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Chapter 46. One hundred million years of predation on ostracods: The fossil record in Texas

Chapter 46. One hundred million years of predation on ostracods: The fossil record in Texas

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638 R. F. MADDOCKS



riously thought to have evolved first in the Late Triassic (Fiirsich and Jablonski, 1984), Albian

(Taylor et al., 1980), or Cenomanian (Fisher, 1964; Sohl, 1969), although the Naticidae and Muricidae themselves date from the Early Jurassic and Albian, respectively. Several palaeobiologists

have analysed drillhole frequencies in Cenozoic mollusc assemblages, attempting to reconstruct

the modes and rate of co-evolution of this predator-prey system (Kitchell, 1982). None of these

investigations has included Ostracoda.

Reyment (1963, 1966, 1967) reported that modern drilling gastropods on the Niger Delta prefer

molluscs (pelecypods, gastropods, scaphopods), but that where molluscs are less abundant, the

juvenile gastropods readily attack either adult or juvenile ostracods. He suggested that large species

with smooth carapaces, pelecypod-like shape, and infaunal habit may be more vulnerable to attack.

Paleocene ostracod assemblages of Nigeria, he also reported, have abundant drillholes, perhaps

because pelecypods are not common in these rocks.

The material for this study was provided by faunal assemblages already in the micropalaeontology collections of the University of Houston. Stratigraphical units not included are non-marine

in outcrop or have few ostracods. Many of these assemblages were originally collected during

preparation of the excursion guidebook for the Eighth International Symposium on Ostracoda

(Maddocks, 1982), in which full stratigraphical documentation and species lists were given for

these localities (Maddocks and Ross, 1982). The faunal assemblages corresponding to the

publications by Garbett and Maddocks (1979), Moysey and Maddocks (1982), Ross and Maddocks

(1983), Chimene and Maddocks (1984), and Maddocks (1985) were also included, as well as other

unpublished recent and fossil samples. The contributions of many former students, especially Alta

Cate, J. B. Chimene 11, Peggy Cole, Elizabeth Garbett, Jo Ann Locklin, David G. Moysey, and

James E. Ross are gratefully acknowledged, as well as the verifications of certain species identifications by Joseph E. Hazel, Alvin Phillips, and W.A. van den Bold. In total, the material encompasses over 400 slides, 80,000 specimens, and 425 species, covering more than 100 million years of

history.

The counts of specimens include both adults and juveniles, single valves and whole carapaces, and

identifiable fragments that are large enough that at least part of any naticid hole would have been

retained. The counts of individuals signify the minimum number of adult animals represented,

calculated as the number of adult whole carapaces plus the number of adult right valves or left

valves, whichever was greater (ignoring juveniles, the less numerous valve, and most fragments).

Mortality is calculated as percentage of total adult individuals bearing drillholes or other presumably fatal predation scars. (For assemblages in which left and right valves have been counted,

this calculation is based on a conservative estimate of the number of individuals as half the number

of specimens. Species of Cytherellu were not counted separately, although most samples contain

two to four species. Tables 1-12 summarize the stratigraphical distribution of evidence for predation,

PLATE1-Fig. 1 . Phenon N. Perutocytherideubrudyi (Stephenson, 1938), RV, Holocene, West Bay, UH2578, X 80.

Fig. 2. Phenon R. Cytherellu sp., RV anterior margin, Navarro Group, UH 4729, x 80. Fig. 3. Phenon Q.

“Cythereis sp. C” of Moysey and Maddocks, 1982, RV anterior margin, Walnut Formation, UH1473, X80.

Fig. 4. Phenon D. Truchylebexis elmuna (Stadnichenko, 1927), LV, Cook Mountain Formation, UH5337,

~ 8 0 Fig.

. 5. Phenon L. Asciocythere rotunda (Vanderpool, 1928). RV, Glen Rose, UH5035, x 125. Fig. 6.

Phenon D. Actinocythereis duvidwhitei (Stadnichenko, 1927), LV anterior margin, Cook Mountain Formation,

UH 5333, X90. Fig. 7. Phenon A. Pterygocythereis sp., LV, Holocene. Gulf of Mexico, UH4125, X45. Fig. 8.

