Brief report
Acta Palaeontologica Polonica 64 (x): xxx–xxx, 2019
The first Triassic vertebrate fossils from Myanmar: Pachypleurosaurs
in a marine limestone
KHAING KHAING SAN, NICHOLAS C. FRASER, DAVIDE FOFFA, OLIVIER RIEPPEL,
and STEPHEN L. BRUSATTE
Institutional abbreviations.—CMLV, Cultural Museum, Lashio,
Myanmar; YDBGLV, Geology Museum, Yadanabon University,
Myanmar.
siltstone and mudstone. Dark grey chert stringers or nodules
are intercalated within the fossil-bearing limestone, and minor
amounts of ore (lead-zinc) mineralization can be observed in
some parts of the bed. This limestone unit apparently grades
into the surrounding dolomites.
The fossil-bearing limestone was previously mapped as
belonging to the Nwabangyi Dolomite Formation (Garson et
al. 1976), considered Late Permian to Middle Triassic in age
based on the occurrence of the foraminiferan Glomospirella
irregularis in the Kyaukme-Longtawkno area of northern
Shan State (Brönnimann et al. 1975). This limestone has been
correlated with the Thigaungtaung Limestone (Early–Middle
Triassic, Induan–Anisian) of southern Shan State (Amos 1975;
Whittaker in Brönnimann et al. 1975; see also the recent review of the “Plateau Limestone” of southern Shan State by Win
et al. 2015). Furthermore, Sahni (1936) compiled other records
of lower-most Triassic units (Scythian, Induan–Olenekian) at
Namhkam (north of Lashio and west of Hsenwi), to which he
gave the informal name of the “Na-hkan Beds”. The detailed
stratigraphy, correlations, and ages of these various units are in
need of revision. Vertebrate fossils may help better constrain
their ages (see below).
Geological and geographical setting
Description
Myanmar is divided into four tectonic provinces. From east
to west, these are the (i) Shan-Tanintharyi Block; (ii) Central
Cenozoic Belt; (iii) Western Fold Belt; and (iv) Rakhine Coastal
Belt (Chhibber 1934; Win Swe 1972; Maung Thein 1973). The
pachypleurosaur fossils were collected immediately west of
the town of Lashio, in northern Shan State, Myanmar (Fig. 1).
They were found in rocks of the Lashio Basin, in the northern
part of the Shan Massif, which is part of the Shan-Tanintharyi
Block. The Lashio area is composed mainly of sedimentary
rocks that date from the Middle Devonian to the Jurassic.
Both specimens were found in a ferruginous, micritic limestone, associated with indeterminate fish bones and scales. The
limestone crops out in a mountain range, locally called the
Yebawhaung Kyauk-taung (UTM map 2297-9; 22°56′04″ N,
97°42′49″ E). The fossil-bearing limestone is sandwiched between thin- to medium-bedded, light to dark grey, hard and
compact limestone and red to purple, uniformly thin-bedded,
Pachypleurosaur morphology is best known from complete
specimens of Anarosaurus and Dactylosaurus from the lower
Muschelkalk (lower Anisian) of the Germanic basin (Sues and
Carroll 1985; Rieppel and Lin 1995; Klein 2009, 2012), the late
Anisian Serpianosaurus mirigiolensis from the southern Alps
(Rieppel 1989), the early and middle Ladinian Neusticosaurus
(three species) again from the southern Alps (Carroll and
Gaskill 1985; Sander 1989), and the Ladinian (possibly early
Carnian) Keichousaurus from southwestern China (Lin and
Rieppel 1998; Cheng et al. 2009; Cheng et al. 2004; Holmes
et al. 2008). Dianopachysaurus dingi is another well-preserved pachypleurosaur from the Anisian of southwestern
China, which has played an important role in pachypleurosaur phylogeny reconstruction (Liu et al. 2011). In this section,
we compare the Myanmar specimens to these taxa, especially
Keichousaurus and Dianopachysaurus given the geographical
proximity of their occurrences (see also Rieppel 2000).
As ecosystems recovered from the end-Permian extinction, many new animal groups proliferated in the ensuing
Triassic. Among these were the sauropterygians, reptiles
that evolved from terrestrial ancestors and transitioned to a
marine environment. The first sauropterygians were small,
marine-adapted taxa such as pachypleurosaurs, which are
known from Middle–Late Triassic deposits, particularly
in the Tethyan realm of Europe, and more recently from
Lagerstätten in southwestern China. Here we report two
pachypleurosaurs from Myanmar, the first Triassic vertebrate fossils from the country. These specimens demonstrate that their entombing rocks in northern Shan State,
which have received less study than terrestrial sediments in
southern Shan State and whose ages have long been uncertain, are Triassic. The specimens may be among the oldest
pachypleurosaurs globally, potentially corroborating biogeographic scenarios that posit an eastern Tethyan origin
for pachypleurosaurs, and raise the potential for future
discoveries of well-preserved Triassic reptiles in Myanmar.
