Conodont ultrastructure : the family Panderodontidae

Life Sciences Contribution 90
Royal Ontario Museum

Conodont
Ultrastructure:

The Family
Panderodontidae

C.R. Barnes, D.B. Sass,
M.L.S. Poplawski

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C. R. BARNES, D. B. SASS,
M. L.S. POPLAWSKI

LIFE SCIENCES CONTRIBUTIONS
ROYAL ONTARIO MUSEUM

NUMBER 90

Conodont Ultrastructure:
The Family Panderodontidae

Publication date: 15 June 1973

Suggested citation: Life Sci. Contr., R. Ont. Mus.

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Chairman, R. L. PETERSON
Editor, J. R. TAMSITT
Associate Editor, D. BARR

Associate Editor, E. J. CROSSMAN

CHRISTOPHER R. BARNES is a Research Associate, Department of Inverte-
brate Paleontology, Royal Ontario Museum, and an Associate Professor
in the Department of Earth Sciences, University of Waterloo, Waterloo,
Ontario.

DANIEL B. SASS is Chairman and Professor in the Department of Geology,
Alfred University, Alfred, New York.

M. L. SILVANA POPLAWSKI is a Research Assistant in the Department of
Earth Sciences, University of Waterloo, Waterloo, Ontario.

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Conodont Ultrastructure:
The Family Panderodontidae

Abstract

The ultrastructure of representative specimens of 11 form
species of the conodont genera Panderodus and Belodina,
comprising the family Panderodontidae, was investigated
using oriented, etched sections examined primarily with the
scanning electron microscope. Selected species are of wide
stratigraphic and geographic occurrence. Details of the form
and pattern of crystallites and lamellae permitted interpreta-
tion of the mode of growth of both genera. Critically impor-
tant is the discovery of radial as well as concentric lamellae.
The former, flanking the longitudinal furrow on the inner
lateral face and having surface expression as coarse striations,
represent a radically different form of element construction
not documented, although suspected in other, simple-cone
genera. Lamellae constructing the basal rim commonly are
restricted to the basal region. Development of white matter
is considered to be functionally advantageous by limiting the
amount of potential element damage to the cusp and denticle
tips. Holes within the lamellar basal filling are considered to
be primary. The similarity in ultrastructure between Pandero-
dus and Belodina supports the recent establishment of the
family Panderodontidae as a natural, taxonomic unit.

Introduction

Ultrastructure is now recognized as an important criterion to define and to
distinguish suprageneric taxa of conodonts. Our earlier work (Barnes et al.,
1970; Barnes, Sass and Monroe, in press) documented the ultrastructure
of a wide range of form taxa of Ordovician conodonts, using geographically
widespread collections from a restricted geological time interval. We estab-
lished certain basic properties of the crystallites, lamellae, white matter,
and surface micromorphology. Differences in ultrastructure were docu-
mented between hyaline, neurodont (a subgroup of the hyaline conodonts),
and cancellate (with extensive white matter) conodonts. The only signifi-
cant feature attributable to geographic variation that we found may have
been the extent to which white matter is developed in certain taxa, in con-
trast to others in which the proportion is constant.

Recently we examined the ultrastructure of certain distinctive conodonts
that appear to be closely related and investigated possible variation in ultra-
structure of examples through their geologic range. We examined Cambrian,

l

Ordovician, Silurian, and Triassic conodonts. This paper (see Barnes et al.,
1970, p. 3) deals with the ultrastructure of one of the most common wide-
spread Ordovician genera, Panderodus Ethington (Fig. 3A-D) and a closely
related genus, Belodina Ethington (Fig. 4F-H). Both originated in the early
Middle Ordovician, Belodina ranging to the end of the Ordovician and
Panderodus continuing into the Devonian. These genera are especially com-
mon in the North American Midcontinent Province, and details of their
stratigraphy and ecology were discussed by Sweet et al. (1971), Seddon and
Sweet (1971), and Barnes, Rexroad, and Miller (in press). Both form
genera were established first by Ethington (1959) and considered in a
multi-element concept by Bergstrom and Sweet (1966), Schopf (1966),
and Webers (1966). Panderodus was emended by Clark and Ethington
(1966) and recently revised by Ziegler and Lindstrom (1971).

In his suprageneric classification of conodonts, Lindstrom (1970, p. 433)
included the following subdivision:

Superfamily Panderodontacea Lindstrom
Family Acanthodontidae Lindstr6ém
Subfamily Acanthodontinae Lindstrom
Subfamily Protopanderodontinae Lindstrom
Family Panderodontidae Lindstroém

Subsequently Lindstrom and Ziegler (1971, p. 12) informally suggested
that the two subfamilies of Acanthodontidae should be treated as separate
families. Although the subdivision may require future revision, the family
Panderodontidae appears to be a valid taxon containing the multi-element
genera Panderodus, Neopanderodus, and Belodina. Lindstrom and Ziegler
(1971) investigated the ultrastructure of members of the superfamily Pan-
derodontacea and based their observations on 15 specimens that were
fractured artificially to reveal aspects of the inner and outer structure. For
the family Panderodontidae they used specimens of the form species Pan-
derodus gracilis (Branson and Mehl), P. simplex (Branson and Mehl),
P. unicostatus (Branson and Mehl), Neopanderodus perlineatus (Ziegler
and Lindstr6m), and Belodina grandis (Stauffer). Results of our concur-
rent work agree generally with their conclusions, but our techniques of
preparation revealed more of the structure, permitted a more detailed inter-
pretation of ultrastructure, and resulted in the discovery of a pattern of
conodont growth not recognized before.

Species of Panderodus and Belodina are common in Middle and Upper
Ordovician faunas from shelf carbonates overlying the Precambrian Shield
in many areas of Canada and are presently under study by Barnes. They
are especially abundant in northern and western Canada, where species
are associated with the Arctic Ordovician fauna (e.g., Nelson, 1959).

