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The Cleveland Member (Late Famennian) fish fauna represents one of the most speciose and well-collected faunas from the Late Devonian; however, our understanding of the fauna's paleoecology is limited. Published interpretations of placoderm paleoecology typically suggest that most species are obligate bottom-living forms or are tied closely to life on the bottom. Dunkleosteus terrelli (Newberry, 1873) (Placodermi, Arthrodira), however, was a pelagic organism. This interpretation is based on an analysis of the nature of the distal Appalachian Basin depositional habitat and the distribution of this species' remains within the basin. The species presence as fossils represents the remains of organisms that lived within the water column in the basin rather than an allochthonous accumulation of floating carcasses. Thus, the disarticulation seen in specimens of Dunkleosteus terrelli is a result of local post-mortem flotation and possible scavenging within the water column. This hypothesis is supported by a Chi-squared statistical analysis for the geographic distribution of Dunkleosteus terrelli fossil remains. Finally, estimates of dry and wet weights for Dunkleosteus terrelli suggest (1) an inability to rest on the bottom given the physical properties of the sediments and (2) the presence of a mechanism for static buoyancy, which would account for post-mortem flotation in a basin with published estimated depths of 30 to 100 meters.
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The Cleveland Museum of Natural History
November 2010 Number 57:36–45
Department of Biological Sciences
Ohio University, Athens, Ohio 45701-2979
The Cleveland Member (Late Famennian) fish fauna represents one of the most speciose and
well-collected faunas from the Late Devonian; however, our understanding of the fauna’s
paleoecology is limited. Published interpretations of placoderm paleoecology typically suggest that
most species are obligate bottom-living forms or are tied closely to life on the bottom. Dunkleosteus
terrelli (Newberry, 1873) (Placodermi, Arthrodira), however, was a pelagic organism. This
interpretation is based on an analysis of the nature of the distal Appalachian Basin depositional
habitat and the distribution of this species’ remains within the basin. The species presence as fossils
represents the remains of organisms that lived within the water column in the basin rather than an
allochthonous accumulation of floating carcasses. Thus, the disarticulation seen in specimens of
Dunkleosteus terrelli is a result of local post-mortem flotation and possible scavenging within the
water column. This hypothesis is supported by a Chi-squared statistical analysis for the geographic
distribution of Dunkleosteus terrelli fossil remains. Finally, estimates of dry and wet weights for
Dunkleosteus terrelli suggest (1) an inability to rest on the bottom given the physical propertiesof
the sediments and (2) the presence of a mechanism for static buoyancy, which would account for
post-mortem flotation in a basin with published estimated depths of 30 to 100 meters.
Our understanding of the paleoecology of Late Devonian fish
faunas is limited by the lack of high diversity faunas available for
analysis. The Cleveland Member (Late Famennian) fauna is one
of the best known, possessing 66 species (Hlavin, 1976; Denison,
1979; Williams, 1990; Carr, 1996; Carr and Jackson, 2010)
consisting of sharks (33), placoderms (28), and osteichthyans (5).
Paleoecological interpretations of the fauna historically were
based on analogy to putatively similar taxa from other faunas.
Coccosteus (Miles and Westoll, 1968), for example, is often used
as an analogue to interpret other arthrodires (e.g., Heintz, 1932,
p. 202, used the form of Coccosteus to interpret ‘‘Dinichthys’’ or
Dunkleosteus, a comparison across subordinal-level distinct taxa).
These analogies often involve the comparison of taxa from
dissimilar facies and faunal compositions.
Hlavin (1976) reviewed sedimentological-depositional models
to interpret the Catskill Delta and the distal black shales (the
Cleveland Member of the Ohio Shale along with other regional
shales and their biostratigraphic relationships) and provided an
up-to-date faunal analysis for the Cleveland Member. However,
he did not consider the impact of sedimentological interpretations
on the potential lifestyles of the fishes associated with the
depositional habitat of the Cleveland Member.
Williams (1990) provided the first comprehensive predator-prey
analysis of the Cleveland Member fish fauna. His analysis
centered on chondrichthyans due to their unusually complete
preservation including stomach contents (53 specimens reported
with identifiable gastric contents, Williams, 1990, p. 280).
Evaluation of non-chondrichthyans was limited to a few cases
of associated remains possibly suggesting predator-prey relation-
ships (the presence of a shark spine imbedded in the oral region of
an arthrodire; associated arthrodire and shark remains suggesting
a possible predator-prey interaction; and a palaeoniscid with
shark and arthrodire stomach contents; Williams, 1990, p. 286–
Despite the vast amount of effort dedicated to the sedimento-
logical history of the Appalachian Basin (e.g., Woodrow and
Sevon, 1985) and the long history of vertebrate paleontology in
the region, little work has addressed the paleoecology of the
vertebrate fauna associated with these sediments. The placo-
derms, which represent the most speciose Devonian clade (Carr,
1995) and numerically the largest part of regional collections,
represent the least known clade in terms of their paleoecology.
As seen in the examples above, estimates concerning the
paleoecology of Paleozoic fishes are based on evidence such as
stomach contents, body form, or indirectly on facies analyses.
Little is known concerning placoderm stomach contents, although
Dennis and Miles (1981) and Miles and Westoll (1968, p. 462)
have evaluated stomach contents in Incisoscutum and Coccosteus,
respectively. The interpretation of gnathal-plate morphology is
used to suggest specializations for various feeding strategies such
as durophagy or piscivory (Denison, 1978, p. 17). However, the
feeding strategies of generalized forms or forms without recent
analogues are difficult, if not impossible, to interpret without
additional supporting evidence.
Lindsey (1978) and Webb (1982) used body form to interpret
locomotor styles among extant fishes. Among placoderms this
approach has found limited application being applied primarily to
laterally and dorsoventrally compressed forms. Miles (1969,
p. 129; see also Moy-Thomas and Miles, 1971, p. 175) suggested
a nektonic lifestyle for the laterally compressed brachydeirid
arthrodires, while several authors (Stensio¨, 1963, p. 13; Miles,
1969, p. 129; Moy-Thomas and Miles, 1971, p. 185, 197–198)
interpreted dorsoventrally compressed forms, such as the raylike
rhenanids, as bottom living. Between these extremes, a vast
number of taxa are interpreted generally as bottom living
(Stensio¨, 1963, p. 13) or specialized for ‘‘life just above the
bottom’’ (Moy-Thomas and Miles, 1971, p. 197).
Finally, analysis of the facies in which placoderm fossils are
found has provided an additional source of information, but relies
on the assumption that fishes lived in a habitat on or above the
accumulating sediments. The utility of this approach is difficult to
evaluate when post-mortem transport is involved and the fishes
lived outside the area of deposition.
