Megistaspis (Greek for "largest shield") is a genus of asaphid trilobites that lived throughout the Early and Middle Ordovician. Megistaspis was common throughout the Early and Middle Ordovician of Baltoscandia, but specimens have also been found in Australia, France, Germany, Morocco, and the United States. Appendages and a digestive system of M. hammondi have been preserved; it likely served as a mixed detritivore, producing Cruziana rugosa trace fossils. Meanwhile, Megistaspis hyorrhina may have burrowed below the sediment, using a swelling on the head to detect pressure changes. Megistaspis is also notable as it displays a large degree of morphological variation as a result of environmental conditions like ocean depth and substrate. The genus plays an important part in the biostratigraphy of Baltoscandia from the Ordovician, with several biozones being named after Megistaspis species.
History of discovery The type species of Megistaspis, M. (Megistaspis) limbata, was described as Trilobites limbatus by Norwegian zoologist Christian Boeck in 1838. In 1851, Swedish paleontologist Nils Peter Angelin moved Trilobites limbatus, along with Entomostracites extenuatus and Asaphus heros, to the new genus Megalaspis and erected several new species, including Megalaspis gigas, Megalaspis explanata, and Megalaspis planilimbata.
In 1956, Estonian-Swedish geologist Valdar Jaanusson recognized that the name Megalaspis was preoccupied by the fish genus Megalaspis, named by Pieter Bleeker earlier in 1851. He therefore erected the genus Megistaspis to replace Megalaspis, choosing the name, which is Greek for "largest shield", to be as similar to the old name Megalaspis as possible. Jaanusson created two subgenera: M. (Megistaspis) to accommodate forms with a strongly convex thoracic rachis and triangular cephalon, and M. (Megistaspidella) to accommodate later forms with a flat thoracic rachis and elongate cephalon, as well as suggesting the existence of a third group, composed of early forms with a somewhat flat thoracic rachis but semicircular cephalon, which he called the M. planilimbata group.
Also in 1956, Scandinavian paleontologist Torsten E. Tjernvik moved Megistaspis planilimbata to the subgenus Plesiomegalaspis (Plesiomegalaspis) and erected the new species Plesiomegalaspis estonica, Plesiomegalaspis norvegica, and Plesiomegalaspis scutata in addition to creating the new subgenus Plesiomegalaspis (Ekeraspis), containing forms with a long pygidial spine and deep posterior border furrow of the cephalon, to accommodate Megalaspis heroides and his new species Plesiomegalaspis armata.
In 1976, Soviet paleontologist E. A. Balashova split the genus Megistaspis into several genera as part of the family Megistaspisidae, which also included Plesiomegalaspis and several other genera. Balashova (1976) created several new genera: she created the genus Paramegistaspis, comprising forms previously within the M. planilimbata group, to accommodate P. planilimbata, P. estonica, P. norvegica, P. scutata and their relatives and the genus Rhinoferus, composed of forms with a large swelling on the glabella, to accommodate M. hyorrhina and its relatives. In addition, M. (Megistaspidella) was split into the genus Megistaspidella and P. (Ekeraspis) was split into the genus Ekeraspis. In 1983, B. T. Wandås created the subgenus M. (Heraspis), containing forms with a wide cephalon and a long pygidial spine, to accommodate two species: M. heroica and M. laticauda.
In 1995, Danish paleontologist Arne Thorshøj Nielsen demoted the genera Megistaspidella, Paramegistaspis, Rhinoferus, and Ekeraspis to subgenera of Megistaspis, giving Megistaspis six subgenera: Megistaspis, Megistaspidella, Paramegistaspis, Rhinoferus, Ekeraspis, and Heraspis. Heraspis was subsequently subsumed into Megistaspidella by Hansen, 2009.
Description
Megistaspis is a large genus of asaphid trilobite, with several species reaching lengths in excess of 25 centimeters. Like other asaphid trilobites, Megistaspis has a cephalon, eight thoracic segments, and a pygidium which may or may not bear a terminal spine.
The cephalon is large and semielliptical or triangular. The glabella is almost rectangular, with a semicircular frontal lobe, and the preglabellar field (the area of the cephalon in front of the glabella) is long and can reach over a quarter of the length of the cephalon. The facial sutures diverge strongly directly in front of the eyes before curving into a point in front of the glabella. The eyes are holochroal, small- to medium- sized, and situated around halfway along the length of the cephalon very close to the axial furrows which differentiate the axial lobe from the pleural lobes. The free cheeks (librigenae) are composed of a narrow anterior process that runs along the preglabellar field, a triangular middle portion designated as the body of the free cheek, and a posterior process that forms the genal spines, which extend backward from the rear corners of the cephalon. In most species of Megistaspis (other than M. (Megistaspidella) gigas and certain other members of the same species group), the peripheral rim of the free cheeks is flattened and the lateral parts of the cephalon are concave, forming a distinct marginal rim and border between the rim and the rest of the cephalon. The anterior body of the hypostome is arched and oval, with a weakly present or absent median furrow, while the posterior body of the hypostome takes the form of a narrow crescent. The lateral body of the hypostome is broad with distinct and rounded lateral projections. The posterior margin of the hypostome can be slightly concave (as in M. (Megistaspis)), evenly rounded, or slightly pointed (as in M. (Megistaspidella)).
The thorax is composed of eight segments with a narrow rachis. The pleural portion of the doublure (a rim around the ventral margin of the body formed by a folding over of the dorsal segments) have straight inner margins and bear Panderian notches. The Panderian notches, which may have contributed to enrollment, take the form of a hole in the doublure and are also present on the doublure of the free cheeks. 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. The rachis is distinctly segmented, with an articular half-ring followed by an articular furrow and several pygidial rings (between seven rings in M. (Ekeraspis) hammondi and up to 27 rings in M. (Megistaspidella) curvispina and M. (Megistaspidella) gigas). The first three rings are usually divided into an anterior and posterior half by a median depression. The pleural lobe of the pygidium bears ribs that may be undeveloped or distinctly furrowed. The pygidial doublure is narrow with a parabolic inner border. The pygidium may (as in M. (Ekeraspis) and some species of M. (Megistaspidella)) or may not (as in M. (Megistaspis), M. (Rhinoferus), M. (Paramegistaspis) and some species of M. (Megistaspidella)) bear a terminal spine.
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".
Digestive system
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.