Apparatuses of both multi-element genera consist of one, two, or three
element types, depending on the species. Most panderodid, multi-element
species (see Fig. 3A-D) have two element types: a round, slender form,
e.g. P. gracilis (Branson and Mehl) s.f. and a broad, more compressed

Z

element whose anterior margin is more regularly curved, e.g. P. compressus
(Branson and Mehl) s.f. (Note that s.f., i.e. sensu formae, is used herein
to distinguish a form taxon from a multi-element taxon, as recommended
by a majority of specialists attending the Marburg Symposium in Conodont
Taxonomy in Germany, September 1971.) Homologues of these two ele-
ment types are recognizable in most belodinid species. For example, B.
compressa is composed of B. grandis (Stauffer) s.f. (Fig. 4G,H) and B.
compressa (Branson and Mehl) s.f. (Fig. 4E,F) but also has a third com-
ponent: a smaller, non-denticulate, strongly recurved element, Eobelodina
fornicala (Stauffer) s.f. Apparatuses of Panderodus and Belodina are thus
similar. Partial, fused apparatuses were described by Pollock (1969) for
the former and by Barnes (1967a) for the latter.

Certain specimens of Panderodus spp., normally a non-denticulate, simple
cone, developed germ denticles along the posterior margin (see, for example,
Schwab, 1969). A more complete gradational series, ranging from pandero-
did to belodinid forms, has been observed in Barnes’ collections. Although
the form genera Belodina and Eobelodina were defined by Ethington (1959)
and Sweet et al. (1959), respectively, to possess bifid basal cavities, this is
not true for all species. Some have a single, deep cavity, as in panderodids.
Morphology is thus similar in Panderodus and Belodina.

It has been known for many years (see Stauffer, 1935) that the outer
surface of specimens of the two genera bear fine striations, which have been
better illustrated with scanning electron microscope (SEM) photographs
(Via, 1970; Lindstrom and Ziegler, 1971). Striations are most prominent
on each side of a deep, narrow, lateral groove (Fig. 3H). Similar fine, longi-
tudinal striations occur in specimens of several Ordovician simple-cone
genera, especially Scolopodus (sensu Lindstrém, 1971, p. 40-41) and
notably S: gracilis Ethington and Clark. Hence, Panderodus and Belodina
share many common features and are clearly closely related. They differ
principally in dentition and, to a lesser extent, in the occurrence of a bifid,
basal cavity in species of Belodina. A similarity in ultrastructure would
further support their close relationship and the validity of the family Pan-
derodontidae.

Techniques and Material

Techniques of study are the same as those described previously (Barnes
et al., 1970; Barnes, Sass and Monroe, in press). Specimens were embedded
in a bioplastic medium, ground to the desired level and orientation, polished
with 0.003 aluminum oxide powder, etched for 20 seconds in 2N HCl,
and then examined with the Cambridge Stereoscan Mark 1A (Cambridge
Instrument Co., Cambridge, England).

Lindstrom and Ziegler (1971), who artificially fractured their speci-
mens and examined the resulting surfaces, criticized etching techniques,
and suggested that secondary fine structures could be simulated that did
not conform to the original structure. A discussion of both methods is perti-
nent. Fracture surfaces should allow study of unaltered structure but present
disadvantages and limitations. Conodont specimens do not readily fracture

s

along all desired directions. The arrangement of the crystallites and lamellae
produces preferred lines of fracture. Hyaline conodonts fracture length-
wise, whereas white matter fractures transversely. Stress applied to produce
alternative lines of fracture may not yield a “clean break” and conceivably
could result in illusory or ill-defined structures.

Etching techniques have been used in ultrastructure studies of other fos-
sil groups, e.g., brachiopods (Sass, 1967; Williams, 1968) and molluscs
(Hudson, 1968). Material removed is undoubtedly selective, but structural
patterns are accentuated. We studied sections using progressively extended
etching times to observe changes occurring from polished, normal, and
fracture surfaces to those intensively etched. Etching is not extreme in
most of our work as is indicated by the persistence of fine polishing grooves.

The greatest advantage of our technique, however, is the control of
orientation of faces. For each form species examined from each locality,
we used at least three embedded specimens to examine longitudinal, trans-
verse, and basal transverse sections. Usually more than three specimens
were studied, and oblique lines of sections were commonly made with
curved specimens. With many conodonts, including examples of the two
genera considered here, we made serial sections to trace structures or to
pass from white to hyaline matter. Illusory features and artifacts, which
may result with electron microscopy (poor coating for example) are avoided
or identified by using numerous specimens and orientations for each form
species. The ultimate justification of the techniques used is in the result.
In the Panderodontidae, we consider that new information has been pro-
vided by our embedding and etching techniques but stress that all methods
(etching, fracturing, thin-sectioning) are useful and should be used to
complement each other.

To obtain wide geographic and stratigraphic coverage, specimens from
several horizons and localities were studied. Specimens of form species that
we examined (all not illustrated here) were as follows:

Belodina compressa (Branson and Mehl): Chaumont Formation, Black
River Group, Middle Ordovician, upper Ottawa Valley, Ontario, locality
2 of Barnes (1967b); Prosser Member, Galena Formation, Middle Or-
dovician, east side, Highway 52, south of Decorah, Iowa (locality 3, Eth-
ington, 1959).

B. sp. A: Farr Formation, Upper Ordovician, Shipyards Quarry (47°29’N,
79°39'/W), Lake Timiskaming outlier, Ontario (Munro and Barnes, un-
published data).

B. sp. B: Farr Formation, Upper Ordovician, Shipyards Quarry (47°29’N,
79°39’W), Lake Timiskaming outlier, Ontario (Munro and Barnes, un-
published data).

Panderodus arcuatus (Stauffer): Farr Formation, Upper Ordovician, Ship-
yards Quarry (47°29’N, 79°39’W), Lake Timiskaming outlier, Ontario
(Munro and Barnes, unpublished data).

