Silurian Encrinurinae (Trilobita) from the central United States

Abstract

Silurian (Llandovery–Ludlow) Encrinurinae from Wisconsin, Indiana, and Illinois include species ofBalizomaHolloway, 1980,CurriellaLamont, 1978,DistyraxLane, 1988,EncrinurusEmmrich, 1844,MackenziurusEdgecombe and Chatterton, 1990a, andNucleurusRamsköld, 1986.Mackenziurus lauriaen. sp. is described from Wenlock strata of the Racine and Sugar Run Dolomites; nine segments in the holaspid thorax are the fewest known for any encrinurid. A square rostral plate is diagnostic ofMackenziurus. New morphological data are provided by Illinois specimens ofBalizoma indianensis(Kindle and Breger, 1904) from the Wenlock–Ludlow Racine Dolomite, and ofCurriella tuberculifrons(Weller, 1907) from the Llandovery Elwood–Wilhelmi Formations.Nucleurusn. sp. is possibly from Llandovery strata in Wisconsin.Distyraxn. sp. occurs in the Brandon Bridge Member (late Llandovery–early Wenlock) of the Joliet Dolomite in Illinois. The type pygidia and possible topotype material ofEncrinurus nereusHall, 1867, confirm assignment to that genus as currently viewed; this reefal species is distinct from the closely related inter-reef speciesE. eganiMiller, 1880, and appears to be distinct from reefalE. reflexusRaymond, 1916. Lectotypes are selected for theseEncrinurusspecies.

Growth and variation in the Silurian proetide trilobiteAulacopleura koninckiand its implications for trilobite palaeobiology

“…This view was based on observations of many species and is supported by the consistency of holaspid thoracic segment numbers within several clades (McNamara 1983;Owen 1985). Examples include the Phacopina with 1 1 segments, Asaphidae with 8 segments (Fortey & Chatterton 1988), Trinculeidae with 6 segments, Phillipsiidea with 9 segments, and the Encrinurinae with 1 1 (excluding the sister taxa Fragiscutum with 10 segments and Mackenziuruswith 9 [Gass et al 1992;Edgecombe & Chatterton 19931). This consistency suggests that the control of thoracic segmentation was tightly constrained in many taxa, even at high taxonomic levels.…”

Not the first leech: An unusual worm from the early Silurian of Wisconsin

“…13, 14). Arthropods dominate—mainly abundant and diverse trilobites (an undescribed dalmanitid and at least 12 other species; Wendruff et al, 2020a; Gass et al, 1992; Gass and Braddy, in press), crustaceans (including three species of the phyllocarid Ceriatocaris Jones, Feldmann, and Schweitzer, 2015, leperditicopid ostracodes, and the thylacocephalan Thylacares brandonensis Haug et al, 2014), a chelicerate (synziphosurine; Moore et al, 2005), and various enigmatic arthropods. These include Parioscorpio venator Wendruff et al, 2020b, originally considered the oldest scorpion, but probably a cheloniellid-like arthropod with a pair of large raptorial appendages (Braddy and Dunlop, 2021; Braddy et al, 2022, but see Anderson et al, 2021, and Van Roy et al, 2022).…”

Phacopid trilobites from the Silurian of Arctic Canada

“…Other groups that have established relative occurrences similar to the phacopids include, for example, Encrinurus sensu strictu, which is very common in southern Laurentia (e.g., Gass et al, 1992;Edgecombe and Chatterton, 1993), but known from only two species in northern Laurentia (Edgecombe and Chat-terton, 1993). Similarly, the otarionine Maurotarion (sensu Adrain and Chatterton, 1995) is the most common aulacopleurid in the southern Laurentian Silurian, but is represented by a single species, M. messieri Adrain and Chatterton, 1995 Hawle and Corda, 1847 Discussion.…”