The Eurypterida of New York/Volume 1/Ontogeny

by John Mason Clarke and Rudolf Ruedemann

IV
ONTOGENY

The collections from the shale beds of the Shawangunk grit at Otisville have furnished an unrivaled series of larval stages of one species each of the genera Eurypterus, Pterygotus, Stylonurus and Hughmilleria. Many of the growth stages measure but 2 millimeters or even less in length and hence so little surpass in size the eggs^[1] of Limulus and probably those of Eurypterus lacustris and Pterygotus that they may be safely considered as representing the first larval or nepionic stage. Our large amount of material has allowed us to connect these larval stages with the adult forms by a fairly continuous series. There have also been obtained at Buffalo and Jerusalem hill youthful stages of Eurypterus remipes, E. lacustris, Pterygotus macrophthalmus and Eusarcus scorpionis which present characters indicating their neanic condition.

All these young individuals have a number of characters in common which distinguish them from the adults and for this reason may be considered as ontogenetic characters of larger scope. We here briefly summarize these general ontogenetic features from lists of specific ontogenetic characters taken from the detailed descriptions of the development given under the respective species.

Eurypterus remipes [pl. 4, fig. 1–4; pl. 5, fig. 1, 2]

The neanic stage is distinguished from the mature by

1. Larger, less quadratic carapace
2. Larger, more anteriorly situated compound eyes
3. Larger and more prominent ocellar mound
4. Larger swimming feet
5. Possibly a more compact, more rapidly contracting preabdomen

E. lacustris

Neanic stage [pl. 11, fig. 1]

1. Larger size of compound eyes
2. Possibly more compact form of preabdomen

E. maria

Nepionic and neanic stages [pl. 21, fig. 1–11]

1. Excessively short and wide, subtriangular carapace
2. Very large, anteriorly situated diverging compound eyes
3. Rapid contraction of abdomen
4. Short, wide preabdominal segments
5. Larger swimming feet

Eusarcus scorpionis

Neanic stage [pl. 36, fig. 1]

1. Wider preabdomen
2. Wider postabdomen

Eusarcus (?) cicerops

Nepionic stage [pl. 36, fig. 9]

1. Carapace triangular with
   2. Spinelike prolongations of frontal and genal angles
   4. Immense, diverging eyes
   6. High median crest or glabella
   8. Broad, thick border

Pterygotus macrophthalmus

Neanic stage [pl. 69, fig. 3, 5]

1. Larger compound eyes
2. Larger ocelli and more prominent ocellar mound
3. More rapid contraction of preabdomen
4. Larger swimming feet
5. More spatulate telson

Pterygotus globiceps

Nepionic and neanic stages [pl. 81, fig. 1–5]

1. Very large carapace and very small abdomen
2. Smaller number of abdominal segments
   The larval eyes are in this form relatively not larger than in the adult, but provided with a prominent ring (orbital ridge)

Hughmilleria shawangunk

Nepionic and neanic stages [pl. 64, fig. 1–13]

1. Larger (longer and broader) carapace
2. Much larger compound eyes
3. Larger and more prominent ocellar mound
4. More slender abdomen
5. Smaller number of (preabdominal) segments

Stylonurus myops

Nepionic and neanic stages [pl. 51, fig. 1–13]

1. Larger carapace
2. Broad, flat border of carapace
3. Distinct glabella
4. Larger eyes (very large ocular nodes)
5. Eyes nearer to margin
6. Segments not differentiated
7. Smaller number of segments
8. Smaller telson
9. No distinct ornamentation

These lists of ontogenetic characters permit the following general inferences:

1 The carapace is relatively larger in the larval stages; often very much so. This is especially marked in the representatives of Pterygotus, Hughmilleria and Stylonurus. It is also recognizable in the young of E. remipes; the few young specimens of E. lacustris are also suggestive of a larger size of the carapace but not conclusively so, since the preabdomen of the specimens is probably accidentally contracted. In E. maria the carapace is distinctly shorter and wider than in the mature stage. This is in accordance with the entire character of this larval stage which in its broad carapace and broad short preabdomen, anterior divergent eyes and rapid contraction of the preabdomen distinctly exhibits the characters of the genus Eusarcus. The young carapace of E. scorpionis indicates no relatively larger size although the single individual available may be already too far advanced to still exhibit this difference in size.