P. compressus (Branson and Mehl), including both P. compressus s.s. and
the gerontic P. feulneri (Glenister) : Prosser Member, Galena Formation,

Middle Ordovician, east side, Highway 52, south of Decorah, Iowa (local-
ity 3, Ethington, 1959). Chaumont Formation, Black River Group, Mid-
dle Ordovician, upper Ottawa Valley, Ontario, locality 2 of Barnes
(1967b).

P. gracilis (Branson and Mehl): Chaumont Formation, Black River Group,
Middle Ordovician, upper Ottawa Valley, Ontario, locality 2 of Barnes
(1967b); Prosser Member, Galena Formation, Middle Ordovician, east
side, Highway 52, south of Decorah, Iowa (locality 3, Ethington, 1959);
Farr Formation, Upper Ordovician, Shipyards Quarry (47°29’N,
79°39'W ), Lake Timiskaming outlier, Ontario (Munro and Barnes, un-
published data).

P. intermedius (Branson, Mehl, and Branson): Prosser Member, Galena
Formation, Middle Ordovician, east side, Highway 52, south of Decorah,
Iowa (locality 3, Ethington, 1959).

P. panderi (Stauffer) : Farr Formation, Upper Ordovician, Shipyards Quarry
(47°29’N, 79°39’W), Lake Timiskaming outlier, Ontario (Munro and
Barnes, unpublished data).

P. unicostatus (Branson and Mehl): Crug Limestone, Upper Ordovician,
Crug Farm, Llandeilo, Wales (Bergstrom, 1964).

P. simplex (Branson and Mehl): 15 cm below top of Hamra Group of
Munthe (1921) (or within Sundre Beds of Martinsson, 1962), Upper
Silurian, Juves, Gotland, Sweden.

P. unicostatus (Branson and Mehl): 15 cm below top of Hamra Group of
Munthe (1921) (or within Sundre Beds of Martinsson, 1962), Upper
Silurian, Juves, Gotland, Sweden.

Specimens of P. gracilis and B. diminutiva (Branson and Mehl) were exa-
mined with the transmission electron microscope (TEM) (Hitachi-11,
Perkin-Elmer Corp., Norwalk, Connecticut), but did not reveal additional
information. All specimens are deposited in the Department of Earth
Sciences, University of Waterloo, Waterloo, Ontario.

Ultrastructure of Panderodontidae

CRYSTALLITES

Distinct, unequivocal crystallites that comprise the lamellae are only rarely
visible. Crystallites that are seen (Figs. 3F,G; 8F) are usually elongate, with
a diameter of 0.1—0.2 um in Panderodus, as found also by Lindstrom and
Ziegler (1971, p. 12). In Panderodus and Belodina, where lamellar surfaces
are exposed inside the basal cavity (Figs. 8c,D; 11C,D), crystallites appear to
be granular. Possibly their appearance is merely a result of viewing their
basal pinacoids, but a trend from linear crystallites in the cusp to granular
crystallites in the base has been noted in other conodonts (in Ptiloconus by
Barnes et al., 1970, and in Cardiodella by Barnes, Sass and Monroe, in
press ).

Crystallites, when elongate, are oriented with the long axes parallel to that
of the long axis of the conodont, but orientation in specimens of species of

Belodina was less easily determined. Alignment of the crystallites at an
angle of 30° to the interlamellar spaces (Fig. 11D) may represent the lami-
nated pattern that was interpreted by Lindstrom and Ziegler (1971, p. 13,
text-fig. 3) as platy crystallites arranged parallel to prism pair I, that is, to
crystallite faces oriented at an angle of 30° to the external surface of the
conodont.

LAMELLAE

In specimens of Belodina spp., lamellae are distinct in hyaline matter and
are either about 0.8 wm thick (Fig. 11F) or 1.2 wm when thickened in
curving towards the basal cavity (Fig. 11B). Species of Panderodus show
two sets of lamellae, one roughly concentric around the basal cavity and a
second, referred to here as radial lamellae, normal to the outer surface.
Thickness of radial lamellae is about 1.0 wm, but increases to 3.0 wm where
they curve near their origin close to the basal cavity (Figs. 6B—D, 7c—E). The
concentric lamellae are usually thinner, ranging from 0.2—0.8 um in thick-
ness (Figs. 3G; 4B).

INTERLAMELLAR SPACES

In most of our illustrations, lamellae are evident and are separated by inter-
lamellar spaces, usually one-half to one-third the thickness of the lamellae.
Such spaces are not so evident on fractured surfaces and doubtless were ac-
centuated by the etching technique. Lindstr6m and Ziegler (1971, p. 12)
did not observe interlamellar spaces in Panderodontacea. But lamellae are
recognized through the presence of planes of parting or by planes separating
them. These planes on fractured surfaces seem to include irregular spaces
(Lindstrom and Ziegler, 1971, pl. 1, fig. 6; pl. 6, figs. 5-7, for example).
Lamellae thus probably are partly fused and partly separated by irregular
voids, a condition found also in Drepanodus homocurvatus by Barnes et al.
(1970, p.4).

In Panderodus specimens, transverse sections revealed widely-spaced
lamellae at various places close to the basal cavity (Figs. 3E, 5C,D,F; 6A,D).
These best developed occur where radial lamellae, normal to the outer sur-
face, curve sharply near their point of origin within a few lamellae of the
basal cavity. Wide, interlamellar spaces would likely appear as voids in
equivalent fractured sections.

WHITE MATTER

The physical properties of white matter found in the tip of specimens of
Panderodus (Fig. 3A—D) and in denticles and the cusp of specimens of
Belodina (Fig. 4E-H) do not differ from those described in many genera of
cancellate conodonts (Barnes, Sass and Monroe, in press). The transition
from lamellar hyaline to porous, finely-crystalline white matter in Panderodus
is shown in Fig. 4B,D. The more widespread distribution of white matter in
Belodina is illustrated in Fig. 10A—G. An outer lamellar zone passes inwards
through incipient white matter to a core of true white matter. Only the ini-
tial core of denticles is lamellar, the remainder being secondarily converted

6

to white matter. But, this conversion does not destroy the identity of the
denticles, which are distinctly defined by sharp planes.

