Introduction
Predation plays an important role in modern and ancient ecosystems. Trophic
relationships based on predation not only shape communities but also affect
the evolution of both predators and their prey. Thus, fossil evidence of
predator–prey interactions is important for the understanding of energy flow
in ancient ecosystems, food webs, and the arms race between predator and
prey. Direct evidence of predation, such as preserved gut contents,
coprolites, or regurgitated masses containing identifiable prey remains are
rare and exceptional findings. The study of the signs and traces preserved
on the hard skeletal parts of prey organisms that reflect predatory behaviour
is therefore of exceptional importance. These traces (praedichnia) include
skeletal breakage, biting traces, borings, but also regeneration and repair
patterns mirroring unsuccessful attacks.
Among marine invertebrates, the study of predation traces found in echinoids
(Echinodermata) proves to be peculiarly rewarding. Echinoids are the diet of
a wide variety of marine predators including gastropods, crustaceans,
starfish, other echinoids, elasmobranch and teleost fish, turtles, birds and
marine mammals (Kowalewski and Nebelsick, 2003). Previously, most authors
focused on drilling predation by cassid gastropods (“helmet conchs”), since
their predatory strategy leaves the echinoid test entirely intact except for
a small but diagnostic borehole that can be easily recognized. In contrast,
the study of shell-breaking predation is more challenging, assuming that a
successful attack would result in the complete destruction of the echinoid
test. However, the stereomic nature of the echinoid skeleton favours the
preservation of biting traces (“tooth marks”) of unsuccessful attacks
(Kähn, 1928; Gripp, 1929; Thies, 1985), allowing conclusions to be drawn
as to the predator's taxonomic identity. Biting trace analysis is based on
the following two concepts or assumptions (Bowers, 2003): (i) the dental
characteristics of teeth involved in biting are unique in species level and
(ii) this asserted uniqueness is transferred and recorded in the biting
trace. As a consequence, ideally preserved biting traces allow conclusions to
be made about the predators' dentition and consequently their identity (e.g.,
Neumann, 2000; Jacobsen and Bromley, 2009). However, the shape and pattern of
biting traces is influenced by many factors, which not only include the
biters tooth morphology and dental arrangement, which in addition may vary
among individuals of one species due to ontogeny or individual wear, but also
by biting angle, power of biting force, and nature of the substrate.
Moreover, echinoids possess an enormous capability to survive and repair
traumatic damage of the test, even when the test is
penetrated (Bonasoro et al., 2004; Carnevali, 2006). Hence, skeletal
regeneration process of non-lethal lesions produced by a biting attack may
alter or even obscure the original shape of the biting trace.
Here, we describe an exceptional biting trace found on the
basal–ambital
region of a holasteroid echinoid Echinocorys ovata Leske, 1778 from the lower Maastrichtian
chalk of Hemmoor. Repair features indicate that the
echinoid survived this unsuccessful attack. The large size, circular outline,
and arc-shaped arrangement of tooth traces suggests that the echinoid was
bitten by a large predator possessing conical pointed teeth, most probably a
marine reptile. We hypothesize that the echinoid was bitten by a mosasaur
and experimentally tested this assumption by producing biting traces by
applying a mosasaur jaw model and echinoid clay dummies.
The large deposit-feeding echinoid Echinocorys
ovata (MB.E. 6565) from the lower Maastrichtian (Late Cretaceous) of
Hemmoor, exposing biting traces: (a) oral surface
(posterior to the right) with four tooth punctures (P1–P4); the anterior
part of the test is not preserved. (b) Enlarged oblique view showing
the broad linear score emanating from P4. (c) Enlarged view showing
details of puncture shape and regeneration features of P1 (left) and P2
(right). Note slightly irregular outline of P2 due to chipping. Note that the
echinoid's periproct is of comparable size and shape and should not be
confused with the punctures.
