Vol.3, No.1, 9-27 (2011) Natural Science
http://dx.doi.org/10.4236/ns.2011.31002
Fossil middle triassic “sea cows” – placodont reptiles
as macroalgae feeders along the north-western Tethys
coastline with Pangaea and in the Germanic basin
Cajus G. Diedrich
Paleologic, Nansenstr, Germany;
Received 19 October 2010; revised 22 November 2010; accepted 27 November 2010.
ABSTRACT
The descriptions of fossil Triassic marine pla-
codonts as durophagous reptiles are revised
through comparisons with the sirenia and basal
proboscidean mammal and palaeoenvironment
analyses. The jaws of placodonts are conver-
gent with those of Halitherium/Dugong or Mo-
eritherium in their general function. Whereas
Halitherium possessed a horny oral pad and
counterpart and a special rasp-like tongue to
grind seagrass, as does the modern Dugong,
placodonts had large teeth that covered their
jaws to form a similar grinding pad. The sirenia
also lost their anterior teeth during many Mil-
lions of years and built a horny pad instead and
specialized tongue to fed mainly on seagrass,
whereas placodonts had only macroalgae availa-
ble. Indirect evidence for Triassic macroalgae is
provided by benthic palaeocommunities from
different layers and extended European regions
in the Germanic Basin. Studies of tooth wear
stages for Placodus indicate that anterior teeth
may have been used in a similar manner to the
procumbent front teeth of modern Dugong.
Paraplacodus and Placodus seem to have used
these teeth as spatulas to dig out seaplants.
Cyamodus and other placodonts such as Pla-
cochelys had smaller or reduced anterior teeth.
The scarcity of highly worn palatine or maxillary
and lower jaw dentary Placodus or Cyamodus
teeth (less then 0.5%) suggests that they had a
relatively soft diet. The seaplants would only
have been squeezed in a similar feeding strat-
egy to that of modern Dugong feeding on sea-
grass without jaw rotation and grinding. The
phylogenetic trend in tooth reduction within the
placodonts Paraplacodus, Placodus, especially
in Cyamodus but also Placochelys , and Henodus
within 11 My appears to have been a result of
this plant-feeding adaptation and may even ex-
plain the origin or at least close relationship of
the earliest Upper Triassic turtles as toothless
algae and jellyfish feeders, in terms of the
long-term convergent development with the si-
rens.
Keywords: Placodont Reptiles; Triassic;
Convergent Evolutionary Ecological Adaptation;
Sirenia; Macroalgae Feeders; NW Tethys Shelf;
Palaeoecology
1. INTRODUCTION
The extinct reptile group of the placodonts found in
Germany and other European sites (Figure 1), a group
of diverse marine diving reptiles, had large teeth cover-
ing the lower and complete upper jaws (Figsure 2-5)
which made those reptiles quite unique within all known
extinct and extant reptiles on Planet Earth. Those popu-
lar named “Triassic sea cows” [1] were found commonly
with their bones and typical large teeth all over Europe
mainly in the Germanic Basin of Central Europe [1-4].
Placodonts were later discovered additionally in the
north-western Tethys of southern Europe [3,5-7], and
were recorded only with few skeletons in the Middle
Triassic in the eastern Tethys in China [8].
The historical first recorded placodont remains of the
world are from Placodus (Figure 3) which were discov-
ered in 1809 near Bayreuth (Bavaria, south Germany) at
the northern Lainecker Höhenzug (Middle Triassic
mountain chain) mainly due to the activities of Graf zu
Münster [9], Agassiz [10] in more than six, today only
partly remained quarries at Hegnabrunn, Bindlach and
Laineck [11] which were recently restudied in the acces-
sible sections and with their facies and bone taphonomy
[13]. Placodus remains were first identified as a “large
Triassic fish” [9], but subsequently named and recog-
nized as a marine reptile [10,12]. Single teeth, bones and

C. G. Diedrich / Natural Science 3 (2011) 9-27 10
(a)
(b)
(c)
Figure 1. (a) Global distribution of Middle Triassic fossil placodont “sea cows” along the north-west Tethys (China, Europe,
North-Africa). (b) Distribution in Central Europe correlating with macroalgae presence in shallow marine carbonate platform envi-
ronments. (c) Middle Triassic marine placodont reptile bone and skeleton localities of Europe of which are most found in the
“Muschelkalk limestones” of Germany.
Figure 2. Paraplacodus cf. broilii Peyer, 1937 skull and tooth remains. 1. Lower jaw anterior tooth from the Jena Fm. (muS substage,
Pelsonian) of Freyburg a. d. U., Germany (MB no. 4736), lateral. 2. Lower jaw anterior tooth from the Jena Fm. (muS substage, Pel-
sonian) of Jena, Germany (MLU.IfG no. 2007.26), a. lateral, b. lingual. 3. Palatinal with five teeth (one damaged) from the Upper
Muschelkalk of Tarnowice, Poland (MB no. R. 4418), a-b. ventral. 4. Upper jaw premaxillary tooth from the Jena Fm. (muS substage,
Pelsonian) of Heimhausen, Germany (MHI no. 1204), lateral. 5. Upper jaw premaxillary tooth from the Jena Fm. (muS substage,
Pelsonian) of Freyburg a. d. U., Germany (MB no. R.4420), lateral. Skeleton reconstruction based on the skeleton from Monte San
Giorgio (Switzerland).