Of the methods that have been used to determine aspects of
placoderm paleoecology, none individually can provide a
complete picture, and thus far, few attempts have been made to
combine analyses. Ideal preservation, having complete organisms
preserved with soft tissue and stomach contents, is an exceedingly
rare occurrence. Within the Devonian, this ideal is achieved
among the chondrichthyan remains found in carbonate concre-
tions from the Cleveland Member (Williams, 1990). For
placoderms, researchers have retrieved some of the best-preserved
fossil material from the Hunsru¨ckschiefer of Germany (Lower
Emsian) and the acid-prepared concretions from the Gogo
Formation of Western Australia (Frasnian). Even in these
conservation Lagersta¨ tten (sensu Seilacher, 1990) information is
lost. However, a total-evidence approach including, e.g., sedi-
mentology, geochemistry, and taphonomic mechanisms, may
recover sufficient information to reconstruct details of life history
and paleoecology (Tasch, 1965; Elder and Smith, 1988).
The Catskill Delta and its associated foreland basin (Appala-
chian Basin, Figure 1C) provide a unique opportunity to analyze
the paleoecology of fishes in the region. The black-shale facies
found within the distal basin (Figure 1D) represent an anoxic
depositional environment, potentially inhospitable to benthic
organisms or capable only of supporting a low-diversity fauna
(see Discussion below). Fossil fishes found in these shales
represent either fishes living and dying within the region, post-
mortem allochthonous accumulations of floating carcasses, or a
mixture (Brett and Baird, 1993, p. 254).
Historically, most placoderms have been seen as obligate
benthic organisms, which accessed the water column only to feed
(Stensio¨, 1963, p. 13; Moy-Thomas and Miles, 1971, p. 197;
Denison, 1978, p. 17), suggesting that the presence of carcasses
within the low-diversity deep basin represents an accumulation of
remains originating in the established benthic communities of the
aerated shallow regions of the basin. To evaluate this hypothesis
in one species, the current study analyses the distribution of
gnathal elements from Dunkleosteus terrelli (Newberry, 1873)
within the Appalachian Basin. Dunkleosteus terrelli (Figure 2A–
C) is a large arthrodire (4.5–6 m in length) found within the
Appalachian Basin. This species is oval in cross-section and
possesses a well-developed dermal skeleton (Figure 2B, C) with
individual bones up to 5–7.5 cm thick along the lateral and
occipital thickenings of the head shield. The gnathal elements (IG,
ASG, PSG, Figure 2B, C) in this species represent bones that
detach relatively early in the disarticulation process (see Scha¨fer,
1972, p. 49–91, and Elder, 1985, for a discussion of disarticulation
patterns) and would be expected to accumulate close to the site of
death (secondary transport after these remains settle is discussed
below). The geographic distribution of the gnathal elements for
Dunkleosteus terrelli does not support a hypothesis where remains
of this arthrodire represent long-distance floating of carcasses.
Methods and Materials
Dunkleosteus terrelli specimens from the Cleveland Member
(Upper Famennian; postera to late expansa–early praesulcata
conodont zones, Zagger, 1995) of the Ohio Shale analyzed in this
study are housed within the collections of the Cleveland Museum of
Natural History. Data were retrieved from the Museum vertebrate
paleontology catalog using those specimens with entries providing
detailed locality information. The presence or absence of individual
plates indicated in the catalog was confirmed from the actual
specimens. The specimen localities included seven north-south river
or creek basins and the excavation site for Interstate-71 in northern
Ohio near Cleveland (Figure 3). These localities were grouped into
five drainage systems representing five north-south transects in
northern Ohio paralleling the ancient Catskill shoreline (from west-
to-east: 1, Huron and Vermillion Rivers and Chance Creek; 2,
Beaver Creek; 3, Black River; 4, Rocky River; 5, Big Creek and
Interstate-71). These drainage basins expose the black shales
(Cleveland Member) of the distal foreland basin. Recorded
specimens included those with and without gnathal elements and
isolated gnathal elements. The relative abundance of specimens
with and without gnathal elements was evaluated for the localities
listed above.
A Chi-squared nonparametric analysis for k independent
samples (Equation 1; Siegel and Castellan, 1988) was conducted
to test the null hypothesis (H
) that the proportion of specimens
with and without gnathal elements was statistically the same in
each geographic sample (i.e., there was no geographic trend and
the distribution of fossils was random). The research hypothesis,
based on published interpretations of placoderm paleoecology
(e.g., Stensio¨ , 1963; Denison, 1978), was that the proportions
differ across samples (an increased concentration of gnathal
elements shoreward, suggesting an allochthonous accumulation
of fossils within the distal basin from an eastward shallower
source). A significance level of alpha 5 0.05 was chosen with a
sample size of n 5 199.
j~ 1
{N ð1Þ
5 number of expected cases in the ith row of the jth column
when the null hypothesis is true
i 5 variables within the r 3 k contingency table
j 5 groups within the r 3 k contingency table
5 number of observed cases categorized in the ith row of the
jth column (Siegel and Castellan, 1988, p. 191)
Dry weight represents the weight of a living specimen out of
water, while wet weight is its weight in water. Body mass (wet
weight) is an important factor influencing buoyancy (static or
dynamic) in active swimmers, equally important to benthic forms
when considering an unstable substrate, and an important
consideration in the post-mortem transport of carcasses. To
evaluate locomotor patterns in Dunkleosteus terrelli, estimates of
its wet and dry weight were calculated based on a conservative
comparison to extant Western Atlantic sharks and ten Pacific
examples for three species with limited or no Atlantic weight data.
A total of 59 length-weight examples from 18 species were used
(Bigelow and Schroeder, 1948, all of their reported length-weights
were included in this analysis). Bigelow and Schroeder (1948)
provided a conversion factor for dry to wet weight in sharks (5.5%
for noncontinuous and 2.6% for continuous swimmers) that was
applied to Dunkleosteus terrelli. The lack of bone in sharks gives a
conservative estimate for dry and wet weight in D. terrelli;
however, the purpose of this estimate was to evaluate its impact
on potential settling in bottom sediments and buoyancy (static or
After Allison et al. (1995, p. 98), the root aerobic is used
here to refer to ‘‘modes of life or biofaces,’’ while oxic refers
to oxygen levels within the environment. The specimen-
number prefix CMNH denotes specimens from the Cleveland
Museum of Natural History Department of Vertebrate
The raw data of specimens with or without gnathal plates are
summarized in an r 3 k contingency table (Table 1). A Chi-
squared analysis for the five independent samples gives a Chi-
square of 6.1 with four degrees of freedom. The critical Chi-
square value for alpha 5 0.05 is 9.49, thus the null hypothesis (H
Figure 1. A, paleogeographic reconstruction with paleolatitudes based on Middle Devonian map of Streel et al., 2000. B, detailed
reconstruction of North American and northern European blocks after Ettensohn (1985, fig. 2).The position of Ohio (OH) is indicated
and probable landmasses are shaded (LL5 mountains). C, close-up of the western Appalachian Basin in Ohio and other regional
basins (from Elliot et al., 2000, fig. 2). States (dash, double-dotted lines), Great Lakes’ boundaries (dotted lines), basin depositional
contours (solid thin lines), hypothesized basin boundaries (dashed lines), and regional geographic highs (solid thick lines) are indicated.
