Figure BH2.
Determination of Clam (Lyrodesma sp.)
Valve Size from Phosphatic Molds of Dentitions.
117 individual phosphatic molds of Lyrodesma hinge
dentitions were extracted from a scoop of clay by water dissolution.
This sample was compared to 14 complete
Lyrodesma sp. hinges with dentitions
preserved in calcite to estimate the size for the valves from which the
phosphatic molds originated. The clay was from the middle Arnheim Formation of
Florence, KY (site FFB). The
Lyrodesma hinges were
from various sources in the Cincinnatian
Series. Only 6 of these hinges had complete valves for this measurement. The
other Lyrodesma hinges provided correlative data.
List of Calcite Clam Hinge
Sources
7 from the Carrolton Beds of Carrollton, KY (Southgate
member of Kope). Only 3 of these had complete valves preserved in calcite.
4
from KY 17 in Ft. Wright, KY (Southgate member of Kope). 2 of these had complete
valves preserved in calcite.
4 measured from images in Pojeta 1971, Plate 3
using noted magnifications. Two of
these included complete valves (USNM 46223 and USNM 33473).
Result
The
result shows that the phosphatic dentition molds represented valves from 6 to 17
mm in length. If 10 ribs per dentition represents a complete dentition, then the
original valves ranged from 11 to 17 mm. By comparison, the Lyrodesma valves
preserved by calcite replacement that were sampled have lengths ranging from 9.0
mm to 16.7 mm. This suggests the phosphatic dentition molds came from a normal
mix of bivalve sizes.
Whether the smallest of these phosphatic molds represented
juvenile specimens of Lyrodesma sp. cannot be determined.
However, the evidence suggests that
adult Lyrodesma sp. specimens were included In the assemblage.
Measurements
The sampling of phosphatic dentition molds were measured
along the open edge having debris visible (Figure BH3b and BH3a) and used to
estimate the total valve length. 117
individual elements of dentition molds were placed on a flatbed scanner with a
millimeter scale. Each element in scanned image was given a number measured with
respect to the scale. (file: dentition_molds_38_scanner_v2.jpg)
From examining the Calcite replacements of complete
Lyrodesma valves, a ratio of 7.14 to 1 between the valve length and the
dentition lengths was found to be the average across all of the dentitions
capable of impressing a mold edge. The cavities between the calcite dentitions
were measured rather than the calcite dentitions themselves so as to compare
correctly to the phosphatic molds. The number of ribs acted as a control in
identifying short, fragmented or poorly preserved phosphatic dentition molds.
(Figure BH1)
Figure BH2.
Four complete valves of
Lyrodesma sp. replaced in calcite
from the “Carrollton Beds” (Southgate member, Kope Formation)
in Carrollton, KY and Ft. Wright, KY were measured
to determine the relative size of the openings between the individual
hinge dentitions to the entire length of the valve. Additionally, two complete
valves from Pojeta 1971, taken from Plate 3 of specimens Lyrodesma USNM 46223
and USNM 33473 were measured in the same way. (Figure
BH2)
Figure BH3
Only hinge dentition cavities large enough to form
phosphatic molds were included (7 or 8 cavities per valve).
(Figure BH3a)
Potential for using Lyrodesma
Dentition Ribbing for Valve Size Estimates
There may be a way to relate the measurements of the ribs
on the sides of phosphatic dentition molds to the ribs on the sides of Lyrodesma
dentition calcite replacements to determine the size of the valves from which
the phosphatic molds originated. Unfortunately,
hinge dentition ribbing is not often preserved well on specimens preserved as
calcite replacements and so insufficient data was available for this study.
Suggested characteristics of dentition ribbing to measure
include degrees of arc, rib length, width and depth,
distance to the next rib and number of ribs on the dentition element. The
degree of arc may be used to indicate the distance to the axis of the hinge.
(Figure BH4)
Figure BH4
The length and width of the phosphatic mold can be compared
directly to the hinges of Lyrodesma valves preserved as calcite replacement or
other types of replacement.
Figure BH1
Figure BH1 uses the number of ribs on each phosphatic
dentition mold to find a relationship to the calculated length of the original
valve. Valve lengths are calculated based on assumption that the valve is 7.14
times longer than the average dentition cavity/mold length.
The result shows some correlation and suggests if we had more complete
calcite Lyrodesma valves with preserved ribbing, we may be able to more
accurately determine the size of the bivalves from which these phosphatic
dentition molds came.
It may be possible to use the number of ribs per millimeter
to estimate the maturity of the original valve. This author has observed that
the dentition phosphatic molds have ribs that are closer together as they are
closer to the hinge axis.
The depth of each rib on Lyrodesma dentitions may also play
a role in determining the maturity of the valve. This datum, as well as distance
between ribs, will require more study into the maturation of Lyrodesma sp.
Limitations to Measurements
·
Molds are
partial. Phosphatic steinkerns and external molds almost never fill the
cavity completely to the outer edge. Our measurements were taken of the outer
edge of each calcite dentition.
With the curved nature of the dentition internals, valve size will be
underestimated with this measurement
(Figure BH3c).
·
Molds are
broken. The individual phosphatic mold elements have a higher incident of
breakage than articulated groups. All of the sampled phosphatic dentition mold
were individual elements, lending themselves to an underestimated whole valve
size(Figure BH3c). The number and orientation of ribs on the mold could be
usable to assess the error. More work is needed to define the rib
characteristics on the sides of Lyrodesma dentitions.
·
What
defines an adult? More work is needed to identify an adult Lyrodesma for the
purposes of this paper.
·
Insufficient sampling. More
complete Lyrodesma valves need to be measured for this analysis. Among the 11
calcite Lyrodesma hinges measured and 4 photos of Lyrodesma valves, only 6 had
relatively complete valves and only 2 of those calcite valves had visible
ribbing on the dentition sides to help determine completeness of the molds.
·
Undetermined species. Species determination for sampled Lyrodesma valves is
needed to identify differences in dentition characteristics critical to accuracy
in this analysis.
·
Potential
sampling size bias. The phosphatic molds were selected from the clay sample
using a 10x lens and a tweezers. Unintentional size bias was possible.
·
Dentition
length depends on hinge position. The seven or eight dentitions of a
Lyrodesma valve may have different lengths. This makes it inaccurate to use the
length of a single phosphatic
dentition mold to estimate the valve length without knowing the ordinal position
of the dentition in the hinge. However, the samples of complete Lyrodesma this
author has collected suggest that there is not very much difference is dentition
length from one end of the hinge to the other (typically ranging from 1.25mm to
2.00mm ). In Pojeta 1971, page 10
it is mentioned “in other (than L.
armoricana) Middle and Late
Ordovician Lyrodesmas (pl. 3, figs. 10, 15, 20) the posterior teeth are not
appreciably different in size from the other teeth.”
·
Images
from publications were measured.
We measured the valve lengths and hinge teeth lengths from four
photographs of Lyrodesma from Pojeta 1971, taken from Plate 3. Magnifications
were indicated as 1X, 2X and 3X, without regard for decimal precision. This can
introduce inaccuracies in the measurements.
Bill Heimbrock
1621 Westwood Ave
Cincinnati, OH 45214
(513) 921-2335
billheim@cinci.rr.com