• Category
  • Length Range
    18.8-37.3 µm
  • Width Range
    13.1-19.4 µm
  • Striae in 10 µm
  • Synonyms
    Biblarium emarginatum Ehrenb. 1845
    Tetracyclus lacustris var. emarginatus (Ehrenb.) Van Heurck 1896



Valves are cruciform. The valve apices are subcapitate and rounded. The central inflation is notched. Primary transapical costae occur in semi-regular intervals of 2-3 in 10 µm and are irregular in length. Striae are faint. The striae radiate slightly toward the valve margins. The axial area is narrow and has a linear sternum that extends the length of the valve ending just before reaching the valve ends. Septa are present.


Tetracyclus emarginatus has been noted in both modern and fossil deposits across several localities. Specimens described and displayed above come from two fossil deposits: the Valles Caldera complex, New Mexico dating to the mid-Pleistocene (~400 ka) and the Nebraska Ashfall Fossil Beds in Antelope County dating to back to the mid-Miocene (~12 Ma). The Miocene-aged deposit from Ashfall is interpreted to be from a subtropical watering-hole environment, much like those found in African savannas today, that became covered in ash from a mega-eruption in the Bruneau-Jarbidge area of Idaho. The Pleistocene-aged deposit from the Valles Caldera straddles Glacial Termination V, where glacial conditions transitioned to interglacial conditions as Southwestern climate began to warm and precipitation decreased.

Original descriptions of T. emarginatus place it in Siberia and Mexico (Ehrenberg 1845) and later it was reported from the Gap of Dunloe, Killarney (Smith 1856). Subsequent records of T. emarginatus (Patrick and Reimer 1966, Williams 2007) are from from New Jersey and Oregon. Multiple authors described T. emarginatus as a freshwater diatom that prefers the cool waters of lakes and ponds found in mountainous regions (Smith 1865, Patrick and Reimer 1966, Williams 2007).

Valles Caldera
Credit: Dr. Peter Fawcett
Valles Caldera, New Mexico - site of Pleistocene-aged core samples.
12 H 2 core drive
Credit: Dr. Peter Fawcett
12H-2 sediment core containing T. emarginatus specimens.

Original Description

B. corpusculorum valvis lateralibus quadrangulis, ad Crucis formam subequaliter profunde angulosis, radiis obtusis duobos oppositis (ventralibus) emarginatis, striis transversis validis laxis in 1/96 ‘’’7, in 1/72‘’’8. Longit. -1/72’’’. Sutura nulla. Valvas s. foliola 19 in singulo libello observavi, duos libellos semel concatenatos vidi. Sibiria. Mexico. Fossile.

  • Basionym
    Biblarium emarginatum
  • Author
    Ehrenb. 1845
  • Length Range
    15.85 µm
  • Width
    18.12 µm
  • Striae in 10µm

Original Images

Original tetracyclus drawings
Screen Shot 2022 06 15 at 7 07 24 AM

Cite This Page

Cutler, S. (2022). Tetracyclus emarginatus. In Diatoms of North America. Retrieved April 23, 2024, from https://diatoms.org/species/tetracyclus-emarginatus


The 15 response plots show an environmental variable (x axis) against the relative abundance (y axis) of Tetracyclus emarginatus from all the stream reaches where it was present. Note that the relative abundance scale is the same on each plot. Explanation of each environmental variable and units are as follows:

ELEVATION = stream reach elevation (meters)
STRAHLER = distribution plot of the Strahler Stream Order
SLOPE = stream reach gradient (degrees)
W1_HALL = an index that is a measure of streamside (riparian) human activity that ranges from 0 - 10, with a value of 0 indicating of minimal disturbance to a value of 10 indicating severe disturbance.
PHSTVL = pH measured in a sealed syringe sample (pH units)
log_COND = log concentration of specific conductivity (µS/cm)
log_PTL = log concentration of total phosphorus (µg/L)
log_NO3 = log concentration of nitrate (µeq/L)
log_DOC = log concentration of dissolved organic carbon (mg/L)
log_SIO2 = log concentration of silicon (mg/L)
log_NA = log concentration of sodium (µeq/L)
log_HCO3 = log concentration of the bicarbonate ion (µeq/L)
EMBED = percent of the stream substrate that is embedded by sand and fine sediment
log_TURBIDITY = log of turbidity, a measure of cloudiness of water, in nephelometric turbidity units (NTU).
DISTOT = an index of total human disturbance in the watershed that ranges from 1 - 100, with a value of 0 indicating of minimal disturbance to a value of 100 indicating severe disturbance.