Megistaspis

{{Short description|Extinct genus of trilobites}} {{Automatic taxobox | fossil_range = {{fossil range|481|465|Early Ordovician-Middle Ordovician<ref name="Nielsen 1995" />}}{{Period fossil range|Ordovician|481|465}} | image = Megistaspis acuticauda (cropped).jpg | image_caption = Fossil of ''Megistaspis (Megistaspidella) acuticauda'' | taxon = Megistaspis | authority = Jaanusson, 1956 | type_species = ''Megistaspis limbata'' | type_species_authority = Boeck, 1838 | subdivision = 56, see text }}

'''''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 Ordovician Baltoscandia, 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''.<ref name="Jaanusson 1956" /><ref name="Angelin 1878" />

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.<ref name="Jaanusson 1956" />

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''.<ref name="Tjernvik 1956" />

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''.<ref name="Nielsen 1995" /><ref name="Balashova 1976" /> 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''.<ref name="Wandås 1983" />

In 1995, Dutch 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''.<ref name="Nielsen 1995" /> ''Heraspis'' was subsequently subsumed into ''Megistaspidella'' by Hansen, 2009.<ref name="Hansen 2009" />

== Description ==

''Megistaspis'' is a large genus of asaphid trilobite, with several species reaching lengths in excess of 25 centimeters.<ref name="Wandås 1983" /><ref name="Corbacho 2010" /><ref name="Bohlin 1960" /> Like other asaphid trilobites, ''Megistaspis'' has a cephalon, eight thoracic segments, and a pygidium which may or may not bear a terminal spine.<ref name="Gutiérrez-Marco 2017" />

The cephalon is large and semielliptical or triangular.<ref name="Angelin 1878" /><ref name="Balashova 1976" /> 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.<ref name="Jaanusson 1956" /><ref name="Corbacho 2010" /><ref name="Bohlin 1960" /> 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 backwards 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.<ref name="Jaanusson 1956" /><ref name="Bohlin 1960" /> 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)'').<ref name="Jaanusson 1956" />

The thorax is composed of eight segments with a narrow rachis.<ref name="Gutiérrez-Marco 2017" /><ref name="Jaanusson 1956" /> 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.<ref name="Jaanusson 1956" /><ref name="bodyparts">{{cite book | last=Bergström | first=Jan | title=Organization, life, and systematics of trilobites | series=Fossils and Strata | publisher=Universitetsforlaget | volume=2 | date=1973-04-27 | pages=1–69 | isbn=978-82-00-09330-5 | doi=10.18261/8200093301-1973-01 | url=https://www.scup.com/doi/book/10.18261/8200093301-1973 | access-date=2025-10-31 }}</ref> 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.<ref name="Jaanusson 1956" /><ref name="Angelin 1878">{{cite book |last=Angelin |first=N. P. |title=Paleontologia Scandinavica: Pt. 1. Crustacea formationis transitionis / auctore N. P. Angelin |publisher=Samson & Wallin |publication-place=Stockholm |date=1878 |doi=10.5962/bhl.title.14890 |doi-access=free |url=https://www.biodiversitylibrary.org/itempdf/51458 |access-date=2025-09-11 |pages=15–16}}</ref><ref name="Balashova 1976" /> 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.<ref name="Angelin 1878" /><ref name="Wandås 1983" /><ref name="Bohlin 1960" /> 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.<ref name="Jaanusson 1956" /> 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.<ref name="Corbacho 2010" /><ref name="Bohlin 1960" /><ref name="Hoel 2002" /><ref>{{cite journal |last1=Hoel |first1=Ole A. |title=Trilobites of the Hagastrand Member (Tøyen Formation, lowermost Arenig) from the Oslo Region, Norway. Part 1: Asaphidae |journal=Norsk Geologisk Tidsskrift |volume=79 |pages=179–204 |url=https://njg.geologi.no/images/NJG_articles/NGT_79_3_179-203.pdf |year=1999 |issue=3 |doi=10.1080/002919699433816 |issn=0029-196X}}</ref>