Phenon K. Clithrocytherideu subpyrifbrmis (Sutton and Williams, 1939), RV, Cook Mountain Formation,

UH5337, x 70. Fig. 9. Phenon 0. Bruchycythere rhomboidulis (Berry, 1925), LV dorsal margin, Navarro,

UH5034, X 70. Fig. 10. Phenon M. “Cythere ornatu” of Vanderpool, 1928, LV, Glen Rose, UH5072, X 80.

Fig. 11. Phenon N. Cythereis puuperu Jones and Hinde, 1889, LV, Del Rio, UH4901, x 80. Fig. 12. Phenon

G. Henryhowella orelliunu (Sutton and Williams, 1939), RV, Cook Mountain Formation, UH4313, X 80.

Fig. 13. Phenon U. Brachycythere crenulutu Crane, 1965, LV, Dessau Formation, UH5327, X80. Fig. 14.

Phenon S. Huplocytherideu globosu (Alexander, 1929), RV, Navarro Group, UH5359, x 80. Fig. 15. Phenon

S. Bruchycythere ovutu (Be@, 1925), LV, Navarro, UH4732, X60.



Predation on Ostracods in Texas 641



together with the total numbers of localities, assemblages, specimens, individuals, and species

examined and names of species.



PREDATION

ICHNOPHENA

Carriker and Yochelson (1968) reviewed the distinct drilling methods used by modern naticids

and muricids, illustrating the diagnostic morphological criteria by which the resulting holes may

be identified. Many authors have stated that the two kinds of holes are easy to distinguish, and

some molluscan studies have tallied them separately with considerable success. However, in small,

thin-shelled, ridged, or reticulate mollusc shells, naticid and muricid drillholes are less distinct,

and the prescribed taxonomic criteria for distinguishing them do not apply easily to Ostracoda.

On the other hand, within any given stratigraphical interval it proved relatively easy to recognize

arbitrary ichnophena (Greek, ichnos=track or trace, plus phainein=to show; for groups of morphologically similar trace fossils), according to such parameters as size, shape, taper, and roughness

of edges. Twenty ichnophena were arbitrarily defined (designated A-U), some of which may represent transitional or aberrant examples of others. Recognition of some of these ichnophena is

subjective, because their appearance varies with size, thickness, and ornament of the ostracod shell.

For the most part in this data set, these phena are consistently recognized and distinguished from

each other within any one stratigraphical interval but less so vertically. Representative examples

are illustrated in Plates 1 and 2.

A : Very small, acute-conical to cylindrical holes with circular to oval circumference, wall planar

and smooth.

B: Small, approximately parabolic holes with near-circular to oval to irregularly oblong circumference, edges slightly irregular or jagged; wall gently to steeply sloping, uneven to nearly smooth,

not planar.

C: Small to medium-sized holes with steep sides and smoothly curved edges, outline roundedtriangular to ovate to subquadrate.

D: Small to medium-sized cylindrical holes with circular to ovoid outlines and vertical to steeply

sloping, smooth walls; may have flat to parabolic inner floors with tiny circular inner hole.

E: Small to medium-sized to large, oblong holes with sharp, more or less jagged edges and

variable shape; not accompanied by cracks, enlarged normal pore-canals, abrasion, or frosting.

F: Irregular holes of all kinds, often accompanied by cracks,that could have resulted from ordinary

PLATE

2-Fig. 1. Phenon P. Malzella sp., RV anterodorsal margin, Holocene, Gulf of Mexico, UH4125, ~ 6 0 .

Fig. 2. Phenon C. Phacorhabdotus formosus (Alexander, 1934), LV dorsal margin, Midway, UH4885, x 120.

Fig. 3. Phenon E. Loxoconchaaff.L. moralesi Kontrovitz, 1978, LV, Holocene, Gulf of Mexico, UH4120, X 80.

