Lirae Spacing

The second part of our study is an analysis of lirae spacing in three of the earliest families of ammonoids: the Mimagoniatitidae, the Anarcestidae, and the Agoniatitidae. In total, seven specimens were studied; three mimagoniatids, two anarcestids, and two agoniatids.

The results are presented in Figs. 2.6-2.11. The horizontal axis is the lirae number beginning with the most adapical lira on the embryonic shell; in some graphs, the first lira measured is on the postembryonic shell. The vertical axis is the distance between lirae (see Fig. 2.5b), given in microns. Data are presented for the three measured positions: ventral, midflank, and dorsal (Fig. 2.5a). They are graphed together using different symbols for each position on the shell. Data are given in Appendices 1-11.

Fig. 2.5 Sketch of the ammonitella of a primitive ammonoid showing the areas where the lirae spaces (distances between successive lirae) were measured. (A) The three areas are marked with different patterns: Ventral = dots; Midflank = horizontal lines; and Dorsal = cross hatch. (B) Close-up of ventral edge of ammonitella with lirae shown raised. The horizontal line (1, 2, 3) indicates how the distances were measured.

Fig. 2.5 Sketch of the ammonitella of a primitive ammonoid showing the areas where the lirae spaces (distances between successive lirae) were measured. (A) The three areas are marked with different patterns: Ventral = dots; Midflank = horizontal lines; and Dorsal = cross hatch. (B) Close-up of ventral edge of ammonitella with lirae shown raised. The horizontal line (1, 2, 3) indicates how the distances were measured.

The ratios of the ventral (V) and dorsal (D) lirae distance are also given, where possible. The horizontal axis is the lirae number and the vertical axis is the ratio (V/D).

Means and standard deviations were calculated for the distance between lirae using data from the midflank position. Data are given in Table 2.1. Because of the extent of the overlap of standard deviations, these data cannot be used to differentiate among the three taxonomic groups. The results of an F-test confirm that the means are significantly different at the 0.01 level.

3.2.1 Mimagoniatitidae

The three specimens from the Mimagoniatitidae show the same general pattern, although there is some variation (Figs. 2.6a, 2.7a, 2.8a). Two of the specimens are Archanarcestes obesus (AMNH 45374, Fig. 2.7, Appendix 2; AMNH 45370, Fig. 2.8, Appendix 1).

Table 2.1 Means of the distance between successive lirae calculated at the mid flank positions. Measurements aie given in microns.

Lirae spacing (lm)

Family

N

Mean ± SD

Range

MIMAGONIATIDAE

AMNH 46645

76

45 ± 16

14-75

AMNH 45374

52

31 ± 9

9-49

AMNH 45370

55

28 ± 10

14-52

AGONIATITIDAE

NYSM 3545

56

40 ± 8

55-25

AMNH 50417

29

47 ± 15

28-126

ANARCESTIDAE AMNH 46646 AMNH 50416

ANARCESTIDAE AMNH 46646 AMNH 50416

70 94

8-70 10-37

The third specimen is Mimagoniatites fecundus (AMNH 46645, Fig. 2.6, Appendix 4). As expected, based on simple geometry, the ventral distance between lirae is greater than the dorsal distance because the venter subscribes a greater arc. The mid-flank position contains measurements midway between the dorsum and venter.

There is a general increase in the distance between lirae over the initial chamber at the ventral, midflank, and dorsal positions. Then at approximately 5-10 lirae from the apex, there is a decrease in the distance. This is generally slight, but in one specimen (AMNH 45374, Fig. 2.7a), it is expressed very strongly. After this initial drop, there is a second increase, followed by an abrupt drop in lirae distance in all three specimens. This decrease marks the end of the initial chamber and occurs at about 25-30 lirae from the apex. Over the course of the ammonitella coil, there is a gentle increase in spacing followed by a decrease at the ventral, midflank, and dorsal positions. This decrease is abrupt and occurs at the end of the ammonitella. In all, there are 70-77 lirae on the ammonitella of the Mimagoniatitidae.

The spacing between lirae can fluctuate within the general trend. There appears to be no pattern to this fluctuation. The result is a graph with a very jagged appearance. These fluctuations are also not perfectly aligned among the three measured positions.

In two of the specimens (AMNH 46645, Fig. 2.6b, Appendix 5; AMNH 45374, Fig. 2.7b, Appendix 3), the postembryonic distance between lirae was measured. Again, the ventral distance was the widest, the dorsal distance was the narrowest, and the midflank distance was in between. There is a rapid increase in spacing from the first measurable postembryonic lira.

The ratio of the ventral distance/dorsal distance (V/D) on the embryonic shell was compared to that of the postembryonic shell (Figs. 2.6c, d, 2.7c, d). The most complete data set (AMNH 46645) shows that there is much greater correlation between the dorsal and ventral distances between lirae on the postembryonic shell than on the embryonic shell (Fig. 2.6c, d).

