Tuesday, April 16, 2024


This blog post is about oligopygoids, cassiduloids, and spatangoids, all of which are irregular echinoids; thus also classified with sand dollars. For examples and details concerning just sand dollars, please see my blog that immediately precedes this present blog.

Echinoids range from the Paleozoic (starting in the Ordovician Period) to modern day. As adults, they crawl about on the ocean floor. Like all the other invertebrates, echinoids underwent significant changes in their modes of life during Mesozoic time because of increasing predation by predators.

Figure 1: Life zones of regular echinoids versus geologic time. 

Haimea bajasurensis Squires and Demetrion, 1994
                              (two views: top and left side)

     Order Oligopygoida

     Family Oligopygidae

Range: Lower Eocene, Baja California Sur, Mexico. THIS IS A FOSSIL SPECIES. It is both the earliest and the westernmost oligopygoid, and the first occurrence of Haimea in North America.

This is a fossil species. Baja California Sur, Mexico.

Habitat: Shallow-marine.

Source of Information: Squires and Demetrion, 1994. 

Dimensions of Figured Specimen: Length 18 mm, width 15.7 mm.

Figure 1a: top view.

Figure 1b: side view.

                    Calilampus californiensis Squires, 1995

                            (two views: top and left side)

     Order Cassiduloida

     Family Cassidulidae

Range: Lower Eocene, Baja California Sur, Mexico. THIS IS A FOSSIL SPECIES.

Habitat: Shallow-marine.

Source of Information: Squires and Demetrion. 1995. 

Dimensions of Figured Specimen: Length 3.5 cm, width 2.9 cm.

Lovenia cordiformis A. Agassiz, 1872
(two views: top and bottom)

     Order Spatangoida [The heart-shaped echinoids].

     Family Loveniidae

Range: Santa Cruz Island, southern California to Panama, Coco Islands, and the Galapagos; rare in Columbia.

Habitat: Infaunal (about 15 cm depth) in sandy sediment, in the low-intertidal zone to a depth of 200 m.

Shell: Surface is covered by long, brown hair-like spines; the shell resembles a small coconut.

Source of Information: Marine Biodiversity Records https:/mbrbiomedicalcentral.com

Dimensions of Figured Specimen: Length 5.4 cm, width 3.6 cm.

                  Micraster cor-anguinum (Leske), a plaster cast.

           (four successive views: top, left side, bottom, posterior)


     Order Spatanogoida

     Family Micrasteridae

Range: THIS IS A FOSSIL SPECIES. Upper Chalk beds, Upper Cretaceous (Senonian), from Pinden, near Dartford, Kent, England. 

Fossil Locality: This genus lived from Late Cretaceous (Cenomanian) to early Eocene. It was moderately widespread, with species in Europe, North America (mainly the Gulf Coast states), Egypt, and Antarctica (rare). 

Source of Information: Wikipedia (2023). 

Dimensions of Figured Specimen: length 6 cm, width 5.3 cm.

                          Meoma ventricosa? (Lamarck, 1816)

                        (three views: top, bottom, and left side)


   Order Spatnagoida

   Family Brissidae

Common Names: Cake urchin and red heart” urchin.

Range: Florida, the Bahamas, Bermuda, Jamaica, Virgin Islands, and West Indies, Caribbean, southern Mexico, northern Venezuela.

Habitat: Shallow waters. They burrow into sand at the rate of about 3 to 6 cm an hour during the day, and twice as fast during the night.

Shell: Spines blackish-red in color.

Source of Information: Wikipedia (2023).  

Dimensions of Figured Specimen: Length 135 cm, width 112.5 cm, thickness 67 cm.

                    Agassizia scrobiculata Valenciennes, 1846

               (four views: left to right--for both rows--top, 

                          bottom, left side, and posterior)


   Order Spantangoida

   Family Prenasteridae

Common Name:

Range: Only two known species. Upper Gulf of California, Baja California, Mexico to Peru and the Galapagos Islands.

Habitat: Largely subtidal, at depths 0 to 76m, on sandy, muddy, or rocky bottoms. 

Shell: Small, normal size 20 to 35 mm length.

Source of Information: Brusca (1980).

Dimensions of Figured Specimen: Length 35 mm, width 30 mm, thickness 26 mm.

References Cited:

Brusca, R. C. 1980. Common intertidal invertebrates of the Gulf of California. Second edition. The University of Arizona Press, Tucson, Arizona, 513 pp. 

Squires, R.L. and R.A. Demetrion. 1994. A new species of the oligolpygoid echinoid Haimea from the lower Eocene of Baja California Sur, Mexico. Journal of Paleontology 68(4):pp. 846–851, figs. 1–3.