Hyaline conodonts, especially neurodonts, tend to fracture lengthwise, i.e.
parallel to the length of the lamellae. White matter, however, usually frac-
tures transversely, often along lines of holes (Barnes, Sass and Monroe, in
press). Lindstrom and Ziegler (1971, p. 16) considered that holes and fine
crystallinity reduced the resistance of white matter to mechanical fracturing.
They also suggested that these features might be functionally advantageous,
in that they would deter a predator by leaving a mass of broken conodont
tips in its throat. White matter likely retained some organic matrix, but its
actual potential strength, as compared to a hyaline matter, must be conjec-
tural. A predator, moreover, presumably would consume the entire conodont
and be undeterred by a few broken tips. The direction of fracturing is the
critical factor. In hyaline forms breakage commonly results in a complete
longitudinal fracturing and subsequent severance of a cusp or process. Con-
versely, in white matter the same stress would probably produce a transverse
fracture that only removes the cusp or denticle tip(s).

BASAL FILLING

The basal filling is commonly present in specimens of taxa pertaining to the
Panderodontidae and may protrude some distance from the basal rim. Its
retention is doubtless promoted by the shape of the deep, narrow basal
cavity. At high magnifications we could not obtain photomicrographs of good
resolution of crystallites of the basal filling but agree with Pietzner et al.
(1968) and Lindstrém and Ziegler (1971) that crystallites develop iso-
metrically with a size range of 0.1 to 0.2 wm.

The structure of the basal filling is lamellar (Fig. 9c—G), with the thick-
ness of lamellae similar to that of the hyaline matter. Only rarely were these
lamellae seen in contact with and matching lamellae of the hyaline matter
(documented for other conodonts by Miiller and Nogami, 1971) due primar-
ily to an apparent contraction of the basal filling away from the hyaline mat-
ter (Fig. 94,B). Lindstrom and Ziegler (1971, text-fig. 5) suggested that the
basal filling had a higher proportion of organic matrix than the hyaline
matter and was flexible (perhaps analogous to cartilaginous tissue) but suf-
fered post-mortem contraction. Holes, averaging 3—4 um in diameter, occur
in our material (Figs. SE; 9c,£,F), but lamellae bend around them, suggest-
ing a primary origin. Primary holes, or tunnels, were confirmed by thin-sec-
tion studies of other genera by Miiller and Nogami (1971, text-fig. 15),
who considered that some may have been filled by the inward secretion of
lamellae. In longitudinal sections, these holes in Panderodus species appear
to be arranged in rows parallel to the long axis of the conodont element (Fig.
OE).

OUTER ORNAMENTATION

Six types of outer ornamentation of the Panderodontacea were described by
Lindstrom and Ziegler (1971). We agree with and have nothing further to
add to their comments concerning smooth surfaces, fine striations, basal

7

wrinkles, and denticle ornamentation but have additional information re-
garding striations, the longitudinal furrow, and coarse striations.

Longitudinal Furrow

In Panderodus and Belodina species a longitudinal furrow or groove is lo-
cated on the inner side close to the posterior margin and extends virtually
the entire length of the element. As was observed also by Lindstrom and
Ziegler (1971), the furrow penetrates almost to the basal cavity as a deep
narrow slit in cross-section. The width of the slit on etched sections is about
2—3 «wm on the outer surface, narrowing rapidly inwards to 1 wm, and then
closing completely within a few microns of the cavity (Figs. 3E; 44,C,D;
5A,C,D). In cross-section the slit is roughly parallel to the outer lateral face
but at its origin usually swings sharply tangential to the basal cavity, near
which it is associated with the curved, widely-spaced, radial lamellae (Figs.
3E; 5C,D,F; 6A,C,D). Lindstrom and Ziegler (1971, fig. 3) demonstrated that
the furrow does not reach the tip of the cusp in Panderodus and broadens to
a shallow, wide groove towards the basal margin. The groove likewise does
not reach the tip of the cusp in Belodina but continues as a narrow structure
to the basal rim. At the rim, the groove produces a constriction in the base
that assists in producing the bifid basal cavity of certain species (Figs. 4F,G;
11A,E).

Coarse Striations

The longitudinal furrow is usually flanked by a zone, 3—5 wm wide, that is
relatively smooth and lacking in striations (Figs. 3H; 6D). Beyond these
smooth zones occur coarse, longitudinal striations or ridges. The ridges,
which are the surface expression of the radial lamellae, are readily observed
with the light microscope and, when well developed, have even been used as
a specific character (e.g., Panderodus striatus Stauffer, 1935).

We observed striations in both longitudinal and transverse sections. On
the inner, lateral face they extend from the anterolateral shoulder, or carina,
almost to the posterior margin. Striations, about 1 wm wide and consistent
in width along their length (Fig. 7C—E), are more regular than those of con-
centric lamellae. From the inner lateral face, they pass inwards to approach
the basal cavity. Striations anterior to the longitudinal furrow originate as
curved, widely-spaced lamellae adjacent to the cavity referred to previously
(Figs. 3E; 5c,D,F). Thus, striations are not merely surface ornamentation
that persist through ontogeny as was suggested by Lindstrom and Ziegler
(1971); they represent lamellae that develop normal to the outer surface
and are thus radial rather than concentric with reference to the basal cavity.
Hence, in conodonts of the family Panderodontacea the method of element
construction is radically different from the simple, concentric, lamellar
growth of most other conodonts (documented, for example, by Miller and
Nogami, 1971).

Radial lamellae intersect the longitudinal furrow at angles of 30° to the
anterior side and 15° to the posterior side. They appear to terminate just
before reaching the furrow, thus producing a smooth, flanking zone. As axes

8

of the radial lamellae and the furrow intersect, and as the furrow follows the
curve of the element, some striations ultimately overlap and terminate. The
anteriormost radial lamella on the inner lateral face extends the most distal
toward the tip of the cusp, and others are added at the edge of the furrow
farther down its length. As seen in longitudinal sections (Figs. 6B—D; 7C—E)
new lamellae develop apically at positions progressively distal from the tip
of the cusp. As many as 20 such lamellae occur anterior to the furrow and
about 10 posterior to the furrow. Some posterior lamellae terminate as the
furrow extends outward during growth.