Material and methods
Our query for biting traces focussed on the echinoid genus
Echinocorys, a common epifaunal deposit-feeding echinoid with a
worldwide distribution known from the Turonian (Upper Cretaceous) to the
Palaeogene. The applicability of Echinocorys as a model taxon for
the ichnofossils-based study of biotic interactions (parasitism, predation)
in deep-time has been emphasized in case studies by Neumann and
Wisshak (2006, 2009) and Wisshak and Neumann (2006). We examined more than
7000 Echinocorys specimens obtained through our own fieldwork in the
chalk of northern Germany as well as from collection specimens stored at the
Natural History Museum, Berlin, the Geological Museum, Copenhagen,
the Swedish Royal Museum of Natural History, Stockholm and the Federal Institute for
Geosciences and Natural Resources (BGR) in Hanover, Germany. Our query
resulted in the recognition of numerous predation traces. Among these, a
peculiar and outstanding biting trace situated on the test of a large
Echinocorys ovata from the Maastrichtian of Hemmoor (MB.E. 6565,
stored in the Natural History Museum, Berlin) has been discovered which is
described in the present study (Fig. 1a–c).
For descriptive purposes, each tooth imprint has been numbered (P1–P4, see Fig. 1a).
Metric measurements of the test dimensions and the biting trace were taken
with the aid of a precision slide caliper. Terms generally used to describe
teeth (labial, lingual, mesial, distal) are used here to describe the tooth
traces, a practice commonly used in forensic bite mark analysis (Bowers,
2003). Photographic documentation of the trace from various angles has been
undertaken with a digital DSLR after coating the specimen with ammonium
chloride in order to bring out details of ornament.
For the execution of the biting experiment, a resin model skull with a
movable jaw and a total length of 53 cm was produced using the skull of the
type specimen of Prognathodon solvayi Dollo, 1889 as a reference
(Fig. 2). We decided to build such a model, because the preservation of the
original Prognathodon skull (isolated and fragile bones in a metal
frame) appeared unsuitable for either producing casts or applying bite
experiments. Our choice of Prognathodon solvayi is
explained by its completeness, and because it matches the requirements of
tooth morphology, tooth position, and stratigraphic and geographic appearance
(Lingham-Soliar and Nolf, 1989). Differences in dental ornamentation
occurring among different species of the genus (e.g., Lingham-Soliar and
Nolf, 1989; Christiansen and Bonde, 2002; Grigoriev, 2013) are negligible in
our experiment. In our experiment, biting traces from various angles were
produced by pressing the lower and upper jaw of the model into artificial
Echinocorys test “dummies” made of modeling clay. The resulting
traces were measured, photographed, and compared with the fossil biting trace.
The original skull of Prognathodon solvayi, IRSNB R33,
holotype, from the lower Maastrichtian of Mesvin, Belgium, which
has been used as a template for the reconstruction of the resin scale model.
Discussion
Predator bite vs. alternative explanations of trace formation
It is necessary to clarify if the observed set of punctures is actually a
biting trace or if other biogenic processes can come into consideration.
Circular holes or pits in echinoids are known to be produced post-mortem by the
drilling activity of bioeroders
(Rahman et al., 2015), whereas
circular pits may be formed syn vivo by the attachment of parasitic
or commensalistic symbionts (Neumann and Wisshak, 2006). A formation of the
trace after the echinoids' death can be excluded: the repair features (new
formation of skeletal tissue including formation of tubercles) clearly prove
that the trace has been formed in the lifetime of the echinoid and that the
lesion was not lethal. The echinoid survived at least long enough to repair
the injuries. Chipping associated with P2 and P3 and especially the linear
score emanating from the well-defined P4 clearly indicate that a physical
impact from below rather than a parasite infestation affected the sea urchin.
The circumstance that all four punctures are in the same state of
regeneration suggests that they were produced at the same time during a
single event. Together with the serial, semi-circular arrangement of the
punctures, it appears very likely that they were produced by the teeth of a
large animal. To produce circular punctures, conical or cylindroconical teeth
with pointed tips are required (Njau and Blumenshine, 2006).
However, it may appear puzzling that only P2 and P3 are circular in outline
and larger while P1 and P4 are smaller and ovoid in outline. A standard
mosasauroid jaw will produce the contrary pattern, as the first pair of teeth
is generally smaller than the following pairs. Why are there no further
punctures preserved? This pattern can be explained by the assumption that the
trace was produced by an animal with a prognathous (forward pointing)
tooth position. In this case, the frontal pair of teeth would penetrate much
deeper and at a steeper angle than the following pair of teeth, thus
producing large and circular punctures in contrast to ovoid and small
punctures by the following teeth, which hit the echinoid in an oblique
angle.
