Megistaspis is a large genus of
asaphid trilobite, with several species reaching lengths in excess of 25 centimeters. The
pygidium is roughly
equal in size to the cephalon, is triangular or semicircular, and, like the cephalon, has concave lateral parts that form a distinct marginal rim.
Appendages Preserved appendages have been found in several specimens of
M. hammondi, a large species of
Megistaspis from the
Fezouata Biota. One specimen in particular, MGM-6756X, preserves a complete set of
endopods. This specimen shows that
M. hammondi has 21 pairs of
limbs: three pairs of
cephalic limbs, eight pairs of
thoracic limbs, and 10 pairs of
pygidial limbs.
M. hammondi has slight
heteropody as its cephalic limbs are larger and heavier than its thoracic and pygidial limbs. The cephalic limbs also bear spines on
podomeres 2-4, with the best-preserved cephalic appendage of specimen MGM-6756X preserving 11 spines dorsally and 7 spines ventrally. No spines are present on the thoracic or pygidial limbs. An additional specimen, MGM-7569X, preserves part of the right
antenna in addition to the
distal portions of the endopods and
exopods. Antennae can also be observed in many commercially sold specimens, but they usually exhibit varying degrees of cosmetic modification, including the addition of extra podomeres, the adding of spines to the
abaxial side of the antenna, and the outlining of the rock surrounding the antennae with "cat ears". s of
M. hammondi specimen MGM-6756X|center
Digestive system s Another specimen of
M. hammondi, MGM-6755X, preserves the digestive system. The
alimentary canal of MGM-6755X consists of an 8-millimeter wide
crop that extends from the
anterior edge of the eyes to the end of the
cephalon, tapering to 4 millimeters by the end. Following the crop is a 3-millimeter wide
intestine that is preserved up to the third thoracic segment before reappearing for 22 millimeters before the
axial end of the
pygidium. There are two
bilaterally symmetrical digestive
caecae anterior to the crop that occupy the whole anterior region below the
glabella. Behind the eyes are at least 4 additional pairs of lobed,
laterally-oriented caecae with longitudinally aligned tips. The posterior caecae lengthen
transversely as the crop tapers. The first three
thoracic segments preserve digestive caecae similar in size and shape to the posteriormost cephalic pair. The alimentary canal may preserve three pairs of small and simple caecae posterior of the pygidial axis; however, the presence of caecae along the entire digestive tract is impossible to determine due to the quality of preservation and the fact that much of the specimen is restored. The crop and intestine are preserved with a positive
relief, while the caecae are preserved as voids but were likely initially permineralized during early
diagenesis, possibly due to being an
enzymatically active region in life. The presence of both a crop (anatomy) and caecae suggests that
M. (Ekeraspis) hammondi had a unique "type 3" digestive system (in contrast to the "type 1" digestive system defined by the presence of caecae but no crop and the "type 2" digestive system found in
Isotelus and
Birmanites defined by the presence of a crop but no caecae).
Ornamentation The
test of
Megistaspis is generally smooth.
Terracing exists only on the
doublure (only on the base of the "snout" in forms like
M. acuticauda with elongated snouts), the
articular facets of the
thorax and
pygidium, and the
genal spines, while light terracing may exist along the margins of the
free cheeks and pygidium. However, small pits, which are indicated on the internal surface of the test by small
tubercles, occur in all
Megistaspis species (but are only preserved in cases where the test has not been
weathered or
corroded). The pits may be in close proximity to each other or widely spaced from each other depending on the species; in addition, the pits are not always evenly spread across the test and may be crowded around the borders of the test but widely spaced closer to the central regions. In some cases, the pits may be of two different sizes, and in these cases, the smaller pits form a background. Below the surface of the test is a perfectly smooth
lamella which can be observed in specimens where the surface of the test is peeled off. The pits are clearly visible on the lamella and can also be seen on the
mold if the
sediment is
fine enough to preserve almost microscopic details. In some preserved
pygidia with a
weathered test, fine
striations can be seen
diverging forwards and outwards to the
dorsal furrow, where they suddenly turn outwards and somewhat backwards. This structure is entirely independent of the
relief of the pygidium, and the weak ribs in the
posterior parts of the pygidium can be
superseded by the
striations. In the
anterior ribs, the
distal portion of the ribs behind the
rib furrow may become diffuse and merge into the network created by the striae. The striations are preserved in both external and internal relief and can sometimes be preserved in an internal
mold of the pygidium. The striations are also not restricted to the pygidium, being observed in
cranidia of
M. (Rhinoferus) lawrowi and
M. (Megistaspidella) heros. These striations have been found in
M. (Megistaspidella) heros,
M. (Megistaspidella) acuticauda,
M. (Megistaspidella) curvispina, and
M. (Megistaspidella) gigas, but indications of them can be seen in other species and they likely exist in all species of
Megistaspis and perhaps other
asaphid genera. == Paleobiology ==