Generic longevity of articulate brachiopods in relation to the mode of stabilization on the substrate

Palaeogeography, Palaeoclimatology, Palaeoecology, 21(1977): 209--226 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

GENERIC LONGEVITY OF ARTICULATE BRACHIOPODS IN RELATION TO THE MODE OF STABILIZATION ON THE SUBSTRATE

RICHARD R. ALEXANDER

Department of Geology, Utah State University, Logan, Utah 84322 (U.S.A.) (Received and accepted June 4, 1976)

ABSTRACT

Alexander, R. R., 1977. Generic longevity of articulate brachiopods in relation to the mode of stabilization on the substrate. Palaeogeogr., Palaeoclimatol., Palaeoecol., 21: 209--226. The geologic ranges of the articulate brachiopod genera which appear in the

Treatise are converted into numerical values with the aid of radiometric dates on the Phanerozoic time scale. Longevity--frequency distributions are plotted for the genera segregated according to their mode of stabilization on the substrate. The categories and their subdivision for the mode of stabilization include: (1) anchorage by spines, (2) cementation (early in and throughout life), (3) unattached, commissure horizontal (questionably mobile and sessile), (4) unattached, commissure vertical (interarea stabilization and umbonal weighting), (5) functional pedicle (tethered and combined with interarea stabilization). Statistical tests (Median and Mann-Whitney U) reveal that the unattached, freelying, questionably mobile genera and the cemented genera among the endemic brachiopods display a significantly greater generic longevity distribution than the unattached, free-lying, stationary and pedunculate genera. Genera cemented throughout their life show a significantly greater longevity distribution than spine-anchored genera. Brachiopod genera which combined interarea stabilization with a pedicle show a significantly greater longevity distribution than genera tethered on a pedicle. Among the cosmopolitan genera, pedunculate and cemented brachiopods display a significantly greater longevity distribution than the unattached, interarea-stabilized genera.

PURPOSE AND PROCEDURE T h e p u r p o s e o f t h i s i n v e s t i g a t i o n is t o d e t e r m i n e , q u a n t i t a t i v e l y , if a n y relationship exists between the mode of stabilization on the substrate and g e n e r i c l o n g e v i t y a m o n g t h e a r t i c u l a t e b r a c h i o p o d s . B e c a u s e n o a priori r e a s o n is e v i d e n t w h y a n y o n e s t r a t e g y s h o u l d b e m o r e c o n d u c i v e t o l o n g e v i t y t h a n a n y o t h e r , t h e n u l l h y p o t h e s i s t e s t e d is t h a t l o n g e v i t y - frequency distributions of the articulate brachiopods segregated according t o m o d e o f s t a b i l i z a t i o n o n t h e s u b s t r a t e a r e e q u a l , i.e. s i m i l a r a c c o r d i n g 209

210 to critical limits of various statistical tests. If this hypothesis is rejected for any comparisons between categories of stabilization, interpretations are offered to explain the statistically significant dissimilarity. The data source utilized for this analysis is the Treatise on Invertebrate Paleontology, part H, Brachiopoda. Although published over a decade ago (1965) and in need of revision, it is currently the most reliable compendium which incorporates the geologic ranges for all the articulates of the Phanerozoic interval of time. The tedious alternative is a codification of all the information on the ranges of articulate brachiopod genera which is dispersed throughout the literature. Recently, Boucot (1975) provided a detailed tabulation of the ranges o f Upper Ordovician--Devonian brachiopods. This work is treated separately (Alexander, in preparation) and not included in this investigation because of the temporal bias (the selected time interval results in an under-representation of cemented and spine-anchored brachiopods). Geologic ranges of the articulate brachiopod genera expressed in the Treatise in terms of systems, series and stages are converted into spans of millions of years with the aid of radiometric dates on the Phanerozoic time scale (Harland et al., 1964). Radiometric dates are provided for the beginning o f each stage from the Devonian Period through the Cenozoic Era with a few exceptions in the Triassic Period. Radiometric dates were assigned to the beginning of the series in the Cambrian, Ordovician and and Silurian Periods. The ranges of many genera of brachiopods from the Mesozoic and Cenozoic Eras are defined to the stage level and can be converted into numerical values with an uncertainty under +2 million years. However, the range of even more genera, particularly Paleozoic representatives, are series- or period-delimited and the conversion of these ranges into spans of millions of years involves a greater magnitude of uncertainty. As advocated by Sepkoski (1975, p. 354), the durations of the taxa are calculated f r o m the midpoint of the stratigraphic interval in which it first appears to the midpoint of the interval in which it last appears. Taxa confined to a single stratigraphic interval are assigned a duration of one half of the interval. Because the limits of resolution for stages and series are 4 and 10 m. y., respectively (Sepkoski, 1975, p. 350), the horizontal axis of the longevity--frequency distributions was divided into 10 million year increments. The genera are segregated according to their m o d e of stabilization on the substrate. The five categories to which genera were assigned are: (1) anchorage by spines (unaided by cementation), (2) cementation (early in and throughout life), (3) unattached, commissure-vertical (interarea stabilization and umbonal weighting), (4) unattached, commissurehorizontal (questionably mobile and sessile), (5) functional pedicle (tethered and combined w i t h interarea stabilization) (Table I). Some genera have species which can be assigned to different categories, e.g.