Acta Palaeontol. Pol. 64 (X): xxx–xxx, 2019
https://doi.org/10.4202/app.00594.2019
2
ACTA PALAEONTOLOGICA POLONICA 64 (X), 2019
A
98°
96°
24°
N
Mu-Se
Laukkaing
Namkhan
MYANMAR
Ta Mong Nye
Mabein
Kutkai
Manton
Kunlong
Momeik
Namtu
Lashio
Mogok
Tang yang
Sa
lw
Mandalay
80 km
98°
96°
Q2
Tr
Q2
g
O
n
Mong Hsu
Pyin-Oo-Lwin
B
ee
22°
Ayeyarwaddy
Hsipa
Kakume
Q1
N
Tr
P-Tr
PC-Ca
m
m
K
Q2
Namkhan
ult
PC-Ca
we
K
g
Tr
a
li F
Sh
Laukkaing
Tr
J
P-Tr
lt
m
g
g
Momeik
23°
Mogok
Fau
Momeik
Ca
m
Thainni
Q1
Q2
Q1
J
J
Lashio
O
P-Tr
Tr
Tr
J
Q1
g
J
S
m
g
m
S
S
P-Tr
S
Q1
g
PC-Ca
Kyaukme
O
Q1
P-Tr
S
PC-Ca
Q1
O
S
Tr
Naungcho
S
O Q1
Q1
S
O
PC-Ca
P-Tr
Tr
O
S
J
S
PC-Ca S
O
Q1
S
S
g
m
Q1-2
O
40 km
faults
uncertain
lithology
granitic
intrusion
g
m
PC-Ca
Upper Precambrian
Lower Cambrian
P-Tr
Middle Permian
Middle Triassic
Ca
Cambrian
Tr
Triassic
metamorphic
O
Ordovician
J
Jurassic
Quaternary
S
Silurian
K
Cretaceous
Fig 1. Geographical (A) and geological setting (B) for the Triassic pachypleurosaurs from Myanmar. Study area indicated by a star. B based on the
2014 geological map of Myanmar from the Myanmar Geoscience Society.
Specimen CMLV_1 (Fig. 2) includes a partial skull, much
of the neck and pectoral girdles, and a substantial portion of
the trunk. Few morphological details are observable on the
skull, but the retroarticular process is clearly distinct, as in
other pachypleurosaurs. The humerus is evenly curved, more
distinctly so than in other pachypleurosaurs, except perhaps
in immature individuals. There appears to be a large number
of ossifications in the left carpus, although some of these may
represent broken bones instead of separate carpals. The dorsal
ribs are not pachyostotic and the thin gastral ribs number at
least two per vertebral segment in the posterior trunk, as in
other pachypleurosaurs.
Specimen YDBGLV_4 (Fig. 3) preserves much of the skeleton, and it has the lizard-like habitus, with a short-snouted
skull and overall body proportions, that are typical for pachy-
pleurosaurs. Also characteristic of pachypleurosaurs is the homodont dentition comprising numerous small, peg-like teeth.
The snout is not constricted and the orbits are large compared
to the postorbital region of the skull, as in other pachypleurosaurs. As in Anarosaurus, Dactylosaurus, and Keichousaurus,
the cheek is deeply embayed. There are no clear supratemporal
fenestrae, and it appears they have been very much reduced.
Possible fragmentary traces of these openings occur in the posterolateral corners of the parietal table, but there are no clear
margins, and they could even be completely absent. Scleral
ossicles appear to be absent in the better-preserved right orbit;
these are variably present in other pachypleurosaurs. The pineal foramen is located in the centre of the parietal skull table,
as is typical for pachypleurosaurs.
In the axial skeleton CMLV_1 exhibits the individual cervical vertebrae more clearly than YDBGLV_4, but unfortunately
the series runs along the edge of the block and only traces
of each of the anterior cervicals remain. In YDBGLV_4 the
outline of the neck is more intact but there is a significant gap
between the posterior margin (occiput) of the skull and the mid
cervical series, which renders it difficult to obtain more than
an approximate cervical count. Moreover, in both specimens it
is difficult to demarcate the cervical series from the first dorsal
vertebra, but we consider there to be at least 20 cervical vertebrae, and perhaps as many as 24. These numbers are within the
range of variation known in other pachypleurosaurs, although
tending towards the higher end (e.g., 25–26 Keichousaurus; 20
Dianopachysaurus; 17 Dactylosaurus; 15–18 Serpianosaurus;
17 Neusticosaurus edwardsii; 18–20 Neusticosaurus pusillus; Carroll and Gaskill 1985; Rieppel and Lin 1995; Sues and
Carroll 1985; Sander 1989; Lin and Rieppel 1998; Liu et al.