Striations were interpreted by Lindstrom and Ziegler (1971, p. 14) as
surface ornamentation whose form was controlled primarily by the orienta-
tion of crystallite faces. In their illustration (1971, fig. 1) striations penetrate
into the element, and hence they interpreted this condition to reflect onto-
genetically-persistent ornamentation of the surface. Probably the space be-
tween the striations in our sections was produced in part by etching, but dis-
tinct planes of separation are also evident in fracture surfaces of material
studied by Lindstrom and Ziegler. Further, if these striations were simply
surface ornamentation, concentric lamellae would be expected to transect
them and to be evident in the region of the inner, lateral face. But concentric
lamellae are absent there. Moreover, as was demonstrated above, striations
may be traced inwards as radial lamellae terminating as widely-spaced,
curved lamellae built upon the few, initial, concentric lamellae that define
the margin of the basal cavity (Figs. 5E,F; 6C,D).

Radial lamellae are especially distinct in hyaline matter (Fig. 7C—E), but
are less evident in areas of incipient white matter (Fig. 4A—D). A sharp plane
of separation, or suture, might be expected where the concentric lamellae
intersect with radial lamellae. A suture should occur inwards from the
anterolateral shoulder, or carina, and near the posterior margin. But, other
than an indistinct interlamellar space or plane separation, we observed no
sharp boundary in this region.

Although coarse striations occur on the outer surface of specimens of
Belodina spp. (Lindstrom and Ziegler, 1971) whether these have the same
structural basis as those in specimens of Panderodus spp. is not known. The
longitudinal furrow is certainly deep and narrow (Figs. 10F; 11E), but if
flanked by striations representing radial lamellae, they are not evident in-
ternally. We consider that the coarse striations illustrated by Lindstrom and
Ziegler (pl. 3., figs. 7, 8) on the anterior margin represent the surface out-
croppings of lamellae or crystallite bundles.

FUNCTION OF RADIAL LAMELLAE AND THE LONGITUDINAL FURROW

Radial lamellae on the inner lateral face are primary lamellae flanking the
longitudinal furrow and are distinct from the concentric lamellae. With for-
mation of concentric lamellae, the outer part of the element must have been
covered by a secretory tissue producing an organic matrix that was mineral-
ized by linear, apatite crystallites. With radial lamellae, it would appear that
lines of secretory cells must have been present on the edges of the lamellae at
the outer surface. The rows of cells gradually retreated outward with growth

9

while retaining their linear form and parallelism. No rhythmic secretion is
apparent within each radial lamella as it extended outwards.

Lindstrom and Ziegler (1971) suggested the longitudinal furrow to be a
possible site for the insertion of muscles termed retractores tentaculorum.
Certainly the furrow is a prominent and characteristic feature of the Pan-
derodontidae and likely to have been functionally important. Few conodonts
display such deep invaginations into their elements, and these furrows may
well have been impossible to produce unless radial lamellae were adopted.
The furrow must have been occupied by tissue during its growth, but we
cannot prove that the tissues remained after the element was complete. In
Panderodus spp. at least, additional, final lamellae were only secreted in the
basal region (Fig. 1) and may represent a basal withdrawal of secretory
tissue. Certainly a theory of muscle insertion seems equally probable to an
alternative hypothesis that the furrow developed on elements functioning as
exposed masticators. But the ultrastructure of all major conodont groups
needs to be investigated before an hypothesis of the function of conodont
elements can be realistically proposed.

MODE OF GROWTH

After an examination of numerous specimens along many different planes of
section, the mode of growth can be described for Panderodus and Belodina.
Figs. 1 and 2 were assembled from illustrations given here and from others
not reproduced in this paper.

Panderodus
A few, initial, concentric lamellae were secreted, outer ones progressively
overlapping inner ones, to provide a downward extension and progressive
deepening of the basal cavity. The longitudinal furrow was then initiated
on the inner lateral side in a direction almost tangential to the basal cavity
wall but not intersecting it. Anterior to this furrow a group of new, radial
lamellae originated. They arose at a high angle; some vertical to the outer-
most initial lamella were widely spaced proximally, then curved sharply to
become radial in orientation. New radial lamellae originated at the anterior
tip of the furrow, with subsequent ones added progressively lower down the
length of the furrow. Others were terminated by extension of the furrow,
with growth over those to the posterior. The anteriormost radial lamella is
thus the oldest and apically overlaps the next, which is younger. At this stage
of growth two sets of lamellae were present: (a) radial lamellae on each
side of the longitudinal furrow, restricted to the inner lateral face between
the anterolateral shoulder or carina and near the posterior margin, and (b)
thinner, concentric lamellae passing from the anterolateral shoulder around
the outer lateral face to the posterior margin or just beyond. Concentric
lamellae were added externally, extending progressively farther basally.
They also appear to extend over the tip of the cusp, but extension of the
element is limited apically, and most of the increase in length was basal.

As the middle phase of growth continued, hyaline lamellar matter was
transformed, secondarily via incipient white matter, to white matter at or

10

above the basal cavity tip. White matter spread outward to ultimately occupy
all the entire tip, save for a few outermost lamellae. White matter may also
be found in small, isolated pods along the posterior margin, especially where
germ or proper denticles developed in certain panderodids.

The end of the middle growth phase is defined as the time at which maxi-
mum length was achieved. The longitudinal furrow had widened from a
narrow, deep slit to a broad, shallow groove. Radial lamellae usually ter-
minated before reaching the basal margin, apparently overlapped by concen-
tric lamellae that did not extend far up the element. Restricted, concentric
lamellae may indicate an eruption of the main part of the element through
the secretory tissue.