C. G. Diedrich / Natural Science 3 (2011) 9-27
1111
Figure 3. Placodus gigas Agassiz, 1833 skulls, jaws and teeth from the “Upper Muschelkalk” (Anisian, Middle Triassic) from the
Germanic Basin (Central Europe). 1. Skull of the Bindlach Fm. (Illyrian, Upper Anisian) from Bindlach (Lainecker Höhenzug),
Germany (UM-O BT 13), ventral. 2. Lower jaw (cast) from the Bindlach Fm. (Illyrian, Upper Anisian) of Bindlach (Lainecker
Höhenzug), Germany (UM-O no. BT 5067.00; original in the BSP no. 1209), a. dorsal, b. lateral. 3. Skull from the Hegnabrunn Fm.
(Fassanian, Lower Ladinian) of Hegnabrunn, Germany (BSP no. 1968 I 75), a-b. ventral. 4. Skull from the Bindlach Fm. (Illyrian,
Upper Anisian) of Bindlach (Lainecker Höhenzug), Germany (UM-O without no.), ventral. 5. Skull of a younger individual from the
Bindlach Fm. (Illyrian, Upper Anisian) from Bindlach (Lainecker Höhenzug), Germany (SNSD without no.), ventral. 6.
Cross-section through a skull palatinal from the Bindlach Fm. (Illyrian, Upper Anisian) of Bindlach (Lainecker Höhenzug), Germany
(SBMF no. R.361). 7. Lower jaw dentary tooth in wear stage 1 with enamel structure from the Bindlach Fm. (Illyrian/Fassanian,
Upper Anisian) of Laineck (Oschenberg, Lainecker Höhenzug), Germany (MB without no.). 8. Palatinal tooth in wear stage 2 with
smooth surface from the Bindlach Fm. (Illyrian/Fassanian, Upper Anisian) of Bindlach or Laineck (Lainecker Höhenzug), Germany
(NMB without no.), occlusal. 9. Palatinal tooth in wear stage 3 with rubbed enamel from the Bindlach Fm. (Illyrian/Fassanian, Upper
Anisian) of Bindlach or Laineck (Lainecker Höhenzug), Germany (MB without no.), occlusal. Placodus skeleton reconstruction
based on the skeleton from Steinsfurt (Germany).

C. G. Diedrich / Natural Science 3 (2011) 9-27 12
skulls of Placodus (Figsure 3.1-6) from the Middle Tri-
assic were recorded mainly from Bindlach near Bayreuth
in southern Germany with descriptions of many different
today non-valid “species” [14-16], but also more re-
cently from many other German and even Dutch locali-
ties [4,5,9,10,17-23]. The first and only European pla-
codont skeleton of the world was discovered then much
later with a skeleton of Placodus gigas Agassiz, 1839 at
Steinsfurt (South Germany; [21]; Figure 5.5). Only a
single Paraplacodus skeleton remain, the most rare in
the Northern Tethys [7], is recorded most recently also
with two more skeletons and isolated cranial remains
from the Germanic Basin in Romania and the Nether-
lands [24,25]. The few Cyamodus holotype skulls (Fig-
ure 4) were collected by Münster from the Lainecker
Höhenzug localities of Germany [9,20,26], whereas
more recent remains were found at other southern Ger-
man localities [27,28]. Four recently valid species of
Cyamodus (Figures 4.7-1.10) have been described: C.
tarnowitzensis Gürich, 1884, C. rostratus Münster, 1839
(Figures 4.7-1.8), C. münsteri (Agassiz, 1839) (= “Pla-
codus laticeps Owen, 1858”) (Figure 4.4) and C.
kuhnschnyderi Nosotti and Pinna, 1993 (Figures 4.9-1.10)
from only a few crania [26], and C. hildegardis Peyer,
1931 from a skeletal remain in the northern Tethys at
Monte San Giorgio [6] .
The systematic position of placodonts within the ex-
tinct sauropterygian reptiles have been discussed by
several authors, mainly on the cranial morphology but
not solely and most recently only in an cladistic way [3,6,
10,29-31] but recently initially in more biostratigraphic,
taphonomic and ecological context [26]. Former inter-
pretations [3,27,32,33] described all placodonts as “shell
crushing durophagous” reptiles, based solely on their
“shell crushing tooth morphology” without taking into
account any other considerations. Most recent studies
have, however, now identified all placodonts as having
been “macroalgae feeders” [1] which theory is presented
here with much more material and extended arguments
including all placodonts in time and space of the Middle
Triassic Pangaean world in a time frame between
247-236 My (11 My in total).
2. MATERIAL AND METHODS
The new “Triassic sea cow” theories are based on in-
terdisciplinary research about the 1) stratigraphic occur-
rence and historic and recent placodont discoveries and
palaeobiogeography, 2) facies including palaeocommu-
nities with evidence of macroalgae, 3) bone taphonomy,
4) osteological and functional morphology, 5) feeding
strategy in comparison with modern Sirenia, and, 6) fi-
nally, types of placodont teeth and their tooth wear and
replacement stages.
1) Stratigraphic occurrence of historic and recent pla-
codont discoveries and palaeobiogeography: Several
about 220 meters in thickness marine carbonate Middle
Triassic “Muschelkalk” sections in the Germanic Basin,
including the famous Winterswijk, Bayreuth, Freyburg a.
d. U. and many other sites such as Bissendorf or Lamer-
den were subdivided more recently with the international
subdivision for exact dating of the historical and new
discovered skeleton finds and isolated bone material. The
dating and correlation between Tethys and Germanic
Basin deposits is essential for understanding the evolu-
tionary trend within the studied 11 My ranging Middle
Triasic “Lower/Middle/Upper Muschelkalk” placodont
evolution and distribution. In this analyses most of the
material was dated in more detail or even for the first
time including famous holotype and original skulls or
skeletons.
Paraplacodus is represented recently with a Longo-
bardian aged skeleton from the Monte San Giorgio
Illyrian lagoon black shale deposits. From the Germanic
Basin the single bone and tooth record from is very lim-
ited with a part of a disarticulated skeleton known from
Winterswijk (NL, Aegean age) and Brașov (Ro, Aegean
age) which both do not include tooth material. Teeth are
instead isolated finds from different sites in Germany
(Figure 1) ranging between Pelsonian to Illyrian age,
whereas a palatine from Tarnowice (Pl) is of Illyrian age
(Figures 2.2-3).
Placodus instead is well-known starting with remains
from the Aegean aged intertidal deposits of Winterswijk.
Many isolated teeth and bones are from the shallow
submarine carbonate sand bar sediments of Freyburg a. d.
U. or Jena sites. Most material was found at the Illyrian
aged shallow marine glauconite-rich carbonates of
Bayreuth including the holotype and many other sulls
and hundreds of teeth or single bones. The only
world-wide record of a P. gigas skeleton from Steinsfurt
(D) is also of Illyrian age (Figure 6.5).
Cyamodus is represented from the Monte San Giorgio
with skeleton remains of Longobardian age, but in the
Germanic Basin only few skulls were found first in his-
toric times and from Illyrian, Fassanian and Longo-
bardian dated marine shallow subtidal layers (Figure 4).
Here a more detailed biostratigraphic occurrence can be
presented after the Bayreuth Cyamodus locality studies
including ceratite cephalopods, which prove a rapid
cyamodontid evolution of the genus within the Illyrian-
Longobardian times. The oldest record of Aegean age of
C. tarnowitzensis is followed by the Illyrian species of C.
rostratus (atavus-compressus ceratie biozones, Upper
Illyrian), by C. muensteri (evolutus-sublaevigatus cerat-
ite biozones, Fassanian), and finally by C. kuhnschnyderi
(praenodosus-semipartitus ceratite biozones, Longo-
bardian). In total a tooth reduction and rostrum shortening

