Fossil Morphology and Data Analysis Lab 3 PDF

EXERCISE 3 Fossil Morphology and Data Analysis ============================================== Introduction ------------ Growth is a fundamental expression of life. It is more than simple increase in size, but involves various complexities such as changes in bodily proportions, and addition of orga...

EXERCISE 3 Fossil Morphology and Data Analysis ============================================== Introduction ------------ Growth is a fundamental expression of life. It is more than simple increase in size, but involves various complexities such as changes in bodily proportions, and addition of organs and skeletal units so that relationships between parts of an organism change as growth progresses. These changes in measurable relationships may be due to such causes as structural necessity, attainment of some minimum size so that new functions can be engaged, or even so that certain relationships can be maintained uniformly. Description and comparison of organisms can be quite cumbersome and subjective if words alone are available to accomplish the task. Even use of the most detailed possible drawings or photographs cannot adequately compensate for description, for each person must either still use words in thinking about the illustration or retreat even further into subjectivity in developing a nonverbal -- though perhaps highly accurate -- feeling or impression of an organism or how sets of organisms compare with one another. Use of numerical descriptions and comparisons is no panacea that necessarily develops completely objective thought and communication, and that can be used arbitrarily, or as a substitute for thought (Fig. 3.1). However, appropriate use of a combination of qualitative data, illustration, and numerical techniques can make thinking and communication much more successful and objective. The purposes of this exercise are: 1. To introduce principles of numerical description of individual fossils and multiple-specimen samples drawn from populations of fossils; 2. To introduce some simple methods of estimating probability of differences between populations; and 3. To introduce some of the gross differences in growth patterns exhibited by invertebrate animals and examine why such radical differences exist. Numerical description --------------------- Numerical description and comparison of organisms requires the scoring of some property (a **variable**) that is observed to differ or potentially differs among individuals. There are five types of variables: 4. Counts of discrete structures such as number of teeth; 5. Measures such as length, area, volume, etc.; 6. Ranking along some scale such as from shortest to longest; 7. Presence and category of some attribute such as a bump, widget, or gismo; and 8. Ratios, such as length: volume. This exercise focuses on description by measurement, which then may be used directly, or to establish rank, or to determine ratios between two different measures. Linear measurements are typically made approximately along or transverse to the principal axis of growth, along the anterior -- posterior and ventral -- dorsal axes, or in some instances -- especially where the principal axis of growth is not inherently obvious -- along the longest and shortest distances, i.e. the major and minor axes of length. Measurements of larger specimens not requiring magnification for study are made most accurately with calipers or by being laid directly on millimeter graph paper for measurement (Fig. 3.1). Commonly, such as for the pelecypods in Fig. 3.1 length and height often are determined as straight-line measurements that at best are roughly parallel with and transverse to the principal growth axis. Many organisms add to their skeletons by **accretion**, which is direct addition of skeletal material onto the pre- existing part(s) so that the skeletal unit is extended or broadened, or both, and commonly thickened. Rate of accretion typically is variable so that along the growing margin, thickness, color, or some other feature varies and results in *growth lines* that record the history of the size and shape of the skeleton as it grew. Growth lines therefore, allow an investigator to identify the earliest-formed part of the skeleton. Measurement can then be made exactly along the principal axis of growth by following a line perpendicular to the growth lines where their broadest widths indicate the most rapid accretion. Measures of central tendency ---------------------------- 1. Even within a single bed there is generally at best mixing of remains from several successive populations that lived at the site of deposition. 2. Various size classes are selectively removed by taphonomic processes; and 3. The sample brought to the laboratory includes only part of the specimens preserved in the bed, and the collecting procedures can strongly influence the representative ness of the sample. Measures of dispersion ---------------------- 1. Bulk collection of sediment or *in situ* weathered residue of rock that is then screened and completely picked in a laboratory. 2. Marking off clearly defined areas from which every visible specimen is collected; and, in densely fossiliferous spots. 3. Marking off a grid-work that is the frame of reference for collecting at points specified by a sequence of random numbers. (There are tables of random numbers readily available, and many statistical computer packages can be used to generate a series of \[almost\] randomly generated numbers.) 