The shieldlike character of the carapace is sometimes still emphasized by a broad flat margin, as in Stylonurus and Eusarcus (?) cicerops.

2 The compound eyes are relatively much larger than in the mature stage; and sometimes borne on more prominent ocular nodes. The larger size of the compound eyes is a prominent feature of all very young stages here investigated and seems to be a general ontogenetic character. In one species, Pterygotus globiceps, the eyes are not notably larger than in the ephebic individual, but in this case they are very large at maturity and obviously a retained larval character. In Eurypterus maria, Pterygotus macrophthalmus and Hughmilleria shawangunk the difference in size is especially notable. In Stylonurus myops the visual area exhibits little difference in relative size, but the ocular nodes are so extended that they occupy nearly the entire dorsal side of the carapace within the flat border.

3 The compound eyes are nearer to the margin; either the anterior margin (Eurypterus), or the lateral (Stylonurus myops). In Pterygotus and Hughmilleria they are already marginal in the nepionic stage and remain so to the ephebic stage.

4 The ocellar mound or tumescence exhibits a distinct tendency to both larger size and greater prominence in the earlier growth stages than in the adult. This ontogenetic feature has been especially noted in Eurypterus remipes, E. maria, Pterygotus macrophthalmus and Hughmilleria shawangunk. There is also evidence that the ocelli themselves are relatively larger.

5 The swimming legs are, as a rule, larger. This has been noted in Eurypterus remipes, E. maria and Pterygotus buffaloensis. The great majority of all larval individuals have lost the appendages. The specimen reproduced on plate 21, figure 5, shows the relative great size and width of the swimming legs in E. maria.

Of much interest in this connection is a larval specimen of Stylonurus myops exhibiting the metastoma in fine preservation [pl. 51, fig. 6]. The latter is of immense size, almost equaling the carapace in length. As the metastoma is correlated in size to the coxae of the swimming legs it would follow that the latter must have been enormously developed in this species and the larvae highly agile. The probable activity of the larvae is further suggested by the enormously bulging eyes.

6 The abdomen lacks distinct differentiation into pre- and postabdomen. This condition is made clear by the nearly uniform length of all segments. In width there prevails considerable difference, most early growth stages appearing to contract more rapidly than the mature individuals, while in Hughmilleria the body is distinctly more slender [pl. 64, fig. 1]. In Eurypterus remipes, E. lacustris and especially in E. maria, as well as in Pterygotus buffaloensis there is evidence of a relatively broader and shorter preabdomen, although the more compact appearance of the preabdomen may be partly due to casual contraction after death.

7 The number of segments is less in the nepionic stage than in the following stages. In some nepionic individuals there are not more than six segments or half the number normal to the adult [see Stylonurus myops, pl. 51, fig. 1]. On account of the smallness of the earliest growth stages and the lack of differentiation of the segments we have not been able to satisfy ourselves as to whether the preabdomen or the postabdomen is first completed. The larva of Hughmilleria shawangunk [pl. 64, fig. 1] where a distinct preabdomen with four segments and a postabdomen of six segments are noticeable, and some other specimens, suggest that the latter was first completed.

8 The telson spine may have been less developed than in adults; at least such a condition is indicated by several specimens. Unfortunately and singularly the telson is nearly always missing in young specimens. In the embryo of Limulus there is no tail spine and in the larval stage the telson is small.

It is now interesting to inquire which of these ontogenetic characters are purely larval acquisitions, and which are palingenetic, or of phylogenetic significance as representing the recapitulation of ancestral characters. Comparison with the larval stages of Limulus has been of much assistance in discerning between these coenogenetic and palingenetic structures.