125 110

Q 50

ventral midflank dorsal ventral midflank dorsal

125 110

Q 50

-i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—r-

-i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—r-

250 170;

250 170;

20 30 40 Lirae Space

Lirae Space

20 30 40 Lirae Space

Lirae Space

Fig. 2.6 Mimagoniatites fecundus (AMNH 46645, Devonian, Morocco). (A) Lirae spacing on embryonic shell for ventral, midflank, and dorsal positions. Symbols given in graph. The X axis is the number of lirae space (see methods and materials for details). The Y axis is the measured distance between two lirae given in microns (mm). Data are given in Appendix 4. (B) Lirae spacing on postembryonic shell for ventral, midflank, and dorsal positions. The X axis is the number of lirae space. The Y axis is the measured distance between two lirae given in microns (mm). Data are given in Appendix 5. (C) Ratio of the ventral lirae space/dorsal lirae space (V/D) on the embryonic shell. Data are given in Appendix 4. (D) Ratio of the ventral lirae space/dorsal lirae space (V/D) on the postembryonic shell. Data are given in Appendix 5.

Lirae Space Lirae Space

Fig. 2.6 Mimagoniatites fecundus (AMNH 46645, Devonian, Morocco). (A) Lirae spacing on embryonic shell for ventral, midflank, and dorsal positions. Symbols given in graph. The X axis is the number of lirae space (see methods and materials for details). The Y axis is the measured distance between two lirae given in microns (mm). Data are given in Appendix 4. (B) Lirae spacing on postembryonic shell for ventral, midflank, and dorsal positions. The X axis is the number of lirae space. The Y axis is the measured distance between two lirae given in microns (mm). Data are given in Appendix 5. (C) Ratio of the ventral lirae space/dorsal lirae space (V/D) on the embryonic shell. Data are given in Appendix 4. (D) Ratio of the ventral lirae space/dorsal lirae space (V/D) on the postembryonic shell. Data are given in Appendix 5.

3.2.2 Anarcestidae

Two specimens from the Anarcestidae were measured, both identified as Latanarcestes sp. (AMNH 46646 and AMNH 50416). The results are presented in

30 40 50

Lirae Space

Dis 50

40 30 20 10

0 10 20

Lirae Space

on a

Lirae Space

Fig. 2.7 Archanarcestes obesus (AMNH 45374, Devonian, Morocco). (A) Lirae spacing on embryonic shell for ventral, midflank, and dorsal positions. Symbols given in graph. X axis is the number of lirae space. Y axis is the measured distance between two lirae given in microns (mm). Data are given in Appendix 2. (B) Lirae spacing on postembryonic shell for ventral, midflank, and dorsal positions. X axis is the number of lirae space. Y axis is the measured distance between two lirae given in microns (mm). Data are given in Appendix 3. (C) Ratio of the ventral lirae space/ dorsal lirae space (V/D) on the embryonic shell. Data are given in Appendix 2. (D) Ratio ofthe ventral lirae space/dorsal lirae space (V/D) on the postembryonic shell. Data are given in Appendix 3.

35 4( Lirae Space

35 4( Lirae Space

Lirae Space

Fig. 2.8 Archanarcestes obesus (AMNH 45370, Devonian, Morocco). (A) Lirae spacing on embryonic shell for ventral, midflank, and dorsal positions. Symbols given in graph. X axis is the number of lirae space. Y axis is the measured distance between two lirae given in microns (mm). (B) Ratio of the ventral lirae space/dorsal lirae space (V/D) on the embryonic shell. Data are given in Appendix 1.

Figs. 2.9-2.10 and Appendices 6-9. Except for a small gap, the data for AMNH 50416 are complete across the ammonitella (Fig. 2.9a). AMNH 46646 has dorsal measurements for only the initial chamber (Fig. 2.10a). Thereafter, the ventral data are the most complete. On the initial chamber, as in the Mimagoniatitidae, the ventral distance is the widest, the dorsal the narrowest, and the midflank somewhere in between. The ventral distance increases, then decreases to the end of the initial

i 30

40 50 60 Lirae Space

80 90 100

10 20

40 50 60 Lirae Space

80 90 100

10 20 30 Lirae Space

30 40 50 6 Lirae Space

10 20

80 90

30 40 50 6 Lirae Space

0 10 20 30 Lirae Space

Fig. 2.9 Latanarcestes sp. (AMNH 50416, Devonian, Morocco). (A) Lirae spacing on embryonic shell for ventral, midflank, and dorsal positions. Symbols given in graph. X axis is the number of lirae space. Y axis is the measured distance between two lirae given in microns (mm). Data are given in Appendix 8. (B) Lirae spacing on postembryonic shell for ventral, midflank, and dorsal positions. X axis is the number of lirae space. Y axis is the measured distance between two lirae given in microns (mm). Data are given in Appendix 9. (C) Ratio of the ventral lirae space/dorsal lirae space (V/D) on the embryonic shell. Data are given in Appendix 8. (D) Ratio of the ventral lirae space/dorsal lirae space (V/D) on the postembryonic shell. Data are given in Appendix 9.