Squires, R.L. and R.A. Demetrion. 1995. A new genus of cassiduloid echinoid from the lower Eocene of the Pacific coast of western North America and a new report of Cassidulus ellipticus Kew, 1920, from the lower Eocene of Baja California Sur, Mexico. Journal of Paleontology 69(3):509–515. 

en. Wikipedia.org

Saturday, April 13, 2024


NOTE: I have three previous posts about echinoderms:

Jan. 7, 2017-the spatangoid Schizaster diabloensis

Feb. 12, 2019- a two-part series: The first part is about regular echinoids (e.g., spiny sea urchins). The second part is a brief review about the infaunalization of irregular echinoids (sand dollars and some other infaunal groups).

This present post deals ONLY with clypeasteorids (= a main component of the echinoids) and provides much more information, as well as some taxa not mentioned in my 2019 posting. 


There are two main groups of this class: echinoids = 1) regular (sea urchins) and 2) irregular (sand dollars and other infaunal groups). The mode of life for each of these two groups is shown in the following figure.  

Geologic time and life zones of regular and irregular echinoids. 

Irregular echinoids range from Mesozoic to modern day. Although some irregular echinoids (sand dollars = clypeasteroids) crawl around (epifaunal animals) on the ocean floor, most of them are shallow-depth burrowers (infaunal animals), in either shallow or slightly deeper waters. Becoming burrowers protected them from predators (fish, crabs, etc.). Other infaunal groups that could burrow even deeper,” namely oligopygoids, cassiduloids, and spantagoids will be discussed and illustrated in my next post.

Clypeasteroids  = SAND DOLLARS (Epifaunal or Shallow-Infaunal Burrowers)

                                    Encope grandis (top view)

     Order Echinolampadacea

     Family Mellitidae

Common Name: Giant Keyhole sand dollar (it has a large hole (keyhole). 

Range: Subtropical, Gulf of California, (e.g., Bahia de Los Angeles, Baja California, Mexico). 

Habitat: Sandy beaches commonly about 2 m depth (intertidal) but can live to 20 m (394 feet) deep.

Shell: Its keyhole (or lunule = a large opening) allows faster movement of the shell as the animal plow through sand.

Sources of Information: WoRMS (online: World Register of Marine Species), and mexican-fish.com

Figured Specimen: length 7 cm, width 7 cm.

                     Mellita quinquiesperforata (Leske) (top view)

     Order Clypeasteroida

     Family Mellitdae

Common Name: The “five-slotted” sand dollar.

Range: Endemic to the western Atlantic Ocean, from Virginia to Florida (e.g., especially common in Naples area, Florida), Gulf of Mexico to Texas, south to Mexico, and also Bermuda, Jamaica, Puerto Rico, and Brazil. 

Habitat: A temperate to tropical sand dollar that lives in shallow waters below tide lines in sandy bottoms; especially in bays. The five “slots” (openings = lunules) allow for passage of sand during shallow burrowing by the animal.

Source of Information: animaldiversity.org

Figured Specimen: length 7 cm, width 7.2 cm.

                   Rotuloidea fimbriata Etheridge, 1872 (top view)

     Order Clypeasteroida

     Family Rotulidae (consists of two extant genera and one extinct genus)

Range: Safi, Morocco, West Sahara [North Africa],

Fossil: Pliocene (approx. 2.5 million years ago).

Habitat: Probably lived in lagoons and estuaries.

Dimensions of Figured Specimen: Length 3.3 cm, width 3 cm.

              Astrodapsis bajasurensis Squires and Demetrion, 1993

                                (two views: top and bottom)

     Order Clypeasteroida

     Family Echinarachiidae

Range: Baja California Sur, Mexico.

Fossil: Late middle Miocene (Isidro Formation) (approx. 13 million years ago).

Habitat: Subtropical.

Source of Information: Squires and Demetrion, 1993.

Dimensions of Figured Specimen: Length 2.9 cm, width 2.65 cm

Other Comments: The clypeasteroid genus Astrodapsis is endemic to California (Hall, 1962) and Baja California, Mexico (Squires and Demetrion, 1993). It is an index fossil used for relative age-dating of Miocene and Pliocene sedimentary deposits in these areas. Specimens are usually abundant and easily identified to the genus level, at least, based their inflated petals (petaloids), which are food grooves on the top surface of their shell. Species identification of Astrodapsis can be “tricky” however, because of gradational morphology caused by subtle paleoenvironmental differences (note: many nearshore fossils [e.g., oysters] have suffered the same fate). It is extremely likely that there has been considerable over-naming of the California species.

                      Dendraster excentricus (Eschscholtz, 1831)

                                   (two views top and bottom)

     Order Clypeasteroida

     Family Dendrasteridae

Range: Alaska to Baja California Sur, Mexico.

Habitat: Tidal channels and protected areas with moderate water movement (e.g., lagoons).

Source of Information: Wikipedia and gbif.org/species/2279084.

