nevada, USA, and are thus from a
relatively small geographic area, limiting the possible impact of
geographic variation on our assessment of intraspecic variation
(polymorphism) in this sample.
In this sample of Gopherus, the shape of the neurals is
highly variable, including every possible morphology in the
character matrix of Vlachos and Rabi (2018, see their characters
86–90). The contacts and shapes of the costals and neurals
vary considerably (Table 1). For example, neural 1 (character
86) is about equally often rectangular and hexagonal, and both
hexagonal short sides facing anterior and short sides facing
posterior were observed. Neural 2 (character 87) is more
consistent, either octagonal or hexagonal in shape with the short
sides facing anteriorly. Neural 3 (character 88) is extremely
variable, with shapes including rectangular, hexagonal, with
short sides anterior and posterior, or octagonal. Neural 4
(character 89) is rectangular, hexagonal, with short sides facing
anteriorly, or octagonal. Despite suggestions that the shape
controls the contacts (Vlachos, pers. com, 2018), the contacts are
more consistent than the shape, mainly varying in the contacts
of neurals 3-5.
It is worth noting that whereas the shape of the neurals
and their contacts with the costals vary, all specimens have
costal wedging present, as described by Hay (1908). Thus, we
conclude that the shape of the neurals and the presence of costal
wedging are two independently varying characters, contrary to
Vlachos and Rabi (2018). Furthermore, the gulars both cover
the entoplastron and not cover it in dierent specimens (Vlachos
and Rabi, 2018, character 123), as seen in Lichtig and Lucas
(2015c). The angle of the posterior corner of the gulars varies
from 75˚ to 135˚, covering two of the character states of Vlachos
and Rabi (2018, character 124). In no specimen we examined did
we nd the costal-peripheral suture and pleural-marginal sulcus
to not coincide, contrary to Vlachos and Rabi (2018, character
97). Indeed, our examination more than doubles the number of
polymorphic characters in Gopherus agassizii from ve to 11.
In addition, juvenile specimens examined have narrower
nuchals and entoplastra. Part of the lengthening of the
entoplastron is the retention of the long posterior process seen
in many ontogenetically older geoemydids and emydids but
absent in older testudinids (personal observation). This may
TABLE 1. Measurements of extant Gopherus agassizii from Clark County, Nevada, USA. Note the high variation in the neural formula as well as in the gular angle tting
all three states described in Vlachos and Rabi (2018). Less marked is the variation in the contacts of the neurals and the presence of an overlap of the gular scutes onto the
number Gender Neural formula Contacts Gulars overlap
N1 N2 N3 N4 N5 N6 N7 N8
MSB 66576 M No carapace preserved + 75°
MSB 66589 M No carapace preserved - 90°
MSB 66612 F 6A-8-4-8-47-8-4-6A 1 1, 2,
77 7, 8 - 130°
MSB 66597 F 4-8-4-8-4-6A-6A-4 1 1, 2,
55 5, 6 6, 7 8 left,
7, 8 right - 135°
MSB 66598 F 4-8-4-8-4-6P-6A 1 1, 2,
77, 8 N8 absent - 114°
MSB 66569 F? 4-8-6A-8-6A-8-6A-6A 1 1, 2,
55 5, 6 6, 7 7, 8 - 123°
MSB 66609 M 6P-8-6P-8-6A-6A-6P-4-4 1 1, 2,
77, 8 8 - Extra neural
MSB 66611 ? 4-8-6P-4-6A-6A-6A-6E-8 1 1, 2,
33, 4 4 4, 5 5, 6 6, 7 7 Extra neural
MSB 66563 ? 4-8-6A-8-4-8-6A-6A 1 1, 2,
76, 7 7, 8 +
and 5 do not
MSB 66554 ? 6P-8-4-8-6A-6P-6P-6A 1 1, 2,
55 5, 6 6, 7 7, 8
MSB 66538 F 4-8-6P-8-6A-6E-6A-4 1 1, 2,
55 5, 6 6, 7 7, 8 +
MSB 66557 M 4-8-6A-8-8-4-6P-6A-6A 1 1, 2,
55 5, 6 6, 7 7, 8 +
MSB 66556 M 6A-8-4-8-6A-6A-6A-6A 1 1, 2,
55 5, 6 6, 7 7, 8
MSB 66567 ? 6P-8-4-8-4-6A-6A-6A 1 1, 2,
55 5, 6 6, 7 7, 8
indicate that some of the traits associated with the Testudinidae
do not appear until later in testudinid ontogeny, while earlier in
ontogeny there is a more geoemydid-like morphology.
Vlachos (pers. com., 2018) states that he has seen similarly
high intraspecic variation in Chelonoidis. Chelonoidis is a
signicantly more derived taxon of testudinid, so, based on
phylogenetic bracketing, we consider this high variation likely
to be a trait of testudinids as a whole. This is not surprising
given that high intraspecic variation has also been noted in
the non-testudinid testundoids Geoemydidae (see Garbin et
al., 2018 and references therein) and Emydidae (e.g., Jasinski,
2018; Vamberger et al., 2020). Furthermore, as stated by
Bever (2008) and others, variability or the tendency to vary in
various anatomical structures is a heritable trait. This variation
was overlooked by Vlachos and Rabi (2018), which scores
only 0.02% of their character-state entries as polymorphic.
