New Echinoid Species in Texas - Tiaromma sp. Pomel 1883

[Bill Thompson]

I collected another new Species with Dr. Stephen Crane last Saturday  - Tiaromma Pomel, 1883.

It is from the Walnut Formation.  Specimens measure 7 mm up to 13 mm.

 

Too bad I did not collect it before I published my new book  Fossil Echinoids of Texas  - A Monograph of Fossil Sea Urchins.

 

 

[erose]

what are the distinguishing characters that make this Tiaromma?

 

[bone2stone]

Yes please, "what is it's characteristics" for this specimen looks very similar to many of the smaller Phymosoma texanum I have in my collection.

[Bill Thompson]

Thank you both for asking.   When I first found them, my first thought was that they were juvenile Phymosomas. Dr. Stephen Crane has collected dozens of this specimen for years and I collected 2 dozen with him last week.   List of other regular Walnut echinoids with pronounced tubercles: Phymosoma, Loriolia, Tetragramma, and Salenia  (the specimen looks nothing like a Salenia)   Diagnosis: Tubercles perforated - thus it is not a Phymosoma. Apical system does not project into the posterior interambulacrum - thus it is not a Loriolia.        (I cleaned the apical system of the specimen photographed here and the additional matrix in the photo was removed.)         I attached another specimen that I photographed. Pore pairs are uniserial throughout - this it is not a Tetragramma.     Other possibilities I ruled out: Pseudodiadema (buccal notches very deep and with short phyllodes). Diplopodia (pore pairs biserially offset adapically and short phyllodes adorally).     It matches all the features listed in the Echinoid Directory Test flattened below and above; ambitus rounded. Apical disc large, subpentagonal, a little less than half test diameter; plating caducous; when preserved monocyclic, with angular interior indicating pavement of polygonal periproctal plates. Ambulacra straight, plating trigeminate at ambitus, becoming irregularly quadrigemminate adapically; pore-pairs uniform and undifferentiated; at ambitus and above more or less uniserial; no phyllodes adorally. Pore-pairs very widely separated on ambital plates. Primary ambulacral tubercle to each plate; plate compounding diadematid, with all three elements reaching the perradius and all overlapped by the primary tubercle at the ambitus. Interambulacral plates a little wider than tall; plates with a single large tubercle, centrally placed, which diminishes in size adapically and adorally. Row of small adradial secondaries developed adorally. A narrow naked interradial zone developed adapically. Ambulacral and interambulacral tubercles similar in size; perforate and crenulate, the mamelon particularly small. Peristome about half test diameter, distinctly invaginated; buccal notches small but clearly marked and with lip. No sphaeridial pits. Further remarks listed in the Echinoid Directory            In having trigeminate plating up to the ambitus and then quadrigeminate plating aborally Tiaromma resembles Tetragramma. However, Tetragramma is immediately distinguished by its  biserial pore zones and by having enlarged secondary tubercles on interambulacral plates that approach in size the primaries. Tiaromma is ike Loriola in form, but that taxon has a more pentagonal disc that is elongate to the posterior and has only trigeminate plating adapically.     I've attached photo new photo, it is a 7 mm specimen.   This Texas species is smaller than the European species - Tiaromma blancheti  and Tiaromma michelini

[erose]

Interesting indeed.  The explanation sounds good. I will say that I do have Loriolia cf. whitneyi specimens without a notch in the apical notch. But the rest plays out for this genus. I will be looking more closely at some of my "small" Phymosomas.   What part of the Walnut Formation were these coming from? 

[Bill Thompson]

Upper Walnut, just below the Comanche Peak.