Fig, 4. Phenon D. Krithe sp., RV, Holocene, Gulf of Mexico, UH4125, X 75. Fig. 5. Phenon J. Loxoconchu

cluibornensis Murray, 1938, RV, Cook Mountain Formation, UH5333, X 120. Fig. 6. Phena A and I. Haplocytherideu nanifaba Crane, 1965, LV, Burditt Formation, UH4829, X 80. Fig. 7. Phenon A. Cytherelloidea crafri

(Sexton, 1951), RV, Bergstrom Formation, UH4868, x 100. Fig. 8. Phenon E. Megucythere repexa Garbett

and Maddocks, 1979, LV, Holocene, West Bay, UH2622, x140. Fig. 9. Phenon L.Asciocythere rotunda

(Vanderpool, 1928), RV, Glen Rose, UH5036, x 70. Fig. 10. Phenon B. Cytherella sp., LV, Navarro Group,

UH4639, x 80. Fig. 11. Phenon L. Buntonia alabamensis (Howe and Pyeatt, 1934), LV, Stone City Formation, UH1931, x80. Fig. 12. Phenon B. Cytherella sp., LV, Midway, UH4049, ~ 8 0 .Fig. 13. Phenon

I. Asciocythere goodlandensis (Alexander, 1929), LV, Walnut Formation, UH1437, X 125. Fig. 14. Phenon

S. Brachycythere plena Alexander, 1934, dorsal view of whole carapace, Midway Group, UH4051, X 60.

Fig. 15. Phenon R. Cytheropteron fossatum Skinner, 1956, LV, Bergstrom Formation, UH4802, X80. Fig. 16.

Phenon T. Cytherelloidea sp., LV, Del Rio Formation, UH4854, ~ 9 0 .

Fig. 17. Phenon H. Perissocytheridea

bruchyformu Swain, 1955, RV, Holocene, West Bay, UH2856, x 140. Fig. 18, Phenon S. Antibythocypris

macropora (Alexander, 1929), LV, Navarro, UH4788, x80. Fig. 19. Phenon S. Haplocytheridea aff. H .

everetti (Berry, 1925), RV. Navarro Group, UH5357, x 80. Fig. 20. Phenon S. Brachycythere ledaformu

(Israefsky, 1929), dorsal view of whole carapace, Navarro Group, UH4786, x 80.



642



R.F. MADDOCKS



post-mortem breakage rather than predation or solution; not further discussed in this report.

G: Medium-sized, irregular holes with oblong, polygonal, or irregular outlines and uneven wall;

edges fairly smooth; surrounding area may be unevenly abraded or sloping.

H: Small to medium-sized holes of variable outline with smoothly curving to slightly irregular to

more or less jagged, sharp edges; neighbouring region scraped, frosted, abraded, crumbling, or

flaking, usually white.

I: Small to medium-sized holes with very irregular to jagged edges; outer edge chipped, gouged,

or unevenly excavated and abraded; walls sloping gently to steeply, uneven.

.

I

Medium-sized

:

to large, oblong holes in thin, transparent shells; edges sharp, smoothly curved

to slightly jagged.

K: Small holes with polygonal outlines, keyhole-shaped, trefoil-shaped, L-shaped, rhombic, or

quadrate; walls vertical to steeply sloping.

L: Lacy clusters of small to medium-sized to large solution(?) holes with irregular outlines and

jagged edges, walls vertical to unevenly sloping; outer edge may be more or less abraded or gouged

or dissolved; some may be located at normal pore-canals.

M: Medium-sized to large, oval to oblong holes with smoothly curving to slightly irregular

edges, walls vertical to steeply sloping, smooth but not planar.

N: Large, parabolic holes with round to oval circumference; walls countersunk and smoothly

beveled, planar; obviously naticid.

0: Small to medium-sized, rounded-triangular to ovoid holes with vertical walls and smooth

edges.

P: Small holes, the shape of and occupying just one fossa of the reticulate ornament.

Q: Very tiny holes with rounded to irregular to scalloped margins, vertical but uneven walls,

outer margin not chipped or abraded.

R: Small to medium-sized, L-shaped to triangular to rhombic or rectangular holes with slightly

irregular to jagged edges; outer edge not chipped, gouged, or abraded; walls vertical; some may

appear to be two or more holes joined.