This results from their low sample sizes of extant testudinids,
averaging 3.85 specimens per species where specied, including
named references, where applicable.
Testudinidae Linnaeus 1758
Hadrianus (Cope, 1872)
Hadrianus majusculus (Hay, 1904)
1874 ?Hadrianus: Cope, p. 36
1904 Hadrianus Hay, p. 271
1906 Hadrianus: Hay, p. 374
1974 Geochelone (Manouria): Auenberg, p. 171
2015a Hadrianus: Lichtig and Lucas, p. 164
Referred material– UCB 76816, an incomplete shell,
consisting of posterior portions of the carapace and plastron,
and missing the anterior and posteriormost ends of the plastron,
as well as most peripherals, the nuchal, and the pygal from the
carapace (Fig. 2).
Diagnosis– Same as for genus.
Localities and age– Early Eocene Wasatchian (Wa-6) of
the San Jose Formation of the San Juan Basin, New Mexico,
USA and the Wasatchian (Wa-5) of the Willwood Formation of
Description– The holotype of Hadrianus majusculus,
YPM 2743 (Fig. 1), was described and illustrated by Hay (1904,
1908, g. 472, plate 59), obviating the need for a detailed re-
description, so only a brief overview is provided. The carapace
is 530 mm long and at least 440 mm maximum width. It is
crushed toward the left, with the midline greatly fractured as to
obscure the shape and proportions of most of the neurals (Hay,
1908). The nuchal is approximately equal in length and width.
Furthermore, the anterior portion of the two posterior neurals
is rectangular, while the posterior one is hexagonal with the
shorter sides facing anteriorly. The posterior of these may have
been what Hay (1908) referred to as a third suprapygal, which
we do not see in YPM 2743. The associated material includes an
additional hexagonal neural of uncertain placement.
The plastron is also 530 mm long; with 180 mm of this
made up by the anterior lobe, and 175 mm by the posterior lobe.
The anterior lobe is slightly constricted at the gular-humeral
sulcus, forming a very slight trilobed shape. The gulars overlap
the entoplastron in this specimen. The entoplastron is relatively
large and bell shaped, with the wider portion posterior. The
posterior border of the entoplastron is closely approached by the
humeral-pectoral sulcus overlapping the entoplastron slightly at
the midline. The shapes of the pectoral scutes are signicantly
dierent than those in Hadrianus corsoni, where the humeral-
pectoral and pectoral-abdominal sulci approach each other much
more closely just medial to the axillary notch. The posterior
lobe has a deep notch along the midline and a rounded posterior
margin of the xiphiplastron. The lateral margin is constricted at
the femoral-anal sulcus in a manner similar to the anterior lobe
at the gular-humeral sulcus.
UCB 76816 (Fig. 2) is an incomplete tortoise shell from the
early Eocene Willwood Formation of Wyoming. It is identiable
as Hadrianus majusculus based on several features, including:
(1) lack of costal wedging seen in other tortoises; (2) the
posteriorly concave pectoral-abdominal sulcus which is absent
in Echmatemys and Bridgeremys; and (3) the narrow base of the
anterior lobe of the plastron relative to the base of the posterior
lobe. The hyoplastron on UCB 76816 measures 20 mm long
on the midline (based on the right side). The hypoplastron is
21 mm long at the midline. The posteriormost portion of the
xiphiplastron is not preserved, so its length cannot be determined.
The hyoplastron has four scute annuli, suggesting that this was
an individual about 4 years old (see Zug, 1991).
The costals thicken lateral to the vertebral-pleural sulcus.
Given the young ontogenetic age of this specimen, it suggests the
absence of signicant ontogenetic variation in costal wedging in
Hadrianus majusculus. In addition, the striated surface sculpture
of this species, which UCB 76816 possesses, is unique among
the turtles of its type stratum, the San Jose Formation, making
recognition of even fragmentary specimens possible.
Remarks– We do not consider crushing or other taphonomic
deformation to be a likely cause of the lack of costal wedging in
this species as that is a change in length, and we see no evidence
of antero-posterior crushing in the individual elements in either
of these specimens.