211 TABLE I Modes of stabilization on the substrate for articulate brachiopods

Category

Subdivision

Anchorage by spines

Cementation

Unattached : commissure-vertical

Unattached : commissure-horizontal

Characteristics

rhizoid, halteroid and clasping spines root or attach brachiopod to substrate without the aid of cementation youthful stages only

umbonal cementation c o m m o n ; adherence to substrate only during portion of life; after which a freelying orientation assumed; spinosity may augment stabilization

throughout life

umbo or entire valve cemented to substrate throughout life. Spines may augment cementation.

umbonal weighting

pedicle lacking or atrophied; beaks commonly incurved and posterior region giobose which weights organism into substrate with the commissure vertical to oblique to the sediment--water interface

interarea stabilization

pedicle absent or atrophied; hinge line elongated (alate) or bordering triangular interarea which stabilizes brachiopod with commissure nearly vertical to sediment--water interface

sessile

pedicle absent or atrophied; brachiopod semi-infaunal with convex, undermost valve, imbedded in substrate; commissure is horizontal or oblique to sediment--water interface

mobile?

thin, small, slightly eoncavo-convex to planar, strophomenid brachiopods that lacked or atrophied a pedicle; mobility via pectenoid-like propulsion (see Rudwick, 1970, p. 91)

212 TABLE I (cont.) Category

Subdivision

Characteristics

Functional pedicle

tether

pediele may provide permanent anchorage which commonly permits rotational movement (tether) or closely appresses umbo to substrate; epiplanktonic habit via attachment to host; organ also used as retractile, probing device for periodic repositioning

interarea stabilization

elongate hinge line (alate) and triangular interarea augment pedicle; commissure positioned nearly vertical; organism has limited swivelling mobility

attached and unattached species of Atrypa (Copper, 1967b). In such cases the genus is plotted in each category. Other explicit ecologic classifications of brachiopods were devised by Grant (1970) and Waterhouse and Piyasin (1970). Assignment of the genera to categories of stabilization is determined from information in several sources in addition to the Treatise. References which influenced the assignments are juxtapositioned with the families and genera in Table II. If alternative assignments have been proposed, these references are also listed. In the process of categorizing articulate genera, a degree of arbitrariness is unavoidable. The most common problems of intercategory gradations, which necessitate a degree of arbitrariness, include plugging of the foramen in adulthood (e.g. Clitambonitidae), development of a stegidium and the sealing off of the delthyrium (Cowen, 1969), stabilization afforded by varying degrees of development of the interarea (e.g. RhipidomeUa, Cooper, 1970, p. 207}, and the support potential of the pedicle egressed from a small foramen (e.g. Billingsella, Cooper, 1970, p. 212). Following segregation of the genera and conversion of their ranges into numerical values, longevity--frequency distributions are graphed (Figs.1 and 2). The position and number of extant and cosmopolitan genera are indicated in the graphs. Extant genera are removed from the statistical analyses because, as stated by Thayer (1974, p. 829), these taxa represent incomplete experiments. Furthermore, cosmopolitan genera are separated from the endemics and each is treated separately in the statistical analyses.