C. G. Diedrich / Natural Science 3 (2011) 9-27
1313
Figure 4. Cyamodus skulls and jaws from the “Upper Muschelkalk” (Anisian, Middle Triassic) from the Germanic
Basin (Central Europe). 1. Cyamodus tarnowitzensis Gürich, 1884 holotype skull (cast) from the Karchowice Fm.
(Terebratula Member, middle Lower Muschelkalk, lower Pelsonian) of Tarnowice, Poland (SBMF without no.), a-b.
ventral, c. lateral. 2. Cyamodus rostratus (Münster, 1839) half lower jaw from the Bindlach Fm. (atavus/pulcher Zone,
Illyrian/Fassanian) of Bindlach (Bindlacher Berg, Lainecker Höhenzug), Germany (SBMF no. 4040), dorsal. 3. Cya-
modus rostratus (Münster, 1839) holotype skull from the Bindlach Fm. (atavus/pulcher Zone, Illyrian, Upper Anisian)
of Bindlach (Lainecker Höhenzug), Germany (UM-O BT 1210a). a. dorsal, b and c. ventral, d. lateral. 4. Cyamodus
münsteri (Agassiz, 1839) skull (cast, with completed part dark) of an adult individual from the Bindlach Fm. (ata-
vus/pulcher Zone, Illyrian-Fassanian) of Bindlach (Bindlacher Berg, Lainecker Höhenzug), Germany (SBMF without
no., original in the BMNH no. R.1644), a. dorsal, b-c. ventral, d. lateral. 5. Cyamodus rostratus (Münster, 1839) lower
jaw from the Bindlach Fm. (atavus/pulcher Zone, Illyrian, Upper Anisian) of Bindlach (Lainecker Höhenzug), Ger-
many (UM-O BT 1210b), a-b. dorsal, c. lateral. 6. Cyamodus kuhnschnyderi Nosotti and Pinna, 1993 holotype skull
from the Erfurt Fm. (nodosus to dorsoplanus Zone, Longobardian, Lower Ladinian) of Tiefenbach, Germany (UM-O
BT 1210b). a. dorsal, b-c. ventral, d. lateral. 7. Cyamodus kuhnschnyderi Nosotti and Pinna, 1993 lower jaw from the
Meissner Fm. (robustus to nodosus Zone, Illyrian/Fassanian, Anisian/Ladinian boundary) of Heldenmühle near Crail-
sheim, Germany (SNSD 18380), a-b. dorsal. Cyamodus skeleton reconstruction based on the articulated skeleton of C.
kunhschnyderi from Monte San Giorgio (Switzerland).

C. G. Diedrich / Natural Science 3 (2011) 9-27 14
is important to understand placodont reptile evolutionary
trends at all.
In Placochelys there are only three skulls from Fas-
sanian to Longobardian to Cordovolian age are available
(Figure 5). In the Germanic Basin, some carapace frag-
ments are the only well identified postcranial remains.
2) Facies including palaeocommunities with evidence
of macroalgae: Literature studies about marine benthic
palaeocommunities and new benthic community studies
at the sites Freyburg a. d. U., Bayreuth, Bissendorf and
Lamerden are the basics to prove or disprove macroalgae
presence/absence in areas of the Germanic Basin, and
the north-western Tethys. Studies and literature results
for other Germanic Basin and north-western Tethys sites
were included to develop three main palaeogeographical
maps showing the stages of the Germanic Basin devel-
opment, which are the basics to understand the pres-
ence/absence of placodonts.
3) Bone taphonomy: At the sites Bissendorf, Lamer-
den, and Bayreuth, but also Winterswijk and many other
sites in the Germanic Basin and the Monte San Giorgio
sites the relation between facies, skeleton or bone pres-
ervation was important to correlate with the facies and
palaeocommunity types. A separate study about the
Middle Triassic marine reptile bone taphonomy was the
base to understand the real possible reptile habitats, their
postmortal carcass transport and finally the bonebed
accumulation genesis in different palaeoenvironments.
Especially the Monte San Giorgio lagoons and skeletons
and the shallow marine Germanic Basin ones were
compared which both together give a clear picture about
the carcass taphonomy stages at all.
4) Osteological and functional morphology: Two
original skeletons of Halitherium schinzii (SMNS, and
MB) from the European Oligocene, were studied for
convergence comparisons.
Figure 5. Placochelys placodonta Jaeckel, 1902. Skeleton reconstruction based on the original bones of the holotype skeleton from
Hungary. 1. Holotype skull (cast) from the Lower Keuper of Veszprém (Jerusalemer Berg) in Hungary (SBMF no. R.288a). a. dorsal,
b-c. ventral, d. lateral. 2. Lower jaw of the holotype skull of Figure 1 (SBMF no. R.288b). a-b. dorsal, c. lateral. 3. Skull from Vesz-
prém (Jerusalemer Berg) in Hungary (MB no. R.1765). a-b. ventral. 4. Skull from the Trochitenkalk Fm. (Illyrian) of Gaismühle near
Crailsheim in southern Germany (SNSD no. 17403). Skeleton reconstruction based on the original cranial and postcranial bone mate-
rial.

