The ID Update

Soft-bodied worms from the Burgess Shale fossil beds in Yoho National Park, British Columbia, Canada have been known form over 100 years. They are known by the name Spartobranchus tenuis and are considered one of the most abundant species in the Walcott Quarry community. However, only recently has a detailed examination of their characteristics been made, leading to the conclusion that they are ancient examples of acorn worms. One member of the research team was Christopher Cameron from the University of Montreal, who studies modern-day acorn worms. Identifying the fossils was not problematic: "Spartobranchus is clearly an acorn worm," he said in an interview. "It's almost like someone took a picture of a modern-day animal - it's absolutely astonishing." These animals are not true worms but belong to a very different phylum: the Hemichordates. The name means "half chordate" indicating that hemichordates share some of the characters associated with chordates. This is relevant to some of the discussion below. Hemichordates comprise three classes: the Enteropneusta or acorn worms; the Pterobranchia - minute colonial tube-dwelling organisms; and the Graptolithina or graptolites. Pterobranchs were already known to be part of the Cambrian Explosion, so the inclusion of acorn worms in this flowering of animals is a significant addition to knowledge. According to Henry Gee in Nature:

"Before this report, the earliest-known fossil enteropneusts were Triassic, between 250 million years and 200 million years old. That Caron et al. extend the fossil record of enteropneusts back to the Cambrian period makes their findings notable, even with no other contribution."

Modern acorn worms live underwater in mud and fine sand (source here)

The find is notable because it reinforces the evidences for stasis in the fossil record. This deserves highlighting because so many think that the fossil record documents Darwinian gradualism - which it stubbornly fails to do. It is notable because it reinforces the phenomenon of the Cambrian Explosion, showing that those who try to fit the evidence into a Darwinian framework are shoehorning data and resistant to the testing of their cherished theory. From the abstract:

"The presence of both enteropneusts and pterobranchs in Middle Cambrian strata, suggests that hemichordates originated at the onset of the Cambrian explosion."

Notwithstanding all this, the commentary on the research has a very strong evolutionary emphasis. Firstly, the fossil acorn worm is presented as an important transitional form.

"According to Dr Cameron and colleagues, Spartobranchus tenuis reveals a crucial evolutionary link between two distinct living groups of animals: enteropneusts and pterobranchs."
"Enteropneusts look very different from pterobranchs in as much as the former are worms, whereas the latter are tiny animals, live in tubes, are colonial and have feeding tentacles," explained Professor Simon Conway Morris from the University of Cambridge, UK, who also worked on the study. [. . .] "By finding enteropneusts in tubes we begin to bridge this evolutionary gap," Dr Morris told BBC Nature. Dr Cameron commented: "It's astonishing how similar Spartobranchus tenuis fossils are to modern acorn worms, except that they also formed fibrous tubes." In many of the fossils, remains of the worms were encased in these tubes, and the team believe they lived in the structures and may have even sealed themselves in a doughnut shape within the tubes. (Source here)

The nature of these tubes is clearly important. Gee refers to "pterobranch-like tubes" and considers this an unambiguous witness to transformation. However, the research paper gives more insight into just how "pterobranch-like" the tubes are. It says that approximately one-quarter of specimens are associated with tubes that have a fibrous composition, and that only one individual is ever associated with a tube. There is no evidence that the worms were attached to the tubes, so they were able to leave them to feed or in response to stress. Apart from the use of the word "fibrous", the only other information on the construction of the tubes is that they are sometimes branched and sometimes show tearing. Whilst the researchers have made a case for an evolutionary link, there seems to be scope for weighing these evidences and considering alternatives.

"Two lines of evidence suggest that the collagenous coenecium of pterobranchs had origins in the organic tubes of S. tenuis. First, the tubes of S. tenuis have a fibrous structure that sometimes shows evidence of tearing. We have no evidence of characteristic growth bands (fuselli), a pterobranch innovation that probably appeared with a reduction in size and the evolution of a shield-shaped secretory organ. Second, the secretory proboscis and cephalic shield are homologous in enteropneust and pterobranchs (both protosoma), and thus the transition from inferred involvement of the proboscis in tube construction by S. tenuis to the secretory role of the cephalic shield is unproblematic. We suggest that in the lineage leading to the Enteropneusta, the tube lining was lost, whereas in pterobranchs it provided a substrate for an astonishing radiation, not least in Palaeozoic planktonic ecosystems."

The scenario these researchers favour is that the common ancestor had a capability for tube construction that was present in the early enteropneusts like S. tenuis but then lost. This means that, if there was a common ancestor, the biological information for tube construction was already present. Descendants either exploited this information (the pterobranchs) or lost it (the enteropneusts). It is worth considering what form of evolutionary change this implies. We have novel biological information in the inferred ancestor, modified information in the pterobranchs, and lost information in the enteropneusts. This does not fit the Neo-Darwinian theoretical model because the relevant biological information already existed in the common ancestor. A much better match to this situation is phenotypic plasticity: where radiations of animals and plants utilize existing biological information in varying combinations and permutations. For more on phenotypic plasticity, go here.

