A reply I was writing in the comments to the previous post on laterals and palatals was getting long, and started to require tying in some related issues. So, after some further expansion, y’all’re getting a full post on approaches to reconstructing the PU “dental spirant” and “palatalized dental spirant”.
I take it as a fact that these were two distinct Proto-Uralic segments, not allophones of anything else. (Honti’s paper I was reviewing in “part one” has a full argument for the skeptic.) Beyond this clarity ends, however, and we enter a battlefield of competing proposals.
To keep track of things in a neutral manner, I shall first set up a temporary notation for these segments: *d₁ for traditional [ð] = Honti’s [ɬ], and *d₂ for trad. [ðʲ] = Honti’s [ɬʲ]. This is quite close to the notation *d, *ď that I have seen used by the more cautious followers of the traditional view. Yet I find simple number subscripts preferrable: these do not imply a palatalized/unpalatalized relationship between the two, which is not a thing that should be taken for granted. This also allows talking about cases where it is not clear which consonant occurred originally: I will use the symbol *dₓ for these.
In addition to the two previous reconstructions, the comment from reader David M. that prompted this post brings up a third:
I’d be interested to learn what you think of the Moscow School proposal that PU *ð, *ðʲ were [rʲ], [lʲ] (I forgot which is supposed to be which)? Unfortunately I don’t have a reference, all I’ve seen is secondary or tertiary literature that takes this for granted.
I don’t think there is any “Moscow School” that seriously proposes those values? I too have only seen the reconstruction only ever covered fairly tangentially. Muscovite scholars doing current work on Proto-Uralic (at minimum, M. Zhivlov and K. Reshetnikov in their recent “Studies in Uralic Vocalism” papers) seem to accept the *ð *ðʲ reconstruction traditionally supported in Finland. As far as I can tell, the *rʲ *lʲ proposal is instead a stray idea going back to Leiden. Frederik Kortlandt explicitly states in The Indo-Uralic verb (2001):
I reconstruct Proto-Uralic palatalized /r′/ and /l′/ instead of Sammallahti’s spirants /d/ and /d′/ because they pattern like resonants and are reflected as *r and *j in Samoyedic and as *l and *l′ in Finno-Ugric (cf. Sammallahti 1988: 485, 511f., 518, 532), cf. also the variation between Proto-Finno-Permic *śülki and Proto-Ugric *sül′ki ‘saliva’.
This is apparently the entire argument he offers on the topic… and it doesn’t help that he skips over the development of both of these consonants to *ð in West Uralic. (A sadly typical example of a poor level of rigor shown in studies on “long-range” language relationships.) But I actually find this idea deserving of a closer look anyway.
The proposals don’t end here, though. Fourth is an idea mainly explored by D. Abondolo: that *d₁ and *d₂ would be the results of a pre-Uralic split from *n and *ń, respectivly.  This idea results from assuming an original fortis : lenis distinction among consonants in the medial position, followed by generalization of the contrast to other positions in various ways. Several further assumptions are required to end up with the PU “fortis” or “lenis” consonants in the correct positions. TBH I find this framework close to ludicrous, but if I’m entertaining Kortlandt’s suggestion, I might as well include this too.
It is quite unclear what this idea implies for the PU values of the consonants though. A straightforward length distinction *n = [nː] versus *d₁ = [n] does not seem plausible, given how the “fortis ex-allophones” (i.e. the PU plain nasals) are found in a wide variety of positions, including word-initially. The lenis members being nasalized approximants such as [ɹ̃], [ȷ̃] or [l̃], [l̃ʲ] might be possible. These also could have been denasalized entirely by PU, leaving traditional [ð], [ðʲ] — or something else.
Fifth: A. Bomhard has suggested the values *d₁ = [tɬ], *d₂ = [tɬʲ], though without any detailed argument on the matter at all.  This, I think, works as a further example on how the exact identity of these consonants has been left so wide open by mainstream Uralistics that outsiders will happily offer their opinions on the matter.
Finally, a “five-and-a-halfth” proposal comes from Janhunen, who accepts *d₁ as [ð], but suggests that a pure palatal spirant [ʝ] might be a more plausible value for *d₂. 
Given such a wide variety of views, it should be evident that we are not bound to any of them. Even entirely new solutions might arise in a closer analysis.
Kortlandt’s “pattern as resonants” claim will be our first step to understanding these consonants. “Pattern” here does not refer to any phonological processes (very few have been reconstructed for Proto-Uralic, yet), but to the phonotactics of these consonants.
Point one: in the CVCV(C(V)) roots of Proto-Uralic, there is distributional asymmetry. The voiceless obstruents *p *t *č *ć *k *s *š *ś all prefer the initial position (some more, some less strongly), while C₂ is usually a sonorant consonant, either of the “dental spirants”, or the “laryngeal”/”velar spirant” *x.  This does not necessarily translate to an aversion of initial sonorants though — *w- is particularly frequent — so this is not yet an argument for the “spirants” being liquids, as much as for them at least not being voiceless. Thus the only suggestions that take a hit here are Honti and Bomhard’s ideas of reconstructing *(t)ɬ *(t)ɬʲ. This might, at first, also sound like evidence in favor of Abondolo’s internal reconstruction — if there wasn’t actually a good number of CVNV roots! 