D, schematic cross-section of the Ohio Shale (Cleveland and Huron Members) and the laterally equivalent shoreward
(eastward) sediments.
38 CARR No. 57
is not rejected (0.10 , P , 0.20). An analysis of three samples
(transects 1, 3, and 5, thus eliminating any potential geographic
overlap of adjacent transects) does not lead to a rejection of the
null hypothesis (X
5 0.64, d.f. 5 2; 0.70 , P , 0.80).
The log of dry weights versus log of body length for 59 extant
sharks (taken from Bigelow and Schroeder, 1948) provided an
estimated dry weight for Dunkleosteus terrelli (Figure 4). Dry
weight for a 4.6 m (15 ft) Dunkleosteus terrelli is estimated at
665.0 kg (1466.3 lbs). A wet weight estimate is 36.6 kg (80.6 lbs;
or using a continuous swimmer conversion factor, 17.3 kg
(38.1 lbs)).
Given the precautions concerning interpreting paleoecology
based on a fragmentary fossil record (e.g., Elder, 1985; Elder and
Smith, 1988; Allison et al., 1991), it is important to evaluate all
available evidence. Elder and Smith (1988) presented a method to
interpret fish ecology from taphonomic evidence. Using principles
of information theory (Tasch, 1965) in their studies of fish
taphonomy within lakes, they noted that taphonomic processes
provide an informational trade-off. As information about an
organism is lost during the taphonomic process, these processes
themselves provide new information about the organism’s
physical and chemical environment and the post-mortem history
of the organism. Taphonomy provides not only information on
the environment of burial, but if explored fully, the post-mortem
history may reveal details about the source of these organisms and
their life habitat. Unlike a laboratory experiment where individual
variables can be controlled, historical events require a thorough
consideration of all taphonomic variables.
The fossil record for placoderms is meager, limiting evaluation
of habitat and life history. Within the Appalachian Basin,
Dunkleosteus terrelli is numerically the predominant Late
Devonian vertebrate fossil. It is one of the best-described
members of the fauna (Heintz, 1932) and is recognized easily
due to its large size and distinctive osteology relative to the other
large members of the fauna (e.g., the thin plates and unique
morphology of Titanichthys clearly distinguish this taxon from
Dunkleosteus within the Cleveland Member fauna). Dunkleosteus
terrelli specimens range in estimated size from 25 cm to 6 m with
some even smaller specimens questionably attributed to the
species. This size range represents a wide range of age classes.
The available Dunkleosteus terrelli material housed within the
Cleveland Museum of Natural History was collected over many
decades. Peter A. Bungart (Hlavin, 1976) collected, from 1923 to
1946, the bulk of the material in the region of Cuyahoga and
Lorain Counties in northern Ohio (Figure 3), diligently and
meticulously recording all material associated with a single
individual even when collecting continued over several years.
This collection possibly represents the best Paleozoic material
amenable to statistical analysis with the least amount of sampling
Although questions remain concerning the origin of anoxic
basins (Hoover, 1960, p. 31; Degens et al., 1986; Wignall, 1994;
Schieber, 1998), a number of factors are common to the various
hypotheses relating to the development of black shales such as the
Cleveland Member, including: (1) minimal or lack of available
oxygen (Hoover, 1960; Heckel, 1972; Degens and Ross, 1974;
Brumsack and Thurow, 1986; Wignall, 1994; Allison et al., 1995);
(2) presence of hydrogen sulfide representing a potential toxin
(Hoover, 1960, p. 34; Heckel, 1972; Jannasch et al., 1974); (3) an
unstable substrate consisting of fine sediments with high porosity
(Keller, 1974); (4) water depths with low light or below the photic-
zone (additionally, below wave base), and (5) limited or lack of
trace fossils and bioturbation (bioturbation in the Cleveland
Member is reported by Lewis, 1988, p. 25; Schieber, 2003, notes
Figure 2. A, life reconstruction of Dunkleosteus terrelli in pursuit of Cladoselache. Redrawn from Carr (1995, fig. 17). B and C, anterior
and right lateral views of D. terrelli ossified skeleton (composite after Heintz, 1932 and 1968). ASG 5 anterior superognathal, IG 5
inferognathal, PSG 5 posterior superognathal, dashed lines 5 sensory canal grooves.
the unrecognized presence of bioturbation in black shales; see
Brett and Allison, 1998, for a review of paleontological
approaches to interpreting the environment of deposition). The
combined effects of the first four factors above clearly impact the
potential for the establishment of a benthic community. Common
to all models are anoxic pore waters (factor 1). What differs is
whether anoxia extends from the sediment-water interface up into
the water column. Published accounts of the benthic community
associated with black shales (factor 5) range from a complete
absence of benthic organisms (Hoover, 1960, p. 32, 42; Conant
and Swanson, 1961, p. 56–62; Heckel, 1972; Hlavin, 1976; Allison
et al., 1995, p. 100) to low diversity communities capable of
tolerating the physical properties of the depositional environment
(anoxic-dysoxic, stable or intermittent; hydrogen sulfide; depths
potentially below the photic-zone; and unstable substrate;
Hannibal et al., 2005). Detailed geochemical (e.g., degree of
pyritization) and published sedimentological data for the Cleve-
land Member are limited, although recent studies of laterally
equivalent black shales, other non-contemporaneous black shales,
or modern examples provide useful analogues for the Cleveland
Member (hopefully similar studies will be expanded to include the
Cleveland Member).
Modern oxygen-minimum zones that intersect the sediment-
water interface are known from numerous regions of high
primary productivity, for example, the northwest Indian Ocean
(Degens et al., 1986) and the Gulf of California (Brumsack and
Thurow, 1986). However, there are only a few examples of
potential analogues for stagnant foreland basins, including the
Black Sea (Degens and Ross, 1974) and Norwegian fjords
(Brumsack and Thurow, 1986). The Black Sea, a potential
modern analogue to the ancient distal Appalachian Basin, is
characterized by a reduced sedimentation rate (variable within the
basin) and high sediment porosity with the sediments containing
greater than or equal to 71% water by volume (Keller, 1974).
Sedimentation occurs within an anoxic water column with toxic
levels of hydrogen sulfide. The upper few millimeters of sediment
Figure 3. Drainage map of northern Ohio showing the seven river or creek drainages that provided specimens in the current study. Inset
map of Ohio indicates the study area (box). Labels: counties, intermediate sized font; and creeks/rivers, small sized font. Compiled from
Ohio Department of Natural Resources, Principal streams and their drainage areas (map), 1985, and U.S. Geological Survey Kipton,
Ohio, 7.5-minute quadrangle topographic map, 1969.