S: Large, saddle-shaped, solution holes. Early stages have a large, circular to oblong hole symmetrically located dorsolaterally on each valve of the carapace. In later stages a crack connects the

two holes across the dorsum, which then is enlarged by further solution to form a broad channel.

The adjacent lateral surface is opaque, frosted, roughened, flaking, or crumbling. Many examples

consist of disarticulated or still-articulated U-shaped valves retaining only the free margin plus a

narrow band of adjacent shell. Assemblages in which such specimens are common usually also

have abundant tiny shards showing the final fragmentation of these weakened shells.

T: Small, rounded-triangular to angulate-triangular to crescentic holes with smooth edges, usually two convex edges and one concave edge; outer edge not frosted, abraded, chipped, or gouged.

U: Elongate to equidimensional grooves, gouges, and shallow pits, usually not penetrating

shell; edges and walls very irregular; mostly affecting just one side of carapace.

X: Triangular to polygonal, straight-edged holes in smooth, thin shells; molds of pyrite crystals

in partly pyritized shells; not caused by predation, not discussed further in this report.

The tunnels of boring algae and fungi were also recognized but not considered further, because

these are parasitic or post-mortem rather than predatory in nature.



INTERPRETATION

OF ICHNOPHENA

The 20 ichnophena recognized may be grouped into 4 intergradational categories according to

their inferred origins:



Predation on Ostracods in Texas 643



Naticid drillholes: Phenon N. They are best expressed in smooth, robust shells, and early examples are less perfectly drilled than younger ones.

Gastropod drillholes: Phena A-D, M, 0. Some may be naticid holes in thin or ornamented

shells; others may have been drilled by muricids, other gastropods, or perhaps by octopus or turbellarians.

Holes made by unknown, less patient predators: Phena E, G , I-K, P-R, T, U. They appear to

have been ripped, gouged, punctured, or dug out rater than drilled.

Solution holes: Phena H, L, S. Some closely resemble the digestive-solution holes illustrated by

Kornicker and Sohn (1971), while others show enlarged normal pore-canals and characteristic

frosting. Because overall preservation in these assemblages is good to excellent, only a few specimens

of a few species in each assemblage show these holes, and the position of the holes corresponds to

the location of soft parts, they are here attributed to digestion rather than to sedimentary or

diagenetic alteration. Solution features also occasionally accompany other phena.



STRATIGRAPHIC

OCCURRENCE

OF ICHNOPHENA

Glen Rose Formation (Lower Albian)

Three phena of predation scars were seen (Table 1). Preservation in these assemblages ranges

from poor to fair, and some shells are recrystallized and bear calcite overgrowths. Because only a

few specimens of a few species in each assemblage show solution holes (I), the latter are tentatively

interpreted as predation rather than diagenetic alteration. One good gastropod drillhole (M) was.

seen. Estimated mortality due to predation was very low, about one percent.

Walnut Formation (Middle Albian)

Specimens of 12 species bear predation scars belonging to 3 phena (Table 2). Many, but not all,

of the remaining species are quite rare. Phenon I occurs in 5 of the 6 facies and in all 3 members,

while M and Q occur only in Facies I11 and IV of the Bee Cave Member, which also have the richest

ostracod and larger invertebrate faunas. Estimated mortality due to predation was low, about 3

percent.

Del Rio Formation (Cenomanian)

Five phena were recognized, including a good naticid drillhole (N) (Table 3). Estimated mortality due to predation was very low, about one percent.

Austin and Lower Taylor Groups (Lower Campanian)

These date are for cytheraceans only. Table 4 shows the occurrence of the nine phena recognized. Likelihood of predation (or of preservation of evidence for predation) appears to have been

controlled by abundance, size, and smoothness of carapace, in declining order of importance.

Naticid drillholes (N) become increasingly common and of better morphologic quality up the section. Only four phena are present in all three formations. There is no significant change in predation

intensity either just above or just below the minor unconformities that mark the formation boundaries. Estimated mortality due to predation was low, about 2 percent.