Looking slightly closer, we find that there is pretty much no reliable evidence for word-initial *d₁-. The same holds for initial *r-, as well as *x- and *ŋ-.
I mentioned *d₁äpd₁ä “spleen” before, reconstructed by Sammallahti as the ancestor of Samic *ðāpðē (as well as Mari *lep, Permic *lop, Hungarian lép, Khanty *leepətnə), but this raises some red flags: this would simultaneously be the only known PU root featuring initial *d₁, and *d₁ preceded by an obstruent. Quite the accident, when the consonant was evidently highly rare! Also even despite being fortified by Scandinavian loans, the entire lexicon of Proto-Samic (Samic being the only Uralic branch where *d₁ is not merged with a more popular consonant such as *l or *t) only contains five examples of *ð-, of which three go back to *d₂ anyway. Perhaps it would be better to assume only one original oddity here and to e.g. reconstruct PU *läpd₁ä, followed by an assimilation *l-ð → *ð-ð in Samic. Other long-range consonant assimilations such as *s-ś → *ś-ś (→ *ć-ć) seem to also have occurred in the subfamily.  It has even been furthermore proposed that the root might be simply *läppɜ and that a derivational *-ðä has been appended in Samic, but AFAIK no such derivational affix exists.
Initial *l-, then, is not particularly abundant, but not rare either; and though the evidence for initial *d₂- is scarce, there are two widespread roots that seem reliable: *d₂ümä “glue”, and *d₂ëmə “bird cherry”,  as well as a few other weaker etyma. Kortlandt’s reconstruction thus seems to hold up so far: the distribution of *d₁ resembles that of *r, while the distribution of *d₂ resembles that of *l. The traditional reconstruction also fares alright. The other proposals seem weaker.
Point two: consonant clusters. Not a lot of evidence is available here, but most of it turns out to support a reconstruction along the lines of Kortlandt’s.
- The cluster *wd₁ is well-establish’d: *käwd₁ə “rope”, *täwd₁ə “full” are widespread and fairly well-behaving roots. A possible third example is *lewd₁ä- “to find”.  The other instances of PU coda *w seem to generally involve a following sonorant: there are examples of *wj, *wl, *wn, *wŋ. The same holds for *j, with good examples of *jw, *jm, *jŋ being found, perhaps also *jr and *jj. Examples of glide + obstruent are however rare to nonexistent (possibly: *kowsə “spruce”?) In this sense, “diphthongal” roots resemble more the general picture of CVCV roots than that of CVCCV roots. In the latter type, medial obstruents are quite common, with *nt, *ŋk, *lk being some the most frequent clusters overall.
- The single other case where a spirant might have occurred as the 2nd part of a consonant cluster is the “spleen” root, which still is anomalous at best. This potentially works as evidence in favor of a voiceless fricative reconstruction; clusters such as *ks *pś are well-supported. But this could just as well be compared to some other roots which point to unusual clusters such as *kupla “bubble”, *śüklä “wart”.
- The cluster *d₂w is also recurring: *käd₂wä “female animal, esp. weasel”  is well-attested, and in addition *kod₂wa “time” (Samic, Finnic, Mari) and *pad₂wə “tinder” (Finnic, Samoyedic) can be set up. As with *wC clusters, the only other examples of *Cw involve an adjacent sonorant: *jw, *lw, *rw. And again, comparision with *j suggests that this was a general phonotactic limitation for glides: examples are known of *lj, *rj, *wj, *jj.
- Two cases of *d₂k seem reliable: *tud₂ka “tip”, *wad₂kə “river bend”. Also, *d₁k could have occurred in *śod₁ka “duck”, but to reiterate a previous suggestion of mine, *-ka here could also be a suffix added in West Uralic. — At any rate, this doesn’t tell much, since pretty much any PU consonant other than the glides *w, *j could precede *k. However, phonetically, it seems somewhat implausible for these clusters to have actually been articulated as a voiced spirant + voiceless stop.
- A spirant + nasal cluster may occur in *śedₓmä “kidney”, only reflected in Samic and Khanty. Examples are known of *l/*r + nasal; sibilant + nasal or nasal + nasal did not occur.
- Various other theoretically possible cluster types are absent, but this doesn’t help in deciding between the main hypotheses on the table: no PU clusters are known that would combine the liquids, sibilants, and spirants in any way with each other (**ll, **sl, **śś, **rd…)
Point three: morphology. Here the evidence is even thinner, particularly if compared to the rich declensional and derivational systems of Proto-Indo-European. (My impression is that Proto-Uralic seems to have been a rather more analytical language than any of its descendants.) Regardless: the spirants play no part here, being entirely restricted to word roots. This could indicate simply their overall rarity, but perhaps also them having been, at some pre-Uralic date, derived from more basic consonants or consonant clusters in some way. No argument for the manner of the “spirant”s’ articulation can be made here, but this is one further point to suggest they were rather marked consonants.