The larger size of the carapace we consider as a merely larval character, for the following reasons: This larger size is principally due to the lack of development of the abdomen in the earlier stages and corresponds to a like antecedence in development of the cephalothorax in Limulus especially manifest in the embryological stages. From geological evidence and theoretical reasons, just the reverse is to be postulated in the phylogeny of the eurypterids, i.e. an earlier weaker development of the cephalothorax. The greater size of the carapace in such trilobites as Harpes is considered as a larval character retained to maturity, but this is no evidence of the primitive character of the genus. Likewise eurypterids with large carapaces, as certain species of Eurypterus, are not at all to be considered as primitive. The philosophy and phylogeny of the arthropods agree in postulating a gradual growth and consolidation of a greater number of segments into the cephalothorax. The smaller cephalothorax is therefore to be considered as the more primitive and the evidence from such eurypterids as the Cambric Strabops and the simpler species of Eurypterus, is not contradictory to this conclusion. Altogether the remarkably small size of the cephalothorax of the eurypterids in comparison with the abdomen is to be considered as one of their most striking primitive characters.

The larger size of the larval eyes is regarded by us as a larval character without phylogenetic significance. It finds its striking analogy in the megalops stage of the crustaceans, well shown in the common crab Callinectes [see text fig. 23].
Figure 23 Megalops-stage of the common crab (Callinectes hastatus). (From Brooks) In this stage the eyes are not only of excessive size, but also situated at the ends of very movable stalks. In the larvae of the eurypterids the lateral eyes were not only of large size but also borne on very high and large ocular nodes (particularly well shown in Stylonurus myops), giving them a like prominence. None of these features is observable in either the larvae of Limulus or of the scorpion and we have probably to regard them as larval features peculiar to the young of the eurypterids.

The closer approximation of the lateral eyes to the margins in the young than in the adult eurypterids is a feature shared to some extent by young Limuli and still more distinctly shown by the embryo of the scorpion [Laurie, 1890, pl. 17, fig. 45]. In Limulus the lateral eyes and ocelli are placed on distinct ridges, termed by Packard [1870, p. 168] the ophthalmic ridges, which separate the cardio-ophthalmic region from the lateral regions of the carapace. These ridges are most prominent in the larva and become obscured in the adult. They are faintly seen in the later embryonic stages and are there distinctly farther away from the median line of the carapace. The ocelli appear first on the under side of the head [Packard, p. 168] just in advance of the chelicerae and in later embryonic life wander to the upper side.

Strabops of the Cambric has the lateral eyes in about the position where they are seen in nearly all our larval eurypterids; E. megalops (Frankfort) and the Clinton E. prominens as well as the primitive Hughmilleria also have submarginal eyes, and we see in this approximation of the eyes to the margin a palingenetic feature which probably points to ancestors with submarginal eyes. In the Frankfort Lower Siluric fauna taken as a whole, the eyes are nearer the margin, a noteworthy fact in this connection. The genera Pterygotus, Slimonia and Eusarcus make a nearer approach to the primitive condition in this respect than Stylonurus and those species of Eurypterus with approximate eyes high up on the carapace. A comparison in point is afforded by the trilobites, in which, according to Beecher's excellent investigations, the eyes appear on the under side in both the ontogenetic and phylogenetic development.

The prominence and great size of the ocellar mound in the larvae of the eurypterids are fully in consonance in function with the early appearance and relative great size of the ocelli or "larval eyes" in the embryo and larva of Limulus and the great size of the central eye in the scorpion. There is no direct evidence to indicate that the prominence of the ocellar mound is of any other than larval or coenogenetic character.

In the primitive trilobite Harpes the ocelli remain functional through life and no compound eyes are developed; we may infer that the early appearance and strong development of the ocelli in the larvae of both Limulus and the scorpion are, in view of their retention in later life, with diminished force, rather suggestive of their having been the first eyes of the ancestral forms.