Figure 5: Dimensions of Figured Specimen: Length 5.8 cm, width 6.2 cm; this specimen was collected from by the author from beach drift on San Quintin Beach, Baja California Norte.

Note: The food-channeling, very narrow grooves on the bottom of this shell have been stained black by natural causes. 

                                    ?Dendraster excentricus 

This sand dollar population inside the "plastic bubble" has a vertical-feeding position. This image was taken of an exhibit at the Cabrillo Beach Museum, southern California. These sand dollars are in their feeding position; namely, they periodically they align their shells vertically and orient their ventral sides (that is where their mouth is located) facing the current caused by an incoming tide, so as to catch any available suspended food. Typical diameter of this  sand dollar is about 8 cm.

                           Clypeaster rosaceus (Linnaeus, 1758).

                              (three views top, bottom, and side)

     Order Clypeasteroida

     Family Clypeasteridae

Common Name: The “flat” sea biscuit.

Range: Western Atlantic Ocean, South Carolina to Barbados, Caribbean, and Panama.

Habitat: Turtle grass and sand fields bordering turtle grass.

Shell: Its variation in shape is great.

Source of Information: Wikipedia.

Dimensions of Figured Specimen: Length 11.4 cm, width 8.6 cm, height 3.5 cm.

Other Comments: The clypeasteroid genus Clypeaster (illustrated above) has considerable variation in the shape of its species. This variation can be in one or more of the following: its petals  can be low or inflated; overall shape can be low conical to highly conical. Such variation has undoubtedly resulted in overnaming of species.  

                                             Clypeaster sp.

                             (three views: top, bottom, and side)     

     Order Clypeasteroida

     Family Clypeasteridae


Shell: This highly conical shell shape is quite a contrast to the previous example of a species of this genus; namely, Clypeaster rosaceus = a flat-shell species of this genus.

Dimensions of Figured Specimen: length 12.5 cm, width 11.5 cm, height 6 cm.

References Cited:

Hall, C.A. Jr. 1962. Evolution of the echinoid genus Astrodapsis. University of California Publications in Geological Sciences 40(2):47–180.

Squires, R.L. and R.A. Demetrion. 1993. A new species of the clypeasteroid echinoid Astrodapsis from the Miocene Isidro Formation, Baja California Sur, Mexico. Journal of Paleontology 67(2):258–263. 

Friday, April 12, 2024


The so-called "killer clams" are classified as:

Subclass Heterodonta

Order Cardiida

Family Cardiidae: 7 genera and 3 subgenera

Genus Tridacna Brugiuère, 1797 (the most common genus)

Geologic Range:

Late Cretaceous?, Eocene to Recent, with occurrences in Europe, India, Africa, So. Pacific; commonly associated with corals reefs.

Tridacna clams (bivalves) are generally large to massive in shell size, with ribs few in number but strong, and lacking anterior lateral teeth .

Some species attach themselves via byssal threads to a coral reef, and, in so doing provides stability for themselves. The byssal threads extend through an opening (along a dorsal area located posteriorly) called the byssal notch. The notch can be narrow or very wide. Some individuals use their weight to hold them in place rather than rely on byssal threads. Some individuals can also be nestled among corals. Tridacna bivalves can close their valves for protection, but the closure is rather slow, especially among the large individuals. Their shells do not shut fast like a mousetrap. I think that was the basis for the misconception that these bivalves could trap divers.

Tridacna gigas is the largest of all living bivalves, reaching lengths of more than four feet. Their mature adults can live for nearly a century, and their shells are very heavy.

The edges of their shells are fluted, there by allowing for the flexible mantle to extend upward out of the shell in order to catch sunlight. The mantle contains algae (zooanthellae), which via photosynthesis provides photosynthetic “food” for the bivalve. 

View into a tank of ocean water containing living Tridacna specimens for the public to see at the California Academy of Sciences in San Francisco. The large dark-green to black individuals of Tridacna are aligned in parallel rows. If you visit this facility, be sure to see this aquarium with live Tridacna in crystal-clear water.

Close-up of a living Tridacna specimen with its blue mantle tissue exposed. This specimen was in the same tank of water mentioned above. The mantle contains symbiotic algae which the bivalve eats.

Hippopus hippopus (Linnaeus, 1758). Three views: exterior and interior of left valve, dorsal view of both valves. Length 7.2 cm, height 5 cm.

Tridacna maxima (Roding, 1798), Indo Pacific, shallow coral reefs

Three views: exterior and interior of left valve; dorsal view of both valves. Length 9 cm, height 6 cm.

Tridacna (Chametracheasquamosa Lamarck, 1819. SW Pacific, common. Three views: exterior and interior of left valve; dorsal view of both valves. Length 8.5 cm, height 6.5 cm. 

References Used:

Reefkeeping.com (this is a very informative website for the marine aquarist)