=== Appendages ===

Preserved appendages have been found in several specimens of ''M. hammondi'', a large species of ''Megistaspis'' from the Fezouata Biota.<ref name="Gutiérrez-Marco 2017" /><ref name="Gutiérrez-Marco 2022">{{Citation |last1=Gutiérrez-Marco |first1=J. C. |last2=Rábano |first2=Isabel |last3=Sá |first3=Artur A. |last4=Poblador |first4=Juan A. |last5=García-Bellido |first5=Diego |contribution=Nuevo trilobites asáfido con conservación de apéndices en la Biota de Fezouata (Ordovícico Inferior de Marruecos) |contribution-url=https://digital.csic.es/handle/10261/281232 |title=Comunicaciones presentadas en la LXXI Sesión Científica / Geoparque Montañas do Courel (Lugo), 26 de noviembre de 2021 |publisher=Sociedad Geológica de España |date=2022-10-19 |hdl=10261/281232 |issn=0213-683X |language=es |access-date=2025-09-11}}</ref> 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.<ref name="Gutiérrez-Marco 2017" /> 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".<ref name="Gutiérrez-Marco 2022" /> [[File:Figure 2 - Megistaspis (Ekeraspis) hammondi with preserved cephalic and thoracic appendages from the late Tremadocian Fezouata Lagerstätte (Moroccan Anti-Atlas).webp|thumb|Cephalic and first thoracic endopods of ''M. hammondi'' specimen MGM-6756X|center]]

=== Digestive system ===

[[File:Figure 1 - Megistaspis (Ekeraspis) hammondi with soft-body preservation from the late Tremadocian Fezouata Lagerstätte (Moroccan Anti-Atlas).webp|thumb|Fossils of ''M. (Ekeraspis) hammondi'' with soft-bodied preservation. (a-d) Specimen MGM-6755X, preserving digestive structures. (e-h) MGM-6756X, preserving a complete set of endopods]] 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).<ref name="Gutiérrez-Marco 2017" />

thumb|center|Digestive system of Megistaspis (Ekeraspis) hammondi with crop and intestine in yellow and digestive caecae in blue. Inferred parts have white scoring.|500x500px

=== Ornamentation ===

The test of ''Megistaspis'' is generally smooth.<ref name="Angelin 1878" /> 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.<ref name="Bohlin 1960" />

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.<ref name="Bohlin 1960" />

== Paleobiology ==

=== Diet and trace production ===

The relief of the crop and intestine of ''M. (Ekeraspis) hammondi'' has the same texture and grain size as the matrix, suggesting active sediment ingestion or rapid sediment infill. This, combined with the slight heteropody of the cephalic appendages (where the cephalic limbs were slightly larger than the thoracic limbs) and the non-forked hypostome, suggests that ''M. (Ekeraspis) hammondi'' was a detritus feeder, a common mode of life for other benthic trilobites. The spinose cephalic appendages of ''M. (Ekeraspis) hammondi'' were likely used to dig for food in a comb-like motion. This behavior would have produced ''Cruziana rugosa'' traces, which were pascichnia (combined feeding and locomotion traces) associated with the food searching strategy of {{ill|asaphoidea|lt=asaphoid|hu||species}} trilobites. In addition, ''Cruziana rugosa'' traces could have reached 26 centimeters wide, which is only consistent with large asaphoid trilobites like ''M. (Ekeraspis) hammondi'' and ''Ogyginus'', the latter of which are frequently found in association with ''Cruziana rugosa'' traces.<ref name="Gutiérrez-Marco 2017" />

''M. (Ekeraspis) hammondi'' may have also produced resting traces similar to ''Rusophycus carleyi'' and ''Rusophycus morgoti'', molting burrows that were previously interpreted as predation traces. The fine, backwards-oriented crests of these ''Rusophycus'' have been attributed to spines on the endopods, and the trace-makers have non-forked hypostomes. These traces have been assigned to ''Ogyginus'' and ''Asaphellus'', but ''M. (Ekeraspis) hammondi'' may have produced similar resting traces other than the long terminal spine, which could have been directed slightly upwards in life.<ref name="Gutiérrez-Marco 2017" /> [[File:Figure 4 - Behavioural traces attributed to certain Ordovician asaphid trilobites, reflecting ventral anatomy and preserved as convex hyporeliefs in sandstone.webp|center|Various trace fossils attributed to asaphid trilobites. (a) ''Cruziana rugosa'' from the early Dapingian of Argentina, specimen MGM-6760X (b) ''Rusophycus morgati'' from the Floian of France, specimen IGR-114954 (c-d) ''Rusophycus carleyi'' from the Floian Upper Fezouata Formation of Morocco, locally associated with the trackmaker ''Asaphellus'', (c) specimen MGM-6759X, (d) specimen MGM-6758X.|thumb|512x512px]]