40 CARR No. 57
are unable to support even the remains of microorganisms. The
next 30–60 cm are reported to have the consistency of a ‘‘slurry’’
(Keller, 1974, p. 333). Shear strength does not improve within the
sediments until a depth of approximately 140 cm (Keller, 1974,
fig. 2). Associated with the anoxic water column and presence of
hydrogen sulfide is the formation and accumulation of iron
sulfides (Berner, 1974; Rozanov et al., 1974). Neither the Black
Sea or fjords (episutural basin of extreme depth or drowned
glacial valley with restricted surface area, respectively) are truly
representative of the large epicontinental basins of the Devonian;
however, the muds deposited in these anoxic environments share
the physical properties of sediments that potentially form black
The Cleveland Member (Nelson, 1955; Hoover, 1960; Mausser,
1982) is typical of black shales with a high organic content
accumulated in an anoxic environment. The shale produces a
petroliferous odor when freshly broken due to this organic
content (Hoover, 1960, p. 23). Its composition of fine sediments,
with little or no carbonates in the bulk of the shale (Mausser,
1982, p. 86; although discontinuous cone-in-cone and carbonate
concretions are present), suggests an unstable high-porosity
substrate at the time of deposition (estimated water content at
the time of deposition for the Cleveland Member is 75–80%, J.
Schieber, personal communication, 2004). The presence of
disseminated pyrite (Mausser, 1982) throughout the formation
indicates the presence of hydrogen sulfide at the time of
In northern Ohio, Upper Devonian Appalachian Basin rocks
are exposed in a number of north-south river basins that drain
into Lake Erie (Figure 3). These river basins represent a series of
north-south transects that roughly parallel the ancient Catskill
Delta (Figure 1B, C) thereby providing a series of proximal-to-
distal samples within the black shales of the Appalachian Basin.
The Cleveland Member represents the distal element of the basin
facies that are laterally equivalent to the eastward slope deposits
of the Chagrin Member (Figure 1D). This shoreward facies
transition is a temporally repeated pattern within the Appalachian
Basin as noted by Baird and Brett (1986) for the Genesee sequence
of New York.
Evaluation of the paleogeographic distribution of Dunkleosteus
terrelli provides two potential hypotheses: (H1) D. terrelli lived,
died, and was preserved within the basin; (H2) D. terrelli lived and
died elsewhere, but was transported into the basin where it was
preserved. The paleogeography of D. terrelli is reflected in the
nature and distribution of its remains within the Cleveland
Member. Hypothesis 1 would suggest the possibility that remains
of this species are either intact (no disarticulation due to post-
Table 1. Contingency tables (r 3 k) for the Dunkleosteus terrelli specimens in the current study. Table for five river/creek drainage
basins represents north-south transects paralleling the ancient Catskill shoreline. The critical X
value for alpha 5 0.05 is 9.49;
therefore, the null hypothesis is not rejected (0.10 , P , 0.20). Table for three samples (drainage basins) eliminates any potential
geographic overlap of adjacent transects. The critical X
value for alpha 5 0.05 is 5.99; therefore, the null hypothesis is not rejected
(0.70 , P , 0.80). Calculations based on Equation 1. Abbreviations: df, degrees of freedom [(r 2 1)(k 2 1)]; E, calculated expected
occurrences; O, observed occurrences; 1, Huron and Vermilion Rivers and Chance Creek; 2, Beaver Creek; 3, Black River; 4, Rocky
River; and 5, Big Creek and Interstate-71.
Contingency table for five drainage basins
Gnathals 1-O 1-E 2-O 2-E 3-O 3-E 4-O 4-E 5-O 5-E Total
With 18 17.48 4 3.35 7 7.44 13 8.18 32 37.56 74
Without 29 29.52 5 5.65 13 12.56 9 13.82 69 63.44 125
Total 47 47.00 9 9.00 20 20.00 22 22.00 101 101.00 N 5 199
/E 18.54 4.78 6.59 20.66 27.26
28.49 4.42 13.45 5.86 75.04
S 205.10
5 S 2 N 6.10
df 4
P value 0.1 , P , 0.2
Contingency table for three drainage basins
With 18 15.95 7 6.8 32 34.27 57
Without 29 31.05 13 20.66 69 66.73 111
Total 47 47 20 20 101 101.00 N 5 168
/E 20.32 7.22 29.88
27.08 12.78 71.34
S 168.64
5 S 2 N 0.64
df 2
P value 0.7 , P , 0.8
Figure 4. Plot of log dry weight versus log body length for extant
North Atlantic sharks (circles). Estimated dry weight for
Dunkleosteus terrelli of 665.0 kg (1466.3 lbs) for a body length
of 4.6 m (15 ft) is indicated by a star.
mortem transport or through other processes) or disarticulated (if
disarticulated in transport then there should be no pattern
suggesting an eastward source of remains from outside the deep
basin). Hypothesis 2 requires transport of D. terrelli remains into
the deep basin. Associated with the latter hypothesis would be a
pattern reflecting the distance traveled from the source (e.g., more
complete specimens proportionally found closer to the source or
elements that disarticulate early in the taphonomic process found
closer to the source).
The remains of Dunkleosteus terrelli within the Cleveland
Member are typically disarticulated with a variable number of
plates missing (only 5.1% of the evaluated specimens possess over
25% of the dermal and perichondral bones). Elder (1985) and
Elder and Smith (1988) noted three potential sources for the
disarticulation and removal of organic remains in an aquatic
environment: flotation, scavenging, or transport by currents (see
also Allison et al., 1991). Within the distal Appalachian Basin, the
latter two mechanisms have little or no influence on organic
remains once they have settled to the bottom. Proposed
mechanisms to account for anoxia and the deposition of black
shales require an environment below wave base that may be
further restricted from mixing by the presence of a density
stratification (formation of a pycnocline; see Murphy et al., 2000,
and Sageman et al., 2003, for alternative models for the formation
of an anaerobic benthos). An anoxic environment (and presence
of hydrogen sulfide in some models) would make the bottom
unsuitable for macroscopic scavengers; however, that does not
preclude scavenging upon floating carcasses within the water
column. The presence of an unsuitable bottom habitat is
supported in part by the limited or lack of bioturbation and
trace fossils. Additionally there is little evidence for the presence
of a diverse macroscopic infauna or epifauna, and no evidence for
the presence of epibionts encrusting bones. Fossil sharks within
the Cleveland Member often are found fully articulated with soft
tissues preserved (Williams, 1990). These fossils have been
reported from carbonate concretion zones in the shales of Big
Creek and Interstate-71. This represents a sampling bias in that
concretions are easily seen in profile, in contrast to flattened
isolated specimens. Shark fossils are found from outside the
concretion zones in westward deposits either associated with
cone-in-cone deposits or isolated within the shale. The skeletons
of these fishes consist of perichondrally ossified prismatic
cartilage that is highly susceptible to mechanical damage and is
destroyed easily by scavengers or currents.