Middle and Upper Taylor Group (Middle and Upper Campanian)

Specimens of Cytherella dominate these faunas but are under-represented on the picked slides.

Although the 13 species bearing 9 ichnophena account for only 53 percent of the fauna (exclusive



644 R.F.MADDOCKS



of Cytherella), most of the remaining species are quite small and thin-shelled, suggesting that predation is less intense or less easily detected on such species (Table 5). Predation intensity was very

low during deposition of the Pecan Gap Formation and increased only slightly for most of the

Bergstrom (estimated mortality about one percent). At the highest locality (Cottonwood Creek

near Kimbro), stratigraphicallyjust below the unconfortnable Taylor-Navarro contact, it increased

dramatically (estimated mortality about 6 percent). While many Taylor species do not continue

into the Navarro, there is insufficient information about the Mollusca to determine whether the

increased predation on ostracods resulted from extinction of other prey taxa, evolution of new

predator taxa, or unusual environmental conditions.



Navarro Group (Maastrichtian)

These localities include three classic Cretaceous-Tertiary boundary exposures (Maddocks,

percent). At the Littig

Quarry, however, the faunas are very nearshore in aspect, dominated by species of Haplocytheridea. The exceptionally high predation rate there (16 percent adult mortality) may result either from

greater abundance of predators in nearshore communities or from greater vulnerability to predation of Cytherideidae. There are no significant stratigraphical trends in predation intensity, except

at the Cretaceous-Tertiary boundary itself. This supports the abrupt nature of the boundary inferred

from other evidence.

1985). In three of them mortality due to predation was moderate (4-8



Midway Group (Danian)

The assemblages examined (Table 7) come from three classic Cretaceous-Tertiary contact

exposures plus additional, younger exposures. At the Brazos River in southern Falls County (now

considered to be a continuous section), a 3 m transitional interval, bounded by very minor scoured

or burrowed surfaces, separates undoubted Navarro (Maastrichtian) below from Midway (Danian) above. Within this transitional interval, the sparse ostracod assemblages are dominated by

Cretaceous species near the base, by Paleocene species near the top, and have mixed species associations in the middle. Maddocks (1985) reported the first appearances of the diagnostic Paleocene

species Bairdoppilata suborbiculata and Brachycythere plena at 20 and 70 cm above the base of this

interval. Predation was quite intense in this interval (adult mortality 8 percent, mostly by naticid

drilling). Because the assemblages are small and most specimens fragmentary, no reliable stratigraphical trends in predation intensity can be detected within this interval, except that the extreme

abundance of fragments suggests that these data probably underestimate the true intensity of

predation. Phena N and S dominate, as they do at the other contact exposures.

Higher in the Midway, other phena appear, although N and S continue to be most numerous.

Adult mortality due to predation declines somewhat (to 7 percent) but remains well above Cretaceous levels. Minor differences in predation intensity at different localities may result from poorly

known environmental differences.

The species distribution ?f these phena shows, once again, that abundance, size, and smooth

exterior control predation intensity, or at least the preservation of evidence for predation. The

intensification of naticid predation is especially conspicuous in species of Cytherella, Bairdoppilata,

and Brachycythere, which have the most pelecypod-like shape. Recorded mortality rates for adult

individuals of Brachycythere plena range as high as 49 percent (transitional interval) and 26 percent

(Paleocene). We know this species survived this intense predation in the earliest days of its existence

and went on to become a common and characteristic member of Paleocene faunas throughout the

Gulf and Atlantic Coastal Plains. It should be pointed out that, because of the time-averaged and

condensed nature of the r y k and fossil record, predation can be recorded only for those species

that successfully tolerate it and continue to reproduce themseves abundantly, generation after gene-



Predation on Ostracods in Texas 645



ration. A species that succumbs to increased predation would probably do so too rapidly to leave

a decipherable record of its last days. It may be, for example, that this increased naticid predation

caused the extinction of Brachycythere ovata and Brachycythere rhomboidalis, but there is no direct

evidence for this, beyond the fact that both are very rare in the lower part of the transitional interval and absent higher. The origin of Brachycythere plena is also unknown. It does not occur in the

Cretaceous, except where burrowed contacts make sampling likely to yield mixed faunas, and its

first appearance marks the base of the Paleocene in this region. In size, smooth exterior, and weakly

alate venter, B. plena is intermediate between B. ovata and B. rhomboidalis. Its amazing tolerance of

predation may have been due to some characteristic not recorded on the skeleton, such as great

fecundity



.