C. G. Diedrich / Natural Science 3 (2011) 9-27
1515
Figure 6. Comparison of osteological convergent development
in 1. Halitherium from the Tertiary (Oligocene) of Europe and
2. Modern mammal Dugong and Moeritherium from the
European Eocene and 4-5. Placodus from the Middle Triassic
of the central European Germanic Basin. (SNSD: Placodus
cast, and original Halitherium skeleton. Both skeletons are
about 2.5-3.0 meters in length).
5) Feeding strategy in comparison with modern Sire-
nia: The tooth wear stages in the skulls were analysed as
well as the related tooth replacement, for evidence in
support of “soft algae” or “shell food” consumption.
Axctualistic comparisons were made between Dugong
populations in the Arabian Gulf and Placodus popula-
tions in the Germanic Basin. The feeding habits and
feeding strategy of Dugong was studied to understand
the tooth use and possible plant sources Placodus might
have fed on.
6) Types of placodont teeth and their tooth wear stages
and replacement stages: About 23 Placodus skulls and
several lower jaws and the only known original skeleton
but also 140 anterior and 420 other teeth, 5 Cyamodus
crania and some lower jaws, 3 Placochelys skulls and 1
Paraplacodus skull fragment and postcranial skeleton,
but also many lower jaws of those placodonts and, sev-
eral hundred bones and teeth collected historically from
the Germanic Basin of Central Europe were used to un-
derstand the tooth replacement in all placodont reptiles.
A palatine of Paraplacodus from Tarnowice (Pl)
shows three different wear stages on the teeth (Figure
2.3). Also the isolated anterior teeth are rubbed on their
tips only in one case (Figure 2.2).
From Bayreuth 23 skulls and about 140 anterior and
420 other teeth revealed that almost all of the highly
worn teeth were anterior teeth. The other teeth of the
maxillaries, palatals or dentaries are in different wear
stages in the skulls. In some skulls (Figures 3.1 and 5)
the teeth are all in a similar wear stage 2, in others (Fig-
ures 3.3-4) those are in replacement modus, whereas in
both this seem to start in the palatine with the posterior
paired teeth initially, which are changed first of all six
palatal teeth, which is also observed at some other skulls.
The skull material gives some insight in possibly more
regular way of tooth replacements, but the material is
still too few for a statistic and clear picture. Single teeth
instead are in 0.5% in wear stage 1 (Figure 3.7), 99%
are in wear stage 2 (Figure 3.8), and again only 0.5%
are in wear stage 3 (Figure 3.9).
In Cyamodus the tooth change is present in nearly all
few skulls (Figure 4), but the material is to few to give a
clear tooth change modus. Also here the same three
types of wear stages are present. The most interesting
skulls of Figures 4.3 and 4.6 show the tooth replacement
at different positions and the erupting replacement teeth.
In both skulls it seems, that in the palatals all the two or
three teeth of one side are changed first, and then the
other side. The postcranial material is few represented in
the Germanic basin with some vertebrae and carapace
fragments.
In one Placochelys skull all over teeth are in wear
stage 1 (Figure 5.3), the second has wear stage 2 in the
preserved teeth (Figure 5.3), whereas the third skull has
all in wear stage 3 (Figure 5.1). None of the skulls are in
any tooth change stages.
The studied placodont skeleton, teeth and bone mate-
rial is from the following collections: the Senckenberg-
museum Frankfurt (abbreviation: SMF), the Geological
Department of Martin-Luther-University, Halle/Saale
(abbreviation: MLU.IfG), the Humboldt-Museum of
Berlin University (abbreviation: MB), the Staatliche