The fine grain size of Cleveland Member sediments further
suggests a relatively low energy depositional environment
(Nelson, 1955; Mausser, 1982) and the absence of currents
sufficient to move the relatively large bones of Dunkleosteus
terrelli. Additionally, there are no sedimentary structures (bed-
ding features) suggesting the presence of higher energy bottom
currents within the Cleveland Member. Sediment winnowing does
occur, demonstrated by the presence of several isolated lag
deposits (invertebrate thanatocoenoses, Hlavin, 1976; silty lags, J.
Schieber, personal communication, 2004); however, winnowing
tends to accumulate bone and pelagic invertebrates rather than
remove them. The large and relatively heavy Dunkleosteus terrelli
bones would preferentially remain within these deposits.
Flotation remains as the most plausible mechanism to account
for the disarticulated remains of Dunkleosteus terrelli. Flotation
could occur in fishes living within the basin or it may represent a
mechanism for the transport of organisms into the basin.
Common to the patterns of disarticulation in both aquatic
(Elder, 1985; Scha¨fer, 1972) and terrestrial (Hill, 1979; Weigelt,
1989) organisms is the loss of elements near sites of access by
scavengers (microscopic and macroscopic). The oropharyngeal
cavity represents such a site with scavenging activity in this region
resulting in the disarticulation and potential loss of gnathal
elements. In teleosts and arthrodires (Figure 2B, C) both upper
and lower gnathal elements, which lack ossified connections to the
axial skeleton, are lost. In contrast, within mammals where the
maxillae and premaxillae are connected intimately with the skull,
the dentaries alone are detached from the skull. Work by Elder
(1985) and others (Scha¨fer, 1972; Hill, 1979; Smith and Elder,
1985; Elder and Smith, 1988) suggests that gnathal elements
(dentaries only in mammals) in particular are lost early in the
disarticulation process, thus presence of specimens with gnathal
elements suggests a relatively short time of exposure to pre-burial
taphonomic processes (including both transport and scavenging).
In the case of Dunkleosteus terrelli within the Appalachian Basin,
the presence of isolated gnathal plates suggests that floating
carcasses may have dropped elements relatively close to the
original life habitat.
Elder (1985), Smith and Elder (1985), and Elder and Smith
(1988) documented the importance of water depth and temper-
ature to the process of flotation. The physical principles outlined
in the gas laws of Charles and Gay-Lusac (k 5 V/T) and Boyle (k
5 PV) clearly delineate limitations for the potential of an
organism to float upon bacterial decomposition within finite
ranges of water depth and temperature. Thus, the volume of
accumulating decomposition gases within the tissues or body
cavities to induce flotation will increase with warming (a
latitudinal or climate factor) and decrease with depth (increased
pressure). Elder (1985) suggested a limiting depth for flotation in
a number of teleosts although her experimental work did not
consider larger species or variable densities. She suggested that
settling below a depth of 10 m even within tropical waters would
severely limit or prevent flotation. In contrast, Allison et al. (1991)
noted the calculated potential of flotation in whales up to a depth
of 1200 m. Unlike the teleosts studied by Elder (1985), whale
specimens may be nearly neutrally buoyant at the time of their
settling to the bottom (some are positively buoyant and float at
the time of death, representing an inherent static buoyancy,
Allison et al., 1991).
The Appalachian Basin in the Late Devonian was located in the
subtropics (30–35u S), suggesting moderate to warm water
temperatures (17–18u C) based on oxygen isotopic data from
brachiopods (Streel et al., 2000, fig. 30, p. 154). Several authors
dispute the water depth within the open Appalachian Basin;
however, all estimates are well within the computed depths of
Allison et al. (1991) that are necessary for flotation (61.0–91.4 m,
Nelson (1955); 30.5–45.7 m, Hlavin (1976); 50–100 m, Brett et al.
(2003); up to 100 m using the depth limits, noted by Ferguson
(1963), for Lingula found within the Cleveland Member; contrast
these estimates with Schieber’s (1998) estimate of 10–20 m for the
Chattanooga Shale of Tennessee, although, the Chattanooga
Shale is characterized by a number of lag deposits and erosional
The presence of Dunkleosteus terrelli remains in the black shales
of the Appalachian Basin can be explained either as a post-
mortem accumulation of organisms that lived within the basin or
a rain of parts falling from carcasses transported via flotation
from aerated regions. The results of a Chi-squared statistical
analysis of these fossils did not support the hypothesis that D.
42 CARR No. 57
terrelli was restricted to the shallow aerated eastern part of the
basin and that its presence in the distal sediments was the result of
carcasses floating into the basin (Table 1; X
5 6.1, d.f. 5 4, 0.10
, P , 0.20). In an additional analysis of non-adjacent transects
(nos. 1, 3, and 5), the null hypothesis was not rejected (X
5 0.64,
d.f. 5 2, 0.70 , P , 0.80). Non-rejection of the null hypothesis is
consistent with a random distribution of D. terrelli within the
basin, thus it is more likely that this species was a pelagic form
(living and dying within the basin) and not an obligate bottom
dweller (restricted to the eastern aerated shallower benthos, e.g.,
the basin slope represented by the Chagrin Member or more
shoreward regions, Figure 1D).
Given our knowledge of the bottom habitat associated with
black-shale formation, Dunkleosteus terrelli would have had to be
a continuous swimmer. The paleontological, sedimentological,
and geochemical evidence all suggest that the bottom environ-
ment was inhospitable; however, even if fishes could reach the
bottom within the basin, the stability of the substrate to support
the weight of these organisms would have proven to be a problem.
At present there are few estimates for the size or weight of
Dunkleosteus terrelli (Heintz, 1932). The large amount of bone
and concomitantly high body mass has led several researchers to
conclude that D. terrelli must be a benthic form (e.g., Denison,
1978; although rejected by later authors, Heintz, 1932, considered
Dunkleosteus to be an active swimmer and predator). Comparison
of Dunkleosteus terrelli (using an adult length of 4.6 m (15 ft) to
modern sharks (Figure 4) suggests a dry weight of approximately
665.0 kg (1466.3 lbs). Bigelow and Schroeder (1948) suggested
that wet weight is 5.5% of dry weight in extant sharks (2.6% for
free or continuous swimming forms). Using this estimate, the wet
weight for D. terrelli can be calculated to be 36.6 kg (80.6 lbs; or
using a free swimming estimate, 17.3 kg or 38.1 lbs). This
represents a conservative estimate due to the lack of bone in
sharks and the conversion to wet weight being based on
organisms possessing lipids for static buoyancy. A wet weight of
36.6 kg (or 17.3 kg for continuous swimmers) would generate
sufficient shear forces to permit the settling of D. terrelli into the
substrate exposing it to the toxic properties of the sediments and
potentially clogging the gills with fine silt (refer to Keller, 1974,
p. 333–334, for estimates of shear strength in black muds). If D.
terrelli lived within the basin it can be assumed that it did not rest
on the bottom, but swam continuously.
