Geology Reference
In-Depth Information
(4
) Number and shape of laterals are essential in
distinguishing species and genera of dasyclads:
Pri-
mary branches arise from the main axis in different ar-
rangements, numbers and forms. The primary laterals
may branch distally, producing segments of second (Pl.
63/6, Pl. 64/4), third (
Palaeodasycladus
, Pl. 62/1) or
higher order. The number of the segments per lateral
and the number of laterals per whorl as well as the shape
of the laterals are used as species- and genus-specific
criteria.
Terms characterizing the shape of the laterals are
(a) phloiophore (branches distinctly expanded termi-
nally; genus-specific; e.g.
Bornetella
;
Eovelebitella
, Pl.
59/1), (b) vesicular (laterals inflated, straight or slightly
bent), (c) pirifere (laterals short, usually straight, in-
flated, widest diameter at their middle; nearly exclu-
sively as a primary lateral), (d) trichophore (laterals
long, pear-shaped, sudden decrease in diameter fol-
lowed by very thin hairlike forms; restricted to higher-
order segments of laterals;
Neoteutloporella
, Pl. 59/3),
(e) acrophore (laterals thin, acute; mainly as higher-
order laterals), and (f) umbrelloform (laterals inflated
forming an umbrella-like whorl (
Clypeina
, Pl. 59/7) or
separated pear-shaped or lanceolate rays; restricted to
primary segments of laterals).
(5)
Position of the reproductive organs
(Fig. 10.15):
Gametes (sexual reproductive cells) are produced in
Box 10.3.
How to determine dasyclad algae?
The table should help in organizing the requisition of data that are neces-
sary to separate dasyclads at least at a generic level.
Basic considerations
Look at the number of dasyclad sections in your sample. Are there many or just a few?
Note that determining genera and
taxa requires combining criteria seen in longitudinal, cross, oblique and tangential sections of variously shaped
three-dimensional objects.
Examine the preservation of the dasyclads.
Are the algae well or poorly preserved? Did recrystallization destroy impor-
tant taxonomic criteria (e.g. arrangement and shape of pores/laterals)? Are the central stem and pores filled with
sediment or cement? Growth of cement may make it difficult to recognize the number of the orders of branches.
Differentiate the sections into longitudinal, tangential, cross and oblique sections
. Select those sections that exhibit a
maximum of morphological criteria.
Morphological criteria
Evaluate the shape of the thallus using Fig. 10.14A.
Differentiate between unbranched and branched thalli. Unbranched
thalli may be tube-like, pear-shaped or stalked. Both unbranched and branched thalli may be segmented.
Typify the subdivision of the thalli (Fig. 10.14B).
The external or internal configuration (undulation, annulation,
intusannulation) is important in separating genera.
Describe the arrangement of the primary branches arising from the main axis (Fig. 10.14C).
Are the branches irregu-
larly arranged with respect to the main axis (aspondyl)? Are they regularly arranged like the spokes of a wheel
(euspondyl), or do they form regular whorls consisting of clusters of primary branches (metaspondyl)? These criteria
serve to distinguish families and genera.
Look at the contact of the primary branches with the main axis.
Laterals may form different angles with the axis; they
may start from a small platform, or two or more branches may have a joint basis.
Typify the shape of the branches/pores (Fig. 10.14E).
Do the diameter and width of the pores increase distally (phloiophore
and vesicular) or decrease distally (acrophore, trichophore)? These criteria are species-specific.
Examine the number of higher-order branches arising from the primary branches (Fig. 10.14D).
The pattern of distal
branching of primary lateral into branches of second, third and higher orders is crucial in distinguishing genera and
species.
Look for reproductive organs within the central stem or in or between branches (Fig. 10.15).
These criteria are impor-
tant in distinguishing dasyclad groups.
Quantitative data
Collect measurements that characterize the range and variability of the dimensional criteria.
Commonly used in
distinguishing species are the diameter of the central cavity of the thallus, the ratio of the outer and inner thallus
diameter (total diameter versus diameter of the main axis), the thickness of the 'wall' (representing the calcified zone
around the central stem), the diameter of primary and other branches, the angle between branches and main axis, and
the size of the whorls and the number of branching participating into whorl formation.
Relevant references
Start with the papers by Berger and Kaever (1992), De Castro (1997) and Bassoullet et al. (1975) in order to under-
stand the terminology and the handling of the morphological criteria. In most cases you will have some idea about the
approximate age of your sample. Look at the list given in 'Basics' at the end of this paragraph and select the relevant
overviews that will lead you to other useful papers.