C. G. Diedrich / Natural Science 3 (2011) 9-27 16
Museum für Naturkunde in Stuttgart (abbreviation:
SMNS), the Museum für Natur und Umwelt in Osnabrück
(abbreviation: NMO), the Urwelt-Museum Oberfranken
in Bayreuth (abbreviation: UM-O), the Muschelkalk-
museum in Ingelfingen (abbreviation: MHI), and the
Naturkundemuseum in Erfurt (abbreviation: NME).
Material from Romania is in the University of Bucharest,
non-figured finds from the Dutch site is in the Natural
History Museum Enschede or Museum Freriks in Win-
terswijk.
3. DISCUSSION
The cranial material of the Germanic Basin Middle
Triassic placodonts Paraplacodus (Figure 2; 5%), Pla-
codus (Figure 3; 75%), Cyamodus (Figure 4; 15%),
Placochelys (Figure 5; 5%) is dominated by Placodus
gigas remains. It is here discussed if this is the result of
taphonomy or primary habitat differences in the Germanic
Basin (e.g. Bayreuth shallow subtidal site) and north-
western Tethys (e.g. Monte San Giorgio lagoon site).
3.1. Placodont Carcass Taphonomy
The taphonomical fossil record of placodont remains
provides an important source of information on their
primary habitats. Paraplacodus was found with a drifted
partial skeleton on the Aegean aged intertidal biolami-
nates of Winterswijk [25], but also in restricted lagoonal
black shale facies [7]. Single teeth are from similar lay-
ers, in which Placodus and Cyamodus remains were
discovered. Placodus and Cyamodus remains, including
many isolated bones and teeth, have been found in cen-
tral Germany, especially in the Lower Muschelkalk
“Saurierkalk” of Jena and the Pelsonian “Schaumkalk”
carbonate sand bar deposits of Freyburg a. d. U. [34].
Other concentrations come from the Upper Muschelkalk
shallow marine terebratulid dominated bioclastic storm
shell or glauconitic marl layers of the Illyrian/Fassanian
Bindlach and Hegnabrunn Formations near Bayreuth [13]
and the Bad Sulza Formation [26], or from recent exca-
vations in sediments of a similar age and terebratulid
dominated marginal basin facies at Bissendorf, near Os-
nabrück in northern Germany [35]. In many cases of
isolated placodont teeth those represent in most cases
replacement teeth, which were dropped during the ani-
mals life explaining the high amount of “black teeth” in
the Upper Muschelkalk sediments. The only skeleton to
have been found in Germany was of Placodus gigas and
came from the Tonplatten facies (Upper Muschelkalk;
[20]). In contrast to these shallow marine deposits, pla-
codont bones and teeth are absent from the terrest influ-
enced sediments of the uppermost Upper Muschelkalk
bonebeds in northern Germany [13]. This taphonomic
distribution therefore provides evidence of a “facies re-
lationship” for these reptile rmains. Placodont bone re-
mains are very abundant in the carbonate sands and bio-
clastic sediments or shell rich palaeocommunities of the
shallow marine shell-rich oolite bar (Lower Muschelkalk)
or Terebratulid storm shell facies (Upper Muschelkalk)
of the Germanic Basin. In contrast, articulated placodont
skeletons from the restricted black shale lagoons of the
Swiss and Italian Monte San Giorgio area (Anisian-
Ladinain boundary) appear to represent carcasses, that
drifted into the lagoons from their original surrounding
habitats of the shallow carbonate platforms [36,37], such
as drifted skeletons which were found on intertidal zones
in the western Germanic Basin [4]. A few carcasses of
terrest coastal living reptiles that were drifted also have
been found in the Monte San Giorgio lagoons [37].
3.2. Osteological Comparisons
The main arguments for describing placodont reptiles
as “Triassic sea cows” that were herbivorous macroalgae
feeders derive from their anatomical and osteological
similarities to the mammalian Sirenia [1]. A very similar
convergence of the body shape in Placodus and the
body-weight enhancements of placodonts in general is
quite obvious when compared to the European Oligo-
cene siren Halitherium schinzii (see Figure 7). Whereas
dugongs developed pachyostotic ribs to enhance their
body weight, placodont reptiles achieved a similar result
in three different ways: Paraplacodus developed
enlarged thoracic ribs; Placodus had pachyostotic thick
gastral ribs (Figure 7), and Cyamodus had thoracic os-
teoderms [1], in the latter similar as in Placochelys or
Henodus. Apart from these body shape adaptations for
aquatic long-term diving, the skull morphology and den-
tition provide the most important evidence for conver-
gences. Placodus is very similar in its skull shape to the
Oligocene basal proboscidean Meoritherium andrewsi,
which was an aquatic animal adapted to feeding on
freshwater aquatic plants ([38]; Figure 7). The func-
tional morphology of the horny oral pads of the Sirenia,
especially those of the modern Dugong [39], compares
well with the dentition of Placodus (Figure 7). The
horny oral pads and reduced dentition in trichechiid Si-
renia suggest that mouthparts and tongues assumed a
major role in food comminution [40,41]. The cheek teeth
in Modern dugongs are largely non-functional whereas
the horny oral pads are important both in the mechanical
breakdown of seagrass and in conveying the seagrass
through the mouth [40,41]. The full-dentition in the Eo-
cene Protosiren let think about absence of horn pads in
early trichiid Sirenia and full function of the lower jaw
[42] (Figure 7), which changed to become finally of
little importance for dugongs since Oligocene times

C. G. Diedrich / Natural Science 3 (2011) 9-27
1717
Figure 7. Comparison of convergent long-term development in the tooth reduction of Triassic
placodonts (reptiles) and Tertiary sirenia (mammals) as result of adaptation onto seaplant feeding
(algae in placodonts, mainly seagrass in dugongs).
(Figure 8). Those lower jaws were certainly more im-
portant for Placodus, with its large lower jaw teeth and
small tongue. The soft mouthparts of the dugongs are
highly modified in the way that the entire oral cavity
functions to crush seagrasses and especially the rhi-
zomes, while in the placodonts a similar function was
fulfilled by the large teeth that covered the entire jaws.
The siren tongue can be seen as convergent in its func-
tion to the lower jaws of Placodus (cf. Figure 7). Both,
sirens (Dugong) and placodonts (Placodus) have a spe-
cial orientation of the teeth or horny pads in the upper
jaw as another important convergence, apparently de-
veloped to facilitate the expulsion of fluids and sand
particles during the food-crushing process (cf. Figure 7).
Middle Triassic placodonts Paraplacodus, Placodus,
Cyamodus, Placochelys or Henodus and others vary in
their morphologies (cf. Figure 7) and their dentition,
apparently as a result of specialization on different types
of macroalgae, but they still perform similar functions.
3.3. Palaeocommunities as Macroalgae
Indicators Food Source
There are several indirect lines of evidence for the ex-
istence of macroalgae, and even for macroalgae mead-
ows, in the Gemanic Basin within the two facies types of
the Jena Fm (Lower Muschelkalk) and the Bad Sulza/
Bindlach/Hegnabrunn and Meissner Fms (Upper

