Platybrissus roemeri, copyright Koghisberg’s Collections.

Belongs within: Irregularia.

The Asterostomatidae are a possibly polyphyletic group of heart urchins characterised by the loss of petals and/or fascioles (Fischer 1966).

Moving to the top when shifting to the depths
Published 25 November 2023

In 1966, the Treatise on Invertebrate Paleontology volumes dealing with the stem-less echinoderms (sea urchins, sea stars, etc.) reached the presses. The role of reviewing the spatangoid heart urchins for these volumes had fallen to Alfred G. Fischer of Princeton University. Fischer (1966) divided the spatangoids between four suborders: the primitive Toxasterina, the major lineages of the Hemiasterina and Micrasterina, and a fourth suborder including just a single family, the Asterostomatidae. But Fischer himself expressed his unhappiness with this set-up, suggesting that this last family might prove to represent a “polyphyletic assemblage”. Nevertheless, he retained it on the grounds that “for the present recognition of the suborder fills a taxonomic need”.

Asterostoma pawsoni, copyright National Museum of Natural History.

As recognised by Fischer (1966), the Asterostomatidae represented some two dozen genera of heart urchins, found over a period from the Eocene to the present day. Asterostomatids were distinguished from other spatangoids by the reduction or loss of fascioles. Most heart urchins live buried in the sediment. Fascioles are bands of closely placed, cilia-covered spines that help the urchin keep its burrow clear. The spines themselves hold sediment away from the surface of the animal, keeping it from getting smothered, and movement of the cilia on the spines keeps water flowing through the burrow and around the urchin.

Whereas earlier authors had regarded asterostomatids as primitive due to their lack of fascioles, Fischer (1966) argued otherwise. He pointed out the derived appearance of other structural features of their test, and noted that asterostomatids appeared in the fossil record much later than fasciole-bearing spatangoids (which extend right back into the Cretaceous). Instead, Fischer suggested that the asterostomatids represented spatangoids that had convergently lost their fascioles as they abandoned a burrowing habit. Kier (1984) later expressed the opinion that the type genus Asterostoma differed enough from all other spatangoids that it should occupy its own exclusive family, though he too continued to use the broader Asterostomatidae in the absence of any better solution. A phylogenetic analysis of spatangoids by Stockley et al. (2005) placed ‘asterostomatids’ on at least six different branches of the spatangoid tree.

Possible Phrissocystis, a modern ‘asterostomatid’, from Okeanos Explorer.

So what could have incited so many heart urchins to lose their fascioles? Living ‘asterostomatid’-type urchins are inhabitants of deep waters that sit openly on the surface of the sediment. My guess is that burrowing is not a practical option in the flocculent ooze that covers much of the deep sea floor. Unable to maintain their old habits without being choked, the various lineages of heart urchins that had moved into this new realm opted for their more exposed lifestyle. And without the need to burrow, they had no further need for the tools of that trade.

Systematics of Asterostomatidae

Characters (from Fischer 1966): Peristome labiate, phyllodes well developed, test generally fragile; radioles usually present; petals weakly developed or absent, fascioles of various types or absent; primary spines present or absent; apical system ethmolytic; plastron mesamphisternous, holamphisternous or ultramphisternous.