However, ''M. (Rhinoferus) gibba'' has an anteriorly inflated glabella which has been suggested to facilitate the predation or scavenging of larger prey. While the hypostome of ''M. (Rhinoferus) gibba'' is currently unknown, its discovery could elucidate the ecological significance of an anteriorly inflated glabella. The hypostome of ''M. (Rhinoferus) hyorrhina'' is not drastically different from that of other species of ''Megistaspis'', but it has the median notch characteristic of ''M. (Megistaspis)'' rather than the median point of ''M. (Megistaspidella)''. However, the glabellar swelling of ''M. (Rhinoferus) hyorrhina'' occurs not on the anterior of the glabella but on the posteriormost lobe of the glabella behind the glabellar tubercule.<ref name="Hoel 2002" />

=== Burrowing and sensation ===

thumb|Dorsal and lateral reconstruction of the visual field of Megistaspis (Rhinoferus) hyorrhina based on PMO 162.658-659. Darker colour indicates overlapping fields. The dotted line marked GN is the outline of the glabella in the most swollen specimens.''M. (Rhinoferus) hyorrhina'' has been suggested to have a shallow infaunal life habit, where it would burrow just below the sediment. This observation is supported by its flattened exoskeleton, elevated holochroal eyes, and the large glabellar node (a swelling on the posteriormost lobe of the glabella). The glabellar tubercule may have acted as a pressure-sensitive organ in asaphid trilobites, and the inflated glabellar node may have served to elevate this organ above the sediment, allowing ''M. (Rhinoferus) hyorrhina'' to detect pressure changes even when burrowing. The eyes are strongly elevated, and like most other asaphid trilobites, ''M. (Rhinoferus) hyorrhina'' has a strongly restricted visual field. In 2002, Hoel and Høyberget estimated that the visual field of each eye covered about 195°, from a point straight in front of the eye to a point behind the rear of the body. Consequently, the visual fields of the left and right eye would have overlapped at the rear of the body, while a zone directly in front of the body could not be seen by either eye. Vertically, the visual field covers a narrow band sloping backwards and upwards at a 20° angle above the horizontal.<ref name="Hoel 2002" />

=== Cuticular thickness and molting ===

In Norwegian specimens of ''M. (Rhinoferus) hyorrhina'', the pygidia are frequently flattened more than the cranidia. While this was suggested to mean that the specimens were newly molted, two specimens only consisting of a pygidium and five thoracic segments, that are probably shed exuviae, are flattened to the same degree as the other Norwegian pygidia. This may suggest that the cuticle of the cranidium was thicker and more robust than that of the pygidium in life.<ref name="Hoel 2002" />

The two specimens of ''M. (Rhinoferus) hyorrhina'' consisting of a pygidium and five thoracic segments, along with a specimen of ''M. (Megistaspidella) triangularis'' consisting of a pygidium and five thoracic segments that was collected by Magne Høyberget from the same beds, may suggest that during molting, the pygidium and posterior five thoracic segments separated as a unit from the anterior parts, which were shed somewhere else. In addition, the suture between the third and fourth thoracic segments may have been a constant zone of weakness. However, Hoel and Høyberget warn against generalizing based on so few specimens.<ref name="Hoel 2002" />

=== Ecological variations ===

Many characters of ''Megistaspis'', including the width of the test, the convexity of the shell, the depth of furrows, and the development of the pygidial rim are affected by the environment. For instance, the pygidial length-with ratio of adult species of ''Megistaspis'' may vary around 15-20% in individual ecosystems but may total 30% or more (40% in ''M. (Megistaspidella) acuticauda'') across all members of a species. Shallow-water species (like ''M. (Paramegistaspis) planilimbata'' and ''M. (Megistaspis)'') tend to be narrower with a faint posterior border furrow on the cephalon while deeper-water species that dwell on muddy substrate like ''M. (Paramegistaspis) estonica'' tend to have broader tests and lack posterior border furrows.<ref name="Nielsen 1995" />