Claiborne Group (Lutetian)

These Middle Eocene assemblages (Table 8) have high species diversity and 10 phena of predation scars. The 33 prey species account for 95 percent of the total fauna and include virtually all

species that are either large or abundant. Naticid drillholes predominate, but other gastropod

drillholes are also common, especially in the younger assemblages. Adult mortality due to predation reached as high as 16 percent and averaged 12 percent, slightly higher than in the Midway and

much higher than at any time in the Cretaceous.

At the Stone City outcrop, molluscan fossils are almost unbelievably abundant. Here Nelson

(1975, Table 4) reported that 11 percent of gastropod, 11 percent of pelecypod, and 10 percent of

scaphopod shells bear naticid drillholes. According to Stanton and Nelson (1980, p. 125), three naticid species account for 16 percent of the total macrofossil assemblage (120 species). They also

reported predatory muricids, cymatiids, octopus, fish, and arthropods in their trophic analysis of

this community. The correspondance of adult mortality (11 percent) by gastropod predation for both

ostracods and molluscs at Stone City is unexpected and perhaps not a coincidence. While it is not

possible to compare the abundance data directly, it appears that juvenile gastropods preyed on

adult ostracods to roughly the same degree (or with the same effect) as adult gastropods preyed on

adult molluscs. It seems that ostracods provided a significant part of the diet of juvenile naticids at

Stone City. The role of ostracods as alternate prey in this predator-prey system may be more important than commonly supposed.

Holocene, Gulf of Mexico

These 10 samples (Table 9) constitute a transect across the continental shelf south of Galveston.

The individual assemblages have more species, better representation of very fragile or very small

species, more numerousjuveniles, and fewer fragments than would be expected in comparable fossil

assemblages. Naticid drillholes are still the most abundant single phenon but do not dominate these

assemblages as they do older ones, accounting for only 2 percent of adult mortality. Average mortality due to predation is about 7 percent, distinctly lower than in Early Cenozoic assemblages but

higher than in the Cretaceous. Both overall mortality and mortality due to naticids decline into

deeper water.

The lower incidence of naticid predation, by comparison with earlier assemblages, may result

from the fact that thick-shelled species (on which such drillholes can be well preserved) are relatively

less abundant. It is also likely that ongoing sedimentary transport, bioturbation, compaction,

lithification, and diagenesis will modify these Holocene assemblages by selectively removing most

specimens of the smaller and more fragile species. In this manner, naticid drillholes may become

more abundant, relative both to other phena and to the total assemblage. Of course, it may also be

that modern naticids more strongly prefer pelecypods over ostracods than did their ancestors.

Kitchelhet al. (1981) demonstrated that living Polinices selects its molluscan prey species in such



646 R.F. MADDOCKS



a way as to maximize the cost-benefit function (searching and drilling energy expended to foodenergy gained), and that Neogene evolutionary trends in molluscan prey species reflect directional

selection to withstand this predation. We should ask whether the miniaturization of many lineages

of Cytheracea and the relative decline of Cythereffu(infaunal in habit) represent strategies by which

late Cenozoic ostracods have adjusted to this predation.



Holocene, Coastal Lagoons

Assemblages from estuarine and lagoonal environments of Sabine Pass, Galveston Bay, West

Bay, the San Bernard River, Tres Palacios Bay, and eastern Matagorda Bay were examined (Table

10). Solution features (H) predominate in upper and midbay biofacies. Naticid drillholes occur only

in very robust species of the inlet and lower bay biofacies. The somewhat nondescript phenon E

is also characteristic of the lower bay biofacies. Estimated adult mortality averages at least 8 percent. The preceding remarks about taphonomic processes also apply here. Solution features were

also conspicous in the fossil assemblages from nearshore settings (Glen Rose Formation, the

Cretaceous part of the Littig Quarry exposure). These solution features may prove to be useful

indicators of coastal environments, and their origin deserves further investigation.