The other evolutionary theme concerns the origin of vertebrates. Henry Gee makes this the headline for his News & Views essay: "The discovery, in 500-million-year-old rocks, of fossil acorn worms that lived in tubes illuminates the debate about whether the ancestor of vertebrates was a mobile worm-like animal or a sessile colony dweller." In this debate, the hemichordates are deemed to be of considerable importance.

"Understanding the origin of vertebrates is fascinating not just because it appeases our self-interest, but because it is inherently difficult. Vertebrates seem to have a qualitatively more complex construction than other animals. This means not only that the origin of distinctive features of vertebrate anatomy, such as the head or the neural crest, are difficult to trace among invertebrates, but also that the evolutionary roots of vertebrates as a whole are hard to fathom. In a paper published on Nature's website today, Caron et al. address a fundamental issue in this regard: whether the ancestor of the deuterostomes (the larger assemblage to which vertebrates belong) was a free-living, worm-like creature or a sessile, perhaps colonial, animal. Their answer, for now, is that it was solitary, worm-like and motile."

Since all the major players in this debate (hemichordates, echinoderms and vertebrates) are all found among the Cambrian Explosion fauna, the fossil record cannot be mapped against the Darwinian gradualist branching model. Instead, we have the primacy of a theoretical paradigm about universal common ancestry that presupposes that ancestral relationships must exist and that uses evolutionary cladism as the tool for interpreting evidences (as in Gee's essay. Also, go here for additional comments). Of course, this debate will continue, but it deserves to be pointed out that Meyer et al. (2006), on the contribution made by intelligent design theory to these important issues, continues to be relevant. One is tempted to add that as fossil discoveries accumulate, the case of intelligent design becomes more and more compelling.

Tubicolous enteropneusts from the Cambrian period
Jean-Bernard Caron, Simon Conway Morris and Christopher B. Cameron
Nature, (Online March 2013) | doi:10.1038/nature12017

Hemichordates are a marine group that, apart from one monospecific pelagic larval form, are represented by the vermiform enteropneusts and minute colonial tube-dwelling pterobranchs. Together with echinoderms, they comprise the clade Ambulacraria. Despite their restricted diversity, hemichordates provide important insights into early deuterostome evolution, notably because of their pharyngeal gill slits. Hemichordate phylogeny has long remained problematic, not least because the nature of any transitional form that might serve to link the anatomically disparate enteropneusts and pterobranchs is conjectural. Hence, inter-relationships have also remained controversial. For example, pterobranchs have sometimes been compared to ancestral echinoderms. Molecular data identify enteropneusts as paraphyletic, and harrimaniids as the sister group of pterobranchs. Recent molecular phylogenies suggest that enteropneusts are probably basal within hemichordates, contrary to previous views, but otherwise provide little guidance as to the nature of the primitive hemichordate. In addition, the hemichordate fossil record is almost entirely restricted to peridermal skeletons of pterobranchs, notably graptolites. Owing to their low preservational potentials, fossil enteropneusts are exceedingly rare, and throw no light on either hemichordate phylogeny or the proposed harrimaniid-pterobranch transition. Here we describe an enteropneust, Spartobranchus tenuis (Walcott, 1911), from the Middle Cambrian-period (Series 3, Stage 5) Burgess Shale. It is remarkably similar to the extant harrimaniids, but differs from all known enteropneusts in that it is associated with a fibrous tube that is sometimes branched. We suggest that this is the precursor of the pterobranch periderm, and supports the hypothesis that pterobranchs are miniaturized and derived from an enteropneust-like worm. It also shows that the periderm was acquired before size reduction and acquisition of feeding tentacles, and that coloniality emerged through aggregation of individuals, perhaps similar to the Cambrian rhabdopleurid Fasciculitubus. The presence of both enteropneusts and pterobranchs in Middle Cambrian strata, suggests that hemichordates originated at the onset of the Cambrian explosion.

See also:

Gee, H. Tubular worms from the Burgess Shale, Nature, (Online March 2013) | doi:10.1038/nature11963

Over recent years, researchers have developed the scenario of fleas feasting on pterosaurs and feathered dinosaurs during the Jurassic Period of Earth history. Fossils of strashilid insects have been suggested to have piercing and sucking mouthparts and hind pincers suitable for attaching to a host's hairs or feathers. Further reading on this interpretation is to be found in Rasnitsyn (1992) and Gao et al. (2012). Other putative flea species have also been found and Poinar's (2012) perspective paper has the title: "the 165-million-year itch" (see also here).
A new interpretation of the strashilids has now been proposed after examining spectacular recent finds from the Daohugou beds, Inner Mongolia. The researchers have concluded that the fossils are the remains of flies, not fleas. They were not ectoparasites living on animals, but highly specialized flies adapted to aquatic environments.