A second point bearing upon the interpretation of placoderm
lifestyles is their mode of locomotion. Placoderms apparently
possessed a low profile and poorly supported heterocercal tail
and have been interpreted to have swum using a sine-wave
undulation of the body (Thomson, 1971; anguilliform locomo-
tion of Lindsey, 1978). They have been compared to extant
macrurid or chimaerid fishes (Stensio¨, 1963, p. 13), which
demonstrate a bottom-dwelling lifestyle. Although anguilliform
locomotion may be considered a relatively inefficient form of
locomotion (e.g., relative velocity between anguilliform and
other forms of locomotion) it does not mean that anguilliform
swimmers are not effective prolonged or continuous swimmers.
One needs only to look at the migratory patterns of extant eels
(Anguilla anguilla), which migrate between North America and
Europe (McDowall, 1988) to recognize this point. Although
placoderms apparently never achieved some of the advanced
forms of locomotion seen in modern teleosts (e.g., carangiform
and thunniform locomotion, Lindsey, 1978), they did develop a
number of locomotor adaptations associated with increased lift
and reduced drag (Carr, 1995).
The taphonomic evidence did not support an interpretation of
Dunkleosteus terrelli as an obligate benthic organism. A Chi-
squared statistical analysis of the distribution for the remains of
this species failed to support a restricted bottom-living hypothesis.
Dunkleosteus terrelli was a free-swimming species living within the
Appalachian Basin, which may help to explain its wide North
American distribution (California to eastern United States; if
synonymised with D. marsaisi (Lehman, 1954) from Morocco
(Ru¨cklin, 2002) the range would extend via the Rheic Ocean to
east of the Old Red Sandstone Continent, Figure 1A). This
interpretation is consistent with the analysis of Carr (1995), which
noted the development of locomotor specializations within
pachyosteomorph arthrodires and was further supported by the
preservation of relatively complete specimens or the distribution
of elements lost early in the disarticulation process throughout the
Potential objections to viewing placoderms as free-swimming
organisms have included their possession of heavy body armor
and an anguilliform form of locomotion. The distribution of
Dunkleosteus terrelli remains in the Appalachian Basin is
consistent with disarticulation due to localized post-mortem
flotation. The presence of post-mortem flotation in a basin with
published depth estimates ranging from 30 to 100 meters suggests
an organism with some level of additional static buoyancy beyond
inherent tissue buoyancy (refer to Allison et al., 1991). An
interpretation of anguilliform locomotion does not necessarily
imply an ineffective form of locomotion. It appears that an
interpretation of bottom life is a consequence of the researcher’s
choice of extant analogue rather than any necessary correlation
between lifestyle and form of locomotion.
The implications of this study are three-fold and form the
basis of continuing work on the Cleveland Member fauna. Life
within the basin raises questions concerning the: (1) reproductive
strategy; (2) life history; and (3) static buoyancy in Dunkleosteus
terrelli. An inhospitable bottom limits potential nesting sites. It is
not possible at this time to determine whether this species was
viviparous or oviparous; although, the presence of putative egg
cases within the Cleveland Museum of Natural History
collections (CMNH 8133–8136, 9461) raises some interesting
questions. Further work is needed to provide information on the
distribution of age classes for D. terrelli within the Appalachian
Basin. Finally, the presence of disarticulation associated with
flotation raises questions concerning static buoyancy in D.
terrelli. Elder (1985) has pointed out the physical limits (depth
and temperature) associated with flotation; however, Allison et
al. (1991) have demonstrated that given sufficient static
buoyancy, flotation may occur up to depths of 1200 m. The
presence of air sacs in placoderms has been questioned (Denison,
1941, recognized the presence of air sacs in Bothriolepis, but did
not consider them to be a feature of placoderms in general,
although this interpretation has never been confirmed in other
specimens of Bothriolepis or in any other placoderm taxa (D.
Goujet, personal communication, 2004); Gardiner, 1984, con-
sidered air sacs to be a derived feature within Osteichthyes).
However, the predominance of disarticulation associated with
floating in D. terrelli strongly suggests the presence of a static
buoyancy mechanism within this species (if not an air sac then
potentially lipids as seen in chondrichthyans). Further work on
taphonomic patterns and flotation within D. terrelli may help to
shed light on these questions.
I would like to thank D. Goujet and G. R. Smith for their early
reviews and the latter for discussions on taphonomy, R. Cox and
D. Dunn for our many discussions, and C. Brett, J. Schieber, and
D. Goujet for review of the final manuscript. Posthumous thanks
and regards are expressed to M. E. Williams for our many
discussions concerning the Cleveland Member fauna over a
twenty-year period. Finally, I want to additionally thank D.
Fisher, C. Gans, and P. D. Gingerich who provided encourage-
ment and reviews on an early version of this work submitted in
partial fulfillment of the requirements for a Doctor of Philosophy
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... Interestingly, both the lifestyle and trophic position of this species are relatively well known regardless of the virtual absence of post-thoracic remains. D. terrelli has been interpreted as a big cruiser with good swimming capabilities, being at the top of the trophic pyramid as active predator (Lamsdell & Braddy, 2009; Carr, 2010). In consequence, both the ecology and trophic position of this species, and probably other big arthrodires, are comparable to those of living active pelagic sharks, being possible to establish an ecological analogy between both groups. ...
... Previous reconstructions of D. terrelli based on the anatomy of the close related placoderm Coccosteus (e. g. Heintz, 1932) could be inappropriate as this implies the comparison of taxa coming from too different facies and taxonomical assemblages and, ultimately, with possibly disparate lifestyles (Carr, 2010). For this reason, we propose that living sharks can be considered as suitable models for predicting characters strongly correlated with the lifestyle in placoderms, in cases where no other closer related taxa with similar ecology are known. ...
... The bias against large taxa notably excludes from the dataset the large arthrodires (e.g., Dunkleosteus and Titanichthys) and the large chondrichthyan Ctenacanthus. The bias in size is unlikely to affect interpretations of competition between agnathans and gnathostomes, because the largest agnathans are estimated to be approximately 1.5 m in length (Janvier 1996), well within an order of magnitude of the taxa measured here, while large Devonian gnathostomes absent from our dataset were estimated to be several meters long (Carr 2010) and therefore unlikely to have competed with agnathans. Agnathans and gnathostomes should be equally affected by biased stratigraphic and paleoenvironmental sampling. ...
