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III. Eggshell identification: Who laid the egg? |
The only way to positively identify who laid an egg is to find an egg with an embryo preserved inside. However, most of the eggshell fossil record consists of eggshell fragments that are often separated from nests and skeletal remains. In these cases, analyses of the general morphology and microstructure of the eggshell are necessary to learn more about which animal may have laid the egg. Comparisons with modern eggs are central to the study of fossil eggs because the taxonomic identity of living egg-layers can be linked to their physiology and the structural characteristics of their eggshell. Unfortunately, many fossil eggshells lack modern analogs. Nevertheless, even in the absence of embryonic remains and modern analogs, eggshell microstructure and pore patterns may be used to differentiate and categorize fossil eggshell.
Historically, the eggshell classification system included "basic types"
and "structural morphotypes," which were based on the structural and organizational
attributes of the eggshell. Fossil eggshell classification also includes a parataxonomic binomial
nomenclature system similar to that used by biologists for modern taxa, except
the Greek prefix "oo" (meaning egg) is added before the name to form oofamilies,
oogenera, and oospecies. These parataxonomic classifications describe
the morphological and structural characteristics of the eggshell and can be used
to distinguish eggs and eggshell fragments that cannot be identified on the basis
of embryonic remains or other reliable methods such as preservation within an
adult. It should be noted, however, that eggs for which the egg layeris known
can have both taxonomic names (pertaining to bones) and parataxonomic names (pertaining
to eggshell).
The historically-used morphotypes are
listed below. However, several researchers have recently advocated discontinuing
the use of "basic types" and "structural morphotypes"
because the relevant diagnostic information is provided
in the descriptions of parataxonomic oofamilies, oogenera, and oospecies. Researchers now use eggshells in cladistic analyses to study evolutionary relationships and reproductive physiology by using only suites of descriptive morphological
and microstructure characters, regardless of which parataxonomic designation has been assigned.
Basic types and structural morphotypes
Five basic eggshell structures have been recognized and were originally correlated
with specific amniote egg-layers. These types were based on analyses of
modern eggshell and comparisons with fossil material, helping to substantiate
that the correlation between eggshell structure and taxonomy has persisted over
time. They can be distinguished by the general arrangement of calcite crystals
(aragonite in turtle eggs). These structures can be further divided into "structural
morphotypes" based on microstructure (the organization of the shell units
and the pore systems). As eggshell analysis has become more refined, these
terms and concepts have become less commonly used and are now generally considered
outdated. They are not covered in detail here, but are listed below with figures
illustrating their structure. (See also "Eggshell morphology and structure.")
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Geckonoid (Fig. 1)
Testudoid (Fig. 2) Crocodiloid (Fig. 3)
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Figure 1. SEM photomicrograph of modern gecko eggshell (Phelsuma madagascarensis)
showing columnar geckonoid organization. Specimen UCM 444, photomicrograph G44.1. Figure 2. Testudoid eggshell histostructure. A. Thin section of recent eggshell from a European pond turtle (Emys orbicularis). Note that the shell unit has concentric growth lines. The fact that the shell unit is wider than it is tall, indicates that this is not a rigid eggshell. Specimen UCM 828-2, photomicrograph taken under polarized light. B. SEM
photomicrograph of fossil turtle eggshell from England clearly shows the spherulitic
pattern with aragonite radial ultrastructure under SEM. Note the more elongate
shell unit indicating a more rigid eggshell structure. Specimen UCM 185, photomicrograph
1704. Figure 3. Crocodiloid eggshell histostructure. A. Light microscope
photomicrograph of modern alligator (Alligator mississippiensis) eggshell in thin section. Note wedge-shaped shell units, and straight pores that are wider at the bottom of the shell. Specimen UCM 604-B. B. SEM
photomicrograph of fossil eggshell from the Eocene of Colorado showing wedge-shaped
shell units that indicate Crocodiloid shell organization. Specimen UCM 446, photomicrograph
1689. C. SEM photomicrograph of modern crocodile (Caiman crocodiles)
showing the basal plate groups and the bottom of the shell unit near the shell
membrane. Specimen UCM 565, photomicrograph G 114.478. |
Dinosauroid and Ornithoid (Figs. 4-6)
Dinosaur eggshells were originally divided into Dinosauroid and Ornithoid types
based, in part, on egg misidentification. Extant birds, on the other hand,
only lay eggs with Ornithoid structure, but this structure varies among species.