2011).
There are at least 16 left dorsal ribs. This would correspond to an unusually low number of dorsal vertebrae for a
pachypleurosaur, although not too dissimilar to Keichousaurus
(18–19) and Dianopachysaurus (19). Other members of the
group have more dorsal vertebrae, with, for example, 22–24 in
Table 1. Key measurements (in mm) of unidentified pachypleurosaurs
from Myanmar (after Sander 1989). Standard length is defined as the
length of the four posterior dorsal centra; na, not available (as the
specimen is incomplete).
skull
neck length
neck length
standard length
incomplete tail
humerus
radius
ulna
femur
fibula
tibia
snout-vent length
CMLV_1
51
151
165
na
na
46
2.7
29
na
na
na
–
YDBGLV_4
50
127
112
27
42
37
22
23
38
22
23
315
BRIEF REPORT
3
A1
A2
retroarticular
process
humerus
humerus
ulna
radius
carpals
gastralia
50 mm
Fig. 2. Unidentified pachypleurosaur (CMLV_1) from the Triassic of Myanmar, in dorsal view. Much of the neck, pectoral girdles, and trunk are preserved,
along with part of the skull and forelimbs. The pelvis, hindlimbs, and tail are not preserved. Photograph (A1) and explanatory drawing (A2). For measurements see Table 1.
Neusticosaurus pusillus and as many as 25–26 in Anarosaurus
(Sander 1989; Rieppel and Lin 1995; Lin and Rieppel 1998).
It is possible that two vertebrae in the cervical/dorsal transitional zone are not cervicals but dorsals, meaning that there
would be approximately 22 cervicals and approximatley 18
dorsals in YDBGLV_4. There are three sacral vertebrae, as
in other pachypleurosaurs except for Keichousaurus, which
has 2 or 3 (Lin and Rieppel 1998). The neural arches of the
dorsal vertebrae are compressed, and apparently pachyostotic.
Pachyostosis is conspicuously absent, however, in the dorsal
ribs. The delicate gastral ribs are numerous in number, and
there are at least two per vertebral segment in the posterior
trunk region.
In the appendicular skeleton, the coracoid is elongate,
plate-like, and biconcave, as is standard for pachypleurosaurs. The scapula is difficult to discern in detail, but appears unremarkable for a pachypleurosaur, and the interclavicle is indistinct. The most salient feature of the appendicular
skeleton is the distinctly and evenly curved humerus, as in
CMLV_1. This differs from other (mature) pachypleurosaurs,
but is a feature the Myanmar pachypleurosaur shares with
Dianopachysaurus (Liu et al. 2011). The entepicondylar foramen seems to be absent, an unusual character state otherwise
known only in Keichousaurus amongst pachypleurosaurs.
The ulna is distinctly broadened proximally, more so than in
most other pachypleurosaurs, but not overall as broad as in
4
ACTA PALAEONTOLOGICA POLONICA 64 (X), 2019
A1
A2
pineal foramen
lateral temporal
fenestra
orbits
humerus
humerus
radius
ulna
carpals
gastralia
gastralia
tibia
tarsals
femur
fibula
fibula
tibia
femur
metatarsals
50 mm
Fig. 3. Unidentified pachypleurosaur (YDBGLV_4) from the Triassic of Myanmar, in dorsal view. Most of the skeleton is preserved, except for the tail.
Photograph (A1) and explanatory drawing (A2). For measurements see Table 1.
Keichousaurus. The number and morphology of the carpal ossifications and phalanges in the manus are difficult to observe.
The pelvis is also difficult to describe, but the femur is slender, weakly sigmoidally curved, and longer than the humerus.
This latter feature is again shared with Dianopachysaurus
(Liu et al. 2011), and also with Anarosaurus, where the femur length exceeds humerus length to an even greater degree
(Rieppel and Lin 1995; Klein 2012). In other pachypleurosaurs
known from a larger number of specimens (Neusticosaurus,
Serpianosaurus, Carroll and Gaskill 1985; Rieppel 1989;
Sander 1989), the ratio of humerus to femur length is subject to ontogenetic variation as well as sexual dimorphism
(Rieppel 1993: fig. 8). The metatarsals of the right foot are
well preserved; the first metatarsal is much shorter than the
others, as is standard for pachypleurosaurs.