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The rise of jawed vertebrates (gnathostomes) and extinction of nearly all jawless vertebrates (agnathans) is one of the most important transitions in vertebrate evolution, but the causes are poorly understood. Competition between agnathans and gnathostomes during the Devonian period is the most commonly hypothesized cause; however, no formal attempts to test this hypothesis have been made. Generally, competition between species increases as morphological similarity increases; therefore, this study uses the largest to date morphometric comparison of Silurian and Devonian agnathan and gnathostome groups to determine which groups were most and least likely to have competed. Five agnathan groups (Anaspida, Heterostraci, Osteostraci, Thelodonti, and Furcacaudiformes) were compared with five gnathostome groups (Acanthodii, Actinopterygii, Chondrichthyes, Placodermi, and Sarcopterygii) including taxa from most major orders. Morphological dissimilarity was measured by Gower's dissimilarity coefficient, and the differences between agnathan and gnathostome body forms across early vertebrate morphospace were compared using principal coordinate analysis. Our results indicate competition between some agnathans and gnathostomes is plausible, but not all agnathan groups were similar to gnathostomes. Furcacaudiformes (fork-tailed thelodonts) are distinct from other early vertebrate groups and the least likely to have competed with other groups.
... This is possible as the ornament of i.e. Compagopiscis, Eastmansoteus and Torostesus does not differ significantly, yet similar ornament does not mean the same size of the specimen (for instance, juvenile Dunkleosteus would be larger than adult Compagopiscis) (Trinajstic & Hazelton, 2007;Carr, 2010). ...
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Dermal bones are formed early during growth and thus constitute an important tool in studies of ontogenetic and evolutionary changes amongst early vertebrates. Ornamentation of dermal bones of terrestrial vertebrates is often used as a taxonomic tool, for instance in Aetosauria, extant lungfishes (Dipnoi) and ray-finned fishes (Actinopterygii), for which it have been proved to be of use in differentiating specimens to species level. However, it has not been utilised to the same extent in placoderms. Several features of the ornamentation of Early Devonian placoderms from Hamar Laghdad (Morocco) were examined using both optical and scanning electron microscopy to determine whether it is possible to distinguish armoured Palaeozoic fishes. Four distinct morphotypes, based on ornamentation of dermal bones, are differentiated. These distinct types of ornamentation may be the result of either different location of dermal plates on the body or of ontogenetic (intraspecific) and/or interspecific variation.
... The discovery of placoderm egg cases in the Cleveland Shale (Carr, 2010a) demonstrates the equal antiquity of oviparity and viviparity among the basal gnathostomes. The pelagic nature of the large arthrodires (Carr, 2010b) associated with these egg cases suggests the possibility of arthrodiran hatcheries in the eastward shallower parts of the Appalachian Basin. An interesting note regarding the diversity of the Cleveland Shale fauna is the lack of any analysis of microvertebrates, although bone beds and lag deposits have been identified in the field. ...
Conference Paper
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In the Famennian of the eastern Anti-Atlas, microremains of gnathostomes are quite common in some strata due to condensed sedimentation, particularly in the Tafilalt. In the latter region, chondrichthyan diversity can be reasonably high (up to nine species in one layer). By contrast, in the Famennian of the southern Maïder Basin, chondrichthyan diversity appears to be lower (four to five species) and genera such as Clairina, Jalodus and Protacrodus have not been found yet although they are documented from the neighboring Tafilalt Basin (Ginter et al. 2002; Derycke et al. 2008). We address the questions for the ecological factors controlling these differences in diversity and fossil preservation. The latter question is of interest because only in the Maïder Basin, chondrichthyans have been discovered preserving cartilaginous body parts as well as soft tissues. Preservation has been examined by analyzing the mineral composition of various Famennian fossils from the Maïder and the Tafilalt using XRD (X-ray Diffraction) at the Swiss Federal Institute of Technology in Zurich (ETHZ). The results show that indeed the chondrichthyan musculature is now preserved in hematite and other ferric minerals. Both placoderm bones and chondrichthyan cartilage are preserved in hydroxylapatite, fluorapatite or francolite (phosphates). Particularly the abundance of ferric oxides and hydroxides points at pyrite, which was altered due to deep weathering in the desert environment. This is corroborated by rare finds of pyritized fossils from the same strata in depths of over 10 m below today’s surface. In turn, this primary abundance of pyrite (now ferric oxides and hydroxides) in combination with the clayey facies and the scarcity of benthos in some strata suggests that the sediments containing exceptionally preserved gnathostomes were deposited under oxygenpoor conditions (Klug et al. 2016). This is supported by the palaeogeographical situation of the Maïder Basin that was closed to the south, west and north by land, while to the east, the shallower regions of the Tafilalt Pelagic Ridge limited water exchange (Wendt 1988; Kaufmann 1998). Hypoxic to anoxic conditions ultimately explain the absence of the protacrodontids, which mainly occur in shallower, better oxygenated waters (Ginter 2000). Clairina and Jalodus on the other hand likely preferred deeper environments than the one in the Maïder Basin. Following Ginter (2000), the taxa present in the Maïder (Phoebodus and cladodonts) point at an intermediate water depth.
In a 140 m-thick sediment core through Upper Devonian shale units in the Central Appalachian Basin, gray shale units (TOC ≤1%) and black shale units (TOC ≤9%) are separated by “intercalated zones” (ICZs) of thinly interbedded gray and black shales. In accordance with Walther's law of facies successions, ICZs imply environments producing black shales were coeval and adjacent to environments producing gray shales. To understand these facies transitions, we studied event layer sedimentology, organic geochemistry, and trace elements. This study found black shales to be shallow-water deposits (with tempestites and wave-modified turbidites), associated with significant terrestrial input (with abundant hyperpycnites, and terrestrial-derived humic and fulvic acids). The shales were deposited under varying redox conditions, with extensively bioturbated intervals alternating with intervals preserving intact micro-lamination, and demonstrating significant variations in trace element V/Cr, Ni/Co, and Th/U ratios. These observations are reconciled by attributing shale TOC values to sediment dynamics of deltas draining the Neo-Acadian Highlands. Progradation of individual delta distributary lobes resulted in localized offshore regions receiving increased terrestrial nutrient input and causing freshwater-induced water column stratification and oxygen stress (similar to the “dead zones” of many modern deltas), producing black shales. In contrast, gray shales formed between deltas or after delta distributary lobe abandonment. The intercalated black shale-gray shale transition intervals (ICZs) are interpreted to represent dynamic water and sediment conditions during the time interval of delta distributary lobe avulsion events. This model contrasts with previous models calling for basin-wide anoxia or assigning black shales to basinal deposits.
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Discoveries from the Late Devonian Gogo Formation, in the Canning Basin, Western Australia have provided insights into the origin and evolution of many unique gnathostome features such as the origins of teeth, internal fertilisation, air-breathing, transitional tissues between bone and cartilage, and insights into the fin to limb transition. Although vertebrate studies have dominated evolutionary work, invertebrate studies have added important insights into the palaeoecology of the site and demonstrated close faunal affinities along the margins of northern Gondwana and China. Geochemical analyses have broadened the understanding of the pathways involved in the exceptional preservation of this Devonian Konservat-Lagerstätte. Fossils from the Gogo Formation show extensive soft tissue preservation through phosphatization recording anatomical details not normally obtained from fossil sites.