In the final growth stage only minor modifications to the element occurred,
all appearing at the basal rim. Secretion of concentric lamellae (e.g., Figs.
SE; 8A,c) continued only on the basal part, becoming first well-defined, basal
wrinkles (Lindstrom and Ziegler, 1971) and eventually a distinctly raised
rim, about 100 wm wide. These basally-restricted lamellae curve inward
toward the basal cavity but disappear on the outer surface as a tangential
feather edge. Thus, circular banding is not evident; the linear crystallite
bundles, which are unequally developed and comprise the lamellae, produced
the basal wrinkles. Expansion of white matter may have continued during
this final growth phase.

The above sequence is based on panderodids that we studied and could
apply to closely related species. Some species of Devonian panderodids may
have had a different growth pattern. Lindstrom and Ziegler (1971) de-
scribed the structure of Neopanderodus perlineatus (Ziegler and Lindstrém),
which exhibits extremely coarse striations on the posteriolateral faces that
also flank a deep, narrow, longitudinal furrow. Whether these, too, represent
a form of radial or semicircular lamellae cannot be judged from their illus-
trations. Unfortunately, the only transverse section (fig. 7D) given by them
is from an area near the tip, entirely in white matter, and with the original
internal structure lacking.

Belodina

Fewer species and specimens of Belodina were examined than for Pandero-
dus spp. Belodina, being denticulate, is more complex, and the lamellar pat-
tern, largely destroyed by the formation of white matter, is more complicated
than in specimens of Panderodus spp. Thus, our remarks on the mode of
growth of Belodina are tentative.

As in Panderodus spp., the initial growth phase is defined as the secretion
of the first few lamellae that created the form of the tip of the basal cavity.
The shape of the lamellae is similar to those in Panderodus. The middle
stage of growth included the development of the deep, narrow longitudinal
furrow. Striations flank the furrow, especially between the furrow and the
denticles, but whether they are homologous to the radial lamellae of Pan-
derodus spp. is not known. Concentric lamellar growth extended the element
only a limited distance basally and, in some species, a bifid cavity is pro-
duced. Most growth, however, was posteriorly, developing a denticle series

Il

and heel, and apically, extending the curve of the main cusp.

Denticles originated as small cores of lamellae and, although confluent,
are sharply set off from each other by sutures. As they grew posteriorly,
their lower parts are covered by lamellae secreted externally on the sides of
the element. New denticles were usually added apically by outgrowths from
the posterior margin of the main cusp (Figs. 4E—H; 10A). In some species,
one or two denticles may have originated from the heel, as this is extended
posteriorly and basally.

We wished to determine whether all lamellae constructing the cusp ex-
tended over most of the element or whether they were limited. Unfortunately
we were unable to study specimens sufficiently devoid of white matter to
adequately demonstrate the true, lamellar pattern. A similar difficulty arises
with the prominent striations on the anterior margin (e.g., Lindstr6ém and
Ziegler, 1971, pl. 3, fig. 7) that intersect the margin at an angle of about 30°,
are as wide as 6 wm, with some overlapping. Lindstrom and Ziegler (1971,
p. 14) offered a partial explanation of the striations in terms of the prism
shape of the apatite crystallites. Our studies are not conclusive. The stria-
tions may possibly represent the outcropping of lamellae that do not extend
completely along the anterior margin. Lamellae below the lower basal cavity
of the belodinid illustrated in Fig. 11A appear to extend from the cavity to
intersect the anterior margin rather than to be parallel to the margin. Al-
ternatively, the area just above the tip of that same cavity reveals the broad,
inner surfaces of individual lamellae, and an interdigitating pattern is visible.
This pattern, presumably of crystallite bundles, is similar to that found on
the outer anterior margin.

In the final growth stage of Belodina spp. length did not increase, but a
raised basal rim that usually lacks basal wrinkles developed as in Panderodus

Spp.-

Summary

Several methods of ultrastructure investigation of conodonts are necessary
and complementary, and the primary technique of etching oriented, polished
sections is one that yields the most information. Size and orientation of crys-
tallites in specimens belonging to species of Panderodontidae agree generally
with data given by Lindstrom and Ziegler (1971). However, not only are
concentric lamellae present, but the coarse striations found on the surface by
Lindstrom and Ziegler (1971) are shown to be radial lamellae that outcrop
on both sides of the longitudinal furrow on the inner, lateral face. They
represent a radical departure in element growth in comparison to other cono-
donts in which only concentric lamellae exist. Known radial lamellae, how-
ever, are suspected in other simple-cone genera that possess coarse, surface
striations. White matter fractures transversely rather than longitudinally,
which is considered functionally advantageous in minimizing damage to the
entire element by restriction to the tip of the cusp. The structure of the basal
filling suggests a post-mortem shrinkage of mineralized tissue that contained
a high proportion of organic matrix. But holes in the basal filling, similar in
size and apparently axially oriented, are probably primary. Lamellae of the

12

basal rim are restricted to that area and do not extend the entire length of
the element. A contraction of the secretory tissue to the basal rim is sug-
gested, with a possible eruption of most of the element. Whether tissue,
possibly muscles, remained in the longitudinal furrow during the life of the
conodont, is open to question. The similarity in ultrastructure of Panderodus
and Belodina further supports the family Panderodontidae as a natural
taxonomic unit.

Acknowledgments

C. R. Barnes gratefully acknowledges the continuing financial support of
the National Research Council of Canada for conodont research. Support
for the research of D. B. Sass was provided by the Alfred University Re-
search Foundation. Miss Carol Frazier kindly assisted in the preparation of
the embedded specimens, and we are indebted to R. L. Ethington of the
University of Missouri, H. L. Jeppson of the University of Lund and I.
Munro of the University of Waterloo for the donation of some of the speci-
mens studied. The research was facilitated by use of the scanning electron
microscope at the University of Waterloo and the transmission electron
microscope at Alfred University.