In summary, the phonotactic structure of Proto-Uralic turns out to be quite compatible with sonorant values for the “spirants”. By contrast these do not seem to show much commonality with the sibilants *s, *ć, *ś, *č, *š at all. I take this as evidence against Honti’s lateral fricatives *ɬ, *ɬʲ (or Bomhard’s affricates). It also strikes me as a point against the traditional reconstruction with dental fricatives — perhaps dental approximants [ð̞], [ð̞ʲ] would work better? Kortlandt’s idea holds up perhaps surprizingly well: the root-medial-focused general distribution and the variety of reconstructed clusters would both make perfect phonetical and phonological sense if the “spirants” were some flavor of non-standard sonorant consonants. All the other proposals have certain difficulties.
Do notice however that I have touched hardly any actual data in this post — this has been more of a synchronic analysis of Proto-Uralic. Actual comparision between the Uralic languages will be a different task yet, one which will highlight different strengths and weaknesses between the different reconstruction proposals. To be continued, then!
 The latest formulation of this, I think, being Abondolo, Daniel (1996): Traces of pre-proto-Uralic nasals and nasal prosodies. In: Finnisch-Ugrische Mitteilungen 18/19, pp. 9-18.
 Bomhard, Allan (2008-2013): Reconstructing Proto-Nostratic. Actually no more than two examples of these Uralic consonants appear among the hundreds of Nostratic etymological comparisions proposed. The first is *d₂ümä “glue” ~ Semitic *ṣ́-m-d “to join” (where *ṣ́ = [tɬʼ]) ~ PIE *gem- “to join”, the second *d₂okka- “to push” (a fairly questionable root) ~ Arabic ḍakka- “to press” (in Arabic *ṣ́ → ḍ) ~ Dravidian *dūkk- “to push”. I’d say that his reconstruction essentially does not find support from the external relationships proposed, and it should therefore be evaluated on its own terms.
 Tangentially mentioned in Janhunen, Juha (2007): The primary laryngeal in Uralic and beyond. In: Mémoires de la Société Finno-Ougrienne , pp. 203-227.
 The exact identity of *x is perhaps even less obvious than that of *d₁ and *d₂, on level with the notorious Indo-European laryngeals, but Janhunen’s paper ↑ takes some good steps towards understanding this.
 E.g. *kuma “turned”, *kämä “hard”, *lumə “snow”, *lämə “broth”, *enä “big”, *puna “hair”, *menə- “to go”, *panə- “to put”, *sënə “vein”, *kuńa “to blink”, *mińä “daughter-in-law”, *ëńə “tame”, *aŋa- “to open”, *jäŋə “ice”, *piŋə “tooth”, *suŋə “summer”… This number of course gets smaller if we insist that only roots with Samoyedic reflexes can be considered Proto-Uralic, which I think mainly shows how artificially limiting the data taken into consideration can distort our conclusions. — An internal reconstruction with *d₁ *d₂ related to something like *t *ć would perhaps fare better, and would go well in line with the suggestion from Janhunen that *x was originally the lenis counterpart or allophone of *k. But; cart, horse, before, etc.
 E.g. *sükśə → *śikśə → *ćëkćë “autumn”; *sVnśa- → *śanśa- → *ćōnćē- “to stand”. Unpalatalized initials are indicated here by e.g. Hungarian ősz, Mansi *toonć-.
 It may or may not be a coincidence that both of these have a medial consonant *-m-. Abondolo thinks it isn’t, but I find this data too flimsy to base conclusions on. Particularly when there are also roots such as *nimə “name” or *nüd₁ə “handle, shaft” about, for which a pre-Uralic nasal dissimilation would seem to predict **d₁imə and **d₁ünə.
 This is reconstructed on the basis of Finnic *lewtä- and Hungarian lel. However, these could also be independant derivatives *lew-tä-, *lew-lə-, from *lewə- “to hit”. I tend to be skeptical of PU roots supposedly only found in Finnic and Hungarian, which are surprizingly frequent given that Hungarian is the second-most lexically innovative branch of Uralic (after Samoyedic). Many examples have been later shown to be IE loans in either or both. And here too IE loan origin has also been suggested for the Finnic word. — It’s amusing how both of the more secure examples of *wd₁ have the vowel framework *ä-ə, but almost certainly this is coincidental.
 A root with surprizingly specific semantics, but words such as Inari Sami käđfi “female weasel”, Northern Khanty /kej/ “female fur animal”, and Mator /kejbe/ “mare” are in perfect correspondence to each other. Hungarian hölgy “dame”; (archaic) “ermine” seems to confirm the specific semantics in Samic, though its general shape is rather puzzling. PU *käd₂wä would at first glance seem to predict ˣkégy — but, the same unexpected ö and l also pop up in *wad₂kə → völgy “valley”, and *tud₂ka → archaic tölgy “breast, udder”, so there seems to have been some kind of a special umlaut development of vowels preceding older *ĺɣ. In the cluster itself, palatality appears to have been “metathesized” from the 1st member to the 2nd: *ĺɣ → *lj → lgy.