=== Pathologies === thumb|Pathological cephalon of ''Megistaspis acuticauda'' Angelin, 1854 showing tumor on left side and deformation of anterior section of facial suture A specimen of ''Megistaspis (Megistaspidella) acuticauda'' from the Baltic region notably displays a tumor on the left side of the cephalon and a "corresponding deformation of the anterior facial suture."<ref name="treatise">{{cite book | last=Harrington | first=H. J. | title=Treatise on Invertebrate Paleontology | publisher=the Geological society of America & the University of Kansas press | publication-place=Boulder (Colo.) Lawrence (Kan.) | date=1959-01-01 | isbn=0-8137-3015-5 | page=}}</ref> In addition, a specimen of ''M. (Paramegistaspis) planilimbata cyclopyge'' from Montana shows a scarred glabella, likely due to parasitism.<ref name="ross"/><ref name="parasitism">{{cite book | last1=De Baets | first1=Kenneth | last2=Budil | first2=Petr | last3=Fatka | first3=Oldřich | last4=Geyer | first4=Gerd | title=The Evolution and Fossil Record of Parasitism: Coevolution and Paleoparasitological Techniques | chapter=Trilobites as Hosts for Parasites: From Paleopathologies to Etiologies | publisher=Springer International Publishing | publication-place=Cham | date=2021 | isbn=978-3-030-52233-9 | doi=10.1007/978-3-030-52233-9_6#doi | pages=173–201 | doi-broken-date=2 November 2025 }}</ref>

==== Boring organisms ====

''Megistaspis'' exuviae from the Vaginatum limestone are rapidly destroyed by seawater and boring organisms if they are not buried immediately after molting. The surfaces of eroded tests frequently bear branching, winding, and anastomosing ridges with a semicircular cross-section without any structures like pores or striations. These ridges are never seen in specimens with smooth tests and likely contributed to the destruction of the shell. While the organism that produced these tubes may never be discovered, a colony of bryozoans has also been seen on a heavily eroded specimen, proving that the bioerosion was caused by an organism contemporaneous with ''Megistaspis''.<ref name="Bohlin 1960" />

== Species ==

* ''Megistaspis ringsakerensis''<ref name="Nielsen 1995">{{cite book |last=Nielsen|first=Arne Thorshøj|chapter=|series=Fossils and Strata|publisher=Scandinavian University Press|volume=38|date=1995-09-20|pages=1–374|isbn=978-82-00-37658-3|doi=10.18261/8200376583-1995-01|chapter-url=|access-date=2025-09-06|title=Trilobite systematics, biostratigraphy and palaeoecology of the Lower Ordovician Komstad Limestone and Huk Formations, southern Scandinavia|url=https://www.idunn.no/doi/book/10.18261/8200376583-1995}}</ref> Skjeseth, 1952 * ''Megistaspis knyrkoi'' Schmidt, 1906<ref name="Nielsen 1995" /> '''Subgenus ''Megistaspis (Megistaspis)'' Jaanusson, 1956<ref name="Nielsen 1995" /><ref name="Megistaspis at Fossilid">[https://fossiilid.info/1730?mode=in_baltoscandia "Megistaspis"] at Fossiilid.info</ref>'''

This subgenus is characterized by a triangular cephalon and pygidium, a notched hypostome, a glabella with lateral grooves, basal lobes and occipital lobes, a convex rachis and pleurae, the presence of a postmarginal furrow, and a pygidial rim that widens slightly towards the posterior end of the pygidium.<ref name="Balashova 1976" /> * Type species ''Megistaspis (Megistaspis) limbata'' Boeck, 1838 – Volkhov stage of Norway, Sweden, and Russia ** ''Megistaspis (Megistaspis) limbata baltica'' Balashova, 1976 – Volkhov stage of the Baltics * ''Megistaspis (Megistaspis) elongata'' Schmidt, 1906 – Kunda stage of Norway and Sweden * ''Megistaspis (Megistaspis) geminus'' Nielsen, 1995 – Volkhov stage of Sweden * ''Megistaspis (Megistaspis) ingeriensis'' Jaanusson, 1956 – Volkhov stage of the Baltics * ''Megistaspis (Megistaspis) lepikuensis'' Jaanusson, 1956 – Arenig of Russia and Estonia * ''Megistaspis (Megistaspis) lawae'' Schmidt, 1906 – Volkhov stage of the Baltics * ''Megistaspis (Megistaspis) planilimbata'' Angelin, 1851 – Latorp stage * ''Megistaspis (Megistaspis) polyphemus'' Brögger, 1882 – Volkhov stage of Norway and Sweden * ''Megistaspis (?Megistaspis) similis'' Burskij, 1970 * ''Megistaspis (Megistaspis) simon'' Tjernvik and Johansson, 1980 – Volkhov stage of Sweden '''Subgenus ''Megistaspis (Megistaspidella)'' Jaanusson, 1956<ref name="Jaanusson 1956">{{cite journal |last=Jaanusson |first=Valdar |year=1956 |title=Untersuchungen über Asaphiden III: Über die Gattungen Megistaspis n. nom. und Homalopyge n. gen. |journal=Bulletin of the Geological Institutions of the University of Uppsala |volume=36 |pages=59–77 |url=https://paleoarchive.com/literature/Jaanusson1956-AsaphidenIII.pdf |access-date=2025-09-08}}</ref><ref name="Bohlin 1960">{{cite journal |author=Bohlin, B. |journal=Bull. Geol. Inst. Univ. Uppsala |title=Trilobites of the genus Megistaspis from the Vaginatum Limestone of Scandinavia |year=1960 |url=https://paleoarchive.com/literature/Bohlin1960-MegistaspisVaginatumLimestone.pdf}}</ref><ref name="Megistaspis at Fossilid" />'''