13

Fig. 1—Diagram of a panderodid, Panderodus compressus, to illustrate external and
internal structure. A) inner lateral view; B) longitudinal section (along axis indicated
by arrows in C) of the middle part of the element marked by breaks; C) transverse
section of the element at a point marked by lower break. Abbreviations: bc, basal
cavity; bcm, basal cavity margin; bf, basal filling; bw, basal wrinkles; cl, concentric

lamellae; hm, hyaline matter; If, longitudinal furrow; rl, radial lamellae; wm, white
matter; about «250.

14

Fig. 2—Diagram of the internal structure of a belodinid, Belodina compressa; \ongi-
tudinal section. Abbreviations in Fig. 1; about « 125.

Fig 3 A-D—Panderodus gracilis, a multi-element species comprising the form species
P. compressus (A,B, inner and outer lateral views) and P. gracilis (C,D, inner and outer
lateral views). Specimens uncoated and unretouched to show extent of basal cavity
and white matter in tip. Bobcaygeon Formation, Middle Ordovician, Great Cloche
Island, Manitoulin, Ontario, x 80.

E—G—Panderodus arcuatus. Scanning electron (SEM) micrographs of a transverse
section near midlength. E) general view. Note radial lamellae flanking longitudinal
furrow, both penetrating inwards from inner lateral face; concentric lamellae in
remainder of specimen. x 290. F) detail of radial lamellae at outer surface of inner
lateral face that penetrate inwards. Small, needle-like crystallites aligned vertical to the
plane of section, 5,750. G) detail of concentric lamellae around anterior part of
basal cavity. Note lateral changes in thickness of lamellae; crystallites linearly arranged
parallel to the axis of the element; overlapping of lamellae on wall of basal cavity;
wide interlamellar spaces immediately anterior to the basal cavity, x 1,175.

H—Detail of striations (radial lamellae) and longitudinal furrow on inner lateral
face of Panderodus gracilis. Note smooth zones on either side of furrow beyond which
are parallel striations extending to anterior shoulder (right) and close to posterior
margin (left). Disregard transverse fracture and secondary surface overgowth.
Chaumont Formation, Middle Ordovician, Upper Ottawa Valley, Ontario, 575.

16

Wij

GY

Y
Ah Z y

Ly
iy

Fig. 4 a-D—Panderodus compressus. SEM micrographs of transverse section just
below tip of basal cavity. Farr Formation, Upper Ordovician, Lake Timiskaming
outlier, Ontario. A) general view. Note small tip of basal cavity; longitudinal furrow
penetrating to cavity area, flanked by striations (radial lamellae) on inner lateral face;
thinner concentric lamellae throughout remainder of section; darker areas in posterior
part representing the lower extensions of white matter, «550. B) detail on inner
lateral face as in Fig. 1. Note longitudinal furrow extending inwards (to upper left)
as a narrow slit. Radial lamellae posteriorly (below) intersect furrow region at 15°,
whereas those anteriorly (above) intersect at 30°. Mostly incipient white matter,
2,750. C) detail of posterior region, specimen rotated 90°. Note longitudinal furrow;
radial lamellae posterior to furrow; concentric lamellae on outer lateral face; white
matter in centre, x 1,100. D) detail as in Fig. 1, close to outer lateral face near posterior
margin. Note transition from lamellar, hyaline matter at margin (lower left) to porous,
white matter inside (upper right), «2,750.

E-H—Belodinid components of the multi-element species Belodina compressa:
B. compressus (£,F, outer and inner lateral views) and B. grandis (G,H, inner and outer
lateral views). Bobcaygeon Formation, Middle Ordovician, Great Cloche Island,
Manitoulin, Ontario, < 80.

18

Mos!
ee

BERG
me

Fig. 5 A-E—Panderodus compressus. Farr Formation, Upper Ordovician, Lake
Timiskaming outlier, Ontario. A) general view of oblique section through the basal
region and cavity just below mid-length, «120. B) Sketch to show orientation of sec-
tion shown in Fig. 5A. C) detail of anterior margin and basal cavity. Note overlapping
of lamellae in basal cavity; radial lamellae extending from inner lateral face (right)
and passing into curved, widely-spaced lamellae just lateral to the basal cavity; longi-
tudinal furrow terminating inside of curved lamellae, «575. D) detail of lower part
of Fig. Sc to illustrate radial lamellae on both sides of longitudinal furrow. Thinner
concentric lamellae to left of cavity, x 1,175. E) detail of basal region. Note distinct
lamellae that appear to terminate at the outer margin of the basal region; basal filling
containing circular holes, « 575.

F—Panderodus arcuatus. Section more proximal through same specimen shown in
Fig. 3E and rotated 180°. Detail of lamellae anterior to furrow. Note abrupt transition
from curved to radial lamellae, « 1,200.

20

Fig. 6 A-D—-SEM micrographs of longitudinal section through Panderodus arcuatus.
Farr Formation, Upper Ordovician; Lake Timiskaming outlier, Ontario. A) general
view of longitudinal section, x 165. B) detail of upper part of section. Note prominent
longitudinal furrow with radial lamellae, nearly parallel, on both sides, 1,100.
C) detail of longitudinal furrow, flanked by radial lamellae, some of which overlap
lengthwise. Anterior (left) third of section is composed of concentric lamellae, but
lamellae not distinct because of oblique intersection with plane of section; separated
from radial lamellae by a poorly-defined, almost vertical parting, x550. D) detail of
area proximal to region shown in Fig. 6c, around basal cavity. On anterior (left) side
of basal cavity concentric lamellae end as overlapping sheets; on posterior (right) side
of cavity, radial lamellae terminate as sharply curved, widely separated lamellae, built
upon concentric lamellae defining the wall of the cavity, x 1,100.