C. G. Diedrich / Natural Science 3 (2011) 9-27 18
Figure 8. Marine carbonate palaeoenvironments during the Illyrian of Europe (Germanic Basin, north-western Tethys). (Palaeo-
communities compiled after: [13]).
Muschelkalk) [40,41], and also in the northern Tethys
Cassina Fm (Upper Muschelkalk equivalent) of the Alps
([43]; Figures 8,9). In the “Lower Muschelkalk” benthic
communities the gastropod genera Wortheniella, Om-
phaloptycha and Polygyrina indicate the presence of
larger macroalgal meadows [1,41,44]. These communities
disappeared from the Germanic Basin during the evaporitic
Middle Muschelkalk, as did all of the placodonts, whereas
bivalves and gastropods that were adapted to hypersaline
conditions remained as potential food sources for “du-

C. G. Diedrich / Natural Science 3 (2011) 9-27
1919
rophagous predators” resisted. During the Upper Muschel-
kalk of the Germanic Basin benthic communities again
provide evidence for the existence of macroalgae in both,
the Germanic Basin and the Northern Tethys, with gas-
tropods such as Nerita ria in the Germanic Basin [44]
and Rhaphistomella and Anoptych ia in the Tethys [43]
suggesting the existence of extensive macroalgal mead-
ows covering the shallow marine Pangaea surrounding
seafloors (Figures 8-10). Their absence of any studies
on these possible Middle Triassic macroalgae species is
because they have not been preserved as fossils. Other
calcareous green algae groups have, however, been
documented from shallow marine habitats in the north-
ern Tethys [45], but these algae with their massive cal-
careous skeletons were most probably not the right plant
food-sources for placodonts, but can not be excluded as
such. A Placodus skeleton from Middle Triassic lagoon-
ary black shale/carbonate changing Pelsonian strata of
the western Tethys lagoons in China [46] has been re-
ported as having no “shell stomach contents”, providing
further evidence against the “durophagy-thesis”, although
a larger number of articulated skeletons must be found
before this evidence can be considered to be conclusive.
3.4. Facies Related Palaeobiogeography
The placodonts Placodus, Cyamodus and Parapla-
codus, together with all other Triassic placodonts, very
clearly tend to be found in benthic macroalgae pa-
laeoenvironments (Figure 8; [26]), providing additional
important support for the hypothesis that placodonts
were algae feeders. The reliance on macroalgae as a food
source may have been a result of the development of
extensive carbonate platform environments around the
Pangaean supercontinent at about the time of the transi-
tion from Lower to Middle Triassic, which then devel-
oped into related carbonate sand facies within the Ger-
manic Basin (Figures 8-10; [35]). This would explain
why Placodus, Paraplacodus and Cyamodus had already
migrated into these areas by the beginning of the Middle
Triassic, as well as explaining the palaeobiogeographic
distribution of placodonts in all of the shallow marine
shelf paeleoenvironments surrounding Pangaea (Figures
10(a-f)). The Germanic Basin was very different in its
palaeoenvironments compared to the north-western
Tethys with its lagoons and carbonate platforms. The
presence or absence of seaplant food sources must have
been the main factor controlling the world-wide distribu-
tion of the placodonts during the Middle Triassic, in the
same way as the presence or absence of seagrass con-
trols the global distribution of the modern sirenian Du-
gong [47]. The palaeobiogeography of Triassic pla-
codonts was therefore dependent on the distribution of
carbonate sands and bioclastic facies, together with a
warm climate, both of which were necessary for exten-
sive growth of macroalgae [48]. Within Europe, various
Oligocene species of Sirenia were clearly restricted to
these types of habitats, with coincident carbonate sand
sea floors and specific vegetation types (algae, seagrass;
[37]), even if a few of the the seagrass species have set-
tled more recently on siliciclastic environments.
3.5. Placodont Tooth Wear Stages and
Tooth Replacement Impact
A study of approximately 140 anterior and 420 other
teeth, and 23 skulls, all from Placodus, revealed that
almost all of the highly worn teeth were anterior teeth
[1]. There are two main problems with the former sug-
gestion that these animals should have been duro-
phagous; firstly, the large spaces between the anterior
teeth and, secondly, if shells were consumed the anterior
teeth would also be expected to show signs of lateral
wear, which was not observed in any of the studied ma-
terial. Dugongs (Dugong) have procumbent teeth which
are also generally worn as a result of their relict function
in the excavation of seagrass rhizomes - a function simi-
lar to that of the Placodus anterior teeth [49]. Judging by
the wear stages of the anterior teeth, the feeding strategy
of Placodus must have been very similar to that of mod-
ern Sirenia in that the broadly spaced front teeth appear
to have been used to dig out macroalgae from the sedi-
ments, complete with their roots, rather than to merely
strip off macroalgae from the surface. A similar feeding
strategy has been observed for modern Dugong feeding
on seagrass and, in particular, on its rhizomes which
were also excavated from the carbonate sands using the
degenerated teeth of their upper jaws [40,41]. The
stomach content of an individual female dugong (Du-
gong dugon) - which is similar in size to Placodus - had
a total dry weight of digesta of about 3,4 kg of which
98,9% was seagrass material [50]. The amount of swal-
lowed sediment in the siren stomach was negligible,
which is astonishing considering the long, deep trails of
feeding depressions that Dugong leave in the sediments
[37,50]). Dugong have clearly developed a strategy to
flush the carbonate sand particles out of their mouths
using their mouthparts and adapted tongue [50]. The
dentition and jaw functions of placodonts (not only of
Placodus) were very similar to those of the modern Du-
gong, but they used the large teeth that covered the
lower and upper jaws instead of horny pads and a spe-
cialized tongue to fulfil a similar function removing wa-
ter or channelling sand from the mouth before swallow-
ing (cf. Figure 6). Chewing of macroalgae was not nec-
essary, and not even possible with the Placodus jaws, as
indicated by the very small proportion (0.5%) of highly
worn palatal, maxillary and dentary teeth with full-

