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