THE ONTOGENY OF TRILOBITES
Authors: WHITTINGTON H. B. 
Published: January 2008

1. Terminology is explained, and the importance emphasized of work, mostly published during the last 10 years, on extraordinarily well‐preserved exoskeletons replaced by quartz. The need for a reconsideration of theories derived from ontogenetic studies is evident.

2. The protaspis is of sub‐hemispherical to spherical form, i mm. or less in length, axis at least partially denned, in larger specimens the glabella may be divided into four or five rings and the shield divided into cephalon and small protopygidium. Size and form of eye lobe and eye ridge vary, and in many examples the sutures in the late protaspis have essentially the holaspid course. Marginal cephalic spines, a narrow doublure, and a relatively large hypostome are characteristic.

3. Present subdivisions of the protaspid period of development are unsatisfactorily denned, and a division into early (smaller) protaspides and late (larger) protaspides in a series is used.

4. Attention is drawn to new evidence suggesting that the smallest olenellid developmental stages known are of meraspid cephala and not protaspides.

5. Protaspides of Paradoxides pinus? and P. rugulosus are known. Raymon's supposed specimen of the latter is a meraspid cranidium.

6. Protaspides of Sao hirsuta, o‐6‐i‐o mm. in length, have opisthoparian sutures, the free cheek carrying the librigenal spine, and a large, spinose hypostome.

7. Re‐investigation does not uphold Lalicker's view that in protaspides of Blainia gregaria? the sutures, eye ridge and palpebral lobe are visible on the anterior part of the cheek. The poorly‐preserved protaspides resemble that of Welleraspis swartzi. In both species the posterior branch of the suture curves back to the extremity of the posterior cephalic margin.

8. It is pointed out that in the protaspides of Olenus the exact position of the facial sutures is not known.

9. The protaspis identified by Beecher as ‘Triarthrus becki’ (properly T. eatoni) is considered not to belong in this species. Two pairs of lateral glabellar lobes are present and the sutures are opisthoparian.

10. Ross's interpretation of the sub‐spherical protaspis of Menoparia is upheld.

11. The morphology of the protaspides of Licnocephala, Shumardia, a lichid, and Diacanthaspis are reviewed.

12. Beautifully preserved protaspides of genera here placed in the superfamily Phacopoidae are described, with particular reference to their similarity of form, shape and subdivision of glabella, position of eye lobe, and border spines.

13. The meraspid period is here accepted as beginning with the appearance of a joint between cephalon and transitory pygidium, and continuing through successive degrees numbered to correspond with the number of freely‐articulating thoracic segments. The final degree is that in which there is one less than the specific number of segments.

14. Meraspid cephala of Paedeumias of length o‐6 mm. upwards are known, and an example of P. yorkensis 1 mm. in length has articulated with it at least five thoracic segments. Anterior cephalic, genal, and intergenal spines are all present at a particular developmental stage of this species.

15. For's original material oiEltiptocephala asaphoides includes only meraspides. There is no specimen revealing the protopygidium. Both genal and intergenal spines (but not anterior cephalic) are present in the smallest cephalon, and the intergenal spine is not composite. The resemblance to small cephala of Holmia kjerulfi (?) is marked.

16. Dorsal facial sutures have not been observed in olenellid meraspides, nor has the pre‐ocular ridge. The significance of the post‐ocular ridge is unknown.

17. During the meraspid period there is an increase in length of 6–14 times.

18. The number of instars during the meraspid period is not known. Within a particular degree a range in size, and addition of segments to the transitory pygidium, suggests that more than one moult may occur. During certain moults in particular Ordovician species several segments, not one, may simultaneously become freely‐articulating.

19. The transitory pygidium is relatively larger than the true pygidium. Anterior segments are the most fully formed, and long median axial or pleural spines appear posteriorly and move forward, suggesting that new somites are budded forward from the hindmost. Growth of parts of some segments may be retarded relative to those following.

20. Changes that take place in the cephalon during the meraspid period are described. In many species the point where the posterior branch of the suture crosses the border does not change during this period, but in certain cheirurids and pliomerids it migrates backward, as has been claimed in Peltura scarabaeoides. Either a fixigenal or a librigenal spine may be enlarged or reduced. In diverse genera a reduction of the fixigenal proceeds as the librigenal is enlarged, and this is analogous to reduction of intergenal and increase of genal spine in olenellids.