Concluding remarks
The two new specimens from Myanmar, CMLV_1 and
YDBGLV_4, can both be assigned to Pachypleurosauria because they possess several diagnostic features of the clade, including: preorbital region of the skull longer or subequal in
length to postorbital region; upper temporal fenestra much
smaller than the orbit; pre- and postzygapophyses pachyostotic; sacral ribs not expanded distally. The two specimens most
likely belong to the same lower-level taxon, due to their shared
presence of an evenly and distinctly curved humerus, which is
unknown in other pachypleurosaurs except Dianopachysaurus
(Liu et al. 2011), and apparently similar cervical and dorsal
vertebral numbers. Obliteration of the upper temporal fenestrae
would be an important autapomorphy of a new taxon, but the
BRIEF REPORT
specimens are too poorly preserved to allow an unequivocal assessment of this trait. We also are unable to confidently include
the Myanmar specimens in a phylogenetic analysis at this time,
as we feel we do not yet have enough morphological information from the limited fossils to render such an analysis informative. Preferring a phylogenetic approach to species delimitation,
we therefore refrain from diagnosing a new species, due to the
incompleteness of the specimens and the potential for finding
new fossils with additional fieldwork. There has recently been
a flourishing of phylogenetic studies of basal eosauropterygian
relationships, some of which have questioned the monophyly of
Pachypleurosauria and other traditional clades (e.g., Rieppel et
al. 2002; Holmes et al. 2008; Liu et al. 2011; Shang et al. 2011,
2017; Neenan et al. 2013; Jiang et al. 2014; Sato et al. 2014;
Shang and Li 2015; Cheng et al. 2016). Addressing this debate
is outside the scope of this paper, but additional material of the
Myanmar pachypleurosaur, analysed in a phylogenetic context,
may contribute important new insights in the future.
The presence of pachypleurosaurs in Myanmar is important for several reasons. First, these reptiles are known only
from the Triassic, so they are a key piece of evidence that the
Nwabangyi Dolomite Formation and coeval rocks in northern
Shan State, whose ages have long been the subject of uncertainty, are Triassic in age. Second, there is a possibility that
the Myanmar specimens may be among the oldest pachypleurosaurs in the world, which might give insight into the origins
of the group and potentially help untangle the thorny phylogenetic relationships of basal eosauropterygians. The current
oldest unequivocal pachypleurosaurs are Anisian (Liu et al.
2011), but if the Nwabangyi Dolomite Formation is indeed correlated with the Thigaungtaung Limestone of southern Shan
State, then it is possible that the Myanmar taxon is anywhere
from Induan to Anisian in age. Third, if the Myanmar taxon is
among the oldest pachypleurosaurs, it might corroborate the
often-discussed hypothesis of an eastern Tethyan origin for the
group (e.g., Rieppel 1999, 2000; Rieppel and Hagdorn 1997),
or alternatively, pending its inclusion in a phylogenetic analysis, hint that the fossil record is currently too poor to strongly
support any biogeographic scenario.
Finally, the discovery of articulated reptilian material in the
Triassic of Myanmar, although currently limited to these two
specimens, suggests that fine preservation of aquatic and nearshore reptiles may be common in particular units, but has gone
unrecognized due to limited collecting. The fossil-bearing
Burmese rocks are not particularly far geographically from the
Lagerstätten (conservation deposits) of Guizhou and Yunnan
in southwestern China, which over the past two decades have
yielded a wealth of spectacularly preserved Triassic reptiles
(e.g., Wang et al. 2008; Zhang et al. 2008). There may be potential for similar Lagerstätten in Myanmar.
Acknowledgements.—We thank Maung Maung Naing (Rector of
Yadanabon University, Amarapura, Myanmar) for his kind help, encouragement, and suggestions. We also thank James Neenan (University of Oxford, UK) and an anonymous referee for their helpful reviews.
DF and SLB were funded by a Marie Curie Career Integration Grant
(CIG 630652).
5
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Nicholas C. Fraser [nick.fraser@nms.ac.uk], Davide Foffa [davide.foffa@ed.ac.uk], Stephen L. Brusatte [stephen.brusatte@ed.ac.uk], School of GeoSciences, University of Edinburgh, Grant Institute, James Hutton Road, Edinburgh EH9 3FE, UK; National Museums of Scotland, Chambers Street,
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Received 9 January 2019, accepted 8 February 2019, available online 26 April 2019.
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