"Although definitive Middle Jurassic ectoparasites (fleas) on vertebrates were reported recently from the Daohugou outcrops, earlier speculation regarding strashilids as terrestrial ectoparasites can be rejected owing to an alternative hypothesis stemming from the discovery of hitherto unknown large membranous wings and females of Strashila, in two cases preserved in copula. Males and females have identical head morphologies but differ strongly in the absence of the hind leg pincers in females, which excludes ectoparasitism on terrestrial vertebrates as these are reinterpreted as sex-related structures for grasping the female rather than a host's integument. In addition, unlike ectoparasitic insect lineages, the body is cylindrical in strashilids, rather than dorsoventrally or laterally flattened." (page 94, Huang et al. 2013)

From Poinar (2012): "(A) Reconstruction of the 150 million year old Jurassic flea-like Strashila incredibilis. Note the incredibly long and powerful hind legs clasping the base of two feathers on a feathered dinosaur and a well-developed proboscis. Strashila may have fed on pterosaurs or feathered dinosaurs. Drawing by the author. (B ) Reconstruction of the Chinese Jurassic pseudoflea, Pseudopulex jurassicus, imagined parasitizing a feathered dinosaur." (Source here)

It is worth noting that our understanding of strashilids has changed as a result of finding well preserved specimens. Previous work was based on meagre fossil evidence and was inherently speculative. It is possible to generalise the lessons that should be learned from this: when fossil evidence is sparse, it is unwise to speculate. Unfortunately, the pressures to publicise finds are substantial and are unlikely to diminish. The media is always looking for stories that create interest, and speculation appears to be preferred above caution.

It is also worth noting that the new interpretation of strashilids locates them in a more familiar context. The researchers note characters that are "virtually identical with those of the recent nematoceran flies", are "comparable in critical details with those of nymphomyiids", and "similar to those of nymphomyiids". "We conclude that the Strashilidae were a highly specialized dipteran clade related to Nymphomyiidae, with an aquatic or amphibious life history." This paragraph summarises the evidence:

"These structures provide evidence that strashilids share several potential apomorphies with modern nymphomyiid flies, including the finer details of the antennae, ocelli, wings, terminalia and legs. Their reduced mouthparts and wings are inefficient for feeding and active flight, respectively, and indicate an ephemeral life history, akin to modern nymphomyiid taxa. Similar to modern nymphomyiids, adult Strashilidae probably mated soon after emergence, shed their wings, crawled beneath the water, mated, and in some cases died in copula, with the superposing male grasping the female below, thereby explaining the discoveries of females grasped by males, and both without their wings." (page 96)

It is not without significance that the initial interpretation of the fossil strashilids considered them to be primitive flea-like insects. There were hints from some characters that made sense in the perspective provided by evolutionary theory. However, what we can now see is a highly specialized insect related to Nymphomyiidae. It is derived rather than primitive. And this specialised insect, similar to modern nymphomyiids, was part of the Jurassic world. Is it possible that the Darwinian mindset colours the interpretations placed on fossilised species? Before this thought is dismissed out of hand, the case of the strashilids serves as a salutary case study to ponder.

Amphibious flies and paedomorphism in the Jurassic period
Diying Huang, Andre Nel, Chenyang Cai, Qibin Lin & Michael S. Engel
Nature, 495, 94-97 (07 March 2013) | doi:10.1038/nature11898

Abstract: The species of the Strashilidae (strashilids) have been the most perplexing of fossil insects from the Jurassic period of Russia and China. They have been widely considered to be ectoparasites of pterosaurs or feathered dinosaurs, based on the putative presence of piercing and sucking mouthparts and hind tibio-basitarsal pincers purportedly used to fix onto the host's hairs or feathers. Both the supposed host and parasite occur in the Daohugou beds from the Middle Jurassic epoch of China (approximately 165 million years ago). Here we analyse the morphology of strashilids from the Daohugou beds, and reach markedly different conclusions; namely that strashilids are highly specialized flies (Diptera) bearing large membranous wings, with substantial sexual dimorphism of the hind legs and abdominal extensions. The idea that they belong to an extinct order is unsupported, and the lineage can be placed within the true flies. In terms of major morphological and inferred behavioural features, strashilids resemble the recent (extant) and relict members of the aquatic fly family Nymphomyiidae. Their ontogeny are distinguished by the persistence in adult males of larval abdominal respiratory gills, representing a unique case of paedomorphism among endopterygote insects. Adult strashilids were probably aquatic or amphibious, shedding their wings after emergence and mating in the water.