This subgenus is characterized by a triangular and elongated cephalon, a subparabolic or triangular pygidium which may be pointed or bear a small terminal spine, an unsegmented glabella, and a relatively flat thoracic rachis.<ref name="Balashova 1976" /> * Type species ''Megistaspis (Megistaspidella) extenuatus'' Sars, 1835 – Norway * ''Megistaspis (Megistaspidella) acuticauda'' Angelin, 1854 – Lower Ordovician of Sweden * ''Megistaspis (Megistaspidella) convexa'' Bohlin, 1960 – Lower Ordovician of Sweden * ''Megistaspis (Megistaspidella) curvispina'' Bohlin, 1960 – Lower Ordovician of Sweden * ''Megistaspis (Megistaspidella) bombifrons'' Bohlin, 1960 – Lower Ordovician of Sweden * ''Megistaspis (Megistaspidella) gigas'' Angelin, 1854 – Lower Ordovician of Sweden * ''Megistaspis (Megistaspidella) giganteus'' Wandås, 1983 – Llanvirn of Norway<ref name="Wandås 1983">{{cite journal |last=Wandås |first=B. T. G |title=The Middle Ordovician of the Oslo Region, Norway, 33. Trilobites from the lowermost part of the Ogygiocaris series |journal=Norsk Geologisk Tidsskrift |volume=63 |issue=4 |year=1983 |issn=0029-196X |pages=211–267 |url=https://pascal-francis.inist.fr/vibad/index.php?action=getRecordDetail&idt=9502212 |access-date=2025-09-08}}</ref> * ''Megistaspis (Megistaspidella) grandis'' Sars, 1835 – Norway * ''Megistaspis (Megistaspidella) isvosica'' Balashova, 1976<ref name="Balashova 1976">{{cite book |last=Balashova |first=E. A. |title=Sistematika trilobitov Asaphina i ih predstaviteli v SSSR |publisher=Nedra |date=1976 |language=Russian |url=https://kirjandus.geoloogia.info/reference/6368}}</ref> * ''Megistaspis (?Megistaspidella) laine'' Jaanusson, 1956 – Volkhov stage of Sweden * ''Megistaspis (Megistaspidella) lamanskii'' Schmidt, 1906<ref name="Balashova 1976" /> * ''Megistaspis (Megistaspidella) longa'' Balashova, 1976<ref name="Balashova 1976" /> * ''Megistaspis (Megistaspidella) maximus'' Wandås, 1983 – Llanvirn of Norway<ref name="Wandås 1983" /> * ''Megistaspis (Megistaspidella) obtusicauda'' Bohlin, 1955 – Lower Ordovician of Sweden * ''Megistaspis (Megistaspidella) obuchovenis'' Balashova, 1976<ref name="Balashova 1976" /> * ''Megistaspis (Megistaspidella) pseudorudis'' Balashova, 1976<ref name="Balashova 1976" /> * ''Megistaspis (Megistaspidella) pugiocauda'' Harrington and Leanza, 1957 * ''Megistaspis (Megistaspidella) saltaensis'' Kayser, 1898 – Lower Ordovician of Sweden * ''Megistaspis (Megistaspidella) spinulata'' Bohlin, 1960 – Lower Ordovician of Sweden * ''Megistaspis (Megistaspidella) triangularis'' Schmidt, 1906 – Lower Ordovician of Sweden * ''Megistaspis (Megistaspidella) heroica'' Angelin, 1854 – Lower Ordovician of Sweden<ref name="Hansen 2009">{{cite book |last=Hansen |first=Thomas |title=Trilobites of the Middle Ordovician Elnes Formation of the Oslo Region, Norway |series=Fossils and Strata |publisher=Wiley-Blackwell |volume=56 |date=2009 |pages=1–215 |isbn=978-1-4051-9884-4 |doi=10.18261/9781405198844-2009-01 |url=https://www.idunn.no/doi/book/10.18261/9781405198844-2009 |access-date=2025-09-06 }}</ref> * ''Megistaspis (Megistaspidella) heros'' Dalman, 1828<ref name="Wandås 1983" /><ref name="Hansen 2009" /> * ''Megistaspis (Megistaspidella) laticauda'' Wandås, 1983 – Llanvirn of Norway<ref name="Wandås 1983" /><ref name="Hansen 2009" /> '''Subgenus ''Megistaspis (Paramegistaspis)'' Balashova, 1976'''<ref name="Balashova 1976" />