Hay (1908) pointed out that Hadrianus majusculus
(YPM 2743) has unusual costal morphology relative to other
Testudinidae, and summarized it in a list of measurements of
the costal dimensions (Table 2). Only costal 7 is signicantly
narrowed proximally, rather than costals 2, 4, and 6, as in other
Testudinidae. We disagree with the assessment that Achilemys
is the most basal testudinid (Claude and Tong, 2004). Instead,
we consider the most basal testudinid to be Hadrianus given
the absence of the derived traits present in Achilemys and other
testudinids. These features include the lack of costal wedging
and the overlap of the pectoral scutes onto the entoplastron,
features not seen in any testudinid other than Hadrianus. It is
noted that more study needs to be done to better understand
the ontogenetic changes of testudinoids, including those in
Hadrianus corsoni (Leidy, 1871)
1871 Testudo corsoni: Leidy, p. 154
1873 Hadrianus corsoni: Cope, p. 631
Revised diagnosis (modied from Lichtig and Lucas,
2015b)– A species of the genus Hadrianus with pectoral scutes
barely posterior to the entoplastron. Abdominal scutes are less
anteriorly elongated along the lateral edge than in Hadrianus
TABLE 2. Costal measurements in millimeters from Hay (1908)
of the costals in the holotype of Hadrianus majusculus (YPM
Costal Width proximally Width distally
2 62 77
3 45 50
4 72 70
5 46 44
6 45 45
7 30 40
8 35 55
FIGURE 1. Hadrianus majusculus, holotype YPM 2743, nearly complete carapace and plastron: A, left lateral; B, dorsal; C,
anterior; D, ventral; and E, posterior views.
FIGURE 2. Hadrianus majusculus, juvenile, posterior portions of the carapace and plastron, UCB 76816: A, dorsal (of carapace)
and B, ventral (of plastron) views; and C–D, line drawings in C, dorsal (of carapace) and D, ventral (of plastron) views.
majusculus. Pectoral scutes are less than half as wide as the
abdominal scutes along the midline. Peripherals are lower
dorsally than in H. majusculus (Hay, 1908, p. 374). Furthermore,
unlike H. majusculus, the posterior sutures of the epiplastron
are nearly straight, and the costals show wedging in line with
the denition of Hay (1908). Additionally, the genus Hadrianus
diers from Echmatemys in the triangular shape of the postorbital
in contrast to its broad, trapezoidal shape in Echmatemys.
New material– A hatchling turtle identied by the authors
as Hadrianus cf. H. corsoni (FOBU 14015, Green River
Formation, Kemmerer, Wyoming, Figs. 3-4) is remarkably
complete, including the skull, carapace, posterior two-thirds of
the plastron, and incomplete limbs.
Skull– The skull (Figs. 3a, c, 4, A1) is 13 mm long and 9.7
mm wide, reaching a maximum width across the quadratojugals.
Due to the preservation of the specimen, the majority of details
in dorsal view are visible, but those laterally are obscured or not
visible. Dorsally, the frontal contributes to the orbit, forming the
posteromedial half of its dorsal margin. The frontals also have
a distinct anteromedial projection between the prefrontals. The
orbits are anteriorly placed just behind the anterior tip of the
rostrum. The parietals are large, forming most of the posterior
portion of the skull roof. The postorbital is large and expanded
laterally relative to Echmatemys, triangular, and sutured to
the parietal and frontal. It forms the entire posterior margin of
the orbit. The prefrontals are roughly rectangular, forming the
dorsal anterior portion of the orbital margin. The maxilla forms
much of the lateral margin of the nasal passage and the oor of
the orbit. The squamosal and quadrate suture lies about halfway
between the posterior margin of the postorbital and the posterior
extremity of the supraoccipital. The details of these bones are
obscured by crushing. The articular of each side of the mandible
can be seen below the skull about 3 mm lateral to either side
of the supraoccipital, with the left one being more abraded. No
retroarticular process is visible on either side of the skull.
Carapace–The carapace (Fig. 3) is 40 mm long and 31.3
mm wide, including most of the bones. It has the distinct medial
ridge seen in many juvenile testudinoids. The posterior three
FIGURE 3 (facing page). Hadrianus corsoni, articulated juvenile skeleton (still partially encased in matrix), FOBU 14015: A,
dorsal view, with image of specimen (left) and line drawing (right); B, only existing image of the plastron in ventral view; and
C, image of skull showing more of right lateral side, particularly region around the orbit. Abbreviations: c, costal; h, humerus; f,
bula; n, neural; nu, nuchal; p, peripheral; py, pygal; r, radius; t, tibia; u, ulna; and xi, xiphiplastron.
FIGURE 4. Hadrianus corsoni, juvenile skull, FOBU 14015, close up dorsal view of skull (left) and line drawing showing sutures of
the dorsal view of the skull (right). Abbreviations: ar, articular; fr, frontal; ma, maxilla; pa, parietal; pf, prefrontal; po, postorbital;
qu, quadrate; and sq, squamosal.
peripherals and the pygal are poorly ossied, and do not contact
each other. This resembles the anterior-to-posterior formation of
the peripheral bones noted in the embryos of Trachemys studied
by Gilbert et al. (2001). Costal wedging is present, and the
neurals and proximal portions of the costals show the typical
striated surface sculpture associated with Hadrianus. The more
distal portions of the costals, as well as the peripherals, show
a more typical juvenile turtle surface sculpture. The sulci are
deeply incised into the carapace. The nuchal is well developed,
contacting neural 1, both rst peripherals, and both rst costals.