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ANALYSES The longevity--frequency distributions are all positively skewed and position the mode in either the 5--10 or 15--20 m. y. interval. The mean l o n g e v i t y f o r e a c h c a t e g o r y r a n g e s f r o m 1 6 t o 2 2 m . y. f o r t h e e n d e m i c g e n e r a a n d f r o m 21 t o 71 m . y . f o r t h e c o s m o p o l i t a n g e n e r a ( T a b l e I I I ) .

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Fig.2. L o n g e v i t y - - f r e q u e n c y distributions o f the b r a c h i o p o d genera listed in the Treatise ( 1 9 6 5 ) and segregated according t o m o d e o f stabilization o n the substrate. C = c o s m o politan; E = e x t a n t . Three o f the c o s m o p o l i t a n c e m e n t e d ( y o u t h f u l stages o n l y ) genera in the 5 0 - - 6 0 m. y . interval have been reassigned in a c c o r d a n c e w i t h interpretation o f S c h u m a n n ( 1 9 6 9 ) (See Table II).

Statistical comparison of the mean longevity values for endemic genera is not justified because the maximum separation of values (6 m. y.) is less than the time increment (10 m. y.) between midpoints of the class intervals, e.g. 15--20 versus the 25--30 m. y. class. Because the data are aggregated into 10 m. y. incremental classes along the time axis and are strongly positively skewed, a parametric t test which requires normal distribution of the data along an interval or ratio scale is inappropriate to t e s t for similarities among the longevity--frequency distributions. Consequently, the Median test, a nonparametric test, is employed to compare the distributions of endemic genera among the various categories of

217 TABLE III Mean longevity values for genera segregated according to m o d e of stabilization on the s u b s t r a t e ( v a l u e s in m i l l i o n s o f y e a r s ) Endemic genera

Cosmopolitan genera

Ct = 22 U C H m = 21 F i s = 19 U C V u w = 19 Ce = 18 U C V i s = 18 F t = 18 U C H s = 18 As = 16

As = 71 Ce = 63 F t = 62 U C V u w = 53 Fis = 50 U C H m = 43 U C H s = 27 Ct = 23 U C V i s = 21

= functional pedicle (tethered); Fis = functional pedicle (interarea stabilization); = u n a t t a c h e d : c o m m i s s u r e - h o r i z o n t a l (sessile); U C H m = unattached: commissureh o r i z o n t a l ( m o b i l e ) ; C t = c e m e n t a t i o n ( t h r o u g h o u t life); C e = c e m e n t a t i o n (early in life); A s = a n c h o r a g e b y s p i n e s ; U C V u w = unattached: commissure-vertical (umbonal weighting); UCVis = u n a t t a c h e d : commissure-vertical (interarea stabilization). Ft

UCHs

stabilization on the substrate. This test compares the proportional distribution for each of the two longevity--frequency histograms above and below the c om bi ne d median for both plots (Table IV). The calculated value is distributed as a )/2 test with one degree of freedom. Of the 36 comparisons of endemic genera, only 9 are statistically significantly different at the 0.05 probability level. If the unattached, questionably mobile (UCHm) and sessile (UCHs) genera are treated as one category because of reservations about the existence of mobile genera proposed by Rudwick (1970), then 7 comparisons are significantly different at the 0.05 probability level. Comparisons among the spineanchored (As), umbonally weighted (UCVuw), interarea-stabilized (UCVis) and unattached, free-lying, stationary (UCHs) genera display no significant difference in their longevity profiles. The 9 significantly dissimilar comparisons include: (1) pedicle-tethered (Ft) versus questionably mobile, unattached genera (UCHm); (2) pedicle-tethered (Ft) versus genera cemented throughout life (Ct); (3) combined pedicle and interareastabilized genera (Fis) versus brachiopods cemented throughout life (Ct); (4) unattached, free-lying and stationary genera (UCHs) versus the unattached, free-lying, questionable mobile brachiopods (UCHm); (5) the unattached, free-lying, sessile genera (UCHs) versus the brachiopods cemented throughout life (Ct); (6) pedicle-tethered (Ft) versus combined pedicle and interarea-stabilized genera (Fis); (7) pedicle-tethered (Ft) versus genera cemented early in life (Ce); (8) unattached, free-lying, sessile genera (UCHs) versus genera cemented early in life (Ce) and; (9) genera anchored by spines (As) versus genera cemented throughout