C. G. Diedrich / Natural Science 3 (2011) 9-27 20
rubbed enamel. In durophagous animals teeth change-
ments especially of the palatinals and maxillaries would
have had strong negative feeding effects with impossi-
bilities to crush well shells. In algae consumers this tooth
replacement would have had not that impact. The rela-
tively small overall numbers of worn teeth, mainly esti-
mated on Placodus teeth, are also indicative of a softer
diet.
3.6. Possible Placodont Population Sizes
Modern dugongs (Dugong dugon) live in large popu-
lations ranging between 1,800 and 7,300 individuals in
relatively small ocean areas such as the Red Sea or the
Arabian Gulf but also in the Indian Ocean [49,51-54].
The Middle Triassic Placodus, at least, may also have
been present in somewhat smaller, but still large popula-
tions, as estimated from the relatively large numbers of
bones and teeth found all over Europe and especially in
Germany. In some bonebeds several placodont teeth and
bones have been found within only a few square meters
in a single layer [35]. Although the large quantities of
Placodus, and fewer Cyamodus, and to a lesser extent,
Paraplacodus bones and teeth in the “Jenaer Saurier-
kalk” (Lower Muschelkalk) and the “Bayreuther
Muschelkalk” (Upper Muschelkalk) of the Germanic
Basin would certainly have accumulated over long peri-
ods of time (Figures 9,10), they nevertheless appear to
support the speculative existence of Placodus popula-
tions comprising between several hundred and several
thousand individuals within the Germanic Basin, with
smaller populations of Cyamodus or other placodonts,
even if taken in account, that many isolated teeth are
replacement teeth.
3.7. Palaeobiogeography and Evolutionary
Trends
In the Germanic Basin the ratio between the Middle
Triassic placodonts is estimated on the studied material
(Bones/Teeth = 1,257 specimens): Placodus (75%),
Cyamodus (20%), Paraplacodus (5%). In the lagoons of
the northwestern tethys (Monte San Giorgio) the fossil
record of those three placodonts is completely different:
Placodus (1%), Cyamodus (70%), Paraplacodus (29%).
Especially the presence and nearly absence of Placodus
is the best argument for the adaptation onto macroalgae
feeding, because those algae were absent in the lagoons
of the Monte San Giorgio [45], which finally explains
also the absence of Placodus there (cf. Figures 9,10).
Placodus gigas seems to have been well distributed
within the Germanic Basin since the marine ingression at
the top of the Lower Triassic (Myophoria/Gogolin
Members, Upper Bunter: [1,34] (Figure 10(a)), and few
remains from the northern Tethys (?Israel; [3]) indicate
that an interchange of these reptiles must have been pre-
sent during the basal Middle Triassic. A new Placodus
skeleton has, however, recently been reported from the
Pelsonian of China [46]. Paraplacodus broilli (Peyer,
1931) remains from the uppermost Upper Bunter (Ae-
gean) to upper Lower Muschelkalk (Pelsonian) of the
Germanic Basin indicate a similarly early presence and
continuing existence [25], and range to the middle Upper
Muschelkalk with a skeleton find in the Monte San
Giorgio lagoon deposits from the Anisian/Ladinian
boundary [7] (Figures 9, 10( d)). In the material from
both of these species no (or few not yet recognized)
evolu- tionary development can be observed over almost
the whole of the Middle Triassic (Aegean to Fassanian,
247- 239 My); this may, however, possibly be due to
gaps in the fossil record. In contrast, Cyamodus shows
an evolu- tionary trend in general tooth reduction during
the Mid- dle Triassic (Figure 9). This form of tooth re-
duction, which also occurred in the Sirenia during the
Tertiary, can be seen in both Cyamodus and generally in
all pla- codonts (Figure 9).
The oldest Cyamodus records in the Germanic Basin
are of Cyamodus tarnowitzensis from the Myophoria/
Gogolin Members (Lower/Middle Triassic boundary).
This species then dissappeared during the Middle
Muschelkalk low stand, together with all other placodonts
(Figure 9). C. rostratus then appeared with a new trans-
gression into the Germanic Basin. C. hildegardis, which
already had fewer praemaxillary and maxillary teeth, be-
came distributed within the Germanic Basin, and also in
the north-western Tethys where the only known skeleton
was found [6] in the lagoonal deposits of Monte San
Giorgio [1]. These cyamodonts were able to migrate and
spread out to the north and south during the maximum
high stand, and were not “endemic” species, in contrast.
Finally, skulls of Cyamodus kuhnschyderi have been
found in the Burgundian Gate and Germanic Basin re-
gions [28]. During the upper Ladinian the marine envi-
ronment of the Germanic Basin became brackish and a
lagoon until it finally disappeared, as did all of the pla-
codonts. In the meantime, other placodont genera (Macro-
placus, Protenodontosaurus, Psephoderma: Jaeckel,
1907 [2,3,33,55] developed from these or other related
placodonts in the shallow marine north-western Tethys
over a long time frame of about 30 My (chronostratigra-
phy: [56]; Figure 9), although their global distribution,
palaeobiogeography, and systematic relationships still
remain unclear as a result of the scarcity of the fossil re-
cord.
Similar trends to the tooth reduction within Triassic
placodonts (Figure 7) have also been reported in the
evolution of mammalian Sirenia [38] presented here for
Tertiary Protosiren, Halitherium and Dusosiren [37,42,
57]. Both, the early placodonts and early Sirenia, were