This subgenus is characterized by a semicircular cephalon, a semicircular or triangular pygidium, a gently notched hypostome, a glabella without lateral grooves, basal lobes and occipital lobes, a slightly convex rachis and pleurae, an unpronounced postmarginal furrow, and a pygidial rim that widens slightly towards the midpoint of the lateral margin pygidium.<ref name="Balashova 1976" /> * Type species ''Megistaspis (Paramegistaspis) planilimbata'' Angelin, 1851 – Tremadocian of Sweden **''Megistaspis (Paramegistaspis) planilimbata cyclopyge'' Harrington, 1938- Lower Ordovician of Montana, USA and Argentina<ref name="ross">{{cite book | last=Ross | first=Reuben James | title=Ordovician Fossils from Wells in the Williston Basin, Eastern Montana | publisher=U.S. Government Printing Office | date=1957 | url=https://books.google.com/books?id=KTmkxvK9hu8C | access-date=2025-10-29 | page=}}</ref><ref name="parasitism"/> * ''Megistaspis (Paramegistaspis) estonica'' Tjernvik, 1956 – Arenig of Russia, Estonia, and Sweden * ''Megistaspis (Paramegistaspis) leuchtenbergi'' Lamansky, 1905 – Tremadoc of Russia * ''Megistaspis (Paramegistaspis) norvegica'' Tjernvik, 1956 – Arenig of Norway and Sweden<ref name="Tjernvik 1956">{{cite journal |last=Tjernvik |first=Torsten E. |title=On the Early Ordovician of Sweden: Stratigraphy and Fauna |journal=Bulletin of the Geological Institutions of the University of Uppsala |volume=36 |pages=107–284 |year=1956 |url=https://paleoarchive.com/literature/Tjernvik1956-EarlyOrdovicianSweden.pdf}}</ref> * ''Megistaspis (Paramegistaspis) popovkiensis'' Balashova, 1966 – Early Ordovician of Russia * ''Megistaspis (Paramegistaspis) putilovensis'' Balashova, 1966 – Arenig of Russia * ''Megistaspis (Paramegistaspis) scutata'' Tjernvik, 1956 – Arenig of Sweden<ref name="Tjernvik 1956" /> '''Subgenus ''Megistaspis (Rhinoferus)'' Balashova, 1976'''<ref name="Balashova 1976" />

This subgenus is characterized by a triangular cephalon, a rounded pygidium which may or may not bear a short spine, a gently notched hypostome, a convex glabella with swellings of varying size in front of the occipital lobes, and eyes located far from the posterior margin of the cephalon.<ref name="Balashova 1976" /> * Type species ''Megistaspis (Rhinoferus) hyorrhina'' Leuchtenberg, 1843 – Baltics, Norway and Sweden ** ''Megistaspis (Rhinoferus) hyorrhina'' var. ''typica'' Leuchtenberg, 1843 – Norway<ref name="Hoel 2002">{{cite journal |last1=Hoel |first1=Ole A. |last2=Høyberget |first2=Magne |title=The Lower Ordovician trilobite Megistaspis (Rhinoferus) hyorrhina (Leuchtenberg, 1843)(Trilobita) in Norway, with notes on its autecology. |journal=Norsk Geologisk Tidsskrift |volume=82 |pages=45–51 |url=http://njg.geologi.no/images/NJG_articles/NGT_82_2_045-051.pdf |year=2002 |issn=0029-196X}}</ref> ** ''Megistaspis (Rhinoferus) ?hyorrhina'' var. ''kolenkoi'' Schmidt, 1906 – Norway<ref name="Hoel 2002" /> ** ''Megistaspis (Rhinoferus) ?hyorrhina'' var. ''mickwitzi'' Schmidt, 1898 – Norway<ref name="Hoel 2002" /> ** ''Megistaspis (Rhinoferus) ?hyorrhina'' var. ''stacyi'' Schmidt, 1906 – Norway<ref name="Hoel 2002" /> * ''Megistaspis (Rhinoferus) explanata'' Angelin, 1851<ref name="Nielsen 1995" /> * ''Megistaspis (Rhinoferus) gibba'' Schmidt, 1904 * ''Megistaspis (Rhinoferus) lawrowi'' Schmidt, 1906 – Lower Ordovician of Russia and Sweden * ''Megistaspis (Rhinoferus) petrowtschinensis'' Balashova, 1976 * ''?Megistaspis (?Rhinoferus) pogrebowi'' Balashova, 1966 '''Subgenus ''Megistaspis (Ekeraspis)'' Tjernvik, 1956'''<ref name="Megistaspis at Fossilworks" /><ref name="Corbacho 2010">{{Citation |last1=Corbacho |first1=Joan |last2=Vela |first2=Joan Antoni |title=Giant Trilobites from Lower Ordovician of Morocco |journal=Batalleria (Barcelona) |volume=15 |pages=3–32 |year=2010 |url=https://www.academia.edu/download/77752292/Giant_Trilobites_from_Lower_Ordovician_o20211230-14906-3bqadr.pdf |issn=0214-7831}}</ref>