1. How wide the range is, because the wider the range of values the higher is the chance that 2. How variable the feature is that is measured, because the more variable it is, then the greater the chance that a different set of specimens would have given a substantially different mean value; and 3. The number of specimens in the sample, because an increasingly larger number of specimens used as a sample makes the mean value less affected by unusual specimens so that the sample's mean "settles" towards the assemblage mean. Functional morphology --------------------- Different pelecypods have evolved life habits that have allowed some members of the group to adapt to virtually all the marine and freshwater benthic habitats. Adaptive modifications of form that enable a pelecypod to live in a particular habitat are often conspicuous and commonly specific to one life habit. The outline that follows summarizes common morphological adaptations for each of the major life habits of pelecypods (Figs. 3.2 & 3.3). The information is summarized in large part from Stanley (1970), supplemented by information from Pojeta in Boardman *et al.* (1998). 1. Byssal attachment. a. Commissure perpendicular to substratum (Fig. 3.3a). i. Equivalved. ii. Elongation of body and shell (length: height ≥ 1.35). iii. Reduction of the anterior end so that the shell beaks are terminal or are nearly terminal (although not true for many *Arca*). iv. Decrease its size of the anterior adductor muscle resulting in anisomyarian or even monomyarian condition. v. Presence of a byssal gape in the ventral commissure or a byssal sinus near the transition from the dorsal to the anterior margin of the shell. vi. Generally, the infaunal and semi-infaunal representatives have more similar-sized adductors, are less tapered anteriorly, and have maximum shell width medial or slightly dorsal; epifaunal representatives are more strongly anisomyarian, more strongly tapered anteriorly, and have maximum shell width near the ventral margin. b. Commissure non-perpendicular (horizontal or oblique to substratum) (Fig. 3.3b). vii. Inequivalved. viii. Commonly have a byssal sinus near the transition from the dorsal to the anterior margin of the shell, or a byssal notch or perforation in the lower valve. ix. Associated with byssal sinus, the hinge is elongated anteriorly into a pronounced auricle. x. Posterior elongation of hinge in one group (the pterrids), possibly to separate inhalant and exhalent currents. xi. Anisomyarian or monomyarian. 2. Attachment by cementation (Fig. 3.3c). c. Inequivalved. d. High external spinosity in tropical cemented species. e. Attached valve may be thick. f. Generally preserved *in situ*, attached to original substratum. g. Commonly monomyarian. 3. Reclining (unattached but resting on or partially buried in soft substrata, Fig. 3.3d). h. Almost always inequivalved, with lower valve convex and upper valve usually flat. i. Lower valve commonly very thick. j. Commonly monomyarian. 4. Intermittent swimming (Fig. 3.3e). k. Commissure non-vertical while swimming. xii. Inequivalved. xiii. Shells relatively thin, commonly radially ribbed or plicated near shore but smoother- shelled below normal wave base. xiv. Anterior and posterior auricles, if present, about equal size. xv. Umbonal angle large. xvi. Permanent gapes adjacent to auricles in most (equivalent gapes absent or very narrow in bysally attached relatives). xvii. Monomyarian. l. xviii. xix. xx. xxi. xxii. 5. m. xxiii. xxiv. xxv. xxvi. n. xxvii. xxviii. xxix. xxx. xxxi. xxxii. o. xxxiii. xxxiv. xxxv. xxxvi. xxxvii. p. xxxviii. xxxix. xl. 6. q. r. s. t. u. 7. v. w. Laboratory Exercise 3: Fossil Morphology and Data Analysis ---------------------------------------------------------- Measure two characters (length and width) for the 15 specimens provided from an assemblage of fossil species. Enter the measurements in the appropriate columns in the table. Note that this is a small sample that under many conditions may be too small to adequately characterize an assemblage of organisms that are highly variable. Record the measurements in millimeters. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Sum --- Determine the arithmetic mean of the two sets of measurements made in Question 1. --------------------------------------------------------------------------------- Record the range and compute the variance, standard deviation, and coefficient of variation for the measurements of the two characters made in Question 1. ---------------------------------------------------------------------------------------------------------------------------------------------------------- Range of *X*: \[0.7; 1.5\]. 0.8 Range of *Y*: [ \[]1.2; 1.8\]. 0.6 Variance of *X*[:] 0.031cm2 Variance of *Y*: [ ] 0.026 cm2 Standard deviation of *X*: [ ] 0.18.cm. Standard deviation of *Y*: [ ] 0.16cm Coefficient of variation of *X*: [ ] 17.3%. Coefficient of variation of *Y*: [ ] 10.06% For the two characters that were measured for Question 1, calculate the 95% confidence interval of the mean for the original fossil assemblage from which the sample was drawn. ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 95% confidence interval for *X*: [ \[0.94;1.13\]] 95% confidence interval for *Y*: [ \[1.51;1.67\]] Two specimens of bivalves are provided that represent two different life habits. For each specimen, give the information requested and determine the life habit. ---------------------------------------------------------------------------------------------------------------------------------------------------------------- Genus: [ ] Tridacna Cross-sectional shape: Height:Width ratio[: 80/215 =0.37209] Lateral view outline: Length:Height ratio (elongation): [ ] 145/80 = 1.8125 Thickness of shell relative to length and height: thin shelled, fine Texture of exterior of shell: [ ] rigid and wavy [ ] Interior shell structures: Muscle scars: [ ] anisomyaryan Inferred life habit: [ ] byssal atachment +-----------------------+-----------------------+-----------------------+ | **Specimen A** | **Specimen B** | **Specimen C** | +=======================+=======================+=======================+ | - Equivaled | - Equivaled | - Inequivalved | | | | | | - Fine shelled, | - Fine shelled, | - The lower valve | | ribbed shell | ribbed shell | is very small | | | | | | - Monomyarien | - Monomyarien | - Monomyarian? | | | | | | A and B are very | Inferred life : | | | similar | | | | | Intermittent Swimming | | +-----------------------+-----------------------+-----------------------+