C. G. Diedrich / Natural Science 3 (2011) 9-27
2121
Figure 9. Palaeoenvironmental relationships of placodonts and evolutionary trends in Europe (Germanic Basin, north-western
Tethys). (Skulls compiled and redrawn from: [1,3,5,23,27-30,31,43,55,58]).
therefore in the early evolutionary stages of animals
adapting to feeding on aquatic seaplants. At the other
extreme, Henodus [58,59] whose systematic position is
unclear but is believed to be closest to Cyamodus [3],
represents the “final stage” of tooth reduction with only
a single tooth remaining in its skull and horny ridges
similar to those developed in turtles. In terms of the
“macroalgae feeding” evolutionary trend Henodus was
the most evolved, possibly even sometimes feeding on
jellyfish (as do turtles) during the Lower Keuper
(Carnian) when macroalgae in the Germanic Basin la-
goon facies were likely to have been either completely
absent or reduced to only a few specialized species. Pos-
sibly Placochelys and Henodus also developed possibly
horny pads, which can not be proven anatomically yet.
New discoveries of cyamodonts from China, such as
Psephochelys polyosteoderma (Chun and Rieppel, 2002)
[60], and other remains of uncertain origin from the
northern Tethys [61], demonstrate the incompleteness of
our knowledge of their phylogeny, particularly with re-
gard to the Upper Triassic fossil record.
The tooth reduction evolutionary trend in placodonid
reptiles is illogical in the context of the previously sug-
gested “durophagy”, but can instead be logically ex-
plained as being a result of the adaptation to macroalgal
plant feeding that started somewhere on the shallow ma-
rine carbonate platforms surrounding Pangaea during the
Lower Triassic (Figures 9, 10(a)). It may even offer an
explanation for the “origin of turtles” – in a similar
manner to the Henodus lineage – as toothless primary
algae and secondary jelly fish consumers that eventually
developed into terrestrial turtles such as the globally
distributed Norian Proganochelys (Gaffney, 1990) [62],
and marine sea-turtles [8,63] such as the Carnian Odon-
tochelys [8] of the western Tethys, from a starting point
in the Longobardian (or even earlier in the Fassanian),
when the palaeogeography of the shallow marine habi-
tats surrounding Pangaea went through dramatic changes
with extended shallow marine habitat disappearances
(Figures 9,10(e-f)).

C. G. Diedrich / Natural Science 3 (2011) 9-27 22
(a) (b)
(c) (d)
(e) (f)
Figure 10. Palaeobiogeography of placodonts in Europe (Germanic Basin, north-western Tethys).

C. G. Diedrich / Natural Science 3 (2011) 9-27
2323
The long period of turtle evolution [64] might be ex-
plained also as a result of these studies on placodonts.
All those reptiles may have fed on macroalgae already in
the Middle Triassic and even earlier up to Late Triassic,
but changes to the distribution of shallow marine areas
[44,45,56,64] and the conditions within them (Figures
9,10(a-f)), as well as to the distribution of the larger ma-
rine basins that formed their habitats within the Ger-
manic Basin [37], all seem to have played a major role in
furthering the evolution of the placodonts, and also of
the marine and terrestrial turtles that today are toothless
plant feeders and, to a lesser extent, jellyfish consumers.
4. RESULTS
The sauropterygian marine placodont reptile evolution
and diversification around Pangaea seem to have its ori-
gin already in the upper Lower Triassic and reached after
11 My (247-236 My) its extinction. Those former be-
lieved “durophagous” divers are presented here as
macroalgae sea plant eating adapted reptiles (Figures
11(a-d)), which were convergent developed in their
body shape and jaw function very close to such as siren
mammals, which evolved during the Tertiary in similar
ways by enhancing their body weights for long-term-
diving and reducing of their dentition to adapt onto sea-
plant feeding. Whereas Triassic placodonts Parapla-
codus (Figure 11(a)), Placodus (Figure 11(b)), Cyamo-
dus (Figure 11(c)), or Placochelys (Figure 11(d)), lived
in macroalage meadow shallow marine carbonate pa-
laeoenvironments in the Germanic Basin and northern
Tethys, extinct Tertiary or Modern Sirania such as
Halitherium or Dugong fed and feed on seagrass in
similar environments. The placodonts were distributed
already within the late Lower Triassic around Pangaea
and emigrated at the Aegean (lowermost Anisian) time
into the marine developing Germanic Basin, whereas
their distribution was depend on shallow marine carbon-
ate platform environments surrounding the Tethys Ocean,
and its marginal basins. Those habitats changed drastic-
cally within the Triassic, whereas an intracratonic Ger-
manic Basin as a main habitat became extinct within the
Late Triassic, and with this the placodonts. As a possible
reaction of the marine habitat extinction and new hyper-
saline lagoon developments at the Middle/Upper Triassic
in the Germanic Basin and northern Tethys those envi-
ronmental changes seem to have been the main reason
for the development from placodonts or to placodonts
parallel evolution of turtles, which are already nearly
(a)

C. G. Diedrich / Natural Science 3 (2011) 9-27 24
(b)
(c)

C. G. Diedrich / Natural Science 3 (2011) 9-27
2525
(d)
Figure 11. (a) Paraplacodus, (b) Placodus, (c). Cyamodus and (d) Placochelys during the Middle Triassic
(Illyrian-Longobardian) Upper Muschelkalk/Lower Keuper in the Germanic Basin as specialized macroalgae
feeders (Illustrations “Rinaldino”- G. Teichmann).
toothless in the early Late Triassic, an adaptation similar
onto algae and possibly even jellyfish consuming, and
end of tooth reduction trend.
5. ACKNOWLEDGEMENTS
I thank many museums and their curators (Senckenbergmuseum
Frankfurt, Naturkundemuseum Osnabrück, Naturkundemudeum Mag-
deburg, Lippisches Landesmuseum Detmold, Humboldt-Museum of
Berlin University, Staatliche Museum für Naturkunde Stuttgart, Mu-
seum für Natur und Umwelt Osnabrück, Urgeschichtliche Museum
Bayreuth, Muschelkalkmuseum Ingelfingen, and Naturkundemuseum
Erfurt) and research institutions (Martin-Luther-University Halle/Saale)
for their collections access and support. The reptile illustrations are pro-
duced with copyrights for the company PaleoLogic (www.paleologic.de)
by G. “Rinaldino” Teichmann, the spelling and general check of the
first manuscript was made by E. Manning.
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