218 TABLE

IV

Comparison (Median test) of categories of stabilization on the substrate among e n d e m i c genera

Ft Fis UCHs UCHm Ct Ce As UCVuw UCVis

Ft

Fis

UCHs*

UCHm*

Ct

Ce

As

UCVuw

UCVis

--

4.38 -

0.01 1.53 -

13.87 2.48 6.55 -

21.40 7.69 11.09 0.05 -

5.63 1.51 4.20 0.01 0.08 -

3.70 0.05 1.74 1.77 5. 79 0.81 -

2.45 0.04 1.39 1.28 2.87 0.55 0.02 -

2.43 0.05 1.42 1.07 3.29 0.47 0.01 0.03

Median test is calculated by the formula: x~ =

N(tAD-

BC I - N/2) 2

(A + B)(C + D)(A + C)(B + D)

where A = n u m b e r of observations in group I above c o m b i n e d median; B = n u m b e r of observations in group II above c o m b i n e d median; C = n u m b e r of observations in group I at or b e l o w c o m b i n e d median ; D = n u m b e r of observations in group II at or b e l o w c o m b i n e d median; N = c o m b i n e d sample size. Distributed a s x 2 with one degree of freedom. For abbreviations see Table III. Figures in italics represent significant v a l u e s (critical x 2 = 3 . 8 4 ) a t 5 % . * I f the subdivision "questionably m o b i l e " is unacceptable and should be c o m b i n e d w i t h the sessile genera (UCHs), the following x 2 v a l u e s will result: U C H s v s . F t = 2 . 6 4 ; vs. Fis = 0.02; vs. Ct = 6.50 ; vs. Ce = 1.44 ; vs. As = 0.07 ; vs. UCVuw

= 0.05 ; vs.

UCVis = 0.06.

life (Ct). In each significant comparison listed above, the category cited second placed a proportionally greater number of genera above the combined median. Concerning the cosmopolitan genera, of which there are far fewer listed in the Treatise, the nonparametric Mann-Whitney U test is utilized because of the sensitivity of the test to location as well as dispersion within the combined ranking of the compared categories. Because of very small sample size, the categories which include the brachiopods cemented throughout life (Ct-2 cosmopolitans), unattached, questionably mobile (UCHm-2 cosmopolitans) and anchored by spines (As-4 cosmopolitans) are excluded from the comparisons. Of the 15 comparisons (Table V) only 4 are significantly different at the 0.05 probability level. Three of these significantly different comparisons involve the category which includes the unattached, interarea-stabilized genera (UCVis) in comparison with the temporarily cemented (Ce), and pedunculate (Fis and Ft) genera, whose longevity distributions are greater.

219 TABLE V Comparison (Mann-Whitney U test) of categories of stabilization on the substrate among cosmopolitan genera Fis Fis Ft UCHs Ce UCVis UCVuw

Ft

UCHs

Ce

UCVis

UCVuw

0.896 -

0.940 0.294 -

0.084* 0.384 0.150

0.032 0.019

0.980 0.582 0.960 0.338 0.352

-

0.114 O. 0 0 2

-

Calculated by the formula: 2 T - n l(n, + n~ + 1) z = ((n,)(n2 )(nl + n2 + 1)/3) ½ where T is the sum of ranks of the smaller sample; n 1 is the size of the smaller sample and r22 is the size of the larger sample. Significant probability values of z in a standard normal distribution appear in italics. For abbreviations see Table III. *If computed as a one-tailed test, i.e., Ce ~ Fis, value is 0.042 and significant.