(a) Echinocorys ovata from the Maastrichtian of
Hemmoor (BGR 389a/3) in oral view showing biting traces induced by a teleost
fish or shark. (b) Detail of (a), showing set of tooth furrows and
regenerated fracture. (c) Aboral aspect of living echinoid
Spatangus purpureus from Hvar, Croatia (MB.E 11453) with a healed
non-lethal fracture affecting large regions of interambulacrum 4.
Thus, this distinctive biting trace pattern suggests that the bite must have
been produced by a predator with large cone-shaped teeth arranged
in a prognathous orientation.
Examples for excluded non-mosasauroid causers of the German
Echinocorys bite traces. Positions which are not congruent with the
finding are in bold.
Taxon
Representative source
Morphology
Age/Record
Elasmobranchii
Reif (1973); Motta (2004)
non-cone-shaped teeth
Cretaceous
Ichthyodectiformes (e.g., Xiphactinus)
Schwimmer et al. (1997b)
not all species possess procumbent anterior teeth
unknown from European Maastrichtian
Euteleostei
Friedman (2009)
often numerous fanglike conical teeth alternated by small teeth
Late Cretaceous
Dyrosauridae
Khosla et al. (2009)
narrow arc of front teeth, different tooth sizes
no contemporary records in Europe
Pliosauridae
Ketchum and Benson (2010); Fischer et al. (2015)
procumbent anterior teeth known
disappeared more than 20 Ma before Echinocorys record
Aristonectes parvidens (and the following = Elasmosauridae)
Cabrera (1941); Gasparini et al. (2003); O'Gorman (2016)
relatively large cranium (0.73 m length); jaw bones containing numerous tiny alveoles
Maastrichtian of the Weddellian Sea
Elasmosaurus platyurus
Sachs (2005)
prognathous dentition
early Campanian of Kansas
Hydrotherosaurus alexandriae
Welles (1943)
skull length only 0.33 m; remarkable irregular dentition, not procumbent
Campanian-Maastrichtian of California
Kaiwhekea katiki
Cruickshank and Fordyce(2002)
small and homodontous needle-shaped teeth; skull length 0.62 m
Maastrichtian of New Zealand
Libonectes morgani
Carpenter (1997, 1999)
premaxillae and dentary with prognathous anterior tooth pairs
early Turonian of Texas
Styxosaurus snowi
Welles (1952); Carpenter(1999)
prognath, but skull too short (0.47 m) and slender teeth
Coniacian–Santonian of Texas
Terminonatator ponteixensis
Sato (2003)
skull length 0.26 m; anteriorly procumbent teeth
late Campanian of Saskatchawan
Tuarangisaurus keyesi
Wiffen and Moisley (1986); Carpenter (1999)
long and narrow teeth, skull length 0.37 m; premaxillary teeth interlocked with dentary teeth which are meagerly prognath
Maastrichtian of New Zealand
Dolichorhynchops herschelensis (= Polycotylidae)
Sato (2005)
extremely narrow arrangement of teeth (in parallel)
late Campanian-earlyMaastrichtian of Saskatchewan
The impact leading to the formation of P4 hit the echinoid marginally at the
ambitus, causing not only a puncture, but also a long and broad score running
along the ambitus towards the lateral side of the echinoid.
The fact that the bite pierced the echinoid without crushing it provides
clues for the attacker's prey handling behaviour and biting mechanics. A
powerful snapping bite would pierce the echinoid, in contrast to a situation involving a careful, squeezing bite. In contrast to mollusc shells, where the
plywood-like arrangement of aragonite or calcite crystals leads to a lateral
deflection of the biting force and thus causes irregular breakage, the
meshwork structure of the echinoderm skeleton prevents such lateral biting
force deflection, leading to punctures mirroring the outline of the biter's
teeth (Thies, 1985; Neumann, 2000). Thus, biting traces observed on echinoid
skeletons are generally excellently preserved as may be illustrated by a
further example of another Echinocorys ovata specimen from the
Maastrichtian of Hemmoor (Fig. 3a, b), which exhibits a set of scores
produced by the teeth of a teleost or shark (Thies, 1985). Moreover, in this
case a large fragment of the plastron has been snapped off during the attack,
but the echinoid survived and was able to completely repair the fracture.