As the swimming legs are clearly a secondary acquirement their larger size and probably that of the other legs also, is a purely larval feature in

Figure 24	Dorsal and ventral views of last larval stage of Limulus before the appearance of the telson. l, liver tubule; I, chelicera; op, operculum. (From Kingsley)

line with the greater size of the lateral eyes, both of these being coenogenetic larval characters developed in connection with the greater agility of the larva. The larvae of Limulus [see text fig. 24] possess relatively longer legs than the adults, and this is especially notable in the last pair that projects considerably beyond the carapace. A still better analogy is found in the lively megalops stage of the crab [see text fig. 23] where immense eyes and long legs are correlated features.

The lack of differentiation of the preabdominal and postabdominal segments is likewise shown in the embryo and larva of Limulus, while in that of the scorpion the two regions are distinguishable almost as soon as segmentation sets in, obviously owing to the very pronounced differentiation between the broad preabdomen and the narrow taillike postabdomen in the adult. In the Cambric Strabops there is less differentiation observable between the two abdominal regions than in any later eurypterid and the more primitive of these later forms distinguish themselves by a more uniform and gradual contraction of the abdomen. It is a wholly proper assumption that the undifferentiated condition is the more primitive. The lack of differentiation of the abdomen in the larvae is hence to be interpreted as an inherited palingenetic feature.

The smaller number of segments in the nepionic stage is, for theoretical and observational reasons, to be considered as a purely larval feature incidental to the growth of the organism. The Cambric Strabops possesses the same number of segments as the Upper Siluric or Carbonic eurypterids and if any change in the number of segments in the development of the higher Arthropoda has taken place, it has generally been a reduction. The trilobites with the smaller number of segments in the immature stages and the greater number of segments in mature conditions of the earlier and more primitive species such as Paradoxides and Harpes, furnish an excellent analogy.

The smaller size of the telson is in accordance with the ontogeny of Limulus and is manifestly a palingenetic character indicating the primitive condition. This view is supported by the short, blunt telson of Strabops and the primitive later eurypterids, such as Hughmilleria.

In summing up the larval characters observed in these immature eurypterids we consider as coenogenetic or purely larval the relatively larger size of the carapace, of the compound eyes and of the swimming legs, and the smaller number of the abdominal segments; as palingenetic and phylogenetic, the approximation of the compound eyes to the margin, the prominence of the ocelli and their tumescences, the lack of differentiation of the abdomen and the smaller size of the telson. In these palingenetic characters the nepionic stage resembles so much the Cambric Strabops, that we shall designate this as the Strabops stage. We shall use these characters in the reconstruction of the primitive ancestor of the eurypterids in the next chapter and the discussion of their taxonomic position.

In reference to this taxonomic position it may be stated here that the ontogeny of the eurypterids fully corresponds to that of Limulus in lacking any indication of a nauplius or zoea stage; that the larvae of the eurypterids (nepionic stage) have in common with that of Limulus:

1. The large size of the carapace
2. The broad border of the latter (in Stylonurus myops)
3. The prominent cardiac region of the carapace, giving it a trilobate character (in Stylonurus myops, Eusarcus (?) cicerops, Pterygotus globiceps)
4. The lack of differentiation of the abdominal segments
5. The undeveloped telson.

They differ from it in:

1. The terete abdomen
2. Megaloptic development of the lateral eyes, and
3. Their nearer approach to the margin
4. Having but nine abdominal segments, while Limulus is hatched with the full number of segments.

The meaning of these agreements and differences in the development of the eurypterids is discussed more advantageously in the chapter on the taxonomic relations of the eurypterids where also the ontogeny of the eurypterids is compared with that of the scorpion.

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1. Henry Woodward described and figured [1872, p. 79, pl. 16, fig. 10, 11] as egg packets (Parka decipiens) of Pterygotus ludensis, masses of small, oval carbonaceous bodies of frequent occurrence in the basal Old Red sandstone. Dawson and Penhallow [1891] after a careful study of these bodies, concluded that they are sporocarps, filled with sporangia, of an aquatic plant. The Bertie waterlime and the shale layers of the Shawangunk grit have both furnished circular to oval carbonaceous bodies that suggest or are comparable to the eggs of Limulus.