This subgenus is characterized by a cephalon with long genal spines, a subtriangular pygidium with a long terminal spine, a short hypostome with a rounded posterior margin, a glabella without lateral grooves and occipital lobes, a convex rachis and pleurae, a wide and deep postmarginal furrow, and a narrow rhachis.<ref name="Tjernvik 1956" />

* Type species ''Megistaspis (Ekeraspis) armata'' Tjernvik, 1956 – Arenig (Floian) of Norway and Sweden<ref name="Megistaspis at Fossilworks">[https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=20055 ''Megistaspis''] at Fossilworks.org</ref><ref name="Corbacho 2010" /> * ''Megistaspis (Ekeraspis) euclides'' Walcott, 1925 – Tremadoc of Tasmania, Australia and British Columbia, Canada<ref name="Megistaspis at Fossilworks" /><ref name="Corbacho 2010" /> * ''Megistaspis (Ekeraspis) filacovi'' Bergeron, 1889<ref name="Corbacho 2010" /> ** ''Megistaspis (Ekeraspis) filacovi filacovi'' Bergeron, 1889 – Lower Ordovician of the Montagne Noire, France<ref name="Corbacho 2010" /> ** ''Megistaspis (Ekeraspis) filacovi bergeroni'' Thoral – Lower Ordovician of the Montagne Noire, France<ref name="Corbacho 2010" /> * ''Megistaspis (Ekeraspis) floweri'' Ross, 1970 – Tulean (late Tremadocian – early Floian) of Nevada, USA<ref name="Megistaspis at Fossilworks" /><ref name="Corbacho 2010" /> * ''Megistaspis (Ekeraspis) gladiator'' Fritsch, 1901 – Arenig of Germany<ref>{{Citation |last1=Marek |first1=Jaroslav |last2=Bartzch |first2=Konrad |last3=Drost |first3=Kerstin |last4=Fatka |first4=Oldrich |last5=Kraft |first5=Petr |last6=Linnemann |first6=Ulf |contribution=Revision of trilobites of the Griffelschiefer Formation (Ordovician, Schwarzburg anticline, Germany): preliminary results |title=Ordovician from the Andes, Proceedings of the 9th International Symposium on the Ordovician System. |publisher=INSUGEO |year=2003 |contribution-url=https://www.insugeo.org.ar/publicaciones/docs/scg_17/56.htm}}</ref> * ''Megistaspis (Ekeraspis) hammondi'' Brögger, 1882 – Tremadocian of Morocco<ref name="Corbacho 2010" /><ref name="Gutiérrez-Marco 2017">{{Citation |last1=Gutiérrez-Marco |first1=Juan C. |last2=García-Bellido |first2=Diego C. |last3=Rábano |first3=Isabel |last4=Sá |first4=Artur A. |title=Digestive and appendicular soft-parts, with behavioural implications, in a large Ordovician trilobite from the Fezouata Lagerstätte, Morocco |journal=Scientific Reports |volume=7 |year=2017 |article-number=39728 |url=https://www.nature.com/articles/srep39728 |doi=10.1038/srep39728 |bibcode=2017NatSR...739728G |hdl=10316/108382 |hdl-access=free|pmc=5223178 }}</ref> * ''Megistaspis (Ekeraspis) heroides'' Brögger, 1882 – Arenig of Norway and Sweden<ref name="Megistaspis at Fossilworks" /><ref name="Corbacho 2010" /> * ''Megistaspis (Ekeraspis) nevadaensis'' Ross, 1970 – Stairsian (Tremadocian) of Nevada, USA<ref name="Megistaspis at Fossilworks" /> * ''Megistaspis (Ekeraspis) roquebrunensis'' Courtessole, Pillet, and Vizcaïno 1985 – Montagne Noire, France<ref name="Corbacho 2010" />