Another example (Fig. 3c) shows a specimen of the extant spatangoid
Spatangus purpureus Müller, 1776 from the Adriatic Sea near Hvar,
Croatia, exhibiting a large traumatic fracture affecting a large part of the
aboral interambulacrum 4. Amazingly, this large injury was not lethal to
the echinoid but has been completely repaired. Echinoids, as with most other
echinoderms possess a high potential to repair traumatic injuries of the
test, a striking adaptive strategy for survival exploited by this phylum
(Bonasoro et al., 2004).
Probable biting trace agents
Various trace makers can be excluded on the basis of the overall morphology of
their dentition. Examples of conceivable suspects of Late Cretaceous marine
vertebrates which were unable to produce similar biting traces are listed in
Table 1. Late Cretaceous large teleost fishes do not match the
requirements needed to produce the kind of traces found in the described Echinocorys specimen. Potential teleost candidates have large,
conical teeth that alternate with small teeth, a condition found in most
predatory teleosts of the Late Cretaceous (see Friedman, 2009). Cretaceous
sharks in question, e.g., the widely distributed Squalicorax, do not
possess the cone-shaped teeth able to induce rounded biting traces as seen in the Echinocorys in question (Reif, 1973; Schwimmer et al., 1997a; Motta, 2004;
Becker and Chamberlain, 2012).
In dyrosaurid crocodylians, the premaxillae are relatively narrow and very
slightly inflated laterally relative to the width of the maxillae, resulting in
a narrower arc of front teeth. Each premaxilla bears four alveoli, the
first tooth usually being the smallest, the third tooth being the largest
(e.g., Jouve et al., 2006). Dyrosaurids are documented from Late Cretaceous
to Eocene sediments along the Atlantic Ocean and Tethys Sea margins (Khosla et
al., 2009). Certain remains of the Dyrosauridae of predominantly
Maastrichtian age have been found on the east coast of North America (Denton et
al., 1997), in Argentina (Gasparini and Spalletti, 1990), Mali (Brochu et al.,
2002), Sudan (Salih et al., 2016), and India (Rana and Sati, 2000). No
contemporary remains are recorded to date from European deposits. Only
uncertain remains of Cenomanian age have been found in Portugal (Buffetaut and
Lauverjat, 1978).
Pliosaurs disappeared more than 20 Ma before the Echinocorys record
described here, with the Turonian Brachauchenius lucasi as the last survivor of the group (Hampe, 2005; Ketchum and Benson,
2010; Fischer et al., 2015). Other sauropterygians can also be excluded with
high probability, although recent discoveries of elasmosaur stomach contents, mainly including
benthic invertebrates, have been reported from the Lower Cretaceous in
Australia (McHenry et al., 2005). The Australian research group suggests a
bottom feeding habitus for these Late Mesozoic long-necked plesiosaurs
usually interpreted as fish- and squid-feeders.
Result of bite mark experiment: (a) Echinocorys
clay dummy with superimposed shadow of a mosasaur upper jaw showing four
biting traces artificially produced by the two anteriormost (premaxillary)
tooth pairs of a mosasaur scale model. (b) Upper jaw of the mosasaur
scale model superimposed over the Echinocorys ovata biting trace
demonstrating conformity of tooth traces and globidensine mosasaur tooth
arrangement. (c) Lateral view of the mosasaur scale model
reconstructing the biting angle which produced the distinct biting trace.
Note the prognathous arrangement of the premaxillary teeth and their
different penetration angle and depth. The shaded area of the echinoid is not
preserved in the original.
Although some elasmosaurs possess prognathous anterior tooth pairs, tooth and
skull sizes in most cases do not correspond to the traces found in the
Echinocorys test. Moreover, these sauropterygians lived in a
different time-slice (see Table 1). The fragility of the elasmosaurid skull and
the needle-like associated teeth are probably more suitable for piscivorous
feeding or soft prey diets (see above, and Massare, 1987; Everhart, 2005).
For the time being we cannot prove the possibility of sauropterygians
producing the biting trace because of the incompleteness of records in the
latest Mesozoic. Several isolated vertebrae and insignificant teeth have been
reported from the latest Cretaceous worldwide (Mulder et al., 2000; Vincent
et al., 2011), but the discussion shows that the type of teeth and the
usually significantly lower degree of prognath anterior dentition
excludes the possibility that the echinoid was bitten by a
plesiosaur.
