Computer-generated reconstruction of the distribution of land masses in the Middle Cambrian. Oblique polar view, showing the component parts of Gondwana and Baltica that became Maritime North America and Europe after the assembly and subsequent breakup of the supercontinent Pangea. These components are shown with present-day coastline contours, to facilitate recognition. The actual contours of the Middle Cambrian coastlines may have differed in details from the present configuration. This illustration was adapted from a custom-generated map kindly provided by Christopher R. Scotese, Paleomap Project, University of Texas at Arlington.
In the following days, determined to explore further the content of the strata that yielded my first Paradoxides, I started quarrying the shale beds layer by layer. In this first attempt at a stratigraphic survey, I could ascertain that I had, in my first encounter, hit upon by accident on a narrow band of layers that contained an extraordinary abundance of complete exuviae. These giant trilobites, up to one foot long and brightly colored in yellow, orange, and red (the colors of iron oxides coating the carapace), were too large for the average size slab to contain. It is a well known theorem of fossil collecting that the most interesting finds seem always to occur near the edges of the slab being uncovered, and continue beyond. With trilobites this large, this frustration became the norm, and it was a scramble to extract and split the adjoining shale slabs, in the hope of finding the other half of a trilobite, and restore the broken specimen to its former integrity (thanks to the fissility of the shale containing large exuviae, this approach was surprisingly successful). I did note then that my complete Paradoxides had a broad pygidium, expanding distally in a spatulalike, trapezoidal shape (see plate Al). At first I did not make much of it, but then I noticed that other specimens, collected above my favorite layer, were somewhat different, with a pygidium that tapered distally instead of broadening (see plate A2). Back home, I poured over the literature to find out what was known about the local fauna.
The work of many paleontologists, reviewed most recently by Hutchinson (1962), seemed to indicate a close correspondence of the three faunal zones of the Middle Cambrian of the Atlantic province (in ascending order, the Paradoxides bennetti, Paradoxides hicksi, and Paradoxides davidis zones), found at Manuels, with those known from, for example, Wales (Howell 1925). In Scandinavia, a further faunal zone, above the P. davidis, (alias the Ptychagnostus punctuosus Zone) is recognized: that of Paradoxides forchhammeri. In Eastern Newfoundland, however, the trilobites associated with the P. forchhammeri zone have been found (Hutchinson 1962), but not Paradoxides forchhammeri itself. I then compared my findings with the reconstructions and photographs of the Paradoxides of England and
Scandinavia (Angelin 1851-78; Brogger 1878; Lake 1935). Definitely, I concluded, I had been digging in the Paradoxides davidis levels, and many of my trilobites matched perfectly with the description given by Salter in 1863 of the trilobites he had discovered on the impervious cliffs at St. David's, in Wales (see plate A3). In particular, the pygidium of P. davidis does taper and is quite slender. However, the abundant complete exuviae I had found concentrated in a narrow band, all exhibiting the trapezoidally shaped pygidium, seemed at first to match reasonably well the description of Paradoxides forchhammeri Angelin, the trilobite that was missing at Manuels. Although I did realize that if indeed I had stumbled upon this trilobite, it did occur in the wrong place (embedded within the P. davidis zone and not above it), I naively resisted the notion that I might have discovered something new. Trying to fit my findings with known species, and ignoring what stratigraphy was telling me, I presented my giant trilobites in the first edition of the atlas, as possible examples of P. forchhammeri. After all, it is not uncommon for deposited sediments to be turned upside down by tectonic events. At Manuels, however, all evidence points to an undisturbed series of sediments. As it turned out, my initial assumption was in error, I had indeed found something else, something new.
The situation was clarified after another field trip to Manuels, this time accompanied by my friend Jan Bergstrom, who was visiting at Memorial University of Newfoundland. Beating the rainy season, we surveyed the Manuels beds together, this time with a yardstick. We located the top of the P. davidis zone, characterized by an ubiquitous layer of phosphatic limestone, and went down layer by layer, recording the depth and nature of all the trilobites we found. After a depth of eight feet, populated primarily by the classical variety of P. davidis, my anomalous beds appeared again and yielded more of the wide-tailed complete Paradoxides (see plates A4, A5), at the exclusion of the previous kind. This time we ascertained that the layer containing this trilobite was only about sixteen inches thick and that the classical variety reappeared again beneath it, after some thickness of barren strata (see plate A6). Another important finding plate Al
PARADOXIDES Brongniart, 1822.