INTERPRETATIONS Several plausible e x p l a n a t i o n s can be t e n d e r e d f o r t h o s e categorical c o m p a r i s o n s w h i c h are significantly different. T a x o n o m i c subjectivity cam~ot be c o m p l e t e l y e x c l u d e d in a n y e v a l u a t i o n o f generic l o n g e v i t y (see B o u c o t , 1 9 7 3 , versus Wright, 1 9 7 3 o n P t y c h o p l e u r e l l a - - G l y p t e r i n a ) . What c o n s t i t u t e s a long-ranging genus in t h e o p i n i o n o f o n e s y s t e m a t i c i s t m a y be a c o m p o s i t e o f t w o s h o r t e r ranging genera f r o m t h e v i e w p o i n t o f a n o t h e r t a x o n o m i s t . F u r t h e r m o r e , t h e data in t h e T r e a t i s e were p u b l i s h e d over a d e c a d e ago a n d s u b s e q u e n t l y t h e ranges o f several genera m a y have been c r o p p e d or e x t e n d e d . S c h o p f et al. ( 1 9 7 5 , p. 63) n o t e d t h a t " t h e degree o f perceived c h a n g e in various lineages m a y be d i r e c t l y related t o t h e i r general m o r p h o l o g i c c o m p l e x i t y : m o r e c o m p l e x f o r m s a p p e a r t o c h a n g e rapidly. ' R a t e s o f e v o l u t i o n ' as c u s t o m a r i l y r e p o r t e d b y p a l e o n t o l o g i s t s m a y t h e r e f o r e be p o o r i n d i c a t o r s o f e v o l u t i o n a r y change in t h e g e n o m e . " With r e s p e c t t o this investigation, t h e a b o v e s t a t e m e n t p r o m p t s the q u e s t i o n as to w h e t h e r the generic longevities are real or m e r e l y t h e artifacts o f t a x o n o m y based o n o b s e r v a b l e m o r p h o l o g i c c o m p l e x i t y or simplicity. C o n s e q u e n t l y , o n e feasible e x p l a n a t i o n f o r t h e significant d i f f e r e n c e in the l o n g e v i t y - - f r e q u e n c y d i s t r i b u t i o n s o f certain c o m p a r e d strategies is t h a t o n e c a t e g o r y includes generically oversplit taxa, possibly t h e result o f discrepancies in m o r p h o l o g i c c o m p l e x i t y b e t w e e n t h e t w o g r o u p s if we a p p l y t h e r e a s o n i n g o f S c h o p f et al. ( 1 9 7 5 ) discussed above. H o w e v e r , this

220

investigation involves interordinal comparisons at the highest taxonomic level and frequently intraordinal comparisons, e.g. the cemented strophomenids versus the unattached, free-lying strophomenids. Significant disparity in morphologic complexity is not apparent between compared categories for which a significant difference exists. Furthermore, the group of articulates most commonly cited as oversplit is the productids of the Permian of West Texas, but the spine-anchored brachiopods (As) are involved in only one of the significantly different comparisons. It is doubtful that this explanation of oversplitting can be substantiated for the other eight significant comparisons. Another conceivable bias which would affect the longevity--frequency distributions is interpretation of the functional morphology and subsequent assignment of genera to categories of stabilization on the substrate. For example, stegidial plates in the delthyrium of spiriferids resulted in an atrophied pedicle (Cowen, 1969, p. 539). These spiriferids are assigned to unattached, interarea-stabilized category (UCVis). However, these thin plates which restrict and sometimes seal off the opening through which the pedicle emerged may not always be preserved. Consequently, additional collecting and description of well-preserved specimens of spiriferids may swell the ranks of the unattached, interarea-stabilized brachiopods (UCVis) at the expense of the pedicle plus interareastabilized group (Fis). Such future shifts in generic assignments between categories may diminish and eliminate any significant differences between Fis and UCVis groups and the categories with which they are compared. However, the spine-anchored (As), cemented (Ce and Ct), unattached, free-lying (UCHm and UCHs), umbonally weighted (UCVuw) and pedicletethered (Ft) categories contain very few questionable generic assignments (Table II) and their deletion does not alter the comparisons of distributions which are significantly different. Any investigation of longevity--frequency distributions of a taxon must take into consideration the incompleteness of the known fossil record (Sepkoski, 1975), especially if one subscribes to the estimate that only 1--33% of the species in the fossil record have been described" (Durham, 1967; Valentine, 1970}. Since the publication of the Treatise volume on brachiopods, approximately 500 genera have been authored (G. A. Cooper, written communication). The assignment of these genera to categories of stabilization on the substrate may significantly modify the longevity-frequency distributions and thereby nullify any present significant difference which exists between compared strategies. The pivotal issue is not the incompleteness of the Treatise but the representativeness of the volume on brachiopods. If the volume does not constitute a representative sample then the above reservation is justified. If the volume constitutes a representative sample, then the generic additions of the last decade would probably not significantly alter the geometry of the histograms. By representativeness, the author contends that among the