<==Asterostomatidae [Antillasterinae, Asterostomatina, Palaeopneustidae, Paleopneustidae]F66
    |--Antillaster Lambert 1909 [=Pseudasterostoma Sánchez Roig 1952 non Duncan 1889]F66
    |    `--*A. cubensis (Cotteau 1871) [=Asterostoma cubensis, *Pseudasterostoma cubensis]F66
    |--Brissolampas Pomel 1883F66
    |    `--*B. conicus (Dames 1877) [=Paleopneustes conicus]F66
    |--Brissomorpha Laube 1871F66
    |    `--*B. fuchsi Laube 1871F66
    |--Cleistechinus De Loriol 1882F66
    |    `--*C. canaverii De Loriol 1882F66
    |--Delopatagus Koehler 1907F66
    |    `--*D. brucei Koehler 1907F66
    |--Elipneustes Koehler 1914 [incl. Eurypneustes Koehler 1914 non Duncan & Sladen 1882]F66
    |    `--*E. denudatus (Koehler 1914) [=Eurypneustes denudatus]F66
    |--Genicopatagus Agassiz 1879F66
    |    `--*G. affinis Agassiz 1879F66
    |--Linopneustes Agassiz 1881F66
    |    `--*L. murrayi (Agassiz 1873) [=Palaeopneustes murrayi]F66
    |--Megapetalus Clark 1929F66
    |    `--*M. lovenioides Clark 1929F66
    |--Moronaster Sánchez Roig 1952F66
    |    `--*M. moronensis Sánchez Roig 1952F66
    |--Palaeobrissus Agassiz 1883 [=Palaeobryssus Meissner 1903]F66
    |    `--*P. hilgardi Agassiz 1883 [=*Palaeobryssus hilgardi]F66
    |--Palaeotropus Lovén 1872F66
    |    `--*P. josephinae Lovén 1872F66
    |--Paleopneustes Agassiz 1873 [=Palaeopneustes Duncan 1889]F66
    |    `--*P. cristatus Agassiz 1873 [=*Palaeopneustes cristatus]F66
    |--Paleotrema Koehler 1914 [=Palaeotrema (l. c.)]F66
    |    `--*P. loveni (Agassiz 1881) [=Palaeotropus loveni]F66
    |--Peripatagus Koehler 1895F66
    |    `--*P. cinctus Koehler 1895F66
    |--Plesiozonus de Meijere 1902F66
    |    `--*P. hirsutus de Meijere 1902F66
    |--Prosostoma Pomel 1883 [=Pseudasterostoma Duncan 1889]F66
    |    `--*P. jimenoi (Cotteau 1870) [=Asterostoma jimenoi, *Pseudasterostoma jimenoi]F66
    |--Pycnolampas Agassiz & Clark 1907F66
    |    `--*P. oviformis Agassiz & Clark 1907F66
    |--Pygospatangus Cotteau 1888F66
    |    `--*P. salvae Cotteau 1888F66
    |--Asterostoma Agassiz 1847F66
    |    |--*A. excentricum Agassiz 1847F66
    |    `--A. dickersoni Sanchez Roig 1949SG93
    |--Argopatagus Agassiz 1879 [incl. Meijeria Döderlein 1906, Phrissocystis Agassiz 1898]F66
    |    |--*A. vitreus Agassiz 1879F66
    |    |--*Phrissocystis’ aculeata Agassiz 1904F66
    |    `--‘Phrissocystis’ humilis de Meijere 1902 [=*Meijeria humilis]F66
    |--Heterobrissus Manzoni & Mazzetti 1877 [incl. Archaeopneustes Gregory 1892]F66
    |    |--*H. montesi Manzoni & Mazzetti 1877F66
    |    |--‘Palaeopneustes’ hystrix Agassiz 1880 [=*Archaeopneustes hystrix]F66
    |    `--H. niasicusF66
    `--Platybrissus Grube 1865 [=Platybryssus Meissner 1903]F66
         |--*P. (Platybrissus) roemeri Grube 1865 [=*Platybryssus roemeri]F66
         `--P. (Eurypatagus Mortensen 1948)F66
              `--P. (*E.) ovalis (Mortensen 1948) [=*Eurypatagus ovalis]F66

*Type species of generic name indicated


[F66] Fischer, A. G. 1966. Spatangoids. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt U. Echinodermata 3 vol. 2 pp. U543–U628. The Geological Society of America, Inc., and The University of Kansas Press.

[SG93] Simms, M. J., A. S. Gale, P. Gilliland, E. P. F. Rose & G. D. Sevastopulo. 1993. Echinodermata. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 491–528. Chapman & Hall: London.

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