Paradoxides davidis trape%Opyge Bergstrom and Levi-Setti, Middle Cambrian, east side of Manuels River, Manuels,
Newfoundland (xl. 8). This specimen represents the first encounter by the author with the Paradoxides fauna at Manuels. It was tentatively identified, in the first edition of this atlas, (Levi-Setti 1975, pi. A4) as Paradoxides forchbammeri A n g e l i n. The trapezoidal spatulated pygidium and the diverging last pair of pleurae are a distinctive feature of this trilobite, latet described by Bergstrom and Levi-Setti (1978)
as a new subspecies of P. davidis S alter Print from Kodachrome slide. (RLS coll., id.; now at FMNH.)
Paradoxides davidis davidis S alter, Mi ddl e Cambrian, west side of Manuels River, Manuels,
Newfoundland (x0.65). This large trilobite, originally discovered at St. David's, Wales (see plate A3), represents the ancestral stock (phenotype) of this genus found in Newfoundland. Note the tapered pygidium. This specimen is thought to represent the vestiges of a living trilobite, in contrast to the more commonly found exuviae. (RLS coll., id.)
Cast of the type specimen of Paradoxides davidis davidis Salter, Middle Cambrian, from Porth-y-rhaw, St. Davids, Pembrokeshire, Wales (x0.52), reproduced from Bergstrom and Levi-Setti 1978, pi. 2, fig. 1 (by permission of Geologica et Palaeontologica, Lahnberge, Marburg/Lahn). This specimen is distorted by tectonic shear, however, the similarities with the Newfoundland specimen of the preceding plate can be ascertained. (Original at the Natural History Museum, London, cast kindly supplied by Richard A. Fortey.)
A complete specimen of
Paradoxides davidis trape%opyge
Bergstrom and Levi-Setti, Middle Cambrian, east side of Manuels River, Manuels, Newfoundland (x0.94). The external impression of a hypostome, possibly belonging to the same individual, is superposed to the posterior left side of the thorax. Minor reconstruction of the left librigena was performed with the aid of cast from external impression. This mutant form of P. davidis is found in a narrow band of strata, preceded and followed by strata bearing the ancestral phenotype. Specimen whitened with magnesium oxide. (RLS coll., id.)
Another example of complete exuviae of Paradoxides davidis trape%0pjge Bergstrom and Levi-Setti, extracted in the same vicinity as the specimen in plate A4 (xl.l). It was formerly tentatively identified as P. forchhammeri (Levi-Setti 1975, pi. A7). Both free cheeks are displaced from their original setting; one is overturned. The cranidium is partly crushed, exposing the underlying hypostoma, also diplaced. This specimen was the prize finding of Emile Levi-Setti at age 9, in September 1974, when he was given the chance to quarry a slab of his own. (RLS coll., id.)
Paradoxides davidis davidis Salter, west side of Manuels Rivet, Middle Cambrian, Manuels,
Newfoundl and (x2.3). This juvenile trilobite, representing a late meraspid stage, originates from the lowermost levels of the P. davidis beds at this locality. It exhibits seventeen thoracic segments, versus the norm of twenty for adult individuals. The latter two segments of the thorax, and the attached pygidium, are displaced forward, overlapping the thirteenth to fifteenth segment. The glabella is crushed, exposing the underlying hypostoma. The surface of the entire exuviae is still partially covered by a bright yellow patina of limonite. (RLS coll., id.)
emerged: a different variant to P. davidis appeared in a narrow layer located at about the six-foot level. This trilobite has very short pleural spines and a coarsely granulose carapace (see plate A7), while all other characters matched those of P. davidis.