Neural 1 is rectangular, measuring 6.3 mm long and 5.8 mm
wide, contacts costal 1, and is crossed by the vertebral 1-vertebral
2 sulcus. Neural 2 is hexagonal, measuring 2.5 mm long and
8.1 mm wide, contacts costals 1 and 2, and is not crossed by
any sulci. Neural 3 is hexagonal with short sides paralleling the
midline near the crossing of the vertebral 2-vertebral 3 sulcus. It
measures 5 mm long and 4.4 mm wide. Neural 4 is hexagonal,
with extremely short anterior lateral sides so as to appear
nearly rectangular, measuring 5 mm long and 5 mm wide. This
neural lacks any crossing sulcus and is in contact with costals
3, 4 and 5, although the contact with costal 5 is quite small.
Neural 5 is rectangular with very slight elongation anteriorly
and posteriorly along the midline. Neural 5 is crossed by the
vertebral 3-vertebral 4 sulcus. All three vertebral-vertebral sulci
in this specimen bow anteriorly as they cross the midline,
resulting in a slight “omega” shape. We consider that neural 6 is
taphonomically tilted nearly vertical, so some of its features are
obscured; but, it appears rectangular, with no apparent division
of the lateral sides. This allows us to conclude that the neural
formula up to neural 6 is 4-6A-6A-6A-4-4
The costals at their medial ends are 6.1, 3.1, 4.5, 3.2, 4
and 4 mm long, respectively. Costal 6 is missing the posterior
margin on both sides, so this is its preserved length rather than
its complete length. The distal rib tips (3.8 mm) are exposed
beyond the costal plates, with a slight elongation near the
inguinal buttress. The lateral margin of the carapace is slightly
concave between peripherals 5 and 6, although this lateral
pinching may be a taphonomic feature. The carapace reaches its
greatest width at the peripheral 6-peripheral 7 suture.
Plastron–The morphology of the hyoplastron and
hypoplastron is known thanks to the preparation of the ventral
side prior to recapping this side and preparing the dorsal side
(Arvid Aase, pers. com., 2017) (Fig. 3B). The plastron is missing
the epiplastron and entoplastron, but is otherwise complete.
Both the hyoplastron-hypoplastron and the hypoplastron-
xiphiplastron fontanelles are still open, with the former opening
being especially large. The midline suture is loose and unfused
in some areas. The inguinal buttress is well ossied between
the distal third of costals 5 and 6 and was pushed through the
carapace during burial and fossilization (Fig. 3a).
Caudal vertebrae–At least 13 procoelous caudal vertebrae
are present, forming a relatively long tail (Fig. A6). These are
slightly longer than tall, with the anterior and posterior processes
less developed in the more posterior vertebrae. This appears to
be almost the complete tail, potentially missing just the distal
caudal vertebrae. However, despite appearances, the tail in
FOBU 14015 is no longer relative to plastron width than that of
an extant male Pyxis planicauda (Bonin, 2007) or Agrionemys
horseldii (personal observation).
Forelimb–The forelimbs are nearly complete, but the
right humerus is partially covered by the anterior peripherals.
Both ulnae are 4.4 mm long with short, 6 mm long manus. The
distal phalanges each have a small exor tubercle (Fig. A2-
A3). The development of this exor tubercle is associated with
walking behavior and is absent in species that primarily move
by swimming (Lichtig and Lucas, in prep.). This corresponds to
some of the observations of patterns of muscle modications of
Abdala et al. (2008). The phalangeal formula based on the left
manus is 0-3-3-3-3.
Hind limb–The left hind limb is relatively complete,
including a 5 mm long tibia and a 6 mm long pes. The distal
phalanges of the pes lack a well-dened exor tubercle. The
phalangeal formula of the pes is approximately ?-?-3-3-3 (Fig.
A4-A5). Neither pes preserves the full digit 1 or 2.
Similar to that noted for Hadrianus majusculus above,
more study is needed to better understand the changes through
ontogeny of testudinids and Hadrianus, which may lead to more
information about what these ontogenetic changes mean for the
taxonomy and biology of these turtles.
Echmatemys Hay, 1906
1871 Emys: Leidy p. 367
1871 Emys: Leidy p. 366
1873 Emys: Cope p. 625
1906 Echmatemys Hay p. 448
1908 Echmatemys: Hay p. 308
Type species– Echmatemys septaria (= E. stevensoniana).
Included species– The type species (Echmatemys septaria),
together with Echmatemys wyomingensis, E. uintensis, E. naomi
and E. haydeni.
Revised Diagnosis– Testudinoid turtles with the following
unique characters: anterior three neurals longer than wide,
anterior lobe of plastron enlarged anterior to the hyo-epiplastral
suture, and musk glands usually present anterior to inguinal
Remarks– Our analysis supports the suggestion of J.
H. Hutchison (pers. com., 2015) that E. stevensoniana and E.
septaria are synonyms, as also suggested by Vlachos (2018).
Echmatemys haydeni (Cope, 1873)
1871 Emys haydeni Leidy p. 123
1871 Emys haydeni Cope p. 366
1871 Emys wyomingensis (in part) Leidy p. 367
1873 Emys wyomingensis (in part) Leidy p. 145 plate 9, g. 6
1902 Emys haydeni Hay p. 448
Revised diagnosis– A species diagnosed from other
Echmatemys by the long contact between marginal 1 and pleural
1 (Vlachos, 2018), the overlap of the posterior sulcus of vertebral
5 over the pygal, the lack of broadening of the suprapygal seen
in E. naomi and the large medial extent of the inguinal buttress.