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Figure 4. SEM photomicrograph of a dinosaur eggshell from the Cretaceous
of China with a general spherulitic pattern falling within the "dendrospherulitic" microstructure
classification. Note the intertwined pattern of calcite columns through the eggshell
and the non-linear pore system that characterized this microstructure. Specimen
UCM 681-1, photomicrograph G186.15. Figure 5. Thin section of a Cretaceous
dinosaur eggshell showing a "filispherulitic" microstructure with elongate calcite
columns and pores stretching non-linearly through the shell. The shell layer
and pores often have irregular walls. Specimen UCM 797(2). Figure 6. Photomicrograph
of a thin section of eggshell from a recently extinct elephant bird (Aepyornis)
showing ornithoid ratite organization. There is an abrupt and distinct transition
between the unorganized calcite prism layer and the radiating crystal structure
near the membrane. Specimen UCM 827-(2), photomicrograph M809.11A. |
Parataxonomy
Parataxonomy has long been used in paleontology as a way to classify structures
associated with an organism but that are not part of the organism itself (e.g.,
footprints, burrows, and eggs). Eggshell parataxonomy describes the structure
and morphology of eggs and eggshell independent of the taxonomic classification
of the egg-layer and divides specimens into oofamilies, oogenera, and oospecies.
Each oofamily is classified by its basic eggshell organization, pore system organization,
and surface sculpturing. Oogenera within oofamilies are divided by egg shape
and additional distinguishing characteristics of the morphology, pore system,
and sculpturing. Further division into oospecies is based on quantitative characteristics
of the egg's external and internal morphology. Thus, parataxonomy provides a
shorthand method for referring to a fossil egg with certain characteristics.
Because the descriptions of types (and morphotypes) and ootaxa are redundant,
more researchers now rely on the use of parataxonomic nomenclature and have dropped
the "basic type" and "structural morphotype" terminology.
Figure 7 shows the associations between parataxa, "structural morphotypes," and pore systems.
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Figure 7. Table showing the inferred relationships between basic types, structural morphotypes, parataxonomy, and taxonomy of fossil and mondern amniote eggs. |
Cladistics
Many eggshell researchers now combine parataxonomic nomenclature (ootaxa) with
cladistic analyses to study the similarities among eggshell structures and egg-layers.
With a character-based classification scheme for fossil eggshell and more discoveries
of embryos that permit taxonomic identification, paleontologists have expanded
how eggshell research can be used to analyze evolutionary relationships among
amniotes. The designations of morphotypes and ootaxa are based on suites of characters
that may or may not be unique to certain groups of organisms, making them unreliable
for determining evolutionary relationships. Furthermore, misidentification of
embryos and eggshell structures has caused confusion in relating parataxonomic
groups to taxonomic groups. Thus, a major advantage of incorporating eggshell
characters into cladistic analyses is the ability to determine which microstructure
characteristics are important for differentiating taxa. While cladistic analysis
works particularly well with eggs of known taxonomy (i.e., eggs found with identifiable
embryos), unidentified eggshell can also be cladistically analyzed to explore
how it compares to identifiable eggshell, and to assign some level of taxonomic
identity. Identifying the best derived characters for distinguishing branches
on phylogenetic trees provides a better understanding of reproductive biology.
This phylogenetic approach is now being applied to non-avian and avian dinosaur
(bird) eggs.
Additional readings
Carpenter, K. 1999. Eggs, Nests, and Baby Dinosaurs: A Look at Dinosaur Reproduction. Indiana University Press. 338 pp. Chapter 8 (pp. 135-144), "How to study a fossil egg," focuses on eggshell classification.
Hirsch, K.F. 1994. The fossil record of vertebrate eggs. Pp. 269-294 in S.K. Donovan (ed.), The Palaeobiology of Trace Fossils. John Wiley and Sons.
Mikhailov, K.E. 1991. Classification of fossil eggshells of amniotic vertebrates. Acta Palaeontologica Polonica 36(2):193-238.
Mikhailov, K.E., E.S. Bray, and K.F. Hirsch. 1996. Parataxonomy of fossil egg remains (Veterovata): Principles and applications. Journal of Vertebrate Paleontology 16(4):763-769.
Mikhailov, K.E. 1997. Fossil and recent eggshell in amniotic vertebrates: Fine structure, comparative morphology and classification. Special Papers in Palaeontology (56):1-80.
Varricchio, D.J., and F.D. Jackson. 2004. A phylogenetic assessment of prismatic dinosaur eggs from the Cretaceous Two Medicine Formation of Montana. Journal of Vertebrate Paleontology 24(4):931-937.
Zelenitsky, D.K., S.P. Modesto, and P.J. Currie. 2002. Bird-like characteristics of troodontid theropod eggshell. Cretaceous Research 23:297-305.
Zelenitsky, D,K., and S.P. Modesto. 2003. New information on the eggshell of ratites (Aves) and its phylogenetic implications. Canadian Journal of Earth Sciences 81:962-970.
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