A picture of successive faunal replacements of P. davidis by mutant forms, intercalated with the reappearance of the ancestral form, began to emerge. To put to rest the previous identification of my wide-tailed beasts with P. forchhammeri, we obtained samples of cranidia and pygidia of this trilobite from collections of Scandinavian type specimens. While the pygidia matched quite closely, the cranidia of the Newfoundland find did lack, as is the case for P. davidis, two cranidial furrows that were present in the true P. forchhammeri. The stratigraphy did not fail. It was time to name our new trilobites as subspecies of P. davidis. The classic type had to be called P. davidis davidis. We did compare the Newfoundland specimens with casts of the British types and we concluded to our satisfaction that indeed they were examples of the same trilobite. We then named the short-spined mutant Paradoxides davidis brevispinus, the wide-tailed mutant Paradoxides davidis trape^opyge, and finally a hybrid occurring at the interface between the P. d. trape^opyge and the P. d. davidis levels as P. d. intermedius (see plate A8). The reconstructions of the three principal subspecies of P. davidis, and that of P. forchhammeri, are shown in figure 19.1 should mention that these assignments were based not simply on the appearance of a selected type specimen, as is often done, but on the basis of a statistical morphometric study of various body parts involving several hundred specimens. The characters of an entire population must be known to assess the range of variability of particular morphological indices (a physicist's input here) and avoid the pitfall of naming a new species or subspecies on the basis of individuals exhibiting characters at the tails of a normal population, but still compatible with it. This meant much collecting, preparing, and measuring trilobite's body parts. Another investigation that I undertook was the determination, by X-ray powder diffraction, of the clay minerals making up the shale at various depths. This could provide some clue regarding changes in the environment that could be related with the abrupt faunal changes that we observed. Indeed, the inferred changes in the depositional environment did correspond to the appearance of our mutants, and also with the repeated reappearance of P. davidis davidis. What was left to be done was to make sense of what we had discovered. It did involve evolution and speciation, in a rather unconventional time sequence, however, since we did find the ancestor at both ends. Can evolution proceed backwards as well as forward? As we discussed at length in our paper (Bergstrom and Levi-Setd 1978), our observations find a natural and convincing interpretation in terms of the evolutionary model of punctuated equilibria, advanced by Eldredge and Gould (1972) and further illustrated by the same authors (Gould and Eldredge 1977). Quoting from these auhors, "The norm for a species during the heyday of its existence as a large population is morphological stasis, minor non-directional fluctuations in form, or minor directional change bearing no relationship to pathways of alteration in subsequent daughter species. In the local stratigraphic section we expect no slow and steady transition, but a break with essentially sudden replacement of ancestor by descendant: this break may record the extinction or migration of a parental species and the immigration of a successful descendant rapidly evolved elsewhere in a small peripheral isolated population." Not only does this prediction of how speciation may appear in the fossil record fit what we have seen at Manuels, we may now add a plausible corollary to the above model: that the replacement of descendant by the ancestor may also occur. In other words, we now have evidence that the mutant species is not always successful and that a local return of the ancestor may wipe out the offspring. Evolution can take steps backwards, sometimes.
Let us examine the scenario that would give rise to our Paradoxides evolutionary drama. Newfoundland at that time was not far off the coast of Spain, along an arc extending to Wales and Maritime North America, as shown in figure 18. The main gene pool of Paradoxides davidis davidis must have populated this extended coastline and, at a deposition time corresponding to the lowest levels explored, it was
Another mutant form of P. davidis at Manuels is represented by P. davidis brevispinus Bergstrom and Levi-Setti. Exuviae of this new subspecies are shown in this plate, characterized by very short pleural spines and enhanced tubercular ornamentation (xl.7). (From Bergstrom and Levi-Setti 1978, pi. 9, fig. 5, by permission of Geologica et Palaeontologica, Lahnberge, Marburg/
Lahn.) Once again, this mutant appears confined to a narrow bed within the layers populated by P. davidis davidis. (RLS coll.; now at FMNH.)
Reconstruction of the principal subspecies of Paradoxides davidis: (a) P. d. davidis, (b) P. d. trape%opyge, (c) P. d. brevispinus. Also shown is a reconstruction of Paradoxides forchhammeri (d). The outline of the thoracic region of the latter is conjectural, hence the interruption in the assumed sequence of twenty tergites. (From Bergstrom and Levi-Setti 1978, by permission of Geologica et Palaeontologica, Lahnberge, Marburg/ Lahn.)