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Large nektonic suspension feeders have evolved multiple times. The apparent trend among apex predators for some evolving into feeding on small zooplankton is of interest for understanding the associated shifts in anatomy and behaviour, while the spatial and temporal distribution gives clues to an inherent relationship with ocean primary productivity and how past and future perturbations to these may impact on the different tiers of the food web. The evolution of large nektonic suspension feeders—‘gentle giants’—occurred four times among chondrichthyan fishes (e.g. whale sharks, basking sharks and manta rays), as well as in baleen whales (mysticetes), the Mesozoic pachycormid fishes and at least twice in radiodontan stem group arthropods (Anomalocaridids) during the Cambrian explosion. The Late Devonian placoderm Titanichthys has tentatively been considered to have been a megaplanktivore, primarily due to its gigantic size and narrow, edentulous jaws while no suspension-feeding apparatus have ever been reported. Here, the potential for microphagy and other feeding behaviours in Titanichthys is assessed via a comparative study of jaw mechanics in Titanichthys and other placoderms with presumably differing feeding habits (macrophagy and durophagy). Finite-element models of the lower jaws of Titanichthys termieri in comparison to Dunkleosteus terrelli and Tafilalichthys lavocati reveal considerably less resistance to von Mises stress in this taxon. Comparisons with a selection of large-bodied extant taxa of similar ecological diversity reveal similar disparities in jaw stress resistance. Our results, therefore, conform to the hypothesis that Titanichthys was a suspension feeder with jaws ill-suited for biting and crushing but well suited for gaping ram feeding.
The Late Devonian was a time of dramatic environmental perturbations affecting marine ecosystems. Both the Kellwasser (latest Frasnian) and the Hangenberg crises (latest Famennian) are primarily reported as phases of drastic decreases in marine diversity while the Hangenberg Crisis is also described as a bottleneck in vertebrate evolution. Fossil-bearing localities with Upper Devonian strata are of great interest to assess variations in the effects of environmental perturbations on biodiversity. For this purpose, we examined changes in alpha diversity and ecospace utilization of 21 Famennian (Late Devonian) and early Tournaisian (Early Carboniferous) invertebrate associations containing 9828 specimens from Madène el Mrakib and Aguelmous (southern Maïder Basin, northeastern Anti-Atlas, Morocco), where some layers yield exceptionally preserved gnathostome remains. Both the invertebrate and vertebrate associations contain predominantly opportunistic and pelagic taxa indicating oxygen depletion near the seafloor in this region. Nevertheless, the ecospace extension was fluctuating and correlated with regional and/or global sea-level changes and oxygenation of bottom waters. In the Maïder Basin, the ecospace was depleted after and during several bioevents such as the Kellwasser and Hangenberg crises, the Annulata event (middle Famennian) as well as during the early Tournaisian. Abiotic as well as biotic changes (instability of the invertebrate ecosystem) are considered to have influenced Famennian vertebrate diversity because they were more or less directly dependent on invertebrates as a food source. Key words: Fossillagerstätte, Famennian, gnathostomes, invertebrates, palaeoecology, sea level
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Our knowledge about the body morphology of many extinct early vertebrates is very limited, especially in regard to their post-thoracic region. The prompt disarticulation of the dermo-skeletal elements due to taphonomic processes and the lack of a well-ossified endoskeleton in a large number of groups hinder the preservation of complete specimens. Previous reconstructions of most early vertebrates known from partial remains have been wholly based on phylogenetically closely related taxa. However, body design of fishes is determined, to a large extent, by their swimming mode and feeding niche, making it possible to recognise different morphological traits that have evolved several times in non-closely related groups with similar lifestyles. Based on this well-known ecomorphological correlation, here we propose a useful comparative framework established on extant taxa for predicting some anatomical aspects in extinct aquatic vertebrates from palaeoecological data and vice versa. For this, we have assessed the relationship between the locomotory patterns and the morphological variability of the caudal region in extant sharks by means of geometric morphometrics and allometric regression analysis. Multivariate analyses reveal a strong morphological convergence in non-closely related shark species that share similar modes of life, enabling the characterization of the caudal fin morphology of different ecological subgroups. In addition, interspecific positive allometry, affecting mainly the caudal fin span, has been detected. This phenomenon seems to be stronger in sharks with more pelagic habits, supporting its role as a compensation mechanism for the loss of hydrodynamic lift associated with the increase in body size, as previously suggested for many other living and extinct aquatic vertebrates. The quantification of shape change per unit size in each ecological subgroup has allowed us to establish a basis for inferring not only qualitative aspects of the caudal fin morphology of extinct early vertebrates but also to predict absolute values of other variables such as the fin span or the hypocercal and heterocercal angles. The application of this ecomorphological approach to the specific case of Dunkleosteus terrelli has led to a new reconstruction of this emblematic placoderm. Our proposal suggests a caudal fin with a well-developed ventral lobe, narrow peduncle and wide span, in contrast to classical reconstructions founded on the phylogenetic proximity with much smaller placoderms known from complete specimens. Interestingly, this prediction gains support with the recent discovery of fin distal elements (ceratotrichia) in a well preserved D. terrelli , which suggests a possible greater morphological variability in placoderm caudal fins than previously thought.
One of the most important and challenging aspects of stratigraphy is the interpretation of the temporal scope of sedimentary units (Schindel, 1980, 1982; Sadler, 1981; Brandt Velbel, 1984). The problem arises at the scale of individual beds and of stratigraphic intervals up to many meters thick. Does a particular bed or interval-represent hours, days, years, centuries, or millennia? Resolution of this question is critical for determination of rates of sedimentation and biotic processes, and in assessing the reliability of the sample for paleoecological or evolutionary analysis. In the absence of a reliable framework of absolute radiometric dates the question can only be answered by indirect inference. Biostratigraphic zonation is a critical first step. But zonation is typically too coarse to resolve temporal scales less than 10 ⁶ years and many zones are not firmly anchored to absolute dates. It is also important to keep separate the issue of temporal duration represented by fossils (as bioclasts) within a given stratum and that of the deposition of the sedimentary unit itself. There are many instances of thin graded beds full of fossils, which would be characterized unambiguously by sedimentologists as deposits of a single event of sedimentation, but in which the fossils may differ in age by thousands or even millions of years. Examples include many condensed, lag deposits of bones and conodonts (Baird and Brett, 1991), and condensed ammonoid beds containing fossils of several ammonite zones (Fürsich, 1971). Sedimentologic criteria provide one avenue of approach to this issue but commonly fall short of unambiguous answers.