chamber, seen most strongly in AMNH 50416 (Fig. 2.9a). The pattern of midflank distance parallels that of the ventral distance. The dorsal distance, in contrast, decreases slowly from the apical end of the initial chamber to its end. A decrease in the distance between lirae marks the end of the initial chamber in the ventral and midflank data. There are 40-45 lirae on the initial chamber.

i 30

10 20

10 20

80 90

10 20 30 40 50

Lirae Space

Lirae Space

Fig. 2.10 Latanarcestes sp. (AMNH 46646, Devonian, Morocco). (A) Lirae spacing on embryonic shell for ventral, midflank, and dorsal positions. Symbols given in graph. X axis is the number of lirae space. Yaxis is the measured distance between two lirae given in microns (mm). Data are given in Appendix 6. (B) Lirae spacing on postembryonic shell for ventral, midflank, and dorsal positions. X axis is the number of lirae space. Y axis is the measured distance between two lirae given in microns (mm). Data are given in Appendix 7. (C) Ratio of the ventral lirae space/dorsal lirae space (V/D) on the embryonic shell. Data are given in Appendix 6. (D) Ratio of the ventral lirae space/dorsal lirae space (V/D) on the postembryonic shell. Data are given in Appendix 7.

10 20 30 40 50

Lirae Space

Lirae Space

Fig. 2.10 Latanarcestes sp. (AMNH 46646, Devonian, Morocco). (A) Lirae spacing on embryonic shell for ventral, midflank, and dorsal positions. Symbols given in graph. X axis is the number of lirae space. Yaxis is the measured distance between two lirae given in microns (mm). Data are given in Appendix 6. (B) Lirae spacing on postembryonic shell for ventral, midflank, and dorsal positions. X axis is the number of lirae space. Y axis is the measured distance between two lirae given in microns (mm). Data are given in Appendix 7. (C) Ratio of the ventral lirae space/dorsal lirae space (V/D) on the embryonic shell. Data are given in Appendix 6. (D) Ratio of the ventral lirae space/dorsal lirae space (V/D) on the postembryonic shell. Data are given in Appendix 7.

Immediately after the decrease in distance, there is an increase in distance on the ammonitella coil. This peaks at about 20 lirae from the end of the initial chamber. Adoral of this, the distance decreases on the ammonitella coil until the end of the ammonitella. This pattern is observed on the dorsal, midflank, and ventral positions (Figs. 2.9a, 2.10a). There are 95 lirae on the ammonitella of these Anarcestidae. The distances of the postembryonic lirae show no initial increase, just wide fluctuations (Figs. 2.9b, 2.10b). Only after about 40 lirae does the distance between the lirae begin to increase.

The V/D ratios again show a greater amount of fluctuation between successive lirae on the embryonic shell when compared to the postembryonic shell (Figs. 2.9c, d, 2.10c, d).

3.2.3 Agoniatitidae

Two specimens from the Agoniatitidae were studied, identified as Agoniatites vanuxemi (NYSM 3545) and Fidelites fidelis (AMNH 50417). The data are presented on Fig. 2.11 and Appendices 10-11. It was not possible to measure the distance between lirae on the postembryonic shell. The data for the two Agoniatitidae are less complete than those for the Mimagoniatitidae and the Anarcestidae. There are gaps in various places at all three measurement positions. In F. fidelis, the lirae can only be measured to just adoral of the initial chamber (Fig. 2.11b). Here again, the ventral distance is the widest, the dorsal the narrowest, and the midflank measurement lies somewhere in between. In both genera, after an initial increase in the distance between lirae (1-10) at all three positions, there is a slight decrease. Then, at the ventral and midflank positions, the distance increases rapidly to the end of the initial chamber. At the dorsal position, however, the distances vary widely, but remain essentially level. There are about 30-35 lirae on the initial chamber. There is a decrease at the end of the initial chamber, followed by a very rapid increase in the distance between lirae at the ventral and midflank position and a less rapid increase at the dorsal position. For the remainder of the ammonitella in A. vanuxemi (Fig. 2.11a), there is a rapid decrease in the distance followed by a more gradual increase, then a leveling off or a slight decrease. Data end just before the end of the ammonitella in both specimens. There are approximately 65 lirae on the ammo-nitella of the Agoniatitdae.

Because of gaps in the data, there are fewer data available for calculation of the V/D ratio. What is evident is that over the initial chamber, the ratio increases rapidly at about 20 lirae (V > > D) and reaches a maximum at this point (Fig. 2.11c, d).

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