A third mutant form of P. davidis is found at the interface between the bed containing P. davidis davidis and the underlying bed with
P. davidis trapezppyge. It was named P. davidis intermedins, to stress the hybrid character of the pygidium, intermediate between those of the two bordering subspecies. This plate shows nearly complete exuviae of this new subspecies (xl.2). (From Bergstrom and Levi-Setti 1978, pi. 6, fig. 6, by permission of Geologica et Palaeontologica, Lahnberge, Marburg/Lahn). (RLS coll.; now at FMNH.)
certainly present at Manuels. At some time, possibly due to a sudden bathymetric shift, the local population was cut off from the main pool. Mutation and natural selection in the Newfoundland isolate led to the complete dominance of the subspecies P. davidis trapezopyge. Subsequently, due to a reverse bathymetric shift, connection with the main gene pool was reestablished. What we see then is a brief period of interbreeding (P. d. intermedins) until the local mutant was replaced completely by the ancestral ilk. Then a period of stasis intervened, followed by a new episode of isolation, giving rise to P. d. brevispinus. But shortly afterwards (we are always talking millions of years), the ancestor reappears and wipes out once again the unfortunate mutant. The end eventually occurs also for the dominant ancestor, at the top of the P. davidis zone, coincidentally with evidence of widespread volcanic activity. Remarkably, several of the characters developed by the two mutant subspecies, namely the trapezoidal pygidium and the granulose carapace surface, will reappear at a later time, incorporated in a veritable new species, Paradoxides forchhammeri. Were the mutants of P. davidis less fit for survival than the ancestor? Possibly so: an unusually high rate of occurrence of genetic malformations (teratologies) was observed for the mutant subspecies. An example is shown in plate A9.
This concludes the case history, except for the Linnaean connection. In the first edition of the atlas, I reproduced the reconstruction of Carl von Linne's Enthomolithus paradoxus, as given by Angelin (1878). I did remark at that time that a remarkable resemblance did exist between this and our new trilobite Paradoxides davidis trapezopyge. In the following years, Jan Bergstrom discovered that an actual photograph of the original Linnaean specimen had been published by Nathorst (1907). Pursuing the quest for a fresh look at this historical trilobite, Jan Bergstrom finally located the specimen at the Mineralogical Museum of the University of Copenhagen, took pictures, and kindly provided me with the photograph reproduced in plate A10. This trilobite has twenty tergites, as P. davidis does, and indeed a trapezoidal pygidium. Its glabella is crushed, showing the underlying labral plate, as in many of our specimens. The cranidium and free cheeks are displaced toward the thorax, overlapping the first two tergites. All visible characters match quite well those of my first giant Paradoxides, found on my first trip to Manuels on a wet afternoon. Was it P. davidis trapezopyge Bergstrom and Levi-Setti 1978 that I stumbled upon, or was it Enthomolithus paradoxus Linnaeus 1753? This we may never establish beyond doubt. Nevertheless I am left with the feeling of having shared with Linnaeus the experience of our first encounter with the same giant trilobite.
Several malformations (teratologies) affect the exuviae of this example of P. davidis trape%opyge Bergstrom and Levi-Setti (xl.8). (From Bergstrom and Levi-Setti 1978, pi. 9, figs. 1-3, by permission of Geologica et Palaeontologica, Lahnberge, Marburg/Lahn.) The deformities involve the fusion of tergites 1 and 2, an anomalous number of tergites (18 instead of 20), a gross malformation of the fight pleura of tergites 7 through 11, and atrophic left 15th and right 18th pleural spines. (RLS coll., id.; now at FMNH.)
Entomolithus paradoxus Linnaeus (x0.46). This is the trilobite specimen described by Carl von Linne (Linnaeus 1759), now regarded as an example of Paradoxides paradoxissimus (Wahlenberg). The trapezoidal pygidium and the strongly diverging last pair of pleurae and pleural spines suggest, however, closer affinity to P. forchhammeri (among the Swedish Paradoxides only). In this example, the entire cranidium is displaced to cover the first few thoracic segments. A photograph of this specimen was originally published by Nathorst (1907). This is a reproduction from a modern Polaroid picture of the original taken by Jan Bergstrom at the Mineralogical Museum of the University of Copenhagen, Denmark.
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