Ontogeny and systematics of Toernquistiidae (Trilobita, Proetida) from the Ordovician of the Argentine Precordillera

Journal of Paleontology, Mar 1998 by Chatterton, Brian D E, Edgecombe, Gregory D, Waisfeld, Beatriz G, Vaccari, Norberto E

17) Genal spine: 0) present; 1) absent or very short and knob like (reduced during ontogeny).

The genal spine usually appears as a short projection during the second metaprotaspid stage. In most species, it then lengthens through ontogeny. In some taxa (particularly Ischyrotoma) it then, secondarily, becomes shorter during ontogeny. In Hystricuridae, the genal spine is well developed.

18) Height of fixigena relative to palpebral lobe: 0) palpebral lobe raised above fixigena; 1) equal in height; 2) palpebral lobe not as raised as fixigena is inflated.

Small growth stages have the palpebral lobe similar to the height of the fixigena (1). This is also case for most Hystricuridae, but some have palpebral lobes slightly raised (0) and some slightly lowered (2).

19) Length of posterior area of fixigena (measured from back of palpebral lobe to middle of posterior border furrow, exsagittally): 0) longer than posterior border; 1) shorter than posterior border; 2) about same length as posterior border.

Juvenile state of character is 0. In Hystricuridae state is usually 0 or 2.

20) Height/width (view perpendicular to outer surface) of librigenal field in relation to height/width of eye (visual surface): 0) lower than eye; 1) equal to eye; 2) greater than eye.

In small stages, this character is usually state 0 or 1. In Hystricuridae, the state is usually 2.

21) Length of genal spine relative to more anterior portions of dorsal portion of free cheek: 0) shorter than more anterior portions of free cheek; I) longer than more anterior portions of free cheek; 2) genal spine and more anterior portions of cheek similar in length (vary between being slightly longer and slightly shorter).

In small free cheeks the character is state 0. In Hystricuridae the character occurs in all three states. We have coded Ischyrotoma as 0 for this character, as this spine is usually apparent but shorter than the rest of the free cheek in small growth stages.

22) Path of genal spine: 0) genal spine straight; 1) genal spine incurved backward; 2) genal spine outcurved backward.

This character is represented by state 0 in small free cheeks. Hystricuridae are variable, showing all states. We have coded this character as ? for Ischyrotoma, unless the state is known for small growth stages, where the character is based upon the path of the genal spine in meraspid and early holaspid stages.

23) Hooked tip to genal spine: 0) absent; 1) present.

This character is represented by state 0 in both small growth stages and Hystricuridae. Codings for Ischyrotoma are based on small free cheeks, where known (for I. blanda and I. ovata are based on unpublished data of writers, and Young, 1973, plate 7, figure 14 = I. ovata).

24) Cephalic sculpture of holaspides: 0) smooth; 1) distinctly granulose; 2) tuberculose; 3) smaller granules or tubercles, with larger tubercles.

The sculpture may change several times during the ontogeny. Hystricuridae show all states.

25) Path of posterior section of facial suture: 0) posterolaterally directed behind palpebral lobe; 1) approximately laterally or anterolaterally directed before curving back.