The Miaolingian is the third Series of the Cambrian Period, and was formally named in 2018. It lasted from about 506.5 to 497 million years ago and is divided in ascending order into 3 stages: the Wuliuan, Drumian, and Guzhangian. The Miaolingian is preceded by the unnamed Cambrian Series 2 and succeeded by the Furongian series.
It is named after the Miaoling Mountains in southeastern Guizhou Province, China.
Pikaia gracilens is an extinct, primitive chordate marine animal known from the Middle Cambrian Burgess Shale of British Columbia. Described in 1911 by Charles Doolittle Walcott as an annelid, and in 1979 by Harry B. Whittington and Simon Conway Morris as a chordate, it became "the most famous early chordate fossil", or "famously known as the earliest described Cambrian chordate". It is estimated to have lived during the latter period of the Cambrian explosion. Since its initial discovery, more than a hundred specimens have been recovered.
The body structure resembles that of the lancelet and it swam perhaps much like an eel. A notochord and myomeres (segmented blocks of skeletal muscles) span the entire length of the body, and are considered the defining signatures of chordate characters. Its primitive nature is indicated by the body covering, a cuticle, which is characteristic of invertebrates and some protochordates. A reinterpretation in 2024 found evidence of the gut canal, dorsal nerve cord and myomeres, and suggested that the taxon was previously interpreted upside down.
In geology and related fields, a stratum (pl.: strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as either bedding surfaces or bedding planes. Prior to the publication of the International Stratigraphic Guide, older publications have defined a stratum as being either equivalent to a single bed or composed of a number of beds; as a layer greater than 1 cm in thickness and constituting a part of a bed; or a general term that includes both bed and lamina. Related terms are substrate and substratum (pl.substrata), a stratum underlying another stratum.
The Paleo-Tethys or Palaeo-Tethys Ocean was an ocean located along the northern margin of the paleocontinent Gondwana that started to open during the Middle Cambrian, grew throughout the Paleozoic, and finally closed during the Late Triassic; existing for about 400 million years.
Paleo-Tethys was a precursor to the Tethys Ocean (also called the Neo-Tethys), which was located between Gondwana and the Hunic terranes (continental fragments that broke off Gondwana and moved north). It opened as the Proto-Tethys Ocean subducted under these terranes and closed as the Cimmerian terranes (that also broke-off Gondwana and moved north) gave way to the Tethys Ocean. Confusingly, the Neo-Tethys is sometimes defined as the ocean south of a hypothesized mid-ocean ridge separating Greater India from Asia, in which case the ocean between Cimmeria and this hypothesized ridge is called the Meso-Tethys, i.e., the "Middle-Tethys".
Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding organisms are known chiefly from fossils found from the Middle Cambrian (Miaolingian, Wuliuan) through the Lower Carboniferous (Mississippian). A possible early graptolite, Chaunograptus, is known from the Middle Cambrian. Recent analyses have favored the idea that the living pterobranch Rhabdopleura represents an extant graptolite which diverged from the rest of the group in the Cambrian.
Fossil graptolites and Rhabdopleura share a colony structure of interconnected zooids housed in organic tubes (theca) which have a basic structure of stacked half-rings (fuselli). Most extinct graptolites belong to two major orders: the bush-like sessile Dendroidea and the planktonic, free-floating Graptoloidea. These orders most likely evolved from encrusting pterobranchs similar to Rhabdopleura. Due to their widespread abundance, planktonic lifestyle, and well-traced evolutionary trends, graptoloids in particular are useful index fossils for the Ordovician and Silurian periods.
Rhabdopleura is a genus of colonial sessile hemichordates belonging to the Pterobranchia class. They are exclusively marine, benthic organisms whose species occur within all major oceans and range in habitat from intertidal to c. 900 m. As one of the oldest living genera with a fossil record dating back to the Middle Cambrian, it is also considered to be the only living genus of graptolites.
Rhabdopleura is the best studied pterobranch in developmental biology. Research in the 2010s by Jörg Maletz and other paleontologists and biologists have demonstrated that Rhabdopleura is an extant graptolite.