A closer look at the morphology of the jaws and dentition of the Late
Cretaceous rulers of the epicontinental seas, the mosasauroids, reveals eliminating factors that leave only a few taxa as candidates for having caused the biting trace on the investigated
Hemmoor echinoid. The Tylosaurinae, for example, could be withdrawn from the
list because they do not possess terminal teeth. Tylosaurinae developed
large, prolonged rostra anterior to the premaxillary teeth (e.g., Russell,
1967; Bullard and Caldwell, 2010). The Plioplatecarpinae, the second group of
derived Russellosaurina have short heads, but a predental rostrum is absent.
As a typical representative, Platecarpus, which occurred from the
Santonian to early Campanian, has recurved, not procumbent teeth (Russell,
1967; Konishi and Caldwell, 2007), and is also an older form.
Plioplatecarpus has proportionally small anteriormost teeth but they
can be somewhat procumbent (Holmes, 1996). Halisaurus has small,
recurved premaxillary teeth with delicate crowns (Bardet et al., 2005a).
Clidastes of the Mosasaurinae could also be released as the biting
trace producer because of its small and not procumbent anterior teeth
(Williston, 1925; Russell, 1967). Species of the eponymous genus
Mosasaurus possess procumbent premaxillary teeth to a certain
degree, but they are proportionally small (see Lingham-Soliar, 1995; Mulder,
1999; Konishi et al., 2014) and certainly not favoured to handle sea urchins
with effortlessness.
Having eliminated the above taxa, we are left with the globidensine
mosasaurs as contenders, in which Prognathodon has large
forward-pointed conical teeth and was widespread in late Campanian to early
Maastrichtian strata of the Cretaceous North Sea basin (Dortangs et al.,
2002; Machalski et al., 2003;
Lindgren, 2004). Early
ontogenetic stages of other Globidensini (Carinodens,
Globidens) may also show prognath dentition (see below) and thus may
also come into consideration as producers of the biting trace. At least,
Carinodens also occurs in the Maastrichtian chalk of the North Sea
basin (Milàn et al., 2017). The likelihood that Mosasaurus hoffmanni (Mulder, 1999), a common inhabitant of the North Sea basin, is the
agent is not very probable as outlined in the paragraph before.
Experimental testing of the globidensine mosasaur hypothesis
We tested our hypothesis using techniques adopted from forensic odontology
(ABFO, 1986; Rai et al., 2006) applying biting experiments with an
original-scale resin model of the skull based on the holotype of
Prognathodon solvayi (Dollo, 1889) and Echinocorys clay
dummies. Casts of original Prognathodon teeth from Maastrichtian
phosphates of Morocco were applied imitating the dentition. We produced a
series of biting traces applying varying biting angles and biting forces. The
tooth traces on the fossil echinoid and those produced by the anteriormost
teeth of the upper jaw (premaxillae) in our experiment show a striking
resemblance (Figs. 1a, 3a) where P1 has been produced by the second left
premaxillar tooth, P2 by the first left, P3 by the first right and P4 by the
second right premaxillar tooth (Fig. 4a–c).
Artist's view of a globidensine mosasaur (based on the
Prognathodon model) at the bottom of the chalk-sea feeding on
benthic Echinocorys sea urchins (artwork: Elke Siebert).
We could state that the producer was very probably a representative of the
Globidensini (Fig. 5). In our experiment, the traces fit in size and
arrangement with the fossil finding. Nevertheless, other globidensine
taxa besides Prognathodon
cannot generally be excluded. Globidens and Carinodens can also
have anterior prognathid dentition (e.g., Bardet et al., 2005b; Schulp,
2005) and there is an ontogentic change in tooth form documented for
Globidens (Polcyn and Bell, 2005), with young animals possessing a
very Prognathodon-like dentition. Regarding size and proportion, a
producer of an early ontogenetic stage is very unlikely in our case.
Palaeoecological and palaeobiological implications
Mosasaur biting traces (including Prognathodon) on invertebrate
skeletons have been documented so far from the shells of ammonitid and
nautilitid cephalopods (Kauffman and Kesling, 1960; Kauffman, 2004),
demonstrating that non-durophagous mosasaurs were able to feed on
hard-shelled, nektonic prey. Although some authors questioned whether these
traces were produced by biting mosasaurs (Seilacher, 1998), further work on
this subject has accumulated much evidence that this interpretation was
correct (Tsujita and Westermann, 2001). So far, no evidence of mosasaur
predation on echinoids exists. Dollo (1913) mentioned a
Hemipneustes echinoid as prey of Prognathodon. The
echinoid was deposited above the mosasaur skeleton at a later time interval,
but was superimposed on the mosasaur skeleton, due to low sedimentation rate
(John W. M. Jagt, personal communication, 2008). Thus, this finding
represents a preservation effect rather than predation evidence. Donovan et
al. (2008) suggested mosasaur predation on Hemipneustes, but the
evidence is poor and must be regarded as dubious since it is merely based on
a single round pit on the echinoid test.