Lz

Fig. 7 A-E—SEM micrographs of longitudinal section through Panderodus compressus.
Farr Formation, Middle Ordovician, Lake Timiskaming outlier, Ontario. A) general
view of longitudinal section, with the depressed central part of the inner lateral face
largely unexposed, x 110. B) sketch to show orientation of longitudinal section given
in Fig. 7A. C) detail of central area of Fig. 7A. Radial lamellae in centre and extreme
right; note that several overlap distally. The longitudinal furrow (arrow) is flanked
immediately by non-laminated zones, «550. p) detail of central part of Fig. 7c.
Radial lamellae, arrow marks where one lamella overlaps another, x 2,750. E) detail
of central part of Fig. 7D. Interlamellar partings, or spaces, well developed; crystallites
not apparent within the lamellae, «5,500.

24

Fig. 8 A-D—SEM micrographs of longitudinal section through Panderodus compressus.
Farr Formation, Upper Ordovician, Lake Timiskaming outlier, Ontario. A) general
view. White matter in tip, inside of inner lateral face exposed in lower part of element,
x65. B) widely spaced, thickened lamellae on anterior side of basal cavity. Compare
with transverse section in Fig. 3G, 1,300. c) overlapping of lamellae that form
inner lateral face. Note that lamellae on left would terminate at the outer edge of
element and not extend over the cusp tip. Lamellae on right are more numerous and
unconformable with those on the left, separated by the lower extension of the longi-
tudinal furrow, 325. D) detail of centre of Fig. 8c. Note unconformity between two
sets of lamellae; crystallites of lamellae are granular, x 1,300.

E-F—SEM micrographs of longitudinal section of Panderodus panderi. Farr Forma-
tion, Middle Ordovician, Lake Timiskaming outlier, Ontario. E) general view of
lemellae intersecting with basal cavity (lower left), x585. F) detail of centre of
Fig. 8£, showing linear crystallites of lamellae arranged parallel to the axis of the
element, «5,850.

26

Fig. 9 A-F—SEM micrographs of basal fillings along longitudinal sections of Pandero-
dus compressus (feulneri type) (C-F) and Panderodus gracilis (A—B). From Farr
Formation, Upper Ordovician, Lake Timiskaming outlier, Ontario, and Prosser
Member, Galena Formation, Middle Ordovician, Decorah, Iowa, respectively. A—B)
views of basal filling lower and higher in the cavity, respectively. Note lamellar struc-
ture of adjacent hyaline matter (anterior margin to left); overlapping of lamellae
down cavity wall; inward contraction of basal filling from hyaline matter. Fig. 9a
<570; Fig. 98 1,140. c) general view. Smooth dark areas represent plastic em-
tedding medium. Internal structure is complicated by contraction of basal filling.
Lamellae visible in places (lower right), 285. D) detail of lower right part of
Fig. 9c to illustrate lamellar structure of basal filling, x570. £) detail of basal filling
in area proximal to that of Fig. 9p. Note lamellar structure and a vertical row of
circular holes, 1,140. F) detail of hole, with surrounding lamellae shown just above
centre of Fig. 92. Hole is hemispherical and does not represent surface expression of
a linear channel, «5,700.

28

LVS ss
; ey “Oa
ie wie
a

bys

OF vant Pg

y

Fig. 10 A—SEM micrograph of weakly etched outer surface of Belodina sp. cf.
B. inclinata. Chaumont Formation, Middle Ordovician, Watertown, New York. Note
linear striations at lower left that pass into white matter (light) with small holes and
transverse linear voids; main cusp bulbous along its posterior edge as new denticles
are being formed within, x 480.

B—G—Belodina compressa. Chaumont Formation, Middle Ordovician, Upper
Ottawa Valley, Ontario. Three specimens. B) general view of longitudinal section,
110. c) detail of central part of Fig. 108. Note section cuts through three denticles,
which are clearly defined by near-vertical planes of separation (arrows); each denticle
has a lower cone of concentric lamellae that is transformed distally into white matter,
< 520. D) detail of cusp from Fig. 10B. Note lamellae along anterior margin passing
into a core of white matter; new denticles defined by planes of separation (arrows)
developing immediately posterior of core of the cusp, x 520. E) general view of longi-
tudinal section through the base of denticle series (heel at bottom), 265. F) detail of
central part shown in Fig. 10£. Note that section is through three denticles; each has a
central crude concentric pattern of the cone of lamellae (open arrow) with white mat-
ter beyond (see Fig. 10c); lamellae on extreme (left) outer surface are separated from
the denticles by the longitudinal furrow (closed arrow), 1,050. G) detail of white
matter in a denticle along longitudinal section similar to that shown in Fig. 10£ but
slightly more distal (see Fig. 10c). White matter shows no crystalline structure at this
magnification but has both circular and irregular linear voids, x 6,000.

30

\
N

Fig. 11 s-D—Belodina sp. A. Farr Formation, Upper Ordovician, Lake Timiskaming
outlier, Ontario. A) general view of longitudinal section revealing basal region with
bifid cavity, heel region at top, anterior margin at bottom, lateral furrow passing
horizontally in centre. Note interdigitating pattern on lamallar surfaces above tip of
lower cavity; lamellae below lower cavity intersect anterior margin, 325. B) detail
of upper cavity illustrated in Fig. 114. Note how lamellae curve toward basal cavity,
becoming several times thicker and developing wider interlamellar spaces, 650.
C) detzil of area above lower cavity shown in Fig. 114. Note incurving of lamellae
towards basal cavity; granular texture (crystallites?) of lamellae but with a diagonal
(upper right to lower left) lamination in some at 30° to interlamellar spaces; inter-
digitating pattern seen at lower right, «1,300. p) details of centre of Fig. 11c to show
granular texture of lamellae and apparent lamination, 3,250.

E-F—Belodina, sp. B. Farr Formation, Upper Ordovician, Lake Timiskaming,
Ontario. E) general view. Longitudinal section, lamellar structure except in denticles;
longitudinal furrow prominent, 115. F) detail of area shown in Fig. 11£ at central
lower edge of basal cavity. Lamellar structure; lamellae progressively overlap earlier
deposited ones as basal cavity is enlarged; some lamellae varying in thickness; apparent
inclined lamination of crystallites within lamellae as shown in Fig. 11c, x 1,125.

a2

SSS

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36

*

7

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