Referred specimen– An articulated skull, shell and limbs
from the Green River Formation of Wyoming (TMP 2008.00.14,
Fig. 5) provides the rst documentation of the morphology of
the skull of Echmatemys haydeni. Identication of this specimen
was based on the long contact of marginal 1 and pleural 1 as well
as the overlap of the posterior sulcus of vertebral 5 over the pygal
not seen in E. lativertebralis. In addition, the large medial extent
of the inguinal buttresses indicated by the protrusion between
the costals, separating the costals nearly to the costal-neural
sutures, is unusual and identical on both sides. This is seen in E.
stevensoniana, E. septaria, and E. haydeni (Hay, 1908). We thus
conclude that this specimen represents E. haydeni.
Skull– The skull is 31 mm long and 21 mm wide at its
broadest point. It is dorsoventrally compressed, with most
elements below the skull table obscured. Conversely, the skull
table is complete with clear sutures. The frontals have greater
exposure along the orbital margin than in Hadrianus corsoni.
Furthermore, the postorbitals are reduced, with another bone
(possibly the jugal) forming much of the ventral posterior margin
of the orbit. The anterior process of the frontal is abbreviated
relative to that of Echmatemys naomi (discussed below). The
frontals lie rostrally, mainly between (medial to) the orbits, and
extend caudally slightly beyond the caudal edge of the orbital
rim. Additionally, the frontals in our specimen of E. haydeni
project posteromedially between the parietals, allowing the
parietals to project anterolaterally toward the orbital rim, also
leading to the reduced postorbitals. This also makes the parietals
prominent, extending between the orbits beyond their posterior
rim, distinctly farther than in H. corsoni and E. naomi (discussed
Carapace– The carapace is complete, measuring 142 mm
long, with the normal complement of scutes and bones seen in
Echmatemys as described by Hay (1908). Five or six growth
annuli are present on costal 1, indicating that this individual was
at least ve years old when it died (see Zug, 1991). Further study
is still needed to better understand the ontogenetic changes of
Echmatemys and what this may mean for these turtles.
Caudal vertebrae– The tail is more slender with smaller
anterior and posterior processes of the neural arches than in
Echmatemys naomi and slightly longer, at 29 mm length.
Limb Bones– The limbs are similar to, but more elongate
in Echmatemys haydeni than in E. naomi. The relative length of
the pes is 17.6% of the total carapace length in E. haydeni (TMP
2008.00.14) compared to 13.6% in E. naomi (FOBU 14014).
The pes in E. haydeni measures 25 mm long, but in E. naomi is
only 15 mm. The manus in E. haydeni is 12 mm long, similar to
11 mm in E. naomi. However, relative to total carapace length,
the manus is shorter in E. haydeni (TMP 2008.00.14) at 8.5%
compared to E. naomi (FOBU 14014) at 10%. This dierence
in hind limb morphology may indicate a dierence in ecology
between these two species. Conversely, the small sample size
may mean this simply reects two individual variants.
Echmatemys naomi (Leidy, 1871)
1908 Echmatemys naomi Hay p. 335, gs. 442-444
Revised diagnosis– A species of the genus Echmatemys
diagnosed from other species of Echmatemys by the presence
of a much longer than wide neural 1, the vertebral 4/vertebral
5 sulcus crosses the last neural, a wide suprapygal (Vlachos,
2018), and the lack of overlap of the pleurals over the nuchal.
Referred specimen– A nearly complete skeleton of
Echmatemys naomi from the Green River Formation of
Wyoming (FOBU 14014, Fig. 6A–C). The identication is
based on the lack of overlap of the nuchal by the pleurals, neural
1 much longer than wide, vertebral 4/vertebral 5 sulcus over the
last neural, and a wide suprapygal (based on Vlachos (2018) in
part). This conrms the association of an isolated skull (USNM
17999, Fig. 7A-D), lacking the lower jaws, from the Bridger
Formation in Wyoming, with this species.
Description of new material– FOBU 14014 is a nearly
complete individual carapace with a length of 110 mm. The
rst costal shows four growth lines, so we estimate this turtle’s
ontogenetic age at death as approximately four years (see Zug,
1991). The carapace is similar to other published Echamtemys
naomi (e.g., Hay, 1908). The caudal vertebrae outline a slightly
shorter tail (28 mm long) than in E. haydeni (TMP 2008.00.14),
but it is more robustly constructed, with large anterior and
posterior processes relative to either E. haydeni (TMP
2008.00.14) or Hadrianus corsoni (FOBU 14015).
The skull is quite distinctive, being more robust (22.7
mm long and 16 mm wide) than Echmatemys haydeni (TMP
2008.00.14), with more pronounced prefrontals. The frontals
have a more pronounced and larger anteromedial process
along the midline reaching between the prefrontals. The right
prefrontal is dislocated toward the left, covering part of the
anterior process of the frontals. Furthermore, the postorbital
is expanded, excluding the jugal from the dorsal margin of
the orbit. The parietals form a posteriorly concave suture
with the frontals, rather than being anteriorly concave with a
posteromedial projection of the frontals between them, as in E.