221 genera authored since 1965, no one group has received a disproportionate amount of additions and no group has been swelled by a disproportionate number of long-lived or short-lived taxa. The most likely candidates for disproportionate emphasis are the spine-anchored and cemented genera in consideration of the exhaustive investigations of the Permian of West Texas. However, the spine-anchored productids are involved in only one of the significantly different comparisons and the cemented genera display a protracted, not telescoped, longevity profile, despite generic splitting of Permian forms from West Texas. Another explanation for the significant difference among some intercategory comparisons is that the differences are real and not the artifact of the selected sample (Treatise), taxonomic subjectivity or questionable interpretations of the mode of life of certain genera. This explanation suggests that certain strategies for stabilization on the substrate increased the probability of survivorship for the brachiopod genera which pursued this strategy. Specifically, the unattached, free-lying, possibly mobile genera and the cemented genera display survivorship curves significantly protracted in comparison to the unattached, free-lying, stationary genera and the pedunculate genera (Table IV). The two strategies which provided an increased likelihood of a longer tenure represented contrasting solutions to the problems of stabilization and survival on consolidated and unconsolidated substrates. For the cemented brachiopods, gravity combined with elevation above the particulate substrate solved many of the problems of coping with sedimentation (Surlyk and Christensen, 1974). However, on muddy substrates, gravitational attraction becomes a problem and a premium is placed upon adaptive strategies that facilitate suspension of the valves on a fluid sediment and compensate for sedimentation. Provisionally accepting the possibility of reorientational capability and limited mobility (suggested by Rudwick, 1970, p. 91, but questioned by D. V. Ager and R. E. Grant, written communications), the slightly concavo-convex strophomenids may have had an advantage over their more globose, umbonally weighted, unattached relatives in adapting to muddy substrates. The combination of a compressed geometry plus a pectenoid-like mobility would aid in keeping the valves suspended on the fluid substrate (Alexander, 1975). Furthermore, geniculation, coupled with a reorientational capability, would aid these brachiopods in coping with sedimentation and shifting substrates (Alexander, 1975). Surlyk (1972) plotted left-skewed size-frequency distributions for the unattached (but not mobile) brachiopods from Cretaceous chalk beds and concluded that these brachiopods grew rapidly through the vulnerable youthful stages and consequently did not suffer high juvenile mortality rates. The cumulative advantages of the unattached, questionably mobile (UCHm) and the cemented (Ce and Ct) genera in comparison to the spine anchored (As) and unattached, sessile genera (UCHs) are itemized in

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223 Table VI. However, the advantages which accrued to various modes of stabilization (Table VI) do not provide a logical explanation why pedunculate genera have significantly different longevity profiles than cemented or questionably mobile genera. Possibly the itemized advantages are not all of equal importance to survivorship and/or possibly the percentage of genera within each category that possess that particular advantage varies significantly between categories. Thus, for example, in a comparison of cemented genera versus pedunculate genera, secure anchorage on the substrate is an advantage common to all the genera which remained cemented throughout life (Ct), but the advantage is variably developed among the pedunculate genera, depending on the strength of the pedicle (Thayer, 1975). Such intracategory variation must be considered in evaluation of the advantages and disadvantages accruing to different strategies of stabilization. Pluses listed under each category for a particular capability should not be interpreted to mean the advantage is ubiquitous within the category and equally developed among its representatives. Among the cosmopolitan genera, the pedunculate brachiopods occupy 11 of the top 12 positions in the combined ranking of all cosmopolitans. Several of the pedunculate brachiopods have successfully coped with the fluctuations, sometimes capriciously, of the sedimentologic regime as evidenced by the number of genera that prolonged their longevity beyond a 100 m. y. Genera such as Terebratu/ina have the ability to continuously lengthen the pedicle and thereby compensate for sedimentation (Davidson, 1886, p. 27). Other pedicle-tethered genera retain considerable orientational or rotational capability (Schumann, 1969, p. 203; Brookfield, 1973). Recently, Richardson and Watson (1975) discussed the retractile, probing ability of the pedicle in Magadina cumingi. If this capability were common to extinct genera, they could assume a free-lying orientation (UCHs) or elevate the commissure when the pedicle was imbedded (Fis or Ft). Other pedunculate genera have escaped the perturbations of the sediment substrate by adopting an epiplanktonic mode of life (Ager, 1962, 1963). The pedicle has enabled brachiopods to anchor themselves in very fine particulate substrates as in the case of Chlidonophora (Schumann, 1969, p. 200).Other genera live in habitats subjected to current intensities of 2 m s-1 and possess a pedicle with the equivalent strength of the bivalve byssus (Thayer, 1975). The above-mentioned capabilities are denied to the unattached brachiopods which positioned their commissure nearly vertical and lacked or atrophied the pedicle. The deficiency of a pedicle, and the advantages which accrue to that mode of stabilization, may explain why the unattached, interarea-stabilized (UCVis) cosmopolitan genera have significantly shorter generic longevities in comparison to the pedunculate categories (Fis and Ft) and the cemented genera (Ce). Significant differences among the comparisons of categories of cosmo-