CONCLUSIONS

Twenty ichnophena of gastropod drillholes and other predation scars were recognized in Albian

to Holocene assemblages of Texas. Younger assemblages have greater diversity of ichnophena. The

oldest gastropod drillholes were found in the Albian, but the oldest definitely naticid holes were

in the Cenomanian. Naticid drillholes increased in abundance and morphological quality during

the Campanian. Abrupt increases in naticid predation on ostracods occurred near the end of the

Campanian and at the Cretaceous-Tertiary boundary. High levels of naticid and other predation

continued to characterize Paleocene and Middle Eocene assemblages. The somewhat lower levels

for naticid predation in Holocene assemblages may be explained by environmental, taphonomic,

and evolutionary factors. These recorded fluctuations in predation intensity have obvious paleoecological and biostratigraphical applications, limited at present by the absence of corresponding data

for molluscs. Although in theory these trends might explain the evolution or extinction of certain

ostracod taxa, in practice, evidence of predation can be recorded only in those populations that

successfully survive it. It follows that extinction resulting from predation is not likely to leave a

decipherable record. Nevertheless, predation should be considered as a potential cause for some

Cenozoic evolutionary trends in ostracods.

Abundance, size, and smoothness of carapace control vulnerability of ostracod species to gastropod predation, or control preservation of the evidence for this predation. The accepted criteria

for distinguishing muricid from naticid drillholes in mollusc shells cannot be applied consistently

to ostracods. Although modern naticids seek infaunal prey, fossil naticid holes occur on some

taxa not likely to have been infaunal (Bairdiidae, Cytherideidae). In the Middle Eocene Stone City

Formation, naticid predation was equally intense on Mollusca and Ostracoda. It appears that

ostracods provided a significant food resource for juvenile naticids.

Three phena are attributed to digestivesolution. They are most abundant in coastal settings and

never occur on infaunal taxa (Cytherelfu).



Predation on Ostracods in Texas 641



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NELSON, P.C. 1975. Community structure and evaluation of trophic analysis in paleoecology, Stone City Formation

(Middle Eocene, Texas). Unpublished M.S. Thesis, Texas A & M University, 168 pp.

REYMENT, R.A. 1963. BohrliScher bei Ostrakoden. Palaontologisches Zeitschrift, 37, 382-391.

-1966. Preliminary observations on gastropod predation in the western Niger Delta.Palaeogeogr., Palaeoclimat.,

Palaeoecol., 2, 81-102.

- 1967. Paleoethology and fossil drilling gastropods. Kansas Acad. Sci. Trans., 70, 33-50.

ROSS, J.E. and MADDOCKS,R.F. 1983. Recurrent species associations and species diversity of cytheracean ostracodes

in the upper Austin and lower Taylor groups (Campanian, Upper Cretaceous) of Travis County, Texas. Gulf.

Coast Assoc. Geol. SOC.,Trans., 33, 397-406.

SOHL, N.F. 1969. The fossil record of shell boring by snails. Amer. Zool., 9, 725-734.

STANTON, R.J. and NELSON, P.C. 1980. Reconstruction of the trophic web in paleontology; community structure in the

Stone City Formation (Middle Eocene, Texas). J. Paleont., 54, 118-135.

TAYLOR, J.D., MORRIS, N.J. and TAYLOR, C.N. 1980. Food specialization and the evolution of predatory prosobranch

gastropods. Palaeontology. 23, 375-409.

CHIMENE, J.B. 11



DISCUSSION

De Deckker: Isn't the change in the percentage of predators across the Cretaceous-Tertiary

boundary simply the result of a change in environment?

Maddocks: No, not in the sections studied. Thor Hansen, who has studied molluscan diversity

at these same Cretaceous-Tertiary sections, reports that many taxa on which molluscs prey disappear at 'or near the boundary, while naticid gastropods continue to be abundant. Perhaps the

intensification of predation on ostracods reflects the decreased abundance of the preferred molluscan prey.