USNM 17999, an isolated skull (Fig. 7), was long suspected
(e.g., Hirayama, 1984) to belong to Echmatemys, but until the
discovery of an articulated specimen this could not be conrmed.
Now that an articulated E. naomi has been identied (FOBU
FIGURE 5. Echmatemys haydeni, articulated skeleton (still partially encased in matrix), TMP 2008.00.14 in dorsal view: Line
drawing of dorsal aspect of skull based on original specimen (upper left); dorsal view of a cast of the specimen (lower left); line
drawing of dorsal view of full specimen (right). Abbreviations: c, costal; fr, frontal; j?, ?jugal; ma, maxilla; nu, nuchal; pa,
parietal; pf, prefrontal; ; p, peripheral; po, postorbital, py, pygal; spy, suprapygal.
14014), USNM 17999 can be referred to an individual of the
same species. It is nearly identical in the overlapping portions to
the skull in FOBU 14014, in the morphology of the skull table,
and conrms some of the features discussed in FOBU 14014.
Based on this cranial material, compared to E. haydeni, E. naomi
has a distinctly enlarged frontal. The frontal extends distinctly
caudal to the orbit, particularly posterolaterally. This also leads
to reduced parietals, mainly in regard to their length, in E.
naomi compared to E. haydeni.
USNM 17999 also preserves additional details of the
remainder of the skull not visible in FOBU 14014. The maxilla
in USNM 17999 has a single triturating surface and forms the
anterior portion of the choana. The premaxillae are small and
do not reach the choana. Instead, the vomer forms the anterior
midline of the choana. The palatines form the posterior end of
the choana and the medial edges of the suborbital fenestra. The
pterygoids form only the posterolateral rim of the suborbital
fenestra. Posteriorly, the pterygoids have a long, oblique suture
with the pterygoid process of the quadrate and the basisphenoid.
The quadrate is restricted to the posterior half of the fossa
temporalis inferior. The basioccipital is well developed, forming
the posteriormost preserved portion of the skull.
Remarks– The limbs of FOBU 14014 are similar to those
described by Hay (1908, p. 297, g. 367–374) for an indeterminate
Echmatemys, (YPM VPPU 011525), and the skull associated
with these limbs (Hay, 1908, p. 297, pl. 45, gs. 11–13) is quite
similar to the isolated adult skull described above (USNM
17999). These specimens were not associated with any shell
material, so they could not previously be condently identied
without an articulated specimen of E. naomi for comparison.
Although the frontal contribution to the orbital rim is slightly
more prominent in YPM VPPU 011525 than in USNM 17999,
other morphologic features agree between the two specimens.
Therefore, we consider the skull and limbs (YPM VPPU 011525)
Hay (1908) reported to be from E. naomi as well. With FOBU
14014 identied as an ontogenetically younger individual, more
study is needed to better understand the ontogenetic changes of
Echmatemys and E. naomi, in addition to comparing these to the
ontogenetic changes in other testudinoids.
ECOLOGY OF EARLY NORTH AMERICAN
Following the end-Cretaceous mass extinction, no terrestrial
families of turtles survived, with the possible exception of
meiolaniids in Australia and South America (Lichtig and
Lucas, 2016). It has also been suggested that lindholmemydids
FIGURE 6. Echmatemys naomi, articulated skeleton (still partially encased in matrix), FOBU 14014, in dorsal view; A, magnied
line drawing of skull; B, Line drawing of specimen , and C, Image of complete specimen . Abbreviations: c, costal; fr, frontal; j?,
?jugal; ma, maxilla; nu, nuchal; pa, parietal; pf, Prefrontal; ; p, peripheral; po, postorbital, py, pygal; spy, suprapygal.
might be an exception to this. We do not consider this likely,
as the only published account of lindholmemydid (Mongolemys
elegans) paleoecology by Cadena et al. (2013) suggests that
these are aquatic to semi-aquatic turtles. Furthermore, based on
the methods in Lichtig and Lucas (2017), we examined other
lindholmemydids based on published reconstructions from
Danilov (2003). This resulted in carapace-width-to-plastron-
width ratios of 2-3, higher than any living terrestrial or even
most semi-aquatic turtles. In addition, based on the statements
and images of Cadena et al. (2013), the paratype of Mongolemys
elegans (IGM 90/11) has the elongate femoral head suggested
by Zug (1971) to be associated with swimming behavior.
Thus, the end-Cretaceous extinction left at least the northern
continents devoid of terrestrial turtles, possibly until the Eocene.
The Paleogene transition by turtles to terrestriality was more
challenging than it may at rst seem (Natchev et al., 2015).
Thus, Natchev et al. (2015) assert that most aquatic turtles nd
it dicult or impossible to swallow in the air (respire) on land.
Furthermore, Natchev et al. (2015) suggest that testudinoids
may have been uniquely preadapted to ll this niche, given their
well-developed muscular tongues. This may be why all extant,
fully terrestrially adapted turtles are testudinoid turtles.