Our hypothesis challenges the common assumption that the nature of the prey
is tightly constrained to the prey type considered optimal for a given
predator tooth morphology. Globidensine mosasaurs have a powerful jaw with
large blunt to pointed conical teeth pre-adapted for large, fleshy but also
osseous prey (see also Massare, 1987). Our example illustrates that
Globidensini such as Prognathodon were also able to feed on benthic
armoured invertebrates such as sea urchins. Indeed, the prognathous alignment
of Prognathodon's anteriormost teeth appears to be suitable for
picking up benthic prey from the sea floor. Moreover, we argue that top
predators such as Prognathodon probably possessed considerable
variation in foraging and feeding techniques, including benthic foraging. We
assume that in the pelagic and nutrient-poor chalk-sea (e.g., Voigt and
Schönfeld, 2010; Engelke et al., 2016; Linnert et al., 2016), advantage
was taken of sea urchins, which were probably the most frequently encountered
prey (Fig. 5). Predatory damage caused by vertebates observed in sea urchin
tests from the chalk (e.g., Gripp, 1929; Thies, 1985; Neumann, 2003) suggests
that they probably represented a major food source for a variety of
predators,
and most likely played an important role in the benthic-pelagic coupling and
the energy flow in the chalk-sea ecosystem. In modern seas, vertebrate
predators such as the Atlantic wolffish (Anarhichas lupus)
depend on sea urchins as a food source when their preferred prey is scarce or
absent; even though the energy content of sea urchins is relatively low (Liao
and Lucas, 2000). In the Late Cretaceous of NW Europe, mosasaur remains are
more common in neritic sediments (Machalski et al., 2003; Lindgren and Jagt,
2005; Hornung and Reich, 2015; Sachs et al., 2015) than in the pelagic chalk,
suggesting a preference for nearshore habitats. Although the European pelagic
chalk-sea was probably not the optimal habitat for mosasaurs, their
capability for opportunistic feeding allowed them to exploit available
nutrient resources in this harsh environment, which otherwise might have
represented a barrier for mosasaur migration and dispersal.
A modern analogy for this hypothesis is documented for other marine top
predators, like tiger sharks (Galeocerdo cuvieri). They are able to
change or modify their foraging tactics with changing habitats and changing
food availability (Compagno, 1984). A large dietary range has also been
reported, for example, from toothed whales (Orcinus orca: Tomilin,
1957; Jefferson et al., 1991; Visser, 2005). Furthermore, juvenile
saltwater crocodiles (Crocodylus porosus) already show opportunistic
food intake (Taylor, 1979). Among mosasaurs, Tylosaurus proriger has
a well documented history of being an opportunistic feeder with stomach
contents revealing teleost remains, other mosasaurs (Clidastes),
plesiosaurs, and Hesperornis (Martin and Bjork, 1987; Everhart,
2004). Cephalopods are also reported from stomach contents of
Tylosaurus and possibly from Progathodon (Konishi et al.,
2014). However, the dentition tells us that Globidens,
Carinodens, and Prognathodon were likely better suited for
crushing hard shelled prey than other mosasaurs (Lingham-Soliar, 1999; Bardet et
al., 2005b; Schulp, 2005; Martin and Fox, 2007), but they could also process
the ordinary mosasaur diet. Other mosasaurs mostly killed their prey with powerful
bites and swallowed it whole (Everhart, 2017). Robbins et al. (2008)
utilized carbon stable isotopes to demonstrate segregation of foraging habits
in mosasaurs at both a taxonomic and ontogenetic level. According to their
results, adult Prognathodon and Globidens both exhibit an
enriched δ13C value, indicating a long submergence, suggesting
bottom feeding for both genera (see for more detailed discussion to effects
regarding body size and diving in Schulp et al., 2013).