Deuterostomia I: Origins and Ambulacraria


Living Members: Living deuterostomes were first identified by "deuterostomous" developmental characters: This identification had little to do with their outward morphology. Indeed, the major groups are an odd assortment:


Cyprinius carpio
Chordata: (C. - Rec.) Including


Leopard sea cucumber Bohadschia argus
Echinodermata: (C. - Rec.) Including


Acorn worm from The Telegraph
Hemichordata: (C. - Rec.) Including

Yet wonderfully, the illumination of Cambrian fossils on the stems of each of these groups has provided significant, if imperfect resolution. Of particular importance:


Vetulicolians by Skelefrog from Deviant Art
Vetulicolia : Known from Chengjiang and Sirius Passet. These animals encompass a range of shapes but have these characteristics in common:

The consensus interpretation (see Vinter et al., 2011) is that:

Vetulicolians (along with a few other oddballs) are either:


Deuterostomia:

(Cambrian - Rec.) Whatever vetulicolians are, exactly, their morphology illuminates the ancestral state of deuterostomes. Vertebrate paleontologists have long predicted that the ancestral vertebrate would be, in essence, a swimming pharynx. This seems to describe deuterostomes in general. This information enables us to propose morphological synapomorphies of Deuterostomia: As we will see, this interpretation meshes nicely with other emerging patterns in deuterostome evolution.

Note: Literature older than ten years has been rendered obsolete by molecular analyses that dispelled once and for all the notions the other creatures with three-part coeloms, including brachiopods, phoronids, and chaetognaths (arrow worms) might also be deuterostomes.

Why we care:

We will spend considerable time on these two major deuterostome groups. First, let's put them in their context.

Deuterostome phylogeny is characterized by two major groups:



Tornaria (left) and auricularia (right) larvae from University of Saskatchewan

Ambulacraria:

Recent molecular phylogenies indicate sister taxon relationship between Echinodermata and Hemichordata. Most members are suspension or deposit feeders, although some obtain food in more interesting ways.

Potential synapomorphies of hemichordates and echinoderms:



From Pinterest.com

Hemichordata:

(Cam - Rec.) Solitary or colonial suspension and deposit feeders who use ciliated appendages to concentrate particle-rich water in pharynx, where it is filtered.

Characteristics:



Enteropneust from Filmati di Mare
Diversity:
Enteropneusta: "Acorn worms": (Middle Cambrian - Quaternary)



Rhabdopleura from Physical and Biological Sciences, U. C. Santa Cruz
Pterobranchia: (Spotty record, Cambrian - Rec, excluding graptolites)

Characteristics:


Reflect. In the year 2000, conventional wisdom held that hemichordates shared the synapomorphy of the pharynx and pharyngeal openings with chordates, but not with echinoderms, and so were viewed as more closely related to chordates. But molecular analyses recovered a strong grouping of hemichordates with echinoderms. Now that we have vetulicolians, we can see that the pharynx is, in fact, a plesiomorphy that has been (apomorphically) lost in echinoderms.)

There was another big surprise about this time.


from Paleobiology and the Fossil Record - Blackwell Publishing

Graptolithina:

(Cam. - Carb.) first known as enigmatic fossils of the Early Paleozoic. Typically compressed into two dimensions and displaying a geometric regularity that gave them the common name "graptolite" - "writing stone."

Observations:


Graptolite diversity: There are two major groups. (We spare you the minor groups.)


The record of well-known hemichordates is unsatisfactory in that it doesn't seem to record transitions between:

Recent discoveries and reinterpretations are beginning to fill those gaps.


Herpetogaster collinsi from Wikipedia
Caron et al., 2010: Described Herpetogaster collinsi (Cambrian, right) from the Burgess Shale, a macroscopic (1 - 2 cm total length) soft bodied creature that resembled a solitary pterobranch with: Caron et al. unite Herpetogaster with a few Burgess Shale problematica in Cambroernida, which they interpret as basal deuterostomes. Herpetogaster certainly looks like what one would expect of a creature on the branch leading from the common ancestor of Hemichordata toward pterobranchs. Some other cambroernids like Eldonia are harder to understand.

Another further step may be represented by Oesia disjuncta of the Burgess Shale (Nanglu et al., 2016.) In this case, the animal looks like an enteropneust, but appears to have secreted a protective proteinaceous tube, and has a posterior "grasping organ" that might represent the first representation of the pterobranch stalk.

Yunnanozoans: A final fossil group from Chengjiang (Early Cambrian), these creatures (including Yunnanozoon (right) and Haikouella) were described in the 1990s and optimistically regarded as stem chordates or even craniates (i.e. "fish"). Recent reinterpretation by Shu et al. suggests something more interesting - that they were stem hemichordates (basal to the enteropneust/pterobranch split) with distinct collars, pharynges, and proboscis, capable os some swimming. Opinion remains divided, with Mallatt and Chen, 2003 maintaining that these are stem chordates. Nevertheless, the closer we get to the stem of Ambulacraria, the more we expect to find members whose overall form resembles that of vetulicolians.

Echinodermata:


Stereom from U C Berkeley Museum of Paleontology
Characteristics:


Echinoderm Systematics

Traditionally all stemmed echinoderms were lumped into "Pelmatozoa", and all non-stemmed to "Eleutherozoa". Only the latter is monophyletic. A more cladistic phylogeny of Echinodermata appears at right.