Basal testudinids have long been assumed to have an
ecology similar to extant testudinids. This has been dicult
to demonstrate given the absence of cranial or articulated limb
material for Hadrianus. This assumption of uniform testudinid
ecology is challenged by two facts. First is that Manouria
emys, the most basal extant testudinid, has a distinct ecology
(Natchev et al., 2015). This includes a distinct mode of food
apprehension, breeding behavior, and a greater willingness to
attempt subaqueous food capture. The second is that Scheyer
and Sander (2007) found the histology of Hadrianus more
in line with that of a semi-aquatic to aquatic turtle. This was
suggested to be a carry over from their aquatic ancestors, but
may reect a more aquatic habitus in Hadrianus than previously
recognized. This histology is similar to that observed for
FIGURE 7. Echmatemys naomi, isolated skull, USNM 17999,
in: A, dorsal and B, ventral views; C, line drawing of skull in
dorsal view showing sutures; and D, line drawing of skull in
ventral view showing sutures. Abbreviations: bo, basioccipital;
bs, Basisphenoid; c, choana; f s-t, foramen stapedio-temporale;
fr, frontal; fpcci, foramen posterius canalis carotici interni; ma
, maxilla; a, parietal; pf, prefrontal; pl, palatine; po, postorbital;
pt, pterygoid; qj, quadratojugal; qu, quadrate; s, suborbital
fenestra; sq, squamosal; vo, vomer.
Rhinoclemmys pulcherima, which is aquatic as a hatchling and
becomes terrestrial later in life (Webb, 2010). This could serve
as a transitional form in the path to a fully terrestrial habitus.
Hadrianus are near the base of a long stem leading to crown
testudinids, so we need not assume they were exactly like extant
or even fossil crown testudinids in their ecology. More material,
particularly of the intermediate-sized individuals, is needed to
further test this hypothesis.
AGE OF THE OLDEST TORTOISE
The oldest tortoises are stem-Testudinidae from the early
Eocene of North America. The rst recognition of Eocene
tortoises in North America was that of Hadrianus majusculus by
Cope (1874) from the late Wasatchian (Wa-5) of North America
(Lichtig and Lucas, 2015a) (Fig. 8). This is the oldest reported
FIGURE 8 . Correlation chart showing the relative ages of
proposed earliest testudinids (Testudinidae) and stem-testudinids.
The tortoises with question marks indicate the possible ages of
the unpublished Mongolian material. Correlations based on
Lucas et al. (2003) and Smith et al. (2014). Abbreviations: Br,
Bridgerian; LMA, land mammal age; MP, Mammal Paleogene
zones; Wa, Wasatchian.
pantestudinid. Echmatemys comes from the same stratum.
Echmatemys has been reported as far back as the base of the
Wasatchian (Holroyd et al., 2001) (Fig. 8).
Purported stem-Testudinidae and -testudinids have been
reported from the Eocene of Asia and Europe. This includes
the recent revision of Anhuichelys (Tong et al., 2016) that
concluded that this genus is a stem testudinid, rather than
an emydid as previously proposed (Yeh, 1979). However,
identifying Anhuichelys as a stem-testudinid was based on
the false assumption that costal wedging is a unique trait of
Testudinidae. In fact, it is known to occur in some emydids
(e.g., some Terrapene specimens), making it more widespread
in Testudinoidea. Furthermore, the shortened pectorals of
Anhuichelys referenced by Tong et al. (2016) are actually
relatively long compared to modern tortoises and not distinctly
shorter than are found in emydids. The reputed hinging of the
plastron is also questionable, as no undisputed testudinid has a
kinetic anterior lobe, though this occurs in some members of both
Emydidae and Geoemydidae, in addition to some other families
such as Kinosternidae. Finally, the phylogenetic analysis of
Tong et al. (2016) is suspect given that, by their own statement,
minus either of two characters in their analysis –“Vertebrals 2-4:
0 wider than long, 1 longer than wide” and “Suture between the
epiplastron and hyoplastron: 0 nearly perpendicular to body axis
or backward laterally, 1 forward laterally”– the analysis nds a
sister group relationship of Anhuichelys with Platysternon. This
clade was found to be sister to both basal testudinoids included
in the analysis (Lindholemys and Mongolemys). Therefore, we
conclude that Anhuichelys has no relevance to the origin of the
Testudinidae. More recently, Vlachos and Rabi (2018) found
that Anhuichelys was clearly not a testudinid, nding some ties
to the Emydidae as a sister taxon to the Trachemys+Chrysemys
clade. This agrees with the original placement suggested by Yeh
Purported stem-Testudinidae have long been mentioned
from Mongolia but have never been published (Holroyd and
Parham, 2003). These come from the “Naran Bulak Formation
of Khaichin Ula IV and the Tsagan Khushu localities” (Igor
Danilov pers. com., 2016). Tsagan Khushu is of late Paleocene
to early Eocene age in the Naran Bulak Svita, whereas Khaichin
Ula IV is of middle Eocene age in the Khaychin Svita (Russell
and Zhai, 1987) (Fig. 8). Without a published record it is dicult
to assess this report.