De Deckker: Are you aware of any papers discussing predation by ostracods on those gastropods, because if ostracods did eat the gastropods, your percentages of gastropods drilling on ostracods would simply be a result of size of the gastropod population (that is less predators, their numbers having been reduced by ostracod predation on juvenile gastropods) being able to predate on

ostracods?

Maddocks: Sohn and Kornicker reported that some freshwater ostracods prey on snails, and



648



R. F. MADWCKS



I have also seen this, but this predation leaves no marks on the shell. I do not see how this could be

recognized in fossils. Stanton and Nelson (1975) did not include ostracods in their elaborate

trophic analysis of the Stone City fauna. Larvalgastropod shells are very common in the microscopic

residues of this formation, so clearly juvenile naticid mortality was very high, but the cause is not

obvious. Stanton and Nelson reported that one very abundant and two rare species of Naticidae

comprise the single most abundant family at this locality.

Hartmann: Were you able to distinguish naticids and other gastropods, and between other

drillers and predators such as Polychaeta?

Maddocks: Good naticid drill holes are easy to recognize in robust shells, but other kinds are

not so easy to distinguish. There is a large literature on boring organisms, but I did not know that

polychaetes drilled ostracods. I would like to learn how to recognize holes made by these other

predators.

Oertli: You are talking in your very interesting paper about mortality rate. Are we sure that

living ostracods were attacked (like a jaguar jumping on his prey) and not specimens just dead,

i.e. still “eatable”?

Maddocks: The bevelled hole always has the larger (outer) perimeter on the exterior of the shell.

As Reyment reported earlier, the holes tend to be located over the “meatier” regions of the animal.

There is a large body of literature on the drilling behaviour of modern naticids. It has been reported

somewhere that metabolites in the water guide them to their prey, and that in very crowded aquaria

where the concentrations of these metabolites are very high, they have some trouble distinguishing

live prey from empty shells. I do not know whether naticids would scavenge recently dead ostracods; someone should test this.

Reyment: Naticids operate below the sediment surface, muricids on the surface. Since the

majority of gastropod holes, by far, found in ostracods are naticid (countersunk) holes, those ostracods must perforce be interstitial forms. That the mobile ostracods can be successfully attacked by

drills, (for which several minutes elapse before sufficiently high grip can be achieved) is an outcome

of the survival strategy of ostracods, whereby the animal tightly shuts its shell and remains still for

a period of time until the danger has presumably passed. Drills seek prey a few minutes after hatching. They have an image-conditioned attack mechanism which can lead them to select unsuitable

organisms (such as foraminifers) until the learning process has been completed. The choice of

ostracods by juveniles is doubtless due to two causes (a) mistaken selection of prey by juveniles or

(b) alternative selection of food in the absence of favourite nourishment (see Moore (1958) Marine

Ecology for references concerning the switch to cirripedes in the absence of bivalves due to excessively heavy predation leading to temporary extinction of an accustomed food-source). Recently, we

(Eva Reyment and I) were surprised to find naticid drill holes in ostracods from DSDP material

from the W. Walvis Ridge, from a depth said to be about 3-4000 m in the Maastrichtian (Piveteau,

1960) Trait6 de Zoologie, Vol, CrustacCes) reports naticids from 6000 m). Referring to your

observations on the improvement of drilling technique over time, my own experience for Maastrichtian to Recent in the South Atlantic does not point to any difference in skill in this respect.

Maddocks: Very few (notie?) of the species that are small enough to be interstitial (in the strict

sense) or that show the morphologic adaptations associated with interstitial life (as described by

Hartmann) ever bear naticid drillholes (in the collections I have examined). Except for Cytherellu,

the commonly attacked taxa do not seem likely (by analogy with living relatives) to be infaunal in

habit. Perhaps naticids attack surface-dwelling prey from below, like submarines attacking a ship.



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Chapter 46. One hundred million years of predation on ostracods: The fossil record in Texas

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