=== Distribution ===

While most species of ''Megistaspis'' were concentrated in the temperate waters of Baltoscandia, in particular Norway, Sweden, Estonia, and Russia, members of the subgenus ''M. (Ekeraspis)'' have a wider distribution, being found in temperate regions like Norway and Sweden (''M. (Ekeraspis) armata'' and ''M. (Ekeraspis) heroides''), Germany (''M. (Ekeraspis) gladiator'', and France (''M. (Ekeraspis) filacovi'' and ''M. (Ekeraspis) roquebrunensis''); tropical regions like Nevada (''M. (Ekeraspis) floweri'') and Tasmania (''M. (Ekeraspis) euclides''); and polar regions like Morocco (''M. (Ekeraspis) hammondi'').<ref name="Balashova 1976" /><ref>{{cite journal |last=Scotese |first=Christopher R. |title=Ordovician plate tectonic and palaeogeographical maps |journal=Geological Society, London, Special Publications |volume=532 |issue=1 |year=2023 |issn=0305-8719 |doi=10.1144/SP532-2022-311 |doi-access=free |pages=91–109}}</ref>

== Biostratigraphy ==

The genus ''Megistaspis'' is one of the four most common genera in the {{ill|Komstad limestone|sv|Komstadkalksten}} of Sweden, (the others being ''Nileus'', ''Symphysurus'', and ''Geragnostus''), and its abundance and diversity allows it to be used as an index fossil for several biozones (rock layers defined by the presence of a certain species of index fossil) of Scandinavia and Estonia.<ref name="Nielsen 1995" />

''Megistaspis'' species act as index fossils for the following biozones in Scandinavia:<ref name="Nielsen 2023">{{cite journal |last1=Nielsen |first1=Arne Thorshøj |last2=Ahlberg |first2=Per |last3=Ebbestad |first3=Jan Ove R. |last4=Hammer |first4=Øyvind |last5=Harper |first5=David Alexander Taylor |last6=Lindskog |first6=Anders |last7=Rasmussen |first7=Christian Mac Ørum |last8=Stouge |first8=Svend |title=The Ordovician of Scandinavia: a revised regional stage classification |journal=Geological Society, London, Special Publications |volume=532 |issue=1 |year=2023 |issn=0305-8719 |doi=10.1144/SP532-2022-157 |pages=267–315 |bibcode=2023GSLSP.532..157N |doi-access=free}}</ref> * the '''''M. (Ekeraspis) armata''''', '''''M. (Paramegistaspis) planilimbata''''', and '''''M.'' aff. ''(Paramegistaspis) estonica''''' zones of the Ottenbyan stage.<ref name="Nielsen 2023" /> * the '''''M. (Paramegistaspis) estonica''''' zone of the Billingenian stage.<ref name="Nielsen 2023" /> * the '''''M. (Megistaspis) polyphemus''''', '''''M. (Megistaspis) simon''''', and '''''M. (Megistaspis) limbata''''' zones of the Volkhovian stage.<ref name="Nielsen 2023" /> * the '''''M. (Megistaspidella) gigas'''''-'''''M. (Megistaspidella) obtusicauda''''' zone of the {{ill|Kunda stage|lt=Kundan|et|Kunda lade}} stage.<ref name="Nielsen 2023" />

== References == {{Reflist}}

Category:Asaphida genera Category:Asaphidae Category:Cambrian first appearances Category:Fezouata Formation fossils Category:Fossil taxa described in 1956 Category:Ordovician Argentina Category:Ordovician Bolivia Category:Ordovician Colombia Category:Ordovician trilobites of Africa Category:Ordovician trilobites of Asia Category:Ordovician trilobites of Europe Category:Ordovician trilobites of North America Category:Ordovician trilobites of South America Category:Ordovician United States Category:Silurian extinctions Category:Tremadocian