Perez-Garcia et al. (2016) revised Europe’s oldest tortoises,
naming Fontainechelon for the species “Achilemys” cassouleti
(Claude and Tong, 2004). Perez-Garcia et al. (2016) suggested
the specimen is from the early Eocene (MP 8–9), roughly
equivalent to the late Wasatchian NALMA. This is counter to the
original description, which placed it in MP 10, or approximately
the Bridgerian NALMA (Fig. 8). There appears to be some
confusion between the sites in that region of France. Thus,
the application of the age of the “La Borie” locality to these
specimens, which are from a dierent locality (“Saint-Papoul”),
is not justied (Danilo et al., 2013). This is compounded by
the fact that the compared North American taxa, Hadrianus
majusculus and H. corsoni, are falsely stated to be signicantly
younger than they are and contemporaneous, although they are
of dierent ages. Thus, Fontainechelon from France is younger
than Hadrianus from North America.
We do not agree that Fontainechelon is the most basal
testudinid, an assessment based on “the presence narrower
and longer gular scutes; “wavy” humero-pectoral sulcus with
medial part being convex anteriorly” (Vlachos and Rabi, 2018).
Achilemys and Hadrianus have similar-sized gular scutes, each
occupying approximately one-third of the length of the midline.
The dierence as coded seems to rely on the overlap (or lack
thereof) of the gulars onto the entoplastron, rather than their
size. Our examination of extant testudinids (see above) suggests
that while the proportions are fairly stable, the overlap of the
entoplastron is highly variable. The “wavy” sulcus is coded as
present in a wide variety of taxa in the Emydidae, Geoemydidae
and Testudinidae. There are a number of distinct morphologies
within this, most of which are not particularly close to what is
seen in Hadrianus. The closest comparison we are able to nd
is Rhinoclemmys pulcherima, a basal geoemydid close enough
to be included in Testudinidae in some past molecular analyses
(e.g., Spinks et al., 2004). It is also worth noting that while this
character is quite distinct in mature individuals, it is completely
absent in a three-month-old hatchling of R. pulcherima that
has a transverse humeral-pectoral sulcus (pers. obs.). The more
derived taxa within Testudinidae coded as possessing this
characteristic are quite distinct, having a much lower degree
of curvature to the humeral-pectoral sulcus and lacking the
recurving to concave curvature anteriorly around the midline.
The hypothesis that Testudinidae entered North America
from Asia during the second thermal maximum of the Eocene
period (~52 Ma) was rst proposed by Hutchison (1980). This
was expanded upon in a subsequent review of Eocene turtle
biostratigraphy and biogeography (Hutchison, 1998) and a
review of the Willwood Formation turtle fauna (Holroyd et
al., 2001). Hutchison (1980, 1998) and Holyroyd et al. (2001)
identify three important events: (1) Echmatemys, in the
broadest taxonomic sense, appears in North America by Wa-
0; (2) Baptemys appears, likely an immigrant from south of
Texas, at Wa-5; and (3) at this same horizon in the Willwood
Formation, Testudinidae make an abrupt rst appearance. We
conclude Echmatemys (sensu Hay 1908) is an amalgamation
of geoemydids including at least three distinct groups. These
groups appeared in North America around the Paleocene-Eocene
thermal maximum and emigrated from North America to Europe
and Asia by the earliest Bridgerian.
1. Complete articulated skeletons are described for
Hadrianus corsoni, Echmatemys haydeni, and E. naomi,
together with an incomplete shell of H. majusculus.
2. The appearance of stem-Testudinidae in North America
does not support the hypothesis of Hadrianus emigrating from
Asia. Furthermore, this helps explain the lack of precursors to
Testudinidae in Paleogene rocks of Asia.
3. Given the earlier records relative to other continents of
undisputed tortoises (e.g., Hadrianus), as well as the presence of
stem members, Testudinidae likely originated in North America,
subsequently immigrating to Europe and Asia during the late
We thank Arvid Aase and the sta of Fossil Butte National
Monument for providing access to their specimens for study. We
also thank Tonia Culver and the University of Colorado Museum
of Natural History for access to their collections for this project.
We thank Evan Vlachos, Edwin Cadena and an anonymous
reviewer for their helpful reviews of an earlier version of this
manuscript. The current version of the manuscript was reviewed
by Daren Riedle, Adrian P. Hunt and Carl J. Franklin whose
comments improved the content and clarity of this manuscript.
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FIGURE A1. Hadrianus corsoni, FOBU 14015: Close up of dorsal view of the skull.
FIGURE A2. Hadrianus corsoni, FOBU 14015: Right forelimb.
FIGURE A3. Hadrianus corsoni, FOBU 14015: Left manus and distal radius and ulna.
FIGURE A4. Hadrianus corsoni, FOBU 14015: Close up of the left pes.
FIGURE A5. Hadrianus corsoni, FOBU 14015: Close up of right pes, tibia, and bula.
FIGURE A6. Hadrianus corsoni, FOBU 14015: Close up of caudal vertebrae, pygal, and the right p11.