21. Knowledge of ontogeny of agnostids has not been added to since Barrande's day. It is reiterated (following Stubblefield) that, if the addition of segments in agnostids takes place in the same way as in other arthropods, then the orientation of the exoskeleton is as Barrande portrayed it.

22. The holaspid period is defined as beginning when the last thoracic segment appears. Additional segments may, or may not, be added to the pygidium during this period. At least a five‐fold, and up to forty‐fold, increase in length may occur. Morphological changes seem to be minor–in outline, relative size, and convexity of exoskeletal parts, smoothing out or deepening of furrows, etc.

23. Re‐investigation of holaspides of Isotelusgigas shows a rectilinear relationship between dimensions, and reveals no size groups that might denote successive instars. The smallest holaspis (length 9/4 mm.) lacks lateral glabellar lobes and furrows, and has only the anterior pleural furrow on the pygidium.

24. The protaspid glabella suggests the presence of five, and perhaps a sixth, cephalic segments. Intersegmental lines cannot be traced unequivocally in the pleural regions. It is reiterated that the assumption that the dorsal facial suture follows an intersegmental boundary is highly subjective.

25. Lateral cephalic spines are longer and more numerous in early developmental stages. The later reduction may be correlated with a change from a pelagic to benthonic habit. One of the functions of the genal spine is to support the cephalon when resting on the sea bottom. No new evidence is adduced in support of Raw's views of the segmental relations of lateral cephalic spines, and of their migration in olenellids.

26. Pleural furrows, and similar furrows on the cephalon (intergenal, etc.), are thought to be intrasegmental. Stermer's theory that the exoskeletal joints are not along intersegmental lines, but at an angle to them, is not favoured. The olenellid intergenal spine, and the fixigenal of Paradoxides and other trilobites, is not considered to belong to the pre‐occipital segment.

27. In a number of examples the sutural condition is opisthoparian or proparian from the earliest developmental stage onwards. The abrupt change of condition in Diacanthaspis argues for the use of these terms in a descriptive sense, and at a particular developmental stage, rather than in a systematic sense. Sutural condition is not the sole arbiter of relationship.

28. Only Peltura scarabaeoides remains as an example showing a migration posteriorly of the suture along the lateral and to the posterior margin, and thus a change from proparian to opisthoparian sutural condition, during the meraspid period. In Diacanthaspis cooperi there is no such migration of the posterior branch of the suture, but an abrupt change from proparian to opisthoparian sutural condition occurs at the outset of the meraspid period. The suggestion that proparian holaspides arose by arrested development from opisthoparians is not supported by any new evidence. It is urged that all available morphological criteria, at all developmental stages, be considered in seeking lines of descent between Cambrian and post‐Cambrian trilobites.

29. The abundance and variety of holaspides of 1 cm. or less in length, their different sutural types, and diverse ages, may imply that they did not all arise by neoteny, but that other factors also controlled their evolution.

30. The operation of a partial neoteny in the evolution of trilobites in the manner envisaged by Størmer is not upheld. A marginal suture is not characteristic of protaspides, and the ontogenies of ‘proparians’ and ‘opisthoparians’ does not show the retardation of the preantennal segment complex as he portrayed it.

31. The subjective nature of both some of the measurements and the systematic groupings used by Hupé in constructing his graphs is pointed out. Thus the claim that these graphs reveal the effects of ‘neotenization’ in trilobite evolution is rendered doubly subjective.

32. In attempting to classify trilobites into larger groups it is urged that account be taken of similarities between protaspides and in mode of subsequent development. Examples of the application of these criteria are given.

I am indebted to Drs W. R. Evitt, A. R. Palmer and C. J. Stubblefield for suggestions, criticism and discussion of some of the ideas set down here. Miss Pat Washer made all the drawings except Fig. 7. I express my thanks to Mr C. F. Kilfoyle, New York State Museum, for the loan of For's material of Elliptocephala asaphoides, and to Prof. K. M. Waage, Yale University, for the loan of protaspides studied by Beecher.