Major Groups:

Early echinoderms represent a strange assemblage of experiments with different body forms. First, we survey the range of diversity, then try to make sense of it.


Helicoplacus guthi from Palaeos
Helicoplacoidea (Early Cam.) (Reconstruction)



Foerstediscus splendens
Edrioasteroidea: (Cam. - Late Carb.)



Gogia spiralis
"Eocrinoids": (Cam - Sil.)



Pentremites pyriformis from Humboldt State University Natural History Museum
Blastozoa:


Castericystis vali

Homolazoa:

(Cam. - Dev.)

"Come with me and you'll be in a world of pure imagination..."

Going farther toward the base of the echinoderm tree you wold expect to find creatures that connect them to other deuterostomes. Instead, things get just ugly. Homalozoa is a problematic group of Early Paleozoic echinoderms. True apples of discord with:


What we DO know:


The following groups have been regarded as "members" of "Homalozoa":


Stylophorans from Palaeos
Stylophora (Cambrian - Devonian). With: At most the creatures only approach being bilaterally symmetrical, and often there is nothing like an obvious plane of symmetry. Comprised of two groups:


Syringocrinus from Palaeos
Soluta (Cambrian - Devonian). Similar to Stylophora but with:


Trochocystites from Eternal Menagerie
Cincta (Middle Cambrian). Again, similar but with:


Ctenocystis from Palaeos
Ctenocystoidea (Cambrian - Ordovician). Strange. No stele or aulacophore. Only an approximation of bilateral symmetry, BUT... Difficult to interpret ecology. These seem to have rested on the bottom with no way to elevate their ambulacra into the water column. Possibly deposit feeders, sifting through soft sediment for food. No indication that they were attached to the substrate, but not obvious how they would have moved around either.

Troubles:

I have only described "homaozoan" features and named a few. No homologies with other organisms have been proposed. This is where the trouble starts. Consider the stylophoran stele. It could be:

Similar things could be said about any of the openings of the theca, which could be mouths, anuses, pharyngeal slits, hypropores, etc. Into this chasm of ignorance steps the human imagination.

During the 1980s, Richard Jeffries of the British Museum interpreted the various homalozoans as ancestral to the vertebrates (making vertebrates a polyphyletic group within Echinodermata). This hypothesis (sort of) rests on his convictions about the homologies of the structures. Consider the stylophoran stele.

Jeffries emphatically views it as a chordate-like tail, with room inside for a notochord and myotomes. He also claims to see pharyngeal slits in the openings of the theca. In some ways, these appendages function differently. For instance, according to Jeffries, the "tail" is used to pull the theca along over the substrate.

His conclusion: Chordates are derived from these primitive echinoderms. Indeed, in his scheme, specific homalozoans gave rise to specific chordate groups. To emphasize the propinquity of the relationship, he coined the term Calcichordate. This hypothesis of "calcichordate" phylogeny was developed in the early days of cladistics, and Jeffries does not seem to have used a parsimony analysis.

Objections to his scheme include:

When you add "calcichordates" to the mix, the basic pattern of deuterostome phylogeny seems completely up for grabs.

As of 2011, echinoderm systematists agreed that Jeffries' phylogeny is wrong. But were all over the map otherwise. Some maintained that his assessment of homology may, in part, be right. The Clausen and Smith, 2005 analysis of the stylophoran appendage suggests that it is, indeed, a locomotor appendage. Others, such as David et al., 2000, asserted that the stylophoran appendage is an ambulacrum on a crinoid-like arm, pure and simple, and that Ctenocystoidea have blastozoan-like brachioles. To them, Homalozoa is polyphyletic and its members belong to better known groups.

In the last five years some illumination came from new fossils:


Ctenoimbricata spinosa
Zamora et al. 2012: Described Ctenoimbricata spinosa (Early Cambrian, right), roughly the size and proportions of a ctenocystoid but with two important differences: Ctenoimbricata is the only known creature with: Cool. It also seems to resolve the question of how echinoderms fed ancestrally. Its ambulacra are invested with small plates that seem to have enabled it to sift through deposits. See restoration.

But was that all? Rahman et al., 2015 modeled the feeding behavior of the cinctan Protocinctus mansillaensis to determine that its feeding apparatus was ineffective unless water was actively propelled through its oral/pharyngeal cavity by ciliary action.


Helicocystis moroccoensis from slate.com
What about the rise of five-part symmetry and the gap between helicoplacoids and edrioasteroids? Smith and Zamora, 2013: Described Helicocystis moroccoensis (Early Middle Cambrian, right), the size and shape of a helicoplacoid but with two important differences:

Now, finally, a coherent speculative picture of early echinoderm evolution is emerging. From a vetulicolian-like "swimming pharynx" ancestor, one can picture the evolution of a deposit-feeding ancestral ambulacrarian with an anterior ciliated feeding appendage for concentrating food that that give rise to:

Now our great wish is for a clearer picture of the evolution of Eleuthrozoa - the non-stalked motile echinoderms.
We turn to crown-group Echinodermata in the next lecture.

Additional reading:

To Next Lecture.
To Previous Lecture.
To Syllabus.