224 politan genera are not always extrapolations from significant comparisons of their endemic counterparts. Comparisons between the two pedunculate categories (Ft and Fis), or between the unattached, free-lying, sessile (UCHs) and the temporarily cemented (Ce) genera, or between the pedicletethered (Ft) and the temporarily cemented iCe) genera are significant at the endemic level but not at the cosmopolitan level. Conversely, the unattached, interarea-stabilized genera (UCVis) show significant difference from the cemented iCe) and pedunculate (Fis and Ft) genera at the cosmopolitan level but not at the endemic level. These distinctions may be a statistical artifact, considering that a different test was used for treatment of the endemics (Median) in contrast to the cosmopolitans iMann Whitney U). Furthermore, the categories entitled cemented throughout life (Ct), anchored by spines (As), and unattached, commissure-horizontal, mobile (UCHm), which were involved in significant comparisons at the endemic level, are deleted from comparisons involving cosmopolitan genera because of their very small sample size. Possibly these discrepancies between endemic and cosmopolitan categorical comparisons reflect the different degrees to which cosmopolitan and endemic genera of a stabilization category exploited the array of potential advantages inherent to a particular strategy. Among the pedicletethered brachiopods, only a small percentage of extant genera have exploited the epiplanktonic mode of life. Among the spine-anchored genera, only a few exploited attachment to an erect, living host (e.g. Linoproductus, Grant, 1963). In other words, the cosmopolitan genera of a stabilization category should not necessarily be regarded as endemics that attained a global distribution but otherwise differ little from their endemic counterparts in their morphological and anatomical adaptations for stabilization on the substrate. CONCLUSIONS Among endemic brachiopod genera, the possibly mobile, free-lying genera display a generic longevity distribution significantly greater than the stationary, unattached, free-lying and the pedicle-tethered genera. Reorientational capability and limited mobility may have increased the survivorship potential among these thin concavo-convex forms. The genera which remained cemented throughout their life and those genera which are cemented early in life display a longevity distribution significantly greater than the unattached, free-lying stationary genera and the pedunculate genera. Unattached, free-lying questionably mobile genera /UCHm) show a significantly greater longevity distribution than unattached, free-lying, sessile (UCHs) genera. Pedunculate, interarea-stabilized genera (Fis) display a significantly greater longevity distribution than pedicletethered genera (Ft). Genera cemented throughout life (Ct) show a significantly greater longevity distribution than spine-anchored genera (As).

225

Again, cementation may have increased the survivorship potential for genera that pursued this strategy for stabilization on the substrate. Among cosmopolitan genera, the pedunculate and cemented genera endured for a significantly greater length of time than the unattached, interareastabilized genera. Absence of a pedicle prohibited the colonization of a variety of substrates to which the pedunculate genera adapted.

ACKNOWLEDGEMENTS

I greatly appreciate the constructive suggestions of D. V. Ager, A. J. Boucot, G. A. Cooper, R. Cowen and R. E. Grant who reviewed this manuscript. Special gratitude is extended to A. J. B o u c o t who assisted in the assignment of some of the brachiopods. Although the assistance of the aforementioned reviewers was most valuable, the a u t h o r nevertheless assumes